Key Concepts
By the end of this section, you will be able to do the following:
- Explain the different ways natural selection can shape populations
- Describe how these different forces can lead to different outcomes in terms of the population variation
Natural selection acts on the population’s heritable traits: selecting for beneficial alleles that allow for environmental adaptation, and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency. Scientists call this process adaptive evolution. If we look at a single individual within a population, it may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce (fecundity), but that same individual might also carries an allele that results in a fatal childhood disease, so that fecundity phenotype will not pass to the next generation because the individual will not live to reach reproductive age. So natural selection occurs when individuals within a population have a particular genotype that is more likely to be passed onto the next generation. Scientists call this an organism’s evolutionary (Darwinian) fitness.
Fitness is often quantifiable and is measured by scientists in the field. However, it is not an individual’s absolute fitness that counts, but rather how it compares to the other organisms in the population. Scientists call this concept relative fitness, which allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.
There are several ways selection can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As natural selection influences the allele frequencies in a population, individuals can either become more or less genetically similar and the phenotypes can become more similar or more disparate.
Link to Learning
Stabilizing Selection
If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo stabilizing selection (Figure 19.8). In a mouse population that live in the woods, for example, natural selection is likely to favor mice that best blend in with the forest floor and are less likely for predators to spot. Assuming the ground is a fairly consistent shade of brown, those mice whose fur is most closely matched to that color will be most likely to survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles that make them a bit lighter or a bit darker will stand out against the ground and be more likely to fall victim to predation. As a result of this selection, the population’s genetic variance will decrease.
Directional Selection
When the environment changes, populations will often undergo directional selection (Figure 19.8), which selects for phenotypes at one end of the spe