Genetics and Evolution

Genetic Variation and Evolution

• Genetic variation occurs between generations within a species due to mechanisms such as Natural Selection, Sexual Selection, Artificial Selection, Genetic Drift, and Gene Flow.

Hardy-Weinberg Equilibrium

• A population in Hardy-Weinberg Equilibrium (HWE) has genotypic frequencies determined by allelic frequencies.
• In HWE, the heterozygote frequency never exceeds 0.5, and when a recessive allele is rare, most individuals who carry the allele are heterozygous.

Types of Natural Selection

• Directional Selection: favors individuals with extreme phenotypes, leading to an increase in the frequency of the favored allele.
• Stabilizing Selection: favors individuals with intermediate phenotypes, leading to a decrease in the frequency of extreme alleles.
• Disruptive Selection: favors individuals with extreme phenotypes, leading to an increase in the frequency of both extreme alleles.

Examples of Natural Selection

• Human babies at birth: stabilizing selection favors babies with average birth weights.
• Pesticide resistance: directional selection favors individuals with resistant phenotypes.
• Peppered moth: directional selection favors individuals with dark-colored wings.
• Finches in West Africa: directional selection favors individuals with large or small beak sizes, leading to specialization in seed cracking.

Limitations of Evolution

• Evolution is limited by historical constraints, such as the lack of certain mutations.
• Mutations are often neutral or harmful, and only beneficial in changing environments.
• Diploidy can hide genetic variation from selection, and dominant alleles can "hide" recessive alleles in heterozygotes.

Balanced Polymorphism

• Balanced polymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes.
• Heterozygote advantage is an example of balanced polymorphism, where heterozygotes have a higher fitness than homozygotes.

Co-evolution

• Co-evolution occurs when there is a reciprocal evolutionary response between two species, such as in parasite-host relationships.
• Example: the survival of one phenotype declines if that form becomes too common, leading to the emergence of new resistant phenotypes.

Neutral Variation

• Neutral variation is a genetic variation that results in no competitive advantage to any individual.
• Neutral variation can be maintained in a population due to genetic drift or other mechanisms.

Natural Selection and Adaptation

• Natural selection is the only agent of microevolution that consistently works to adapt organisms to their environment.
• Natural selection leads to adaptation through the survival and reproduction of individuals with favored phenotypes.

Example of Natural Selection

• Resistance to antibacterial soap: natural selection leads to an increase in the frequency of the resistant allele over generations.

Understand the principles of genetics and evolution, including mutation, genetic drift, natural selection, and non-random mating. Learn how alleles combine to form genotypes and how these processes shape the evolution of species.