Putting it All Together: Videos & Practice Problems

The Hardy-Weinberg principle predicts genotype frequencies in a population using the equations p2 + 2pq + q2 = 1. Key assumptions include random mating, no mutation, no natural selection, large population size, and no gene flow. Deviations from these assumptions can lead to changes in allele frequencies, impacting genetic variation through mechanisms like natural selection, genetic drift, and gene flow, which are essential for understanding evolution and population dynamics.

Putting it All Together

The Hardy-Weinberg principle is a foundational concept in population genetics that allows us to predict genotype frequencies within a population. According to this principle, the frequencies of genotypes can be calculated using the equations: p^2 for homozygous dominant individuals, 2pq for heterozygotes, and q^2 for homozygous recessive individuals. Here, p represents the frequency of the dominant allele (A), while q represents the frequency of the recessive allele (a). The relationship p + q = 1 holds true, as these two alleles account for all genetic variation in the population. Consequently, the equation p^2 + 2pq + q^2 = 1 summarizes the total genotype frequencies.

However, real populations often deviate from Hardy-Weinberg equilibrium due to several assumptions that, if violated, can lead to changes in allele frequencies, which is a key indicator of evolution. The assumptions include random mating, no mutations, no natural selection, a large population size, and no gene flow. Each of these factors can influence genetic variation and the overall genetic structure of a population.

Non-random mating occurs when certain genotypes preferentially mate, affecting genotype frequencies but not allele frequencies, thus not driving evolution. Inbreeding, a common form of non-random mating, increases homozygosity and can lead to inbreeding depression, where the fitness of the population decreases due to the increased likelihood of deleterious recessive alleles pairing.

Mutations introduce new alleles into a population, thereby increasing genetic variation. While mutations are rare and often have minimal immediate impact on allele frequencies, they are essential for the process of evolution. Specific types of mutations include point mutations, duplications, and horizontal gene transfer, which can introduce genetic material from different species.

Natural selection is a critical mechanism of evolution, favoring alleles that enhance survival and reproduction. It generally reduces genetic variation by eliminating alleles with low fitness. Different types of natural selection include directional selection, which shifts the population average; stabilizing selection, which favors intermediate phenotypes; disruptive selection, which favors extreme phenotypes; and balancing selection, which maintains multiple alleles in the population, often through frequency-dependent selection or heterozygote advantage.

Genetic drift refers to random changes in allele frequencies, particularly in small populations, leading to a loss of genetic variation. The founder effect occurs when a new population is established by a small number of individuals, resulting in reduced genetic diversity. A population bottleneck is a sharp reduction in population size, which can also increase genetic drift and reduce genetic variation, even if the population later grows large again.

Gene flow, the transfer of alleles between populations through migration or gamete exchange, increases genetic variation within a population and reduces differences between populations. This exchange can reintroduce alleles that may have been lost, enhancing the genetic diversity of the receiving population.

Understanding these concepts and their implications for genetic variation and evolution is crucial for studying population genetics and the dynamics of species over time.

Which of the following statements about the assumptions of the Hardy-Weinberg equilibrium are true?


I) Non-random mating will not cause a change in allele frequency on its own.
II) Infinite population size counteracts the effects of natural selection.
III) Both deleterious and beneficial mutations will affect the Hardy-Weinberg equilibrium.

• A. I & II only.
• B. I & III only.
• C. II & III only.
• D. I, II, & III.

Answer: (B)

Which mechanisms most reliably increase genetic variation?

• A. Natural selection & random mating.
• B. Genetic drift & mutation.
• C. Gene flow & mutation.
• D. Random mating & genetic drift.

Answer: (C)

Imagine that you are a researcher studying the beaks of Galapagos finches, and you measure a sudden shift in beak size in the population over a short time period. What additional information below would best indicate that the change was due to Natural Selection and not genetic drift?

• A. There was a loss of genetic variation over the same time period.
• B. There was a large reduction in population size, with only a few individuals surviving to create the new breeding population.
• C. Allele frequency changed in the population along with the phenotypic change of beak size.
• D. The shift coincided with an environmental change that had coincided with similar previous shifts in phenotype in the past.

Answer: (D)

Here’s what students ask on this topic:

What is the Hardy-Weinberg principle and how is it used to predict genotype frequencies?

The Hardy-Weinberg principle is a fundamental concept in population genetics that predicts the genotype frequencies of a population under certain conditions. It uses the equation p2 + 2pq + q2 = 1, where p and q represent the frequencies of two alleles in the population. The principle assumes random mating, no mutation, no natural selection, a large population size, and no gene flow. By knowing the allele frequencies, we can predict the frequencies of homozygous dominant (p2), heterozygous (2pq), and homozygous recessive (q2) genotypes in the population.

What are the key assumptions of the Hardy-Weinberg principle?

The Hardy-Weinberg principle relies on five key assumptions to accurately predict genotype frequencies in a population: 1) Random mating, where individuals pair by chance without regard to their genotypes. 2) No mutation, meaning the alleles do not change from one form to another. 3) No natural selection, so all genotypes have equal chances of survival and reproduction. 4) A large population size, which minimizes the effects of genetic drift. 5) No gene flow, indicating no new alleles are added or lost through migration. Deviations from these assumptions can lead to changes in allele frequencies and disrupt Hardy-Weinberg equilibrium.

How does non-random mating affect genetic variation in a population?

Non-random mating occurs when certain genotypes are more likely to mate with each other, affecting genotype frequencies but not allele frequencies. This can push a population out of Hardy-Weinberg equilibrium. Non-random mating, such as inbreeding, increases homozygosity, leading to excessive homozygotes. This can result in inbreeding depression, where deleterious recessive alleles become more common, reducing the population's fitness. However, non-random mating does not directly increase or decrease genetic variation, as it does not add or remove alleles from the population.

What is genetic drift and how does it impact genetic variation?

Genetic drift is the change in allele frequencies due to random chance, particularly in small populations. It occurs because the probability of allele transmission does not always match expected frequencies. Genetic drift reduces genetic variation as alleles can be lost from the population. Two specific types of genetic drift are the founder effect, where a new population is established by a small number of individuals, and population bottleneck, where a sudden reduction in population size increases the rate of genetic drift. Both result in a loss of genetic diversity, even if the population size later increases.

How does natural selection influence genetic variation in a population?

Natural selection influences genetic variation by favoring alleles that increase an organism's fitness, leading to their increased frequency in the population. Generally, natural selection decreases genetic variation as alleles with low fitness are removed. However, certain types of natural selection, such as balancing selection, can maintain or even increase genetic variation. Balancing selection includes frequency-dependent selection, where rare phenotypes are favored, and heterozygote advantage, where heterozygotes have higher fitness than either homozygote. These mechanisms ensure multiple alleles are retained in the population.