a. The roles of populations in communities are important factors in their evolution.

b. Coevolution is defined here as the evolution of two interacting species.

c. Sidebar 24.1 addresses pollination and coevolution.

d. Figure 24.1 illustrates a story originated by Charles Darwin regarding the interrelatedness of evolutionary changes.

a. The bases for evolution are genetic variation and selection.

b. Organisms are shaped into functional forms by small changes generation after generation.

a. Methods 24.1 covers radioisotope dating of rock and the fossils found in them.

b. Fossils provide a record of the organisms living in the past.

c. Biologists once thought that evolution was a series of straight-line changes in each group of organisms, such as in this drawing for Equus (Figure 24.2).

d. The modern view (Figure 24.3) recognizes a more haphazard course for evolution, and includes three kind of events:

e. A fourth feature, stasis, indicates no change at all, and is represented in a phylogenetic tree by lines that continue straight on until they branch or end.

f. For the purposes of this book, speciation will be restricted to a process in which one species divides into two or more.

g. Populations of the past are conveniently broken into segments called paleospecies.

h. Different patterns of evolution must result from different modes of selection.

i. Figure 24.4 shows that natural selection operates on a population three ways:

j. Extinction has been shown to be the dominant feature of evolution, as a typical species lasts about 100,000 to a few million years, then disappears.

k. George Gaylord Simpson estimated that 99.9 percent of all species that ever lived have become extinct.

l. Human interference has caused the extinction of many species.

m. Stabilizing selection is a conservative process that keeps a species well adapted to its niche by eliminating genotypes far away from the norm.

n. A spot of light to represent the niche (Figure 24.5) shows that some individuals will fall at the edges and be eliminated.

o. Extinction does not appear to favor one species over another and does not apparently depend on the length of time for which a species has existed.

p. Van Valen's Red Queen Hypothesis states that an ever-changing environment, like a moving spot of light, challenges each species just to keep pace and well adapted (Figure 24.6).

q. Selection can only change a population to the extent of its genetic variability.

a. Many groups show consistent trends in their evolution.

b. Consistent changes in one direction could result either from phyletic evolution or from repeated speciation.

c. Figure 24.7 shows both phyletic changes and sudden changes, referred to as punctuated speciation, that follow periods of stasis or equilibrium.

d. Punctuated equilibrium is the term given to evolution that consists only of speciation and extinction, with little or no change in a species between its origin and extinction.

e. Evidence for punctuated equilibrium has been presented, as in the case of the molluscs shown in Figure 24.8.

f. Some fossils appear to fit this model, and it makes sense in view of the randomness of genetic variation and environmental change.

g. Figure 24.9 shows that some species may become extinct while other species in a population may survive.

h. Though controversy over punctuated equilibrium revolves around the claim that speciation cannot occur too rapidly, such events in the geological record have taken place over about 50,000 years, a small amount of time from a geological viewpoint.

a. The northern flickers of the United States (Figure 24. 10) are woodpeckers that were once called different species, but are actually one species with intermediate forms in areas where the two overlap geographically.

b. Gene flow, the exchange of genes between two populations, occurs regularly between flickers in the overlap regions.

c. The case of the flickers' geographic variation and the intermediate form are good evidence for geographic isolation (Figure 24.11) as a mode of speciation.

d. Barriers such as islands, water, glaciers, mountains, and even rivers, are responsible for geographic isolation.

e. Ernst Mayr developed this theory in studies of birds, and suggested that, while separated, populations change into distinct groups that eventually are termed species.

f. Recall, two populations are separate species only if they are reproductively isolated from one another.

g. In order for reproductive isolation to be meaningful, the populations in question must at least have the potential to interbreed.

h. Two populations are sympatric if their ranges overlap; they are parapatric if their ranges are adjacent; and they are allopatric if their ranges are separate.

i. Discussions of reproductive isolation must be limited to either sympatric or parapatric species.

j. Closely related allopatric populations may be in the process of becoming different species.

k. The species status of any two populations that can interbreed is uncertain, and terms like semispecies, allospecies, or superspecies (Concepts 24.1) are used to describe them.

l. Biologically interesting intermediate stages between species are often found (Figure 24.12), as with the monkey flowers of genus Diplacus (Figure 24.13) and the buckeye butterfly (Figure 24.14).

m. While populations are separated, they develop reproductive isolating mechanisms that prevent interbreeding; these are outlined in Section 24.6.

n. Two newly formed species must occupy different niches if they are to be sympatric again, due to competition for resources.

o. Niche differentiation sometimes involves character displacement, where two species are more dissimilar when sympatric than they are when allopatric (Figure 24.15).

a. Reproductive isolation means two individuals cannot produce viable offspring that can also reproduce and pass on their genes.

b. Below, various forms of premating reproductive isolation are listed:

c. Here, forms of postmating isolation are listed:

d. Though hybrids are often at a disadvantage in adapting to the niches occupied by the two species that made them, each population carries some "hybridizers" with genes that tend to promote mating with the other population (Figure 24.16).

a. The Hawaiian honeycreepers (Figure 24.17) are examples of closely related species that occupy very different niches and have enormous external differences.

b. These birds are examples of opportunists that took advantage of many ways to make a living across several islands (Figure 24.18).

c. The pattern of evolution in the honeycreepers is called adaptive radiation; one original population radiated out in several different biological directions to different ways of life (Figure 24.19).

d. An adaptive zone is a general way of life to which a species becomes adapted.

e. The Hawaiian honeycreepers radiated into three adaptive zones: insect-eating, nectar-eating, and fruit-eating.

f. Verne Grant identified an adaptive radiation in one plant family (Figure 24.20) that adapted to different modes of pollination.

g. Populations are most likely to fail during times of transition, as illustrated by the niche-spotlight model (Figure 24.21).

h. Genetic drift may carry a species quickly from one state to another, and it is an important source of change in small populations such as those founding new populations.

i. A special case of genetic drift is thus known as the founder effect.

j. Table 24.1 reviews situations involving population structure and evolutionary flexibility.

a. Many populations consist of parapatric local populations (demes) that interbreed with one another so that there is some gene flow between them.

b. Questions have arisen about the amount of gene flow between these populations, which are separated by no geographical barriers.

c. Ehrlich and Raven have argued that individuals within species move around very little, and have suggested that animal and plant populations have few opportunities for gene flow, even in the absence of geographical barriers.

d. In spite of this, differences between local demes can be large, as in the case of the meadow brown butterfly (Figure 24.22).

e. A series of parapatric demes could therefore develop into separate species.

f. John Endler demonstrated parapatric speciation with Drosophila and artificial selection, in the presence of no geographical barriers.

a. Hybridization is an important evolutionary mechanism in plants.

b. Introgression is a process in which some genes of one species get into the genome of another species through hybridization.

c. Though F1 hybrids are often less viable than their parents, repeated backcrossing can produce viable and hardy varieties.

d. New plant species also develop through the creation of polyploid individuals, those which have more than a diploid number of chromosomes.

e. Autopolyploidy results from an error in meiosis (Figure 24.23).

f. Allopolyploidy (or amphiploidy) results when two diploid sets of chromosomes come from different species (Figure 24.24).

g. Allopolyploids may be more successful in nature than their parents, as they have a combination of both parents' characteristics (see Section 23.7 on heterosis).

h. Roughly half the known species of flowering plants are thought to have originated through polyploidy.

i. Modern bread wheat (Figure 24.25) also arose through hybridization.

j. These examples illustrate that sympatric speciation can occur without geographic isolation.

a. Though some biologists have argued that relatively small differences created during speciation are not enough to produce the variety of taxa currently known, the fallacy in their argument could be that the taxa are artificially created by humans and do not really exist in nature.

b. Archaeopteryx (Chapter 35), for example, could have been just an unusual reptile, but it happened to give rise to a large taxon through adaptive radiation.

c. This process gave rise to about 9,000 species of birds, each species meeting its environment independently, but arising through relatively small, gradual steps.

a. Simple changes in form may cause dramatic changes in organisms, as illustrated by D'Arcy Thompson in his book On Growth and Form.

b. Thompson based his drawings (Figures 24.27 and 24.28) on those made by a German artist (Figure 24.26), who showed that merely bending a basic form could result in notable changes.

c. Though these drawings may not have anything to do with actual evolutionary changes, they illustrate the concept of changes in structure through heterochrony: a change in the timing or rate of developmental processes, relative to those of an ancestor.

d. We now know these changes depend on genetic variation and change, as illustrated for the basic arthropod body plan (Figure 24.29), which is thought to have arisen through changes in critical homeotic genes.