Core Case StudyWhy Are Amphibians Vanishing? Learning Objective LO 4.1List three reasons

Core Case StudyWhy Are Amphibians Vanishing?
Learning Objective
LO 4.1List three reasons why we need to care about the growing rate of amphibian extinctions.
Amphibians are a class of animals that includes frogs (chapter opening photo, toads, and salamanders. Amphibians were among the first vertebrates (animals with backbones) to leave the earth’s waters and live on land. They have adjusted to and survived environmental changes more effectively than many other species, but their environment is changing rapidly.
An amphibian lives part of its life in water and part on land. Human activities such as the use of pesticides and other chemicals can pollute the land and water habitats of amphibians. Many of the more than 8,000 known amphibian species (90% of them frogs) have problems adapting to these changes.
Since 1980, populations of hundreds of amphibian species have declined or vanished (Figure 4.1). According to the International Union for Conservation of Nature (IUCN), about 33% of known amphibian species face extinction. A 2015 study by biodiversity expert Peter Crane found that 200 frog species have gone extinct since the 1970s, and frogs are going extinct 10,000 times faster than their historical rates.
Figure 4.1
Specimens of some of the nearly 200 amphibian species that have gone extinct since the 1970s.
Joel Sartore/National Geographic Image Collection
No single cause can account for the decline of many amphibian species, but scientists have identified a number of factors that affect amphibians at various points in their life cycles. For example, frog eggs lack shells to protect the embryos they contain from water pollutants and adult frogs ingest the insecticides contained in many of the insects they eat. We explore these and other factors later in this chapter.
Why should we care if some amphibian species become extinct? Scientists give three reasons. First, amphibians are sensitive biological indicators of changes in environmental conditions. These changes include habitat loss, air and water pollution, ultraviolet (UV) radiation from the sun, and a warming climate. The growing threats to the survival of an increasing number of amphibian species indicate that environmental conditions for amphibians and many other species are deteriorating in many parts of the world.
Second, adult amphibians play important roles in biological communities. They eat more insects (including mosquitoes) than do many species of birds. In some habitats, the extinction of certain amphibian species could lead to population declines or extinction of animals that eat amphibians or their larvae, such as reptiles, birds, fish, mammals, and other amphibians.
Third, amphibians play a role in human health. A number of pharmaceutical products come from compounds found in secretions from the skin of certain amphibians. Many of these compounds have been isolated and used as painkillers and antibiotics and in treatments for burns and heart disease. If amphibians vanish, these potential medical benefits and others that scientists have not yet discovered would vanish with them.
The threat to amphibians is part of a greater threat to the earth’s biodiversity. In this chapter, we discuss biodiversity, how it arose on the earth, why it is important, and how it is threatened. We will also consider possible solutions to these threats.
4.1aEarth’s Organisms Are Many and Varied
Every organism is composed of one or more cells. Based on their cell structure, organisms can be classified as eukaryotic or prokaryotic. All organisms except bacteria are eukaryotic. Their cells are encased in a membrane and have a distinct nucleus (a membrane-bounded structure containing genetic material in the form of DNA) and several other internal parts enclosed by membranes (Figure 4.2, right). Bacterial cells are prokaryotic, enclosed by a membrane but containing no distinct nucleus or other internal parts enclosed by membranes (Figure 4.2, left).
Figure 4.2
Comparison of key components of a eukaryotic cell (left) and prokaryotic cell (right).
Scientists group organisms into categories based on their varying characteristics, a process called taxonomic classification. The largest category is the kingdom, which includes all organisms that have one or several common features. Biologists recognize six kingdoms. Two are different types of bacteria (eubacteria and archaebacteria) with single cells that are prokaryotic (Figure 4.2, left). The other four kingdoms are protists, plants, fungi, and animals (Figure 4.2 right).
Protists are mostly many-celled eukaryotic organisms such as golden brown and yellow-green algae, and protozoans. Most fungi are many-celled organisms such as mushroom, molds, mildews, and yeasts.
Plants include certain types of algae (including red, brown, and green algae), mosses, ferns, trees, and flowering plants whose flowers produce seeds. Flowering plant species make up about 90% of the plant kingdom. Some flowering plants such as corn and marigolds are annuals that live for one growing season, die, and have to be replanted. Others are perennials, such as roses and grapes, which can live for two or more seasons before they die and have to be replanted.
Animals are many-celled eukaryotic organisms. Most are invertebrates, with no backbones. They include jellyfish, worms, insects, shrimp, snails, clams, and octopuses. Other animals, called vertebrates, have backbones. Examples include amphibians (Core Case Study), fishes, reptiles (alligators and snakes), birds (robins and eagles), and mammals (humans, whales, elephants, bats, and tigers).
Kingdoms are divided into phyla, which are divided into subgroups called classes. Classes are subdivided into orders, which are further divided into families. Families consist of genera (singular, genus), and each genus contains one or more species. Figure 4.3 shows the detailed taxonomic classification for the current human species: Homo sapiens sapiens.
Figure 4.3
How the current human species got its name: Homo sapiens sapiens.
A species is a group of living organisms with characteristics that distinguish it from other groups of organisms. In sexually reproducing organisms, individuals must be able to mate with similar individuals and produce fertile offspring in order to be classified as a species.
Estimates of the number of species range from 7 million to 100 million, with a best guess of 7 million to 10 million species. Biologists have identified about 2 million species.
2 Million
Number of species that scientists have identified out of the world’s estimated 7 million to 100 million species
Well over half of the world’s identified species are insects that play important ecological roles in sustaining the earth’s life. For example, pollination is a vital ecosystem service that allows flowering plants to reproduce. When pollen grains are transferred from the flower of one plant to a receptive part of the flower of another plant of the same species, reproduction occurs. Many flowering species depend on bees and other insects to pollinate their flowers (Figure 4.4, left). In addition, insects that eat other insects—such as the praying mantis (Figure 4.4, right)—help to control the populations of at least half the species of insects that we call pests. This free pest control service is another vital ecosystem service. In addition, insects make up an increasing part of the human food supply in some parts of the world.
Figure 4.4
Importance of insects: Bees (left) and numerous other insects pollinate flowering plants that serve as food for many plant eaters, including humans. This praying mantis, which is eating a moth (right), and many other insect species help to control the populations of most of the insect species we classify as pests.
Klagyivik Viktor/; Dr. Morley Read/
Some insect species reproduce at an astounding rate and can rapidly develop new genetic traits such as resistance to pesticides. They also have an exceptional ability to evolve into new species when faced with changing environmental conditions.
Research indicates that some human activities are threatening insect populations such as honeybees. We discuss this environmental problem more fully in Chapter 9.
Learning from Nature
Even the lowly mosquito provides benefits to humans by serving as a model for a new type of hypodermic needle, based on the mosquito’s use of a multi-part mouth to work its way through a layer of skin without creating pain. (The pain of a mosquito bite comes from a chemical injected into the skin after it has been penetrated.)
Biodiversity, or biological diversity, is the variety of life on the earth. It has four components, as shown in Figure 4.5.
Figure 4.5
Natural capital: The major components of the earth’s biodiversity—one of the planet’s most important renewable resources and a key component of its natural capital (Figure 1.3).
Right side, top left: Laborant/; right side, top right: leungchopan/; right side, top center: Elenamiv/; bottom right: Juriah Mosin/
One is species diversity, the number and abundance of the different kinds of species living in an ecosystem. Species diversity has two components, one being species richness, the number of different species in an ecosystem. The other is species evenness, a measure of the comparative abundance of all species in an ecosystem.
A species-rich ecosystem has a large number of different species. However, this tells us nothing about how many members of each species are present. If it has many of one or more species and just a few of others, its species evenness is low. If it has roughly equal numbers of each species, its species evenness is high. For example, if an ecosystem has only three species, its species richness is low. However, if there are roughly equal numbers of each of the three species, the species evenness is high. Species-rich ecosystems such as rain forests tend to have high species evenness. Ecosystems with low species richness, such as tree farms, tend to have low species evenness.
Species diversity can enhance the stability of ecosystems. For example, a forest with many different tree species is more stable than a forest with just one tree species, which is the case with a tree farm.
The species diversity of ecosystems varies with their geographical location. For most terrestrial plants and animals, species diversity (primarily species richness) is highest in the tropics and declines as we move from the equator toward the poles. The most species-rich environments are tropical rain forests, large tropical lakes, coral reefs, and the ocean-bottom zone.
The second component of biodiversity is genetic diversity, which is the variety of genes found in a population or in a species (Figure 4.6). Genes contain genetic information that give rise to specific traits, or characteristics, that are passed on to offspring through reproduction. Species whose populations have greater genetic diversity have a better chance of surviving and adapting to environmental changes.
Figure 4.6
Genetic diversity in this population of a Caribbean snail species is reflected in the variations of shell color and banding patterns. Genetic diversity can also include other variations such as slight differences in chemical makeup, sensitivity to various chemicals, and behavior.
The third component of biodiversity, ecosystem diversity, refers to the earth’s diversity of biological communities such as deserts, grasslands, forests, mountains, oceans, lakes, rivers, and wetlands. Biologists classify terrestrial (land) ecosystems into biomes—large regions such as forests, deserts, and grasslands characterized by distinct climates and certain prominent species (especially vegetation). Biomes differ in their community structure based on the types, relative sizes, and stratification of their plant species (Figure 4.7). Figure 4.8 shows the major biomes found across the midsection of the United States. We discuss biomes in detail in Chapter 7.
Figure 4.7
Community structure: Generalized types, relative sizes, and stratification of plant species in communities or ecosystems in major terrestrial biomes.
Figure 4.8
The variety of biomes found across the midsection of the United States.
First: Zack Frank/; second: Robert Crum/; third: Joe Belanger/; fourth: Protasov AN/; fifth: Maya Kruchankova/; sixth: Marc von Hacht/
Large areas of forest and other biomes tend to have a core habitat and edge habitats with different environmental conditions and species, called edge effects. For example, a forest edge is usually more open, bright, and windy and has greater variations in temperature and humidity than a forest interior. Humans have fragmented many forests, grasslands, and other biomes into isolated patches with less core habitat and more edge habitat that supports fewer species.
Natural ecosystems within biomes rarely have distinct boundaries. Instead, one ecosystem tends to merge with the next in a transitional zone called an ecotone. It is a region containing a mixture of species from adjacent ecosystems along with some migrant species not found in either of the bordering ecosystems.
The fourth component of biodiversity is functional diversity—the variety of processes such as energy flow and matter cycling that occur within ecosystems (Figure 3.9) as species interact with one another in food chains and food webs. This component of biodiversity includes the variety of ecological roles organisms play in their biological communities and the impacts these roles have on their overall ecosystems.
A more biologically diverse ecosystem with a greater variety of producers can produce more plant biomass, which in turn can support a greater variety of consumer species. Biologically diverse ecosystems also tend to be more stable because they are more likely to include species with traits that enable them to adapt to changes in the environment, such as disease or drought.
We should care about and avoid degrading the earth’s biodiversity because it is vital to maintaining the natural capital (Figure 1.3) that keeps us alive and supports our economies. We use biodiversity as a source of food, medicine, building materials, and fuel. Biodiversity also provides natural ecosystem services such as air and water purification, renewal of topsoil, decomposition of wastes, and pollination. In addition, the earth’s variety of genetic information, species, and ecosystems provide raw materials for the evolution of new species and ecosystem services, as they respond to changing environmental conditions. Biodiversity is an ecological life insurance policy. When we celebrate, protect, and enhance the earth’s biodiversity, we are helping to preserve our own species and economic systems, which depend on the natural capital that biodiversity provides. We owe much of what we know about biodiversity to researchers such Edward O. Wilson (Individuals Matter 4.1).
Individuals Matter 4.1
Edward O. Wilson: A Champion of Biodiversity
Jim Harrison
As a boy growing up in the southeastern United States, Edward O. Wilson became interested in insects at age 9. He has said, “Every kid has a bug period. I never grew out of mine.”
Before entering college, Wilson had decided he would specialize in the study of ants. He became one of the world’s experts on ants and then widened his focus to include the entire biosphere. One of Wilson’s landmark works is The Diversity of Life, published in 1992. In that book, he presented the principles and practical issues of biodiversity more completely than anyone had to that point. Today, he is recognized as one of the world’s leading experts on biodiversity—often referred to as “the father of biodiversity.”
Wilson continues actively writing and lecturing about the importance of species and the need for global biodiversity discovery and inventory in order to better understand our planet and identify conservation priorities. In 2016, he published Half-Earth: Our Planet’s Fight for Life, a call to conserve half the Earth’s lands and seas in order to ensure species have the space they need to thrive in perpetuity. The E. O. Wilson Biodiversity Foundation is now bringing this vision to life through the Half-Earth Project.
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4.3aEach Species Plays a Role
Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’ ecological niche. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its habitat, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.
Ecologists use the niches of species to classify them as generalists or specialists. A generalist species such as a raccoon has a broad niche (Figure 4.9, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.
Figure 4.9
Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).
In contrast, a specialist species, such as the giant panda, occupies a narrow niche (Figure 4.9, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (Figure 4.10).
Figure 4.10
Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.
Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (Figure 4.9, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.
Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.
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4.3aEach Species Plays a Role
Each species plays a role within the ecosystem it inhabits. Ecologists describe this role as a species’ ecological niche. It is a species’ way of life in its ecosystem and includes everything that affects its survival and reproduction, such as how much water and sunlight it needs, how much space it requires, what it feeds on, what feeds on it, how and when it reproduces, and the temperatures and other conditions it can tolerate. A species’ niche differs from its habitat, which is the place, or type of ecosystem, in which a species lives and obtains what it needs to survive.
Ecologists use the niches of species to classify them as generalists or specialists. A generalist species such as a raccoon has a broad niche (Figure 4.9, right curve). Generalist species can live in many different places, eat a variety of foods, and often tolerate a wide range of environmental conditions. Other generalist species are flies, cockroaches, rats, coyotes, and humans.
Figure 4.9
Specialist species such as the giant panda have a narrow niche (left curve) and generalist species such as the raccoon have a broad niche (right curve).
In contrast, a specialist species, such as the giant panda, occupies a narrow niche (Figure 4.9, left curve). Such species may be able to live in only one type of habitat, eat only one or a few types of food, or tolerate a narrow range of environmental conditions. For example, different specialist species of some shorebirds feed on certain crustaceans, insects, or other organisms found on sandy beaches and their adjoining coastal wetlands (Figure 4.10).
Figure 4.10
Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows for sharing of limited resources.
Because of their narrow niches, specialists are more likely to become endangered or extinct when environmental conditions change. For example, China’s giant panda (Figure 4.9, left) is vulnerable to extinction because of a combination of habitat loss, low birth rate, and its specialized diet consisting mainly of bamboo.
Is it better to be a generalist or a specialist? It depends. When environmental conditions undergo little change, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. Under rapidly changing environmental conditions, the more adaptable generalist species usually is better off.
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4.3cIndicator Species
Species that provide early warnings of changes in environmental conditions in an ecosystem are called indicator species. They are like biological smoke alarms. In this chapter’s Core Case Study, you learned that some amphibians are classified as indicator species. Scientists have been working hard to identify some of the possible causes of the declines in amphibian populations (Science Focus 4.1).
Science Focus 4.1
Causes of Amphibian Declines
Scientists who study amphibians have identified natural and human-related factors that can cause the decline and disappearance of these indicator species.
One natural threat is parasites such as flatworms that feed on certain amphibian eggs. Research indicates that this has caused birth defects such as missing limbs or extra limbs in some amphibians.
Another natural threat comes from viral and fungal diseases. An example is the chytrid fungus that infects a frog’s skin and causes it to thicken. This reduces the frog’s ability to take in water through its skin and leads to death from dehydration. An even deadlier fungal disease is Batrachochytrium dendrobatidis (or Bd), which invades skin cells and multiplies, causing the frog’s skin to peel away. Such diseases can spread easily, because adults of many amphibian species congregate in large numbers to breed.
Habitat loss and fragmentation is another major threat to amphibians. It is mostly a human-caused problem resulting from the clearing of forests and the draining and filling of freshwater wetlands for farming and urban development.
Another human-related problem is higher levels of UV radiation from the sun. Ozone  that forms in the stratosphere protects the earth’s life from harmful UV radiation emitted by the sun. During the past few decades, ozone-depleting chemicals released into the troposphere by human activities have drifted into the stratosphere and have destroyed some of the stratosphere’s protective ozone. The resulting increase in UV radiation can kill embryos of amphibians in shallow ponds as well as adult amphibians basking in the sun for warmth. International action has been taken to reduce the threat of stratospheric ozone depletion, but it will take about 50 years for ozone levels to recover to levels that existed before this threat arose.
Pollution from human activities also threatens amphibians. Frogs and other species are exposed to pesticides in ponds and in the bodies of insects that they eat. This can make them more vulnerable to bacterial, viral, and fungal diseases and to some parasites. Amphibian expert and National Geographic Explorer Tyrone Hayes, a professor of biology at University of California-Berkeley, conducts research on how some pesticides can harm frogs and other animals by disrupting their endocrine systems.
Climate change is also a concern. Amphibians are sensitive to even slight changes in temperature and moisture. Warmer temperatures may lead amphibians to breed too early. Extended dry periods also lead to a decline in amphibian populations by drying up breeding pools that frogs and other amphibians depend on for reproduction and survival through their early stages of life (Figure 4.A).
Figure 4.A
This golden toad lived in Costa Rica’s high-altitude Monteverde Cloud Forest Reserve. The species became extinct in 1989, apparently because its habitat dried up.
Charles H. Smith/U.S. Fish and Wildlife Service
Overhunting is another human-related threat, especially in areas of Asia and Europe, where frogs are hunted for their leg meat. Another threat is the invasion of amphibian habitats by nonnative predators and competitors, such as certain fish species. Some of this immigration into habitats is natural, but humans accidentally or deliberately transport many species to amphibian habitats.
According to most amphibian experts, a combination of these factors, which vary from place to place, is responsible for most of the decline and extinctions of amphibian species. This amounts to a biological “fire alarm.”
Critical Thinking
Of the factors listed above, which three do you think could be most effectively controlled by human efforts?
Birds are excellent biological indicators. They are found almost everywhere and are affected quickly by environmental changes such as the loss or fragmentation of their habitats and the introduction of chemical pesticides.
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4.3dKeystone Species
A keystone is the wedge-shaped stone placed at the top of a stone archway. Remove this stone and the arch collapses. In some communities and ecosystems, ecologists hypothesize that certain species play a similar role. A keystone species has such a large effect on the types and abundance of other species in an ecosystem that without it, the ecosystem would be dramatically different or might cease to exist.
Keystone species play several critical roles in helping to sustain ecosystems. One is the pollination of flowering plant species by butterflies, honeybees (Figure 4.4, left), hummingbirds, bats, and other species. In addition, top predator keystone species feed on and help to regulate the populations of other species. Examples are wolves, leopards, lions, some shark species, and the American alligator (see the following Case Study).
Case Study
The American Alligator—A Keystone Species That Almost Went Extinct
The American alligator (Figure 4.12) is a keystone species in wetland ecosystems where it is found in the southeastern United States. These alligators play several important ecological roles. They dig deep depressions, or gator holes. These depressions hold freshwater during dry spells and serve as refuges for aquatic life. They supply freshwater and food for fishes, insects, snakes, turtles, birds, and other animals.
Figure 4.12
Keystone species: The American alligator plays an important ecological role in its marsh and swamp habitats in the southeastern United States by helping support many other species.
Arto Hakola/
The large nesting mounds that alligators build provide nesting and feeding sites for some herons and egrets, and red-bellied turtles lay their eggs in old gator nests. In addition, by eating large numbers of gar, a predatory fish, alligators help maintain populations of game fish that gar eat, such as bass and bream. When alligators excavate holes and build nesting mounds, they help keep vegetation from invading shorelines and open-water areas. Without this ecosystem service, freshwater ponds and coastal wetlands where alligators live would fill in with shrubs and trees, and dozens of species could disappear from these ecosystems.
In the 1930s, hunters began killing American alligators for their exotic meat and their soft belly skin, used to make expensive shoes, belts, and pocketbooks. Other people hunted alligators for sport or out of dislike for the large reptile. By the 1960s, hunters and poachers had wiped out 90% of the alligators in the state of Louisiana, and the Florida Everglades population waswas near extinction.
In 1967, the U.S. government placed the American alligator on the endangered species list. By 1987, because it was protected, its populations had made a strong comeback and the alligator was removed from the endangered species list. Today, there are well over a million alligators in Florida. The state now allows property owners to kill alligators that stray onto their land.
To conservation biologists, the comeback of the American alligator is an important success story in wildlife conservation. Recently, however, large and rapidly reproducing Burmese and African pythons released deliberately or accidently by humans have invaded the Florida Everglades. These nonnative invaders feed on young alligators, and could threaten the long-term survival of this keystone species in the Everglades.
Critical Thinking
The American Alligator and Biodiversity
What are two ways in which the American alligator supports one or more of the four components of biodiversity (Figure 4.5) within its environment?
The loss of a keystone species in an ecosystem can lead to population declines and, in some cases, to extinctions of other species that depend on them for certain ecosystem services. This is why it important for scientists to identify keystone species and work to protect them.
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4.4aEvolution Explains How Life Changes Over Time
How did the earth end up with such an amazing diversity of species? The scientific answer is biological evolution or simply evolution—the process by which the genes of populations of species change genetically over time. According to this scientific theory, species have evolved from earlier, ancestral species through natural selection—the process in which individuals with certain genetic traits are more likely to survive and reproduce under a specific set of environmental conditions. These individuals then pass these traits on to their offspring.
A huge body of scientific evidence supports this idea. As a result, biological evolution through natural selection is the most widely accepted scientific theory that explains how the earth’s life has changed over the past 3.8 billion years and why we have today’s diversity of species.
Most of what we know about the history of life on the earth comes from fossils—the remains or traces of past organisms. Fossils include mineralized or petrified replicas of skeletons, bones, teeth, shells, leaves, and seeds, or impressions of such items found in rocks (Figure 4.13). Scientists have discovered fossil evidence in successive layers of sedimentary rock such as limestone and sandstone. They have also studied evidence of ancient life contained in ice core samples drilled from glacial ice at the earth’s poles and on mountaintops.
Figure 4.13
This fossil shows the mineralized remains of an early ancestor of the present-day horse. It roamed the earth more than 35 million years ago. Notice that you can also see fish skeletons on this fossil.
Ira Block/National Geographic Image Collection
This body of evidence is called the fossil record. It is uneven and incomplete because many past forms of life left no fossils and some fossils have decomposed. Scientists estimate that the fossils found so far represent probably only 1% of all species that have ever lived. There are still many unanswered scientific questions about the details of evolution by natural selection, and research continues in this area.
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4.4bEvolution Depends on Genetic Variability and Natural Selection
The idea that organisms change over time and are descended from a single common ancestor has been discussed since the early Greek philosophers. There was no convincing explanation of how this could happen until 1858 when naturalists Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913) independently proposed the concept of natural selection as a mechanism for biological evolution. Darwin gathered evidence for this idea and published it in his 1859 book, On the Origin of Species by Means of Natural Selection.
Biological evolution by natural selection involves changes in a population’s genetic makeup through successive generations. Populations—not individuals—evolve by becoming genetically different.
The first step in this process is the development of genetic variability: a variety in the genetic makeup of individuals in a population. This occurs primarily through mutations, or changes in the coded genetic instructions in the DNA in a gene. During an organism’s lifetime, the DNA in its cells is copied each time one of its cells divides and whenever it reproduces. In a lifetime, this happens millions of times and results in various mutations.
Most mutations result from random changes in the DNA’s coded genetic instructions that occur in only a tiny fraction of these millions of divisions. Some mutations also occur from exposure to external agents such as radioactivity, ultraviolet (UV) radiation from the sun, and certain natural and human-made chemicals called mutagens.
Mutations can occur in any cell, but only those that take place in the genes of reproductive cells are passed on to offspring. Sometimes a mutation can result in a new genetic trait, called a heritable trait, which can be passed from one generation to the next. In this way, populations develop genetic differences among their individuals. Biologists refer to the genes of a population as a gene pool.
The next step in biological evolution is natural selection, which explains how populations evolve in response to changes in environmental conditions by changing their genetic makeup. Through natural selection, environmental conditions favor increased survival and reproduction of certain individuals in a population. These favored individuals possess heritable traits that give them an advantage over other individuals in the population. Such a trait is called an adaptation, or adaptive trait. An adaptive trait improves the ability of an individual organism to survive and to reproduce at a higher rate than other individuals in a population can under prevailing environmental conditions.
An example of natural selection at work is genetic resistance. It occurs when one or more organisms in a population have genes that can tolerate a chemical (such as a pesticide or antibiotic) that normally would be fatal. The resistant individuals survive and reproduce more rapidly than the members of the population that do not have such genetic traits. Genetic resistance can develop quickly in populations of organisms such as bacteria and insects that produce large numbers of offspring. For example, some disease-causing bacteria have developed genetic resistance to widely used antibacterial drugs, or antibiotics (Figure 4.14).
Figure 4.14
Evolution by natural selection: (a) A population of bacteria is exposed to an antibiotic, which (b) kills all individuals except those possessing a trait that makes them resistant to the drug; (c) the resistant bacteria multiply and eventually (d) replace all or most of the nonresistant bacteria.
Through natural selection, humans have evolved traits that have enabled them to survive in many different environments and to reproduce successfully. If we think of the earth’s 4.6 billion years of geological and biological history as one 24-hour day, the human species arrived about a tenth of one second before midnight. In that short time, we have dominated most of the earth’s land (Figure 1.9) and aquatic systems. Evolutionary biologists attribute our ability to dominate the earth to three major adaptations (Figure 4.15):
Strong opposable thumbs that allowed humans to grip and use tools better than the few other animals that have thumbs
The ability to walk upright, which gave humans agility and freed up their hands for many uses
A complex brain, which allowed humans to develop many skills, including the ability to communicate complex ideas.
Figure 4.15
Homo sapiens sapiens: Three advantages over other mammals have helped us to become the earth’s dominant species within an eye blink of time in the 3.8-billion-year history of life on the earth.
To summarize the process of biological evolution by natural selection: Genes mutate, individuals are selected, and populations that are better adapted to survive and reproduce under existing environmental conditions evolve.
Evolutionary biologists study patterns of evolution by examining the similarities and differences among species based on their physical and genetic characteristics. They use this information to develop branching evolutionary tree, or phylogenetic tree, diagrams that depict the hypothetical evolution of various species from common ancestors (Figure 4.16). They use fossil, DNA, and other evidence to hypothesize the evolutionary pathways and connections among species.
Figure 4.16
Simplified phylogenetic tree (or tree of life) diagram showing the hypothesized evolution of life on the earth into six major kingdoms of species over 3.8 billion years.
On an evolutionary timescale, as new species arise, they have new genetic traits that can enhance their survival as long as environmental conditions do not change dramatically. The older species from which they originated and the new species evolve and branch out along different lines or lineages of species that can be recorded in phylogenetic tree diagrams (Figure 4.16).
4.4cLimits to Adaptation through Natural Selection
In the not-too-distant future, will adaptations to new environmental conditions through natural selection protect us from harm? For example, will adaptations allow the skin of our descendants to become more resistant to the harmful effects of the sun’s UV radiation, enable their lungs to cope with air pollutants, and improve the ability of our livers to detoxify pollutants in our bodies?
Scientists in this field say not likely because of two limitations on adaptation through natural selection. First, a change in environmental conditions leads to adaptation only for genetic traits already present in a population’s gene pool, or if they arise from random mutations.
Second, even if a beneficial heritable trait is present in a population, the population’s ability to adapt may be limited by its reproductive capacity. Populations of genetically diverse species that reproduce quickly often can adapt to a change in environmental conditions in a short time (days to years). Examples are dandelions, mosquitoes, rats, bacteria, and cockroaches. By contrast, species that cannot produce large numbers of offspring rapidly—such as elephants, tigers, sharks, and humans—take thousands or even millions of years to adapt through natural selection.
4.4dMyths about Evolution through Natural Selection
There are a number of misconceptions about biological evolution through natural selection. Here are five common myths:
Survival of the fittest means survival of the strongest. To biologists, fitness is a measure of reproductive success, not strength. Thus, the fittest individuals are those that leave the most descendants, not those that are physically the strongest.
Evolution explains the origin of life. It does not. However, it does explain how species have evolved after life came into being around 3.8 billion years ago.
Humans evolved from apes or monkeys. Fossil and other evidence shows that humans, apes, and monkeys evolved along different paths from a common ancestor that lived 5 million to 8 million years ago.
Evolution by natural selection is part of a grand plan in nature in which species are to become more perfectly adapted. There is no evidence of such a plan. Instead, evidence indicates that the forces of natural selection and random mutations can push evolution along any number of paths.
Evolution by natural selection is not important because it is just a theory. This reveals a misunderstanding of the concept of a scientific theory, which is based on extensive evidence and is accepted widely by the scientific experts in a particular field of study. Numerous polls show that evolution by natural selection is widely accepted by over 95% of biologists because it best explains the earth’s biodiversity and how populations of different species have adapted to changes in the earth’s environmental conditions over billions of years.
4.5aHow Do New Species Arise?
Under certain circumstances, natural selection can lead to an entirely new species. Through this process, called speciation, one species evolves into two or more different species. For sexually reproducing organisms, a new species forms when a separated population of a species evolves to the point where its members can no longer interbreed and produce fertile offspring with members of another population of its species that did not change or that evolved differently.
Speciation, especially among sexually reproducing species, happens in two phases: first geographic isolation, and then reproductive isolation. Geographic isolation occurs when different groups of the same population of a species become physically isolated from one another for a long time. Part of a population may migrate in search of food and then begin living as a separate population in an area with different environmental conditions. Winds and flowing water may carry a few individuals far away where they establish a new population. A flooding stream, a new road, a hurricane, an earthquake, or a volcanic eruption, as well as long-term geological processes (Science Focus 4.2), can also separate populations. Human activities, such as the creation of large reservoirs behind dams and the clearing of forests, can also create physical barriers for certain species. The separated populations can develop different genetic characteristics because they are no longer exchanging genes.
Science Focus 4.2
Geological Processes Affect Biodiversity
The earth’s surface has changed dramatically over its long history. Scientists discovered that huge flows of molten rock within the earth’s interior have broken its surface into a number of gigantic solid plates, called tectonic plates. For hundreds of millions of years, these plates have drifted slowly on the planet’s mantle formed today’s continents (Figure 4.B).
Figure 4.B
Over millions of years, the earth’s landmasses have moved very slowly on several gigantic tectonic plates to different locations and formed today’s continents (right).
Critical Thinking:
How might an area of land splitting apart cause the extinction of a species?
Rock and fossil evidence indicates that 200–250 million years ago, all of the earth’s present-day continents were connected in a supercontinent called Pangaea (Figure 4.B, left). About 175 million years ago, Pangaea began splitting apart as the earth’s tectonic plates moved. Eventually tectonic movement resulted in the present-day locations of the continents (Figure 4.B, right).
The movement of tectonic plates has had two important effects on the evolution and distribution of life on the earth. First, the locations of continents and oceanic basins have greatly influenced the earth’s climate, which plays a key role in where plants and animals can live. Second, the breakup, movement, and joining of continents have allowed species to move and adapt to new environments. This led to the formation of a large number of new species through speciation.
Along boundaries where they meet, tectonic plates may pull away from, collide with, or slide alongside each other. Tremendous forces produced by these interactions along plate boundaries can lead to earthquakes and volcanic eruptions. These geological activities can also affect biological evolution by causing fissures in the earth’s crust, which can isolate populations of species on either side of the fissure. Over long periods, this can lead to the formation of new species as each isolated population changes genetically in response to new environmental conditions.
Volcanic eruptions that occur along the boundaries of tectonic plates can also affect extinction and speciation by destroying habitats and reducing, isolating, or wiping out populations of species. These geological processes are further discussed in Chapter 14.
Critical Thinking
The earth’s tectonic plates, including the one you are riding on, typically move at about the rate at which your fingernails grow. If they stopped moving, how might this affect the future biodiversity of the planet?
In reproductive isolation, mutation and change by natural selection operate independently in the gene pools of geographically isolated populations. If this process continues long enough, members of isolated populations of sexually reproducing species can become different in genetic makeup. Then they cannot produce live, fertile offspring if they rejoin their original populations and attempt to interbreed. When that is the case, speciation has occurred and one species has become two (Figure 4.17).
Figure 4.17
Geographic isolation can lead to reproductive isolation, divergence of gene pools, and speciation.
4.5bArtificial Selection, Genetic Engineering, Gene Editing, and Synthetic Biology
For thousands of years, humans have used artificial selection to change the genetic characteristics of populations with similar genes. First, they select one or more desirable genetic traits that already exist in the population of a plant or animal. Then they use selective breeding, or crossbreeding, to control which members of a population have the opportunity to reproduce to increase the numbers of individuals in a population with the desired traits (Figure 4.18).
Figure 4.18
Artificial selection involves the crossbreeding of species that are close to one another genetically. In this example, similar fruits are being crossbred to yield a pear with a certain color.
Learning from Nature
Artificial selection is a classic case of learning from nature. It involves learning how natural processes produce a particular trait in a fruit or vegetable and then using crossbreeding to enhance that trait.
Artificial selection is not a form of speciation. It is limited to crossbreeding between genetic varieties of the same species or between species that are genetically similar to one another. Most of the grains, fruits, and vegetables we eat are produced by artificial selection. Artificial selection has also given us food crops with higher yields, cows that give more milk, trees that grow faster, and many different varieties of dogs and cats. However, traditional crossbreeding is a slow process.
Scientists have learned how to speed this process of manipulating genes in order to get desirable genetic traits or eliminate undesirable ones. They do this by transferring segments of DNA with the desired trait from one species to another through a process called genetic engineering. In this process, also known as gene splicing, scientists alter an organism’s genetic material by adding, deleting, or changing segments of its DNA to produce desirable traits or to eliminate undesirable ones. Scientists have used genetic engineering to develop modified crop plants, new drugs, pest-resistant plants, and animals that grow rapidly.
There are five steps in this process:
Identify a gene with the desired trait in the DNA found in the nucleus of a cell from the donor organism.
Extract a small circular DNA molecule, called a plasmid, from a bacterial cell.
Insert the desired gene from step 1 into the plasmid to form a recombinant DNA plasmid.
Insert the recombinant DNA plasmid into a cell of another bacterium, which rapidly divides and reproduces large numbers of bacterial cells with the desired DNA trait.
Transfer the genetically modified bacterial cells to a plant or animal that is to be genetically modified.
The result is a genetically modified organism (GMO) with its genetic information modified in a way not found in natural organisms. Genetic engineering enables scientists to transfer genes between different species that would not interbreed in nature. For example, scientists can put genes from a cold-water fish species into a tomato plant to give it properties that help it resist cold weather. Recently, scientists have learned how to treat certain genetic diseases by altering or replacing the genes that cause them. Genetic engineering has revolutionized agriculture and medicine. However, it is a controversial technology, as we discuss in Chapter 12.
In 2012, scientists developed a new gene editing technique called RISPRCRISPR. This easy and cheap technique allows researchers to snip, insert, delete, or modify genetic material at targeted spots in DNA molecules with increased precision.
Gene editing has great promise for correcting disease-causing mutations in DNA molecules. It could be used treat cancers and other human diseases and to modify the DNA in human embryos to remove disease-causing mutations. It has been used to cure mice with HIV and hemophilia and to engineer pigs to make them suitable organ donors for humans. Gene editing can be done to cells outside the human body. Then the modified cells can be implanted in the human body. These genetic changes can then be passed on to future generations.
One worry is that gene editing could become so easy and cheap (a gene editing kit can be ordered online for $150) that anybody could do it and not be subject to regulations. Individuals could possibly alter cells in human embryos, eggs, and sperm to come up with “designer babies” with genes that favor certain traits. This could lead to discrimination against certain groups and a host of other major ethical challenges.
A new and rapidly growing form of genetic engineering is synthetic biology. It enables scientists to make new sequences of DNA and use such genetic information to design and create artificial cells, tissues, body parts, and organisms not found in nature. Synthetic biology can bypass the long process of evolution by natural selection and create new forms of life in a short time.
This process starts with a computer code of an organism’s entire genetic sequence (genome). Then engineers insert new sequences of the four nucleotide bases, adenine (A), cytosine (C), guanine (G), and thymine (T) (Figure 2.9), to create a new and different genetic sequence, or genome. Next, they transplant the new genome into the cell of a bacterium to transform it into a different, human-created species of bacteria. This technology uses science and engineering to alter the planet’s life by reducing the cell to a machine that can assemble forms of life like products in a factory.
Proponents of synthetic biology want to use it to create bacteria that can use sunlight to produce clean-burning hydrogen gas, which can be used to fuel motor vehicles. This could help us reduce our dependence on fossil fuels. Synthetic biology might also be used to create bacteria and algae that break down oil, industrial wastes, toxic heavy metals, pesticides, and radioactive waste in contaminated soil and water. It could be used to create vaccines to prevent diseases and drugs to combat parasitic diseases such as malaria. It might be used to develop instructions for three-dimensional printers to print human body parts, car parts, and clothing.
Scientists are a long way from achieving such goals, but the potential is there. If used properly and ethically, this new technology could help us live more sustainably. The problem is that, like any technology, synthetic biology can be used for good or bad. For example, it could be used to create biological weapons such as deadly bacteria that spread new diseases, to destroy existing oil deposits, or to interfere with the chemical cycles that keep us alive. It might also end up hindering the ability of decomposers to breakdown and recycle wastes, or it might add new pollutants to soil and water. This is why many scientists call for increased monitoring and regulation of this new technology to help control its use.
Learning from Nature
Scientists are applying synthetic biology by studying how organisms in nature operate. For example, some bacteria can consume substances that are harmful to humans, and scientists hope to create a bacterium that can be used to cleanse the human body of such substances.
4.5cExtinction Eliminates Species
Another factor affecting the number and types of species on the earth is biological extinction, or simply extinction, which occurs when an entire species ceases to exist. When environmental conditions change dramatically, a population of a species faces three possible futures: adapt to the new conditions through natural selection, migrate (if possible) to another area with more favorable conditions, or become extinct in the area where they are found.
Species found in only one area, called endemic species, are especially vulnerable to extinction. They exist on islands and in other isolated areas. For example, many species in tropical rain forests have highly specialized roles and are vulnerable to extinction. These organisms are unlikely to be able to migrate or adapt to rapidly changing environmental conditions. Many of these endangered species are amphibians (Core Case Study).
Extinction is a natural and ongoing process. Fossils and other scientific evidence indicate that 99.9% of all species that have existed on the earth are now extinct. Throughout most of the earth’s long history, species have disappeared at a low rate, called the background extinction rate. Based on the fossil record and analysis of ice cores, biologists estimate that the average annual background extinction rate has been about 0.0001% of all species per year, which amounts to 1 species lost for every million species on the earth per year. At this rate, if there were 10 million species on the earth, about 10 of them, on average, would go extinct every year.
Evidence indicates that life on the earth has been sharply reduced by several periods of mass extinction during which extinction rates rise well above the background rate. In such a catastrophic, widespread, and often global event, 50-95% of all species are wiped out because of major, widespread environmental changes such as long-term climate change, massive flooding because of rising sea levels, huge meteorites striking the earth’s surface, and gigantic volcanic eruptions. Such events can trigger drastic environmental changes on a global scale, including massive releases of debris into the atmosphere that block sunlight for an extended period. This can kill off most plant species and the consumers that depend on them for food. Fossil and geological evidence indicates that there have been five mass extinctions (at intervals of 25–60 million years) during the past 500 million years (Figure 4.19).
Figure 4.19
Scientific evidence indicates that the earth has experienced five mass extinctions over the past 500 million years and that human activities have initiated a new sixth mass extinction.
A mass extinction provides an opportunity for the evolution of new species that can fill unoccupied ecological niches or newly created ones. Scientific evidence indicates that each past mass extinction has been followed by an increase in species diversity as shown by the wedges in Figure 4.19). However, this recovery process takes millions of years.
As environmental conditions change, the balance between speciation and extinction determines the earth’s biodiversity. The existence of millions of species today means that speciation, on average, has kept ahead of extinction. However, evidence indicates that the global extinction rate is rising sharply. Many scientists see this is as evidence that we are experiencing the beginning of a new sixth mass extinction caused mostly by human activities (Figure 4.19). We examine this issue and ways to deal with it in Chapter 9. The Case Study that follows discusses the threat of extinction for the monarch butterfly because of human activities.
Case Study
The Threatened Monarch Butterfly
The beautiful North American monarch butterfly (Figure 4.20 and the front cover of this book) is in trouble. This species is known for its annual 3,200- to 4,800-kilometer (2,000- to 3,000-mile) migration from the northern United States and Canada to a small number of tropical forest areas in central Mexico. They arrive on a predictable schedule and later return to their North American home. Another monarch population in the Midwestern United States makes a shorter annual journey to coastal northern California and then returns home.
Figure 4.20
Monarch butterflies in Mexico.
Melinda Fawer/
During their annual round-trip journeys, these two populations of monarchs face serious threats from bad weather and numerous predators. In 2002, a single winter storm killed an estimated 75% of the monarch population migrating to Mexico.
During their migration, the monarchs need access to milkweed plants to lay their eggs. Once the butterfly larvae hatch, the caterpillar survives to become a butterfly by feeding on the milkweed plant. Without milkweed, the monarch butterfly cannot reproduce and faces extinction.
Once the monarchs reach their winter forest destinations in Mexico and California, they cling to trees (Figure 4.20) by the millions as they rest. Each year, biologists estimate the monarch’s population size by measuring the total areas of forest they occupy at these destinations.
The overall estimated monarch population varies from year to year, mostly because of changes in weather and other natural conditions. However, the U.S. Fish and Wildlife Service estimates that since 1975, this overall population has dropped by nearly a billion. The size of Monarch butterfly populations wintering in Mexican forests varies from year to year, often because of weather and other environmental factors. However, since 1996, there has been an overall decline in their annual populations.
The monarchs face three serious threats from human activities in addition to the historic natural threats. One threat is the steady loss of their winter forest habitat in Mexico, due to logging (most of it illegal), and loss of their northern California habitat due to coastal development. A second threat is reduced access to milkweed plants essential for their survival during their migration. Almost all of the natural prairies in the United States, which were abundant with milkweed plants, have been replaced by croplands where milkweed plants grow much more sparsely only as weeds between rows of crops and on roadsides.
A third threat over the past decade is the explosive growth of cropland in the American Midwest planted with corn and soybean varieties genetically engineered to resist herbicides that are used to kill weeds, including milkweed. Some of these herbicides are thought to be killing monarchs as well as their food source.
So why should we care if the monarch butterfly becomes extinct, largely because of human activities? One reason is that monarchs provide an important ecological service by pollinating a variety of flowering plants (including corn) along their migration routes as they feed on the nectar from the blossoms of such plants. Another reason for many people is the belief that it is ethically wrong for us to cause the premature extinction of the monarch butterfly or other species.
What can we do to reduce the threat to this amazing species? Researchers call for protecting their migratory pathways and for the government to protect the monarch by classifying it as a threatened species. They propose that we sharply reduce the use of herbicides to kill milkweed. In addition, many people are trying to help by planting milkweed and other plants that attract pollinators such as butterflies and honeybees (whose populations are also decreasing, as we discuss in Chapter 9).
Learning from Nature
Scientists are studying the Monarch butterfly to find out how they are able to navigate their age-old annual migration routes and arrive at the same places in Mexico and California on the same day of each year. This knowledge could have benefits for human aviation.
Big Ideas
Each species plays a specific ecological role, called its niche, in the ecosystems where it is found.
As environmental conditions change, the genes in some individuals mutate and give those individuals genetic traits that enhance their abilities to survive and to produce offspring with these traits.
The degree of balance between speciation and extinction in response to changing environmental conditions determines the earth’s biodiversity, which helps to sustain the earth’s life and our economies.
Tying It All TogetherAmphibians and Sustainability
Robert King/
This chapter’s Core Case Study describes the increasing losses of amphibian species and explains why these species are important ecologically. In this chapter, we studied the importance of biodiversity—the numbers and varieties of species found in different parts of the world, along with genetic, ecosystem, and functional diversity.
We examined the variety of niches, or roles played by species in ecosystems. For example, we saw that some species, including many amphibians, are indicator species that warn us about threats to biodiversity, ecosystems, and the biosphere. Others such as the American alligator are keystone species that play vital roles in sustaining the ecosystems where they live.
We also studied the scientific theory of biological evolution through natural selection, which explains how life on the earth changes over time due to changes in the genes of populations and how new species can arise. We learned that the earth’s species biodiversity is the result of a balance between the formation of new species (speciation) and extinction of species due to changing environmental conditions.
The ecosystems where amphibians and other species live are functioning examples of the three scientific principles of sustainability in action. These species depend on solar energy, the cycling of nutrients, and biodiversity. Disruptions in any of these forms of natural capital can result in degradation of these species’ populations and their ecosystems.
Chapter Review
Critical Thinking
What might happen to humans and a number of other species if most or all amphibian species (Core Case Study) were to go extinct?
How might a reduction in species diversity affect the other three components of biodiversity?
Is the human species a keystone species? Explain. If humans become extinct, what are three species that might also become extinct and what are three species whose populations might grow?
Why should we care about saving the monarch butterfly from extinction caused by human activities? Do you care? Why or why not?
How would you respond to someone who tells you:
We should not believe in biological evolution because it is “just a theory.”
We should not worry about air pollution because natural selection will enable humans to develop lungs that can detoxify pollutants.
How would you respond to someone who says that because extinction is a natural process, we should not worry about the loss of biodiversity when species become extinct largely because of our activities?
List three aspects of your lifestyle that could be contributing to some of the losses of the earth’s biodiversity. For each of these, what are some ways to avoid making this contribution?
Congratulations! You are in charge of the future evolution of life on the earth. What are the three most important things that you would do? Explain.
Chapter Review
Doing Environmental Science
Study an ecosystem of your choice, such as a meadow, a patch of forest, a garden, or an area of wetland. (If you cannot do this physically, do so virtually by reading about an ecosystem online or in a library.) Determine and list five major plant species and five major animal species in your ecosystem. Which, if any, of these species are indicator species and which of them, if any, are keystone species? Explain how you arrived at these hypotheses. Then design an experiment to test each of your hypotheses, assuming you would have unlimited means to carry them out.
Chapter Review
Data Analysis
The following table is a sample of a very large body of data reported by J. P. Collins, M. L. Crump, and T. E. Lovejoy III in their book Extinction in Our Times—Global Amphibian Decline. It compares various areas of the world in terms of the number of amphibian species found and the number of amphibian species that were endemic, or unique to each area. Scientists like to know these percentages because endemic species tend to be more vulnerable to extinction than do non-endemic species. Study the table and then answer the following questions.
Number of Species
Number of Endemic Species
Percentage Endemic
Pacific/Cascades/Sierra Nevada Mountains of North America
Southern Appalachian Mountains of the United States
Southern Coastal Plain of the United States
Southern Sierra Madre of Mexico
Highlands of Western Central America
Highlands of Costa Rica and Western Panama
Tropical Southern Andes Mountains of Bolivia and Peru
Upper Amazon Basin of Southern Peru
Fill in the fourth column by calculating the percentage of amphibian species that are endemic to each area. 
Which two areas have the highest numbers of endemic species? Name the two areas with the highest percentages of endemic species.
Which two areas have the lowest numbers of endemic species? Which two areas have the lowest percentages of endemic species?
Which two areas have the highest percentages of non-endemic species?

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