Topic 4- Biol 213

Genetic Variation

The raw material required for evolutionary change

Heterozygosity

A population state where individuals have a high proportion of heterozygous loci, containing more genetic variation

Homozygosity

A population state where individuals have a high proportion of homozygous loci, containing less genetic variation

Evolution

Changes in allele frequencies in a population over time

Mechanisms that change allele frequencies over time

selction, genetic drift, gene flow, mutation

Mechanisms that change genotype frequencies over time

Non-random mating

Mutation

The ultimate source of all genetic variation, introducing novel differences in DNA sequences in offspring compared to their parents

SNP (Single Nucleotide Polymorphic)

A variation at a single position in a DNA sequence among individuals in a population

Genetic Drift

Unpredictable changes in allele frequencies in a population over generations due to chance, offspring represent a random sample of the gene pool of prev gen

Genetic drift is minimized

with increasing population size

Neutral Evolution

Evolutionary changes in the frequency of selectively neutral variation (which doesn't affect survival or reproduction), frequency of this not affected by natural selection but is affected by genetic drift

Effects of Genetic Drift

Loss of heterozygosity(reduction in genetic variation), change in allele freq, stronger effect in small pop, losee or fixation of alleles, and divergence over time

Loss of heterozygosity

alleles are lost at loci until only one remains and allele frequencies drift towards fixation or loss

examples of genetic drift

bottleneck effect, and founder effect

Bottleneck Effect

An extreme form of genetic drift that occurs when a population's size is sharply reduced by a sudden event, drastically lowering genetic variation

Founder Effect

An extreme form of genetic drift that occurs when a small group of individuals splits off from a larger population to establish a new colony, resulting in lower genetic diversity [cite: 144, 190]

Gene Flow

The transfer of alleles into (immigration) and out of (emigration) a population

Bidirectional gene flow

The transfer of alleles into and out of a population through both immigration and emigration.

Unidirectional gene flow

The transfer of alleles into a population without any corresponding emigration of alleles out of the population.

Effects of gene flow

genetic difference between pop(homogenizes pop), can restore genetic variation lost to genetic drift in small pop, can slow adaptation to local conditions

Natural Selection

A process where heritable variations in phenotypic traits lead to differences in fitness (differential survival and reproduction) among individuals

Natural Selection evolution Conditions

variation in phenotypic traits, variation must be heritable, variation lead to difference in fitness between individuals

Types of natural selection

directional, stablilzing, disruptive/divergent,

Directional Selection

A type of natural selection where trait values shift progressively toward one direction over time, trait value increases or decreases

Stabilizing Selection

A type of natural selection where intermediate trait values are the most fit, narrowing the trait's frequency distribution

Disruptive (Divergent) Selection

A type of natural selection where trait values shift toward both extremes, decreasing the intermediate values and splitting the population into distinct groups

Frequency-Dependent Selection

A selection process where an individual's fitness is directly tied to how common or rare its phenotype/genotype is in the population

Positive Frequency-Dependent Selection

A form of selection where the most common genotypes or phenotypes have the highest relative fitness

Negative Frequency-Dependent Selection

A form of selection where rare genotypes or phenotypes have the highest relative fitness (e.g., host-parasite immune dynamics)

Negative Frequency-Dependent Selection (Host-Parasite Example)

Parasites target the common host phenotype, driving hosts to mate with rare immune genotypes so offspring have diverse antigen proteins

Beak Depth Evolution (Daphne Major)

During the 1977 drought, finches with deeper beaks survived better because they could crack the remaining large, hard seeds, leading to the evolution of larger average beak size

1977 Drought

A severe environmental event on Daphne Major that caused a sharp decline in seed abundance and finch populations, leaving mostly large, hard seeds

Peter and Rosemary Grant

Researchers who conducted a famous long-term case study on natural selection using Darwin's finches

Natural Selection (Three Conditions)

1. Variation in phenotypic traits, 2. Heritability of the variation, 3. Variation leads to differences in fitness (differential survival/reproduction)

Adaptation

A heritable trait that increases an organism's fitness in a particular environment

Adaptive Evolution

Consistent evolutionary change driven by natural selection that makes an organism more suitable to its habitat

Sexual Selection

Differential reproductive success driven specifically by variation in success at obtaining mates

Two types of Sexual selection

intersexual, and intrasexual

Intersexual Selection

A form of sexual selection where individuals of one sex (usually females) exercise choice over their mates from the opposite sex

Intrasexual Selection

A form of sexual selection involving direct competition between individuals of the same sex (usually male-male competition) to gain access to mates

Non-Random Mating

A phenomenon where the likelihood of mating depends on phenotypic traits, which changes genotype frequencies without altering allele frequencies on its own

Assortative Mating

Mating patterns based directly on phenotypic similarity or dissimilarity

Positive Assortative Mating

Mating between phenotypically similar individuals, which increases homozygosity at the associated loci

Negative Assortative Mating

Mating between phenotypically different individuals, which increases heterozygosity at the associated loci

Inbreeding

Mating with oneself (selfing) or with closely related individuals, which is highly inevitable in small populations and increases homozygosity

Inbreeding Depression

The reduction in fitness of inbred individuals caused by increased homozygosity expressing harmful recessive alleles

Condition 1: Phenotypic Variation (Finch Case Study)

Daphne Major birds in 1976 showed clear phenotypic variation in beak depth, ranging from 6 mm to 14 mm

Condition 2: Heritability (Finch Case Study)

Finch beak depth is highly heritable, showing a strong positive correlation between midparent and midoffspring beak depth (h² = 0.74)

Condition 3: Fitness Differences (Finch Case Study)

Variation in beak depth directly influenced survival during the drought

Evidence of Finch Evolution (1976 vs. 1978 offspring)

The offspring of drought survivors (hatched in 1978) had significantly larger average beak sizes than the generation hatched before the drought (1976), proving the trait evolved

Long-term Beak Evolution Trends

Daphne Major finch traits are dynamic

Asymmetry of Sex (Females vs. Males)

Females are limited by resource-intensive, expensive, and few eggs (not mates)

Bateman's Principle

The prediction that sexual selection is more potent in driving phenotypic evolution in males than females because males are limited by mates and females are limited by eggs

Male Competition vs. Female Choosiness

Two evolutionary outcomes of Bateman's principle where males actively fight for access to females and females are highly selective in picking the "best" males