Population Genetics III

Review of Hardy-Weinberg Equilibrium

Overview of Hardy-Weinberg Principles

Today's lecture marks the final discussion segment dedicated to the principles of Hardy-Weinberg equilibrium. A recap of key concepts will be provided, with a particular focus on the implications that arise when Hardy-Weinberg principles are violated. It is important to note that such violations can often lead to a cascade effect, where one breach of the principles influences another.

Application of Hardy-Weinberg to Population Decline

The discussion will also cover the historical decline of the bison population. Maps illustrating the historical extent of the bison range and its subsequent reduction demonstrate this drastic decline. From an estimated 60 million bison in 1800, the population plummeted to approximately 300 by 1890, representing a severe population bottleneck that significantly reduced genetic diversity. The implications of this bottleneck are notable: it resulted in high genetic drift due to the drastically reduced population size, and low gene flow as geographically separated herds had limited mating opportunities, leading to exacerbated inbreeding issues. Consequently, conservation efforts frequently involve the trading of individuals between herds to actively combat inbreeding.

Non-Random Mating and Its Implications

Assortative Mating

The concept of assortative mating is introduced, which describes non-random mating patterns, especially those based on genotypes and phenotypes. In this scenario, the probabilities typically assumed for random combinations of alleles in mating no longer apply, as mating becomes selective based on specific preferences.

Two Patterns of Assortative Mating:

1. Positive Assortative Mating: This pattern involves mating between individuals who share similar genotypes or phenotypes.

2. Negative Assortative Mating: This pattern describes a mating preference between individuals possessing contrasting genotypes or phenotypes.

Distinction Between Inbreeding and Assortative Mating

It is crucial to differentiate between inbreeding and assortative mating. Inbreeding specifically refers to mating among closely related individuals, whereas assortative mating is based on preferences for shared or differing phenotypes, irrespective of kinship. For example, individuals with blood type A might pair due to positive assortative mating without necessarily being closely related, as blood types are common across a large population. Regarding terminology, inbreeding generally has a more pronounced impact on genetic diversity across the entire genome, leading to greater effects compared to the typically lesser effects observed with assortative mating.

Inbreeding Effects on Genotypic Frequencies

The consequences of inbreeding are illustrated using a hypothetical flower population, with assumptions for homozygous (RR and rr) and heterozygous (Rr) genotypes. At initial equilibrium, the expected allele frequencies based on Hardy-Weinberg principles are 0.250.25 for RR, 0.250.25 for rr, and 0.500.50 for Rr. A severe inbreeding scenario, such as self-fertilization, is discussed, which leads to a reduction of heterozygote frequencies by half in each successive generation, potentially leading to the extinction of the heterozygous phenotype. The complexities of botanical reproduction methods and their significant implications for genetic diversity in natural settings are also acknowledged.

Natural Selection and Genotype Changes

Selection Mechanisms

The focus then shifts to how natural selection impacts allele and genotype frequencies. A thought experiment examines the peppered moth and the profound effect of environmental changes, specifically rapid industrialization, on the survival rates of its color variants. This analysis contextualizes the environmental transformations since the Industrial Revolution and the subsequent shifts in phenotypic advantages.

Case Study: Peppered Moth (Biston betularia)

In the case study of the Peppered Moth (Biston betularia), two primary phenotype colors are observed: Typica, which is recessive and light-colored, and Carbonaria, which is dominant and dark-colored. Expected allele frequencies are significantly influenced by environmental camouflage. A scenario demonstrates fitness values for each phenotype based on environmental benefits: the homozygous dominant (Carbonaria) has a fitness of 1, while the homozygous recessive (Typica) has a fitness of 0.5. This illustrates a rapid change in allele frequencies when the dominant phenotype confers a strong environmental advantage.

Historical Context and Impact of Pollution

The historical event of the "Great Smog of London" is highlighted to illustrate the detrimental consequences of pollution, which ultimately instigated legislative changes and heightened ecological awareness. A follow-up discussion reveals how subsequent pollution reduction influenced the moth populations, observing a delayed genetic response to these environmental improvements. The study of these moths underscores the necessity for long-term perspectives on selection and adaptation in animal species.

Genetic Response Delays

The disparity in the removal rates of beneficial phenotypes after environmental improvements emphasizes the extended timeline required for genetic adaptations. Empirical observations show that overall phenotypic changes lagged behind environmental shifts, thereby demonstrating the inherent complexities of evolutionary responses following drastic ecological changes.

Implications and Applications of Selection in Populations

Impact of Selection on Genotype Frequencies

A summary of outcomes under selective pressures indicates that dominant alleles generally fare better in terms of representation compared to recessive alleles, as selection processes yield varied outcomes depending on genetic visibility. Empirical data analyzed across generations highlight the critical importance of the genotype-phenotype relationship and the implications of recessive and dominant trait dynamics on population changes.

Practical Applications Regarding Hardy-Weinberg Violations

Discussion of population data aims to examine deviations from expected Hardy-Weinberg frequencies. Notable patterns observed include increased homozygote frequencies coupled with reduced heterozygote frequencies, which are indicative of potential inbreeding scenarios. Key considerations for future population studies involve classifying genetic impacts on allele frequencies and identifying potential environmental adaptations, while also ensuring the avoidance of technical mismatches during pattern analysis.