Origins of Biodiversity
What is evolution?
The basic idea of biological evolution is that populations and species of organisms change over time. Today, when we think of evolution, we are likely to link this idea with one specific person: the British naturalist Charles Darwin.
In the 1850s, Darwin wrote an influential and controversial book called On the Origin of Species. In it, he proposed that species evolve (or, as he put it, undergo "descent with modification"), and that all living things can trace their descent to a common ancestor. Darwin also suggested a mechanism for evolution: natural selection, in which heritable traits that help organisms survive and reproduce become more common in a population over time.
Over the course of his travels, Darwin began to see intriguing patterns in the distribution and features of organisms. We can see some of the most important patterns Darwin noticed in distribution of organisms by looking at his observations of the Galápagos Islands off the coast of Ecuador.
What is evolution?
The basic idea of biological evolution is that populations and species of organisms change over time. Today, when we think of evolution, we are likely to link this idea with one specific person: the British naturalist Charles Darwin.
In the 1850s, Darwin wrote an influential and controversial book called On the Origin of Species. In it, he proposed that species evolve (or, as he put it, undergo "descent with modification"), and that all living things can trace their descent to a common ancestor. Darwin also suggested a mechanism for evolution: natural selection, in which heritable traits that help organisms survive and reproduce become more common in a population over time.
Over the course of his travels, Darwin began to see intriguing patterns in the distribution and features of organisms. We can see some of the most important patterns Darwin noticed in distribution of organisms by looking at his observations of the Galápagos Islands off the coast of Ecuador.
Darwin found that nearby islands in the Galápagos had similar but nonidentical species of finches living on them. Moreover, he noted that each finch species was well-suited for its environment and role. For instance, species that ate large seeds tended to have large, tough beaks, while those that ate insects had thin, sharp beaks. Finally, he observed that the finches (and other animals) found on the Galápagos Islands were similar to species on the nearby mainland of Ecuador, but different from those found elsewhere in the world.
Darwin didn't figure all of this out on his trip. In fact, he didn't even realize all the finches were related but distinct species until he showed his specimens to a skilled ornithologist (bird biologist) years later! Gradually, however, he came up with an idea that could explain the pattern of related but different finches.
According to Darwin's idea, this pattern would make sense if the Galápagos Islands had long ago been populated by birds from the neighboring mainland. On each island, the finches might have gradually adapted to local conditions (over many generations and long periods of time). This process could have led to the formation of one or more distinct species on each island.
If this idea was correct, though, why was it correct? What mechanism could explain how each finch population had acquired adaptations, or features that made it well-suited to its immediate environment? During his voyage, and in the years after, Darwin developed and refined a set of ideas that could explain the patterns he had observed during his voyage. In his book, On the Origin of Species, Darwin outlined his two key ideas: evolution and natural selection.
Evolution
Darwin proposed that species can change over time, that new species come from pre-existing species, and that all species share a common ancestor. In this model, each species has its own unique set of heritable (genetic) differences from the common ancestor, which have accumulated gradually over very long time periods. Repeated branching events, in which new species split off from a common ancestor, produce a multi-level "tree" that links all living organisms.
Darwin referred to this process, in which groups of organisms change in their heritable traits over generations, as “descent with modification." Today, we call it evolution.
Natural Selection
Importantly, Darwin didn't just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn't be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: natural selection. This mechanism was elegant and logical, and it explained how populations could evolve (undergo descent with modification) in such a way that they became better suited to their environments over time.
Darwin's concept of natural selection was based on several key observations:
Darwin didn't figure all of this out on his trip. In fact, he didn't even realize all the finches were related but distinct species until he showed his specimens to a skilled ornithologist (bird biologist) years later! Gradually, however, he came up with an idea that could explain the pattern of related but different finches.
According to Darwin's idea, this pattern would make sense if the Galápagos Islands had long ago been populated by birds from the neighboring mainland. On each island, the finches might have gradually adapted to local conditions (over many generations and long periods of time). This process could have led to the formation of one or more distinct species on each island.
If this idea was correct, though, why was it correct? What mechanism could explain how each finch population had acquired adaptations, or features that made it well-suited to its immediate environment? During his voyage, and in the years after, Darwin developed and refined a set of ideas that could explain the patterns he had observed during his voyage. In his book, On the Origin of Species, Darwin outlined his two key ideas: evolution and natural selection.
Evolution
Darwin proposed that species can change over time, that new species come from pre-existing species, and that all species share a common ancestor. In this model, each species has its own unique set of heritable (genetic) differences from the common ancestor, which have accumulated gradually over very long time periods. Repeated branching events, in which new species split off from a common ancestor, produce a multi-level "tree" that links all living organisms.
Darwin referred to this process, in which groups of organisms change in their heritable traits over generations, as “descent with modification." Today, we call it evolution.
Natural Selection
Importantly, Darwin didn't just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn't be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: natural selection. This mechanism was elegant and logical, and it explained how populations could evolve (undergo descent with modification) in such a way that they became better suited to their environments over time.
Darwin's concept of natural selection was based on several key observations:
- Traits are often heritable. In living organisms, many characteristics are inherited, or passed from parent to offspring. (Darwin knew this was the case, even though he did not know that traits were inherited via genes.)
More offspring are produced than can survive. Organisms are capable of producing more offspring than their environments can support. Thus, there is competition for limited resources in each generation.
Offspring vary in their heritable traits. The offspring in any generation will be slightly different from one another in their traits (color, size, shape, etc.), and many of these features will be heritable.
Based on these simple observations, Darwin concluded the following:
Darwin's model of evolution by natural selection allowed him to explain the patterns he had seen during his travels. For instance, if the Galápagos finch species shared a common ancestor, it made sense that they should broadly resemble one another (and mainland finches, who likely shared that common ancestor). If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored, such as different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of distinct species on each island.
Natural selection depends on the environment
Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
Natural selection acts on existing heritable variation
Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation (differences among individuals) for that feature. Also, the differences have to be heritable, determined by the organisms' genes.
Heritable variation comes from random mutations
The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation (changes in DNA sequence). Random mutations that are passed on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms. Sexual reproduction "mixes and matches" gene variants to make more variation.
The underlining theme is that different individuals in the same population have varying degrees of success in reproducing based on genetic differences. Those with genetic differences that are suited to the current environment will successfully reproduce, have offspring, and pass on their genes. Those individuals whose genetic differences do not suit the current environment will die and will not pass on their genes. The distinction between the two groups of individuals is called differential reproduction.
Differential reproduction results in the predominance of certain genes and therefore certain phenotypes. Typical populations have a normal curve for each phenotype where some individuals have extreme characteristics, but most are between the extremes. Stabilizing selection occurs when under certain environmental conditions, the extreme phenotypes are not favorable, and therefore those individuals in the extreme region will die, and their genes do not continue in the population. Directional selection involves removal of the genes and individuals from one of the extremes, and therefore pushes the particular phenotype toward one end of the spectrum. By contrast, diversifying or disruptive selection somehow selects against those in the middle of the normal curve, therefore favoring the genetic variants of the extreme phenotypes.
- In a population, some individuals will have inherited traits that help them survive and reproduce (given the conditions of the environment, such as the predators and food sources present). The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.
- Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common (present in a larger fraction of the population) in the next generation.
- Over generations, the population will become adapted to its environment (as individuals with traits helpful in that environment have consistently greater reproductive success than their peers).
Darwin's model of evolution by natural selection allowed him to explain the patterns he had seen during his travels. For instance, if the Galápagos finch species shared a common ancestor, it made sense that they should broadly resemble one another (and mainland finches, who likely shared that common ancestor). If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored, such as different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of distinct species on each island.
Natural selection depends on the environment
Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
Natural selection acts on existing heritable variation
Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation (differences among individuals) for that feature. Also, the differences have to be heritable, determined by the organisms' genes.
Heritable variation comes from random mutations
The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation (changes in DNA sequence). Random mutations that are passed on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms. Sexual reproduction "mixes and matches" gene variants to make more variation.
The underlining theme is that different individuals in the same population have varying degrees of success in reproducing based on genetic differences. Those with genetic differences that are suited to the current environment will successfully reproduce, have offspring, and pass on their genes. Those individuals whose genetic differences do not suit the current environment will die and will not pass on their genes. The distinction between the two groups of individuals is called differential reproduction.
Differential reproduction results in the predominance of certain genes and therefore certain phenotypes. Typical populations have a normal curve for each phenotype where some individuals have extreme characteristics, but most are between the extremes. Stabilizing selection occurs when under certain environmental conditions, the extreme phenotypes are not favorable, and therefore those individuals in the extreme region will die, and their genes do not continue in the population. Directional selection involves removal of the genes and individuals from one of the extremes, and therefore pushes the particular phenotype toward one end of the spectrum. By contrast, diversifying or disruptive selection somehow selects against those in the middle of the normal curve, therefore favoring the genetic variants of the extreme phenotypes.
Natural selection and the evolution of species
In the example of Darwin's finches, we saw that groups in a single population may become isolated from one another by geographical barriers, such as ocean surrounding islands, or by other mechanisms. Once isolated, the groups can no longer interbreed and are exposed to different environments. In each environment, natural selection is likely to favor different traits (and other evolutionary forces, such as random drift, may also operate separately on the groups). Over many generations, differences in heritable traits can accumulate between the groups, to the extent that they are considered separate species.
Evolution by natural selection and other mechanisms underlies the incredible diversity of present-day life forms, and the action of natural selection can explain the fit between present-day organisms and their environments.
Evolution Happens on Large and Small Scales
Broadly speaking, evolution is a change in the genetic makeup (and often, the heritable features) of a population over time. Biologists sometimes define two types of evolution based on scale:
Microevolution and macroevolution aren’t really two different processes. They’re the same process – evolution – occurring on different timescales. Microevolutionary processes occurring over thousands or millions of years can add up to large-scale changes that define new species or groups.
Evidence for Evolution
Biogeography
The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the breakup tend to appear uniquely in smaller regions of Earth. For instance, there are unique groups of plants and animals on northern and southern continents that can be traced to the split of Pangaea into two supercontinents (Laurasia in the north, Gondwana in the south).
In the example of Darwin's finches, we saw that groups in a single population may become isolated from one another by geographical barriers, such as ocean surrounding islands, or by other mechanisms. Once isolated, the groups can no longer interbreed and are exposed to different environments. In each environment, natural selection is likely to favor different traits (and other evolutionary forces, such as random drift, may also operate separately on the groups). Over many generations, differences in heritable traits can accumulate between the groups, to the extent that they are considered separate species.
Evolution by natural selection and other mechanisms underlies the incredible diversity of present-day life forms, and the action of natural selection can explain the fit between present-day organisms and their environments.
Evolution Happens on Large and Small Scales
Broadly speaking, evolution is a change in the genetic makeup (and often, the heritable features) of a population over time. Biologists sometimes define two types of evolution based on scale:
- Macroevolution, which refers to large-scale changes that occur over extended time periods, such as the formation of new species and groups.
- Microevolution, which refers to small-scale changes that affect just one or a few genes and happen in populations over shorter timescales.
Microevolution and macroevolution aren’t really two different processes. They’re the same process – evolution – occurring on different timescales. Microevolutionary processes occurring over thousands or millions of years can add up to large-scale changes that define new species or groups.
Evidence for Evolution
Biogeography
The geographic distribution of organisms on Earth follows patterns that are best explained by evolution, in combination with the movement of tectonic plates over geological time. For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the breakup tend to appear uniquely in smaller regions of Earth. For instance, there are unique groups of plants and animals on northern and southern continents that can be traced to the split of Pangaea into two supercontinents (Laurasia in the north, Gondwana in the south).
The evolution of unique species on islands is another example of how evolution and geography intersect. For instance, most of the mammal species in Australia are marsupials (carry young in a pouch), while most mammal species elsewhere in the world are placental (nourish young through a placenta). Australia’s marsupial species are very diverse and fill a wide range of ecological roles. Because Australia was isolated by water for millions of years, these species were able to evolve without competition from (or exchange with) mammal species elsewhere in the world.
The marsupials of Australia, Darwin's finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings, but have distant relationships to ancestral species on mainlands. This combination of features reflects the processes by which island species evolve. They often arise from mainland ancestors – for example, when a landmass breaks off or a few individuals are blown off course during a storm – and diverge (become increasingly different) as they adapt in isolation to the island environment.
Fossil record
Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record is not, alas, complete or unbroken: most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Nonetheless, the fossils that humans have collected offer unique insights into evolution over long timescales.
Fossils document the existence of now-extinct species, showing that different organisms have lived on Earth during different periods of the planet's history. They can also help scientists reconstruct the evolutionary histories of present-day species. For instance, some of the best-studied fossils are of the horse lineage. Using these fossils, scientists have been able to reconstruct a large, branching "family tree" for horses and their now-extinct relatives. Changes in the lineage leading to modern-day horses, such as the reduction of toed feet to hooves, may reflect adaptation to changes in the environment.
The marsupials of Australia, Darwin's finches in the Galápagos, and many species on the Hawaiian Islands are unique to their island settings, but have distant relationships to ancestral species on mainlands. This combination of features reflects the processes by which island species evolve. They often arise from mainland ancestors – for example, when a landmass breaks off or a few individuals are blown off course during a storm – and diverge (become increasingly different) as they adapt in isolation to the island environment.
Fossil record
Fossils are the preserved remains of previously living organisms or their traces, dating from the distant past. The fossil record is not, alas, complete or unbroken: most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. Nonetheless, the fossils that humans have collected offer unique insights into evolution over long timescales.
Fossils document the existence of now-extinct species, showing that different organisms have lived on Earth during different periods of the planet's history. They can also help scientists reconstruct the evolutionary histories of present-day species. For instance, some of the best-studied fossils are of the horse lineage. Using these fossils, scientists have been able to reconstruct a large, branching "family tree" for horses and their now-extinct relatives. Changes in the lineage leading to modern-day horses, such as the reduction of toed feet to hooves, may reflect adaptation to changes in the environment.
Direct Observation of Microevolution
In some cases, the evidence for evolution is that we can see it taking place around us! Important modern-day examples of evolution include the emergence of drug-resistant bacteria and pesticide-resistant insects.
For example, in the 1950s, there was a worldwide effort to eradicate malaria by eliminating its carriers (certain types of mosquitos). The pesticide DDT was sprayed broadly in areas where the mosquitoes lived, and at first, the DDT was highly effective at killing the mosquitos. However, over time, the DDT became less and less effective, and more and more mosquitoes survived. This was because the mosquito population evolved resistance to the pesticide.
In some cases, the evidence for evolution is that we can see it taking place around us! Important modern-day examples of evolution include the emergence of drug-resistant bacteria and pesticide-resistant insects.
For example, in the 1950s, there was a worldwide effort to eradicate malaria by eliminating its carriers (certain types of mosquitos). The pesticide DDT was sprayed broadly in areas where the mosquitoes lived, and at first, the DDT was highly effective at killing the mosquitos. However, over time, the DDT became less and less effective, and more and more mosquitoes survived. This was because the mosquito population evolved resistance to the pesticide.
Emergence of DDT resistance is an example of evolution by natural selection. How would natural selection have worked in this case?
In parts of the world where DDT has been used extensively in the past, many of the mosquitoes are now resistant. DDT can no longer be used to control the mosquito populations (and reduce malaria) in these regions.
Why are mosquito populations able to evolve rapid resistance to DDT? Two important factors are large population size (making it more likely that some individuals in the population will, by random chance, have mutations that provide resistance) and short lifecycle. Bacteria and viruses, which have even larger population sizes and shorter lifecycles, can evolve resistance to drugs very rapidly, as in antibiotic-resistant bacteria and drug-resistant HIV.
Speciation
New species arise through a process called speciation. In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.
For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed.
Geographic Isolation
In geographic isolation, organisms of an ancestral species evolve into two or more descendant species after a period of physical separation caused by a geographic barrier, such as a mountain range, rockslide, or river. Sometimes barriers, such as a lava flow, split populations by changing the landscape. Other times, populations become separated after some members cross a pre-existing barrier. For example, members of a mainland population may become isolated on an island if they float over on a piece of debris.
Once the groups are reproductively isolated, they may undergo genetic divergence. That is, they may gradually become more and more different in their genetic makeup and heritable features over many generations. If the reproductive barriers that have arisen are strong—effectively preventing gene flow—the groups will keep evolving along separate paths. That is, they won't exchange genes with one another even if the geographical barrier is removed. At this point, the groups can be considered separate species.
Case study: squirrels and the Grand Canyon
The Grand Canyon was gradually carved out by the Colorado River over millions of years. Before it formed, only one species of squirrel inhabited the area. As the canyon got deeper over time, it became increasingly difficult for squirrels to travel between the north and south sides. Eventually, the canyon became too deep for the squirrels to cross and a subgroup of squirrels became isolated on each side. Because the squirrels on the north and south sides were reproductively isolated from one another due to the deep canyon barrier, they eventually diverged into different species.
- Before DDT was applied, a tiny fraction of mosquitos in the population would have had naturally occurring gene versions (alleles) that made them resistant to DDT. These versions would have appeared through random mutation, or changes in DNA sequence. Without DDT around, the resistant alleles would not have helped mosquitoes survive or reproduce (and might even have been harmful), so they would have remained rare.
- When DDT spraying began, most of the mosquitos would have been killed by the pesticide. Which mosquitos would have survived? For the most part, only the rare individuals that happened to have DDT resistance alleles (and thus survived being sprayed with DDT). These surviving mosquitoes would have been able to reproduce and leave offspring.
- Over generations, more and more DDT-resistant mosquitoes would have been born into the population. That's because resistant parents would have been consistently more likely to survive and reproduce than non-resistant parents, and would have passed their DDT resistance alleles (and thus, the capacity to survive DDT) on to their offspring. Eventually, the mosquito populations would have bounced back to high numbers, but would have been composed largely of DDT-resistant individuals.
In parts of the world where DDT has been used extensively in the past, many of the mosquitoes are now resistant. DDT can no longer be used to control the mosquito populations (and reduce malaria) in these regions.
Why are mosquito populations able to evolve rapid resistance to DDT? Two important factors are large population size (making it more likely that some individuals in the population will, by random chance, have mutations that provide resistance) and short lifecycle. Bacteria and viruses, which have even larger population sizes and shorter lifecycles, can evolve resistance to drugs very rapidly, as in antibiotic-resistant bacteria and drug-resistant HIV.
Speciation
New species arise through a process called speciation. In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.
For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed.
Geographic Isolation
In geographic isolation, organisms of an ancestral species evolve into two or more descendant species after a period of physical separation caused by a geographic barrier, such as a mountain range, rockslide, or river. Sometimes barriers, such as a lava flow, split populations by changing the landscape. Other times, populations become separated after some members cross a pre-existing barrier. For example, members of a mainland population may become isolated on an island if they float over on a piece of debris.
Once the groups are reproductively isolated, they may undergo genetic divergence. That is, they may gradually become more and more different in their genetic makeup and heritable features over many generations. If the reproductive barriers that have arisen are strong—effectively preventing gene flow—the groups will keep evolving along separate paths. That is, they won't exchange genes with one another even if the geographical barrier is removed. At this point, the groups can be considered separate species.
Case study: squirrels and the Grand Canyon
The Grand Canyon was gradually carved out by the Colorado River over millions of years. Before it formed, only one species of squirrel inhabited the area. As the canyon got deeper over time, it became increasingly difficult for squirrels to travel between the north and south sides. Eventually, the canyon became too deep for the squirrels to cross and a subgroup of squirrels became isolated on each side. Because the squirrels on the north and south sides were reproductively isolated from one another due to the deep canyon barrier, they eventually diverged into different species.
Reproductive Isolation
In reproductive isolation, organisms from the same ancestral species become reproductively isolated and diverge without any physical separation.
One classic example is the North American apple maggot fly. As the name suggests, North American apple maggot flies, like the one pictured below, can feed and mate on apple trees. The original host plant of these flies, however, was the hawthorn tree. It was only when European settlers introduced apple trees about 200 years ago that some flies in the population started to exploit apples as a food source instead.
In reproductive isolation, organisms from the same ancestral species become reproductively isolated and diverge without any physical separation.
One classic example is the North American apple maggot fly. As the name suggests, North American apple maggot flies, like the one pictured below, can feed and mate on apple trees. The original host plant of these flies, however, was the hawthorn tree. It was only when European settlers introduced apple trees about 200 years ago that some flies in the population started to exploit apples as a food source instead.
The flies that were born in apples tended to feed on apples and mate with other flies on apples, while the flies born on hawthorns tended to similarly stick with hawthorns. In this way, the population was effectively divided into two groups with limited gene flow between them, even though there was no reason an apple fly couldn't go over to a hawthorne tree, or vice versa.
Over time, the population diverged into two genetically distinct groups with adaptations, features arising by natural selection, that were specific for apple and hawthorne fruits. For instance, the apple and hawthorne flies emerge at different times of year, and this genetically specified difference synchronizes them with the emergence date of the fruit on which they live.
Some interbreeding still occurs between the apple-specialized flies and the hawthorne-specialized flies, so they are not yet separate species. However, many scientists think this is a case of reproductive isolation in progress.
Extinction
After speciation, the second process affecting the number and types of species on Earth is extinction When environmental conditions change, a species must (1) evolve (become better adapted), (2) move to a favorable area (if possible), or (3) cease to exist (become extinct).
The Earth's long-term patterns of speciation and extinction have been affected by several major factors: (1) large-scale movements of the continents (continental drift) over millions of years, (2) gradual climate changes caused by continental drift and slight shifts in Earth's orbit around the sun, and (3) rapid climate change caused by catastrophic events (such as large volcanic eruptions, huge meteorites and asteroids crashing into the Earth, and release of large amounts of methane trapped beneath the ocean floor. Some of these events create dust clouds that shut down or sharply reduce photosynthesis long enough to eliminate huge numbers of producers and, soon after, the consumers that fed on them.
Over time, the population diverged into two genetically distinct groups with adaptations, features arising by natural selection, that were specific for apple and hawthorne fruits. For instance, the apple and hawthorne flies emerge at different times of year, and this genetically specified difference synchronizes them with the emergence date of the fruit on which they live.
Some interbreeding still occurs between the apple-specialized flies and the hawthorne-specialized flies, so they are not yet separate species. However, many scientists think this is a case of reproductive isolation in progress.
Extinction
After speciation, the second process affecting the number and types of species on Earth is extinction When environmental conditions change, a species must (1) evolve (become better adapted), (2) move to a favorable area (if possible), or (3) cease to exist (become extinct).
The Earth's long-term patterns of speciation and extinction have been affected by several major factors: (1) large-scale movements of the continents (continental drift) over millions of years, (2) gradual climate changes caused by continental drift and slight shifts in Earth's orbit around the sun, and (3) rapid climate change caused by catastrophic events (such as large volcanic eruptions, huge meteorites and asteroids crashing into the Earth, and release of large amounts of methane trapped beneath the ocean floor. Some of these events create dust clouds that shut down or sharply reduce photosynthesis long enough to eliminate huge numbers of producers and, soon after, the consumers that fed on them.