B.7CDE Natural Selection

Student Expectation

The student is expected to analyze and evaluate how natural selection produces change in populations, not individuals AND analyze and evaluate how the elements of natural selection, including inherited variation, the potential of a population to produce more offspring than can survive, and a finite supply of environmental resources, result in differential reproductive success AND analyze and evaluate the relationship of natural selection to adaptation and to the development of diversity in and among species.

Key Concepts

• Natural selection is a process by which the gene frequencies in a population change over time because of the differential survival and reproductive success of various genotypes. An organism’s survival depends on its inherited traits, which influence its ability to compete for resources, procure mates, and avoid danger to itself and its offspring. The overall ability to survive and pass its genes on to the next generation is called fitness. In each generation, the individuals with the highest fitness are more likely to reproduce and pass on their traits to their offspring.
• Changes in the gene pool of a population due to natural selection take many generations. Natural selection does not occur within individuals; it occurs within and among populations. Within a diverse population, different individuals of the same species have unequal fitness; this is called within-population diversity. In addition, different species may have unequal fitness in a particular environment; this is called among-species diversity.
• The frequencies of alleles within a population change due to different levels of reproductive success among individuals, as some produces more successful offspring than others. Allelic frequencies can also change when more individuals are produced than the environment can support and strong competition for resources results.
• Species are usually defined as groups of organisms that can interbreed and produce viable, fertile offspring (species are defined differently for asexual organisms). Speciation occurs when the gene pools of populations diverge to the point where individuals from different populations can no longer produce viable, fertile offspring together; their gene pools then become permanently isolated. Speciation can be allopatric (with geographic isolation between populations) or sympatric (without geographic isolation between populations). Over many generations following speciation, each newly diverged species accumulates its own adaptations, and the species slowly become more and more different from one another. In this way, natural selection and adaptation create species diversity.

Fundamental Questions

• What is the relationship between overpopulation and natural selection?
• Why does natural selection take place within populations and not within individuals?
• How do differing degrees of reproductive success result in natural selection?
• How do natural selection and adaptation relate to diversity of species?
• What factors might influence the rate of natural selection?