Chapter Five

1. Chapter Five Natural Selection and the Origin of Species

2. Natural Selection • Natural selection is one of the processes that acts to change the frequencies of alleles in a population. • An important element to natural selection is the variability of populations. There are several mechanisms of variation, including mutation, independent assortment, recombination, and crossing over. • Adaptation is always relative to the environment. Environment can be classified as physical, biological, or cultural. • Other important distinctions include microenvironment, habitat, and niche.

3. The Mechanisms of Natural Selection • Any factor that brings about a difference in fertility among members of a population is termed a selective agent. A selective agent places a selective pressure on certain individuals in the population, resulting in a change in allele frequencies in the next generation. • Individuals, or populations, that have higher fertility rates in a particular ecological niche are said to display a greater fitness to that niche. • Fitness is a measure of how well-adapted a particular individual or group is to the requirements of the niche.

4. Types of Natural Selection • The results of natural selection differ in various situations. • Directional selection: mean of the variation is moving in a particular direction. • Stabilizing selection: selection is against organisms at both ends of the curve. • Disruptive selection: natural selection favors both extremes.

5. Natural Selection in Humans • One example is sickle-cell anemia. The fitness of the individual with sickle-cell anemia is effectively zero. As expected, natural selection is operating to eliminate the allele HbS in many areas, such as the U.S. • However, populations in many parts of Africa, northern Europe, and the Middle East have very high frequencies of HbS. • The high frequencies of HbS are found in areas characterized by high incidences of falciparum malaria. In this case, the fitness of the heterozygote is relatively high. • This situation is one form of balanced polymorphism.

6. Sexual Selection • A male’s reproductive success is limited by his access to females. Characteristics that increase his success will increase in frequency over time; this is called sexual selection. • Intersexual selection is selection for traits that make males more attractive to females. • Intrasexual selection occurs in cases where males directly compete with one another. Sexual selection will favor traits that make the male a stronger fighter. Thus larger size is favored in males, resulting in sexual dimorphism.

7. Kin Selection • Since relatives share many of the same genes, an individual who assists a relative in reproducing and passing on its genes is also assisting in passing on its own genes. This is called kin selection. • Inclusive fitness refers to the number of offspring produced plus the offspring created by relatives who share some of the same genes. • Kin selection explains cases of altruism, a behavior that decreases the reproductive success of the individual while simultaneously benefiting the reproductive success of other members of the population.

8. The Origin of Species • Macroevolution is the evolution of new species and higher taxa such as genera and families. • All natural populations of plants and animals vary. Individuals within a local population, or deme, tend to resemble one another more closely than individuals living in adjacent demes. • As a result of gene flow, nearby demes resemble one another more than demes that are further away. • The development of subspecies often occurs when something is present that hinders or prevents gene flow between groups of demes.

9. The Evolution of Species • Geographical isolation is a primary initiator of the evolution of new species in animals. • Species that occupy mutually exclusive geographical areas are called allopatric species. • Sympatric species are different species that live in the same area. There are reproductive isolating mechanisms that serve to separate closely related species living side by side. These include ecological isolation, seasonal isolation, sexual isolation, mechanical isolation, gametic mortality, hybrid inviability, and hybrid sterility.

10. Competition • The ability to adapt to new niches varies from population to population. One factor is competition, when two populations occupy the same or parts of the same niche. • Preadaptation refers to the potential to adapt to a new niche. Organisms do not adapt due to necessity, but because by chance they have the potential to adapt. • A specialized species exhibits little variation and can tolerate minimal change within its niche. A generalized species exhibits greater diversity and therefore can survive in a variety of niches. Specialized and generalized can also refer to specific traits.

11. The Tempo of Evolutionary Change • Phyletic gradualism regards evolution as a slow process characterized by a gradual transformation of one population into others. • Punctuated equilibrium sees evolution as having shifts in the tempo of change. From Postlethwait, John H., and J. L. Hopson, The Nature of Life (New York: McGraw-Hill, 1989), p. 344. Used by permission of Janet L. Hopson and John H. Postlethwait, Ph.D.

12. Adaptive Radiation and Extinction • The evolution of new species is most likely to occur in certain situations -- for example, when a species enters an uninhabited environment or one in which competition does not exist. Such a proliferation of new species is called an adaptive radiation. • However, in many cases the organisms involved are too specialized to adapt to changing situations. As a result, they become extinct.