BIOL 286: Lecture #2, Pt. 2 Review (Population Genetics & Hardy Weinberg Equilibrium)

Genetics, genomics, and population genetics can offer insights into: (5)

(1) speciation; (2) relatedness; (3) genetic diversity within a population; (4) gene flow between populations; (5) if there was a reduction in genetic diversity (e.g. bottleneck effect or founder effect)

How do ecologists and evolutionary biologists study genetic variation within populations? (5)

investigate (1) the whole genome; (2) organelle genomes (e.g. mitochondrial or chloroplast genes); (3) DNA sequence of one gene; (4) RNA (patterns of gene expression); (5) epigenetics (e.g. patterns of methylation and acetylation)

Populations have both . . . and . . . variation

phenotypic; genotypic

We can track the proportions of . . ., . . ., and . . . in whole populations

alleles; genotypes; phenotypes

gene pool

The collection of alleles in a population

Hardy-Weinberg population equilibrium

States that a population's allele and genotype frequencies are inherently stable—unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. Assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population. While no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes.

Assumptions of Hardy-Weinberg population equilibrium: (4)

(1) there are no mutations--no new alleles are introduced to the population; (2) there is no migration (i.e. no gene flow)--no new alleles are added to or removed from the population; (3) there is no selection--a given genotype is not favored over another; (4) there is an infinitely large population; (5) mating is random--there is no preferential mating based on genotype or phenotype

Hardy-Weinberg principle equations: Allele frequency

For a population with two alleles p (dominant allele) and q (recessive allele), where p and q represent frequencies of occurrence, it follows p + q = 1

Hardy-Weinberg principle equations: Genotype frequency

For a population with two alleles p (dominant allele) and q (recessive allele), where p and q represent frequencies of occurrence, it follows p^2 + 2p*q + q^2 = 1. This equation represents the three possible genotypes (homozygous dominant, heterozygous, and homozygous recessive) and their respective likelihoods of realization.

Terms of Hardy-Weinberg principle equations: Homozygous dominant

p^2 (p*p): the chance of inheriting two dominant alleles

Terms of Hardy-Weinberg principle equations: Heterozygous

2p*q (p*q + p*q): the chance of inheriting one dominant allele and one recessive allele (two possible permutations)

Terms of Hardy-Weinberg principle equations: Homozygous recessive

q^2 (q*q): the chance of inheriting two recessive alleles

If most populations fail to meet the assumptions of Hardy-Weinberg equilibrium, how is it a useful concept?

We can compare the proportions of genotypes and alleles in a population to what we would expect if the population was in Hardy-Weinberg equilibrium and, accordingly, formulate and test hypotheses proffering evolutionary explanations for the observed phenomena.

Mechanisms that can change allele/genotype frequencies: (5)

(1) mutations (introduce new alleles); (2) gene flow (introduces or removes alleles from populations); (3) non-random mating; (4) genetic drift, founder effects, and bottleneck effects; (5) selection

gene flow

The movement of individuals, and their alleles, between different populations

Causes of gene flow: (5)

(1) animals moving; (2) male mammals leaving their mother and either joining or starting another group; (3) pollinators travelling with pollen; (4) seeds dispersing long distances; (5) movement of larval fish in oceans

Explain how humans can contribute to gene flow in wildlife, using the Florida panther as a case study.

Humans can facilitate gene flow when natural movement between wildlife populations is blocked by habitat loss, roads, or fragmentation. The Florida panther is a classic example: by the 1990s, the population was so isolated that inbreeding caused heart defects, kinked tails, low sperm quality, and reduced survival. To restore genetic diversity, wildlife managers introduced several female Texas pumas—members of a closely related subspecies—into Florida. Their successful breeding brought new alleles into the population, increased genetic variation, and reversed many inbreeding‑related problems. This human‑mediated gene flow helped stabilize and partially recover the Florida panther population.

non-random mating

Refers to a situation where individuals in a population do not pair up by chance. Instead, they choose mates based on specific traits, behaviors, or preferences, which can influence the genetic structure of the population. This means that the probability of mating is not the same for all individuals, leading to unequal chances of mating among them

Types of non-random mating:

(1) positive assortative mating; (2) negative assortative mating

Types of non-random mating: positive assortative mating

A type of non-random mating that occurs when similar individuals prefer to mate with each other (e.g. aggressive bluebirds prefer to mate with other aggressive blue birds, while non-agggressive blue birds prefer to mate with other non-aggressive blue birds)

How does positive assortative mating impact the frequency of heterozygotes in the population?

it decreases the frequency of heterozygotes

Types of non-random mating: negative assortative mating

A type of non-random mating that occurs when dissimilar individuals prefer to mate with each other (e.g. in the white-throated sparrow population, tan-striped birds prefer to mate with white-striped birds)

How does negative assortative mating impact the frequency of heterozygotes in the population?

it increases the frequency of heterozygotes in the population

Types of non-random mating: inbreeeding

A type of non-random mating that occurs when closely related individuals mate (can be a problem in small populations)

How does inbreeding impact population heterozygosity?

it decreases heterozygosity and increases homozygosity

Types of non-random mating: self-fertilizing

A type of non-random mating that occurs when an individual reproduces with itself rather than with another genetically distinct individual, and this pattern is not random with respect to the population's genetic composition (e.g. Arabdopsis thaliana)

How does self-fertilization impact population heterozygosity?

it decreases heterozygosity and increases homozygosity

genetic drift

random fluctuations in gene and allele frequencies in a population (i.e. not caused by selection)

Genetic drift often occurs in . . .

small populations

Genetic drift may be caused by . . . and . . .

bottleneck effects; founder effects

bottleneck effect

Occurs when there is a sudden decrease in population size, potentially resulting in a loss of alleles or a change in allele frequencies. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation significantly, even if the bottleneck itself doesn't last many generations.

Explain how the bottleneck effect impacts population genetics, using Northern elephant seals as a case study.

The bottleneck effect occurs when a population is suddenly reduced to very few individuals, leaving only a small, unrepresentative sample of the original gene pool. Northern elephant seals experienced this in the 1800s, when hunting reduced them to fewer than 100 animals. Although the population has since rebounded to well over 100,000, their genetic diversity remains extremely low because all modern seals descend from that tiny group of survivors (very low heterozygosity). This loss of variation limits the population's ability to adapt to environmental change and increases vulnerability to disease and other stressors.

founder effect

In population genetics, the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population

Explain how the founder effect impacts population genetics, using Mexican wolves as a case study.

The founder effect occurs when a new population is established by a very small number of individuals, carrying only a fraction of the genetic variation present in the original group. Mexican wolves illustrate this clearly: the entire modern population descends from just seven surviving founders taken into captivity in the late 20th century. Because all living wolves trace back to this tiny genetic base, the population shows low genetic diversity, higher inbreeding risk, and reduced adaptive potential. Even as numbers increase through recovery programs, the genetic constraints created by the founder event persist.