BIOL 120: Chapter 23 Activity Review
BIOL 120: Chapter 23 - Genetic Variation Mechanisms and Effects
Overview of Genetic Variation Processes
The following outlines various processes and mechanisms that affect genetic variation in populations, including their definitions, examples, and effects on variation.
Processes and Mechanisms of Genetic Variation
1. Disruptive Selection
Effect on Variation: Decrease
Description/Example: Selects for both extreme phenotypes. This type of selection increases the variance in the population as it favors individuals at both ends of the phenotypic spectrum, leading potentially to speciation.
2. Founder Effect
Effect on Variation: Decrease
Description/Example: An example of the effect is the incidence of retinitis pigmentosa on Tristan de Cunha, or the prevalence of Huntington’s Disease in Venezuela. The founder effect occurs when a small group establishes a new population, and the genetic variation of this new population is limited compared to its source population.
3. Heterozygote Advantage
Effect on Variation: Maintain
Description/Example: Heterozygotes have greater fitness than homozygotes in certain environments, fostering genetic variation as both alleles are preserved in the population. An example is the preservation of the sickle-cell allele in malaria-endemic regions due to the advantage provided by heterozygosity.
4. Intrasexual Selection
Effect on Variation: Decrease
Description/Example: Individuals within one sex (typically males) compete with each other for mating opportunities. This competition can lead to the dominance of certain traits in males that enhance their mating success, thus reducing variation among males who do not possess those traits.
5. Migration
Effect on Variation:
Immigration = Increase
Emigration = Decrease
Description/Example: Movement of humans from war-torn countries around the globe to the United States highlights how migration can introduce new genetic variations into a population or alter existing genetic structures through changes in allele frequencies.
6. Stabilizing Selection
Effect on Variation: Decrease
Description/Example: Selects against extreme phenotypes, favoring average traits. This kind of selection reduces variation by favoring the mean phenotype within a population, as seen in human birth weights where extreme weights (both low and high) are less favorable for survival.
7. Balancing Selection
Effect on Variation: Maintain
Description/Example: Balancing selection includes frequency-dependent selection and heterozygote advantage, maintaining genetic diversity within a population by ensuring multiple alleles persist, as occurs in certain cases of predator-prey interactions affecting coloration.
8. Directional Selection
Effect on Variation: Decrease
Description/Example: Shifts the average of a particular trait in one direction, favoring one extreme phenotype. An example includes the increase in size of beaks in finches during periods of drought that favor birds with larger beaks.
9. Bottleneck Effect
Effect on Variation: Decrease
Description/Example: A severe drop in population size leads to a loss in genetic diversity. Events such as natural disasters can drastically reduce population sizes, consequently reducing genetic variability.
10. Intersexual Selection
Effect on Variation: Decrease
Description/Example: Type of selection where, often, a female selects for a high-quality or showy male. This selection can limit genetic variation if certain traits become overrepresented in the gene pool.
11. Natural Selection
Effect on Variation: Decrease
Description/Example: When the environment causes selection for some phenotypes and against others, it can reduce genetic variation by favoring the traits that enhance survival and reproductive success in a given environment.
12. Mutation
Effect on Variation: Increase
Description/Example: Genetic errors that result in new alleles can contribute to the genetic diversity of a population. Mutations provide the raw material for evolutionary changes, potentially leading to new adaptations and traits.