Question

Give examples of how each of the following can alter allele frequencies from Hardy-Weinberg equilibrium: a. nonrandom mating b. migration c. a population bottleneck d. mutation

Short answer

Nonrandom mating changes allele frequencies by increasing the proportion of certain alleles due to mate preference. Migration can alter allele frequencies by introducing new alleles into a population or removing existing ones. A population bottleneck can lead to a drastic reduction in genetic diversity and alter allele frequencies by disproportionately killing off certain genotypes. Mutation introduces new alleles into a population, potentially changing allele frequencies, particularly if the mutation provides a major advantage or disadvantage for survival or reproduction.

Step-by-step solution

Nonrandom Mating

Nonrandom mating occurs when individuals with certain genotypes or phenotypes are more likely to mate with each other than at random. This can both reduce and increase the variants of a certain gene in a population. For example, if individuals with a certain trait like tall height are preferred as mates, the allele for tallness will tend to increase in frequency - this is an example of positive assortative mating. Negative assortative mating, on the other hand, promotes heterozygosity and thus increases genetic diversity.

Migration

Migration refers to the movement of individuals from one population to another. If the allele frequencies in the two populations differ, then migration can alter the allele frequencies in both the source and destination populations. For instance, if individuals possessing a particular allele migrate out of a population, that allele's proportion would decrease in the original population. Similarly, if individuals possessing a particular allele migrate into a population, that allele's proportion would increase in the destination population.

Population Bottleneck

A population bottleneck occurs when a large percentage of the population dies off or is otherwise prevented from reproducing. This can greatly reduce the genetic diversity and thus the allele frequencies in the remaining population. In severe cases, some alleles may be lost entirely, and the genetic diversity of the population can take a long time to recover.

Mutation

Mutation is a change in DNA sequence. If these changes occur in the germ line, i.e., in the cells that give rise to eggs or sperm, they can be passed to offspring. Because a mutation is a novel change to the DNA, it introduces new alleles into the gene pool. Hence, these new alleles can alter allele frequencies in the population. However, the effect of a single mutation on allele frequencies is typically very small unless the mutation offers a significant survival or reproduction benefit.

Key concepts

Hardy-Weinberg equilibrium

The Hardy-Weinberg equilibrium is a foundational principle in population genetics. It describes a state where allele frequencies in a population remain constant from generation to generation, provided that certain conditions are met. These conditions include:

• Random mating
• No mutations
• No migration
• Large population size
• No natural selection

When these conditions are not met, the population may experience changes in allele frequencies. This makes it a useful model to understand real-world genetic variations.
Learning the conditions under which a population remains stable helps in predicting how various forces like selection, mutation, and migration can alter genetic makeup.

Nonrandom mating

Nonrandom mating occurs when certain individuals in a population are more likely to mate with each other than with others. This disrupts the randomness needed for Hardy-Weinberg equilibrium. There are different patterns of nonrandom mating:

• Positive assortative mating: Individuals with similar phenotypes are more likely to mate. This can lead to an increase in homozygosity, where offspring are more likely than not to have two identical alleles for a trait. For instance, if tall individuals preferentially mate with other tall individuals, the population will see an increase in frequency of the tallness allele.
• Negative assortative mating: Individuals with dissimilar phenotypes are more likely to mate. This results in increased heterozygosity, contributing to greater genetic diversity.

Both assortative and disassortative mating can dramatically shift allele frequencies over successive generations, impacting the genetic variation in populations.

Population bottleneck

A population bottleneck occurs when a large portion of a population is suddenly reduced. This could happen due to catastrophic events like natural disasters, diseases, or human activities. The consequences are profound:

• The genetic diversity plummets, often significantly reducing the variety of alleles present.
• Allele frequencies can change sharply, with some alleles potentially becoming extinct if no remaining individuals carry them.

A bottleneck effect results in a reduced gene pool, which can make populations vulnerable to inbreeding and less adaptable to environmental changes. Recovery of genetic diversity can take many generations, if it happens at all. Despite this, bottlenecks provide insight into how populations adjust to stress and fluctuating environmental conditions.

Mutation

Mutations are changes in the DNA sequence that occur naturally in all organisms. They serve as a crucial source of genetic variation and can occasionally lead to significant alterations in allele frequencies:

• Mutations can introduce new alleles into a population. Even though individual mutations are random and rare events, over time, they provide the raw material for evolution.
• The impact of a single mutation on the allele frequency is relatively minimal unless the mutation significantly affects an organism's survival or reproductive capabilities.

The effect of mutations accumulates over time, potentially leading to evolutionary changes. Understanding mutations helps in appreciating the dynamic nature of genetic information within populations.

Migration

Migration involves the movement of individuals from one population to another. It is a powerful force in altering allele frequencies and can have several consequences:

• When individuals migrate, they bring their alleles with them, potentially introducing new genetic material to the destination population.
• If the origin and new population have different allele frequencies, migration can increase genetic variation within the new population.
• The loss or gain of alleles due to migration can disrupt Hardy-Weinberg equilibrium and lead to genetic divergence between populations.

Migration can foster genetic exchange between populations, influencing evolution and adaptation by altering the genetic structure of populations.