biol 1p92 (lec 2)

Population Genetics

• Population genetics studies genes and genotypes within a population.
• It merges natural selection, Mendelian inheritance, and modern molecular genetics.
• Population geneticists examine genetic variation within gene pools, how variation changes between generations, and explanations for this variation.

Key Concepts

• Genes in populations
• Natural selection
• Sexual selection
• Genetic drift
• Migration and non-random mating

Key Terminology

• Gene: A DNA sequence coding for RNA or protein; contributes to organism traits.
• Gene locus: The specific location of a gene on a chromosome.
• Allele: Different variants of a gene.
• Genotype: The combination of alleles for a specific gene (e.g., Tt).
• Homozygous: Having identical alleles for a gene (e.g., AA).
• Heterozygous: Having different alleles for a gene (e.g., Aa).
• Gene pool: All the alleles for every gene in a given population.

Genotype vs. Phenotype

• Genotype: The combination of alleles an organism possesses at a specific locus or several loci.
• Phenotype: The observable characteristics of an organism. (E.g., homozygous dominant = AA, heterozygous = Aa, homozygous recessive = aa)

Populations

• A group of individuals of the same species occupying the same environment, able to interbreed.
• Some species span wide geographic areas and are divided into smaller populations.

Genes Are Usually Polymorphic

• Many traits display variation (polymorphism) within a population.
• A polymorphic gene has two or more alleles, with an allele frequency greater than 1%.
• Most variation stems from single nucleotide polymorphisms (SNPs).
• Large, healthy populations show high genetic diversity.

Polymorphism in Horse Coat Colour

• Chestnut vs. black coat colour in horses (and many other species) is due to different alleles of the MC1R gene.
• This gene affects eumelanin (black) vs. pheomelanin (red) pigment accumulation.

Allele and Genotype Frequencies

• Allele frequency: The proportion of a specific allele in a population.
• Genotype frequency: The proportion of a specific genotype in a population.

Example: Four o'Clock Flower

• A population of 100 four o'clock plants has different flower colors based on genotypes.
• Genotypes (and corresponding numbers) include: CRCR (49), CRCW (42), CWCW (9).

Allele frequency calculation

• Frequency of CW = [(CRCW) + 2(CWCW)] / 2(CRCR) + 2(CRCW) + 2(CWCW)

Hardy-Weinberg Equilibrium

• A mathematical relationship between allele and genotype frequencies within a population.
• Under certain conditions, allele and genotype frequencies will not change between generations (equilibrium).
• Conditions including: no new mutations, no natural selection, large population size, no migration between populations, random mating.

Hardy-Weinberg Equilibrium Predictions

• p² and q² represent frequencies of homozygous genotypes.
• 2pq represents frequency of heterozygous genotypes.
• This equation assumes that no other evolutionary factors like mutations, natural selection, or genetic drift affect the population.

Microevolution

• Evolution of a population by changes in gene pool in successive generations.
• It includes mechanisms including mutation, gene duplication, horizontal gene transfer, natural selection, genetic drift, migration, and non-random mating.

Factors That Govern Microevolution

• New mutations introduce new alleles with varying traits.
• Gene duplication creates extra copies of genes.
• Horizontal gene transfer occurs when a gene from one species moves to another.
• Natural selection leads to the adaptation and survival of organisms based on traits.
• Genetic drift results in chance changes in allele frequencies that are prominent in small populations.
• Migration of individuals or alleles into or out of a population changes their frequencies.
• Non-random mating can influence the balance of genotypes in a population.

Natural Selection

• Beneficial heritable traits consistently become more common over successive generations.
• It results in increased fitness within a population.
• It has various patterns (directional, stabilizing, diversifying, balancing).

Reproductive Success

• The likelihood that a particular genotype will contribute to the next generation compared to other genotypes.
• It depends on a number of traits (influences survival and breeding).

Fitness

• The measure of reproductive success.
• It's not always connected to physical attributes alone.
• A genotype's fitness is relative to other genotypes' values within a population.
• Mean fitness measures the population's average reproductive success.

Fitness Example

• Example: A hypothetical gene with alleles A and a shows different reproductive success (AA = 5 offspring, Aa = 4 offspring, aa = 1 offspring).

Natural Selection Patterns

• Directional selection.
• Stabilizing selection.
• Disruptive/Diversifying selection.
• Balancing selection.

Directional Selection

• Selection for one extreme phenotype.
• Driven by environmental changes or introduction of novel, highly fit alleles.
• Can lead to genetic monomorphism.

Example: Directional Selection (Mice).

• Dark brown mice have higher fitness than light-colored mice in a dark forest environment leading to an increase in their frequency in the population.

Example: Directional selection (antibiotic resistance).

• Individuals resistant to antibiotics tend to survive and reproduce more frequently in populations exposed to antibiotics.

Stabilizing Selection

• Selection for intermediate phenotypes.
• Extreme phenotypes either don’t survive well enough or do not reproduce sufficiently.
• Example includes clutch size or birth weight in offspring

Diversifying Selection (Disruptive Selection)

• Selection for two or more extreme phenotypes.
• Population occupies two environments, and fitness values are high in one environment but low in the other and vice versa.
• Members of the population need to be able to interbreed.

Balancing Selection

• Selection that maintains genetic diversity.
• Two common mechanisms: heterozygote advantage and negative frequency-dependent selection.

Heterozygote Advantage(Balancing Selection)

• Heterozygotes exhibit higher fitness compared to homozygotes.
• Example: Sickle cell disease in malaria-prone areas.

Negative Frequency-dependent Selection (Balancing Selection)

• Rare genotypes possess higher fitness than the common ones.
• Example: Predator-prey relationships, where rare prey is less susceptible to predation.

Sexual Selection

• Individuals with certain traits are favored for reproductive success.
• Often influences males more than females.
• Traits related to mate attraction (secondary sex characteristics).
• Two types: intrasexual selection and intersexual selection

Sexual Dimorphism

• Significant difference in appearances between the two sexes in a species.

Intrasexual Selection

• Male-to-male competition for access to mates.
• Examples include horns in sheep or antlers in moose or enlarged claws in fiddler crabs .

Intersexual Selection

• Mate choice based on the characteristics of the male.
• Example: the brightly colored plumage of peacocks.

Cryptic Female Choice

• Females choose which male's sperm fertilizes eggs.
• This can lead to differential fertilization success for males.

The Cost of Reproduction

• Traits favored by sexual selection may decrease survival.
• The cost is higher if these traits make organisms more conspicuous to predators, which could reduce their fitness.

Genetic Drift

• Random changes in allele frequencies due to chance events, not fitness.
• Strongest in smaller populations.
• Can lead to loss or fixation of alleles.

Bottleneck Effect (Genetic Drift)

• Population size suddenly decreases, and survives but loses genetic diversity.
• Remaining individuals randomly determine the allele frequencies of the next population.

Founder Effect (Genetic Drift)

• A small group separates from a larger population and starts a new colony.
• Allele frequencies in the new population may differ significantly from the original one.

Gene Flow

• Transfer of alleles into or out of a population.
• Happens when individuals with different allele frequencies migrate between populations.
• Tends to reduce differences between populations and increase genetic diversity within a single population.

Non-random Mating

• Individuals choose mates based on genotype or phenotype.
• This can alter genotype frequency in populations.
• Two forms of non-random mating are assortative and disassortative.

Assortative Mating

• Individuals with similar phenotypes are more likely to mate and increase the proportion of homozygotes.

Disassortative Mating

• Individuals with dissimilar phenotypes are preferentially matched, resulting in increased heterozygosity.

Inbreeding

• Mating of genetically related individuals.
• Increases homozygosity and decreases heterozygosity potentially impacting populations' overall fitness.

Inbreeding Depression

• Reduced fitness in a population due to inbreeding.
• Occurs when offspring of close relatives have lower fitness than those not related.

Genetic Restoration of Populations

• Conservation strategies may aid populations that have gone through inbreeding depression and gene flow problems by introducing new alleles.