(10-7) - Notes Changing Population Genetics and Mechanisms of Evolution

Bottleneck

A bottleneck occurs when a population’s gene pool is drastically reduced due to environmental events or human activities, leaving a small, less diverse set of alleles. This reduction in genetic variation can make populations more vulnerable to environmental stressors.

• Example: A population originally has alleles A, a, B, b, C, c, D, d, E, and e. If certain alleles are lost (e.g., through fire or migration), diversity decreases.

• Cheetahs have undergone two historical bottlenecks, leading to non-random mating and small population sizes, both of which violate Hardy-Weinberg (H-W) equilibrium assumptions and result in inbreeding depression and reduced population health.

Genetic Drift

Genetic drift is the random change in allele frequencies in a population over time due to chance events during reproduction (random mating).

• Example: If allele A is 60% and a is 40%, random mating might shift frequencies to 70% A and 30% a, then later 55% A and 45% a.
  Key characteristics:
  a. Drift is random.
  b. Allele frequency changes occur by chance, not adaptation.
  c. Frequencies fluctuate up and down over generations.
  d. Non-adaptive process — not driven by natural selection.
  e. Strongest in small populations (e.g., after bottlenecks or founder effects).
  f. Can lead to fixation (only one allele remains) or loss of alleles.
  g. Does not increase allele diversity — it only alters existing allele frequencies.
  h. Drift within expected randomness does not violate H-W equilibrium since random mating is an assumption of the model.

Gene Flow

Gene flow is the movement of individuals and their alleles between populations.

• It tends to equalize allele frequencies between populations, making them genetically more similar.

• Increases genetic diversity in the population receiving migrants but can decrease diversity in the population losing individuals.

• Gene flow and mutation are the two primary ways to increase allele diversity in a population.

Nonrandom Mating

In the Hardy-Weinberg model, mating is assumed to be random, but in most species (insects, vertebrates, plants), mate selection is nonrandom.
Even in species that release gametes broadly (like wind-pollinated grasses), nearby individuals are more likely to mate than distant ones.
Nonrandom mating can occur through:

1. Assortative Mating

2. Inbreeding

3. Sexual Selection

Assortative Mating

Occurs when individuals prefer mates that are phenotypically similar to themselves (positive assortative mating) or dissimilar (negative/disassortative mating).

• Common examples include humans, plants, blister beetles, and wolves.

• This type of mating can influence genotype frequencies without necessarily changing overall allele frequencies.

Inbreeding

Inbreeding is mating between individuals that share a recent common ancestor, increasing the likelihood of offspring inheriting identical alleles.

• “Recent” depends on species and how quickly new alleles are introduced or maintained.

• Example: Cheetahs are so genetically uniform that almost all matings are inbred, even between individuals separated for generations.

• Plant example: Some pea plants can inbreed for many generations with little effect.

• Researchers track genetic lineages (especially in conservation breeding programs) to maximize diversity and avoid excessive inbreeding.

Inbreeding Depression

The reduction in biological fitness resulting from increased homozygosity and decreased heterozygosity due to inbreeding.

• Caused by recessive deleterious alleles being expressed in homozygous form.

• Leads to reduced survival, fertility, and adaptability.

• Does not change allele frequencies, so evolution is not occurring — only genotype frequencies shift.

• Observed in both humans and plants (e.g., cardinal flower).

Sexual Selection

A special form of natural selection that favors individuals with traits that increase mating success.

• Typically acts more strongly on males than females because females usually invest more in offspring (eggs, gestation, parental care).

• Females tend to be choosy, selecting mates based on traits that signal health, strength, or resources.

• Males, with lower investment, compete for access to females and often display showy traits.

• Examples: Peacocks (pea-fowl) and guppies.

Female Choice

Females select males with traits that indicate good alleles or high fitness.

• Bright coloration, elaborate displays, or large body size suggest health and genetic quality.

• Healthy males can “afford” to invest energy in showy traits, while unhealthy males cannot.

• Females may also choose based on non-physical factors such as a male’s ability to provide resources, care for young, or defend territory.

• Example: Peahens choosing brightly colored peacocks.

Male–Male Competition

Occurs when males compete directly for mating opportunities.

• Typically, stronger, larger, or more dominant males win access to females.

• Example: White-tailed deer, where males fight for breeding rights.

• Although it may seem like “female choice,” this form of sexual selection is driven by male competition, not preference.

• Both male competition and female choice can result in sexual dimorphism—distinct differences in appearance between males and females.

Sexual Dimorphism

The condition where the two sexes of a species appear different in size, coloration, or morphology.

• Often a result of sexual selection, though not always.

• Examples include brightly colored male birds or larger male mammals compared to females.