1. 19-2 Patterns and Processes of Evolution
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Macroevolution
Macroevolutionary patterns refer to large-scale evolutionary patterns and processes that occur over long periods of time.
The ways new species emerge through speciation, and the ways species disappear through extinction, are both examples of macroevolutionary patterns.
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Macroevolution
Five important topics in macroevolution are:
extinction
adaptive radiation
convergent evolution
coevolution
punctuated equilibrium
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Extinction
Extinction
More than 99% of all species that have ever lived are now extinct.
Usually extinctions occur through natural selection.
Species compete for resources.
If an environment changes, some species adapt and survive.
If a species cannot adapt, it will die out.
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5. Patterns of Extinction
Species are always evolving and competing—and some species become extinct because of the slow but steady process of natural selection, referred to as background extinction.
In contrast, a mass extinction affects many species over a relatively short period of time.

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In the past, there have been several mass extinctions that wiped out entire ecosystems.
Species became extinct because their environment was collapsing around them.
Extinction was not related to ordinary natural selection.
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Most researchers looked for a single, major cause for each mass extinction.
Many paleontologists now think that mass extinctions were caused by several factors.
During several mass extinctions, for example, volcanoes were erupting, sea levels were changing, and land masses were moving.
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Extinction
Mass extinctions have had an effect on the history of life. They have
provided ecological opportunities (like new habitats, for example) for organisms that survived.
resulted in bursts of evolution that produced many new species.
The extinction of dinosaurs, for example, cleared the way for the evolution of modern mammals and birds.
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9. Punctuated Equilibrium
Rate of Evolution
Darwin felt that biological change was slow and steady, an idea known as gradualism.
In many cases, the fossil record does show a gradual change over time. This is called gradualism.
Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.
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There is also evidence that this pattern does not always hold.
Some species, like horseshoe crabs, have changed little over time. These species are in a state of equilibrium.
Every now and then, though, something may happen to upset a species in equilibrium.
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11. Punctuated Equilibrium
At several times in evolutionary history, changes in organisms occurred in a “short” time period relative to a period of equilibrium.
Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of more rapid change.
Biologists have considered two different explanations for the rate of evolution, as illustrated in these diagrams. Gradualism involves a slow, steady change in a particular line of descent. Punctuated equilibrium involves stable periods interrupted by rapid changes involving many different lines of descent.
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12. Rapid Evolution After Equilibrium
Rapid evolution may occur after a small population becomes isolated from the main population. This small population can evolve faster than the larger one because genetic changes spread more quickly among fewer individuals.
Rapid evolution may also occur when a small group of organisms migrates to a new environment. That’s what happened with the Galápagos finches.

13. Punctuated Equilibrium
Also, following a mass extinction, new opportunities arise for species that survive. These organisms will then evolve relatively rapidly to fill these niches.
Evolution has often proceeded at different rates for different organisms at different times during the history of life on Earth.
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Adaptive Radiation
Adaptive Radiation
Adaptive radiation is the process by which a single species or a small group of species evolves into several different forms that live in different ways.
For example, in the adaptive radiation of Darwin's finches, more than a dozen species evolved from a single species.
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Adaptive Radiation
Adaptive radiations can occur on a much larger scale.
Dinosaurs were the result of the adaptive radiation of ancient reptiles.
The disappearance of dinosaurs then resulted in the adaptive radiation of mammals. This radiation produced a great variety of mammals.
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Adaptive Radiation
Adaptive Radiation of Mammals
This diagram shows part of the adaptive radiation of mammals, emphasizing current hypotheses about how a group of ancestral mammals diversified over millions of years into several related living orders. Note that the dotted lines and question marks in this diagram indicate a combination of gaps in the fossil record and uncertainties about the timing of evolutionary branching.
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Convergent Evolution
Convergent Evolution
Different organisms may undergo adaptive radiation in different places or at different times but in similar environments. The organisms may start out quite different from each other but similar environmental demands (for moving through water, for example) may modify body structures (like arms & legs) into similar modified forms (like wings or flippers).
The process by which unrelated organisms come to resemble one another is called convergent evolution.
Convergent evolution has resulted in sharks, dolphins, seals, and penguins.
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Convergent Evolution
Structures that look and function similarly but are made up of parts that do not share a common evolutionary history are called analogous structures.
A dolphin’s fluke and a fish’s tail fin are analogous structures.
(Recall homolgous structures. These are different structures that originated from the same embryonic tissue.)
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Coevolution
Coevolution
Sometimes organisms that are closely connected to one another by ecological interactions evolve together.
The process by which two species evolve in response to changes in each other over time is called coevolution.
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Flowering plants and their insect pollinators often coevolve. For example, Darwin discovered an orchid whose flowers had a 40-centimeter-long structure called a spur with a supply of nectar at the bottom. Darwin predicted that some pollinating insect must have some kind of feeding structure that would allow it to reach the nectar. Darwin never saw that insect. About 40 years later, researchers discovered a moth with a 40-centimeter-long feeding tube that matched Darwin’s prediction.
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21. Plants and Herbivorous Insects
Plants and herbivorous insects also demonstrate close coevolutionary relationships.
Once plants began to produce poisons, natural selection on herbivorous insects favored any variants that could alter, inactivate, or eliminate those poisons.
Milkweed plants, for example, produce toxic chemicals. But monarch caterpillars not only can tolerate this toxin, they also can store it in their body tissues to use as a defense against their predators.