Chapter 19: Genetic Diversity in Populations

Darwin and Wallace

Charles Darwin and Alfred Wallace proposed that species change over time through natural selection:

- Individuals with traits better suited to their environment are more likely to survive and reproduce.

- These individuals pass on favorable traits to the next generation.

Population

A group of organisms of the same species that live together in a defined area and time. Example: Ground squirrels in a field one summer, or wild rose bushes in a valley. Individuals in a population usually interbreed, though occasional breeding with nearby populations can occur. Fragmentation of habitats (e.g., by roads) can create separate populations.

Gene

are carried on chromosomes and determine traits like flower or fur color. A gene can exist in different forms, called alleles.

Example: Black coat allele (B) and white coat allele (b) in mice.

Genotype Frequency

The proportion of a population with a specific genotype.

Phenotype Frequency

The proportion of a population with a specific observable trait.

For recessive traits like white coat color, phenotype frequency equals the genotype frequency of bb.

Allele Frequency

The frequency of a specific allele within the population.

Diploid organisms have 2 alleles per gene, so total alleles = 2 × number of individuals.

Gene Pool

The gene pool is the total of all alleles for all genes in a population. Represents all the genetic variation that can be passed to the next generation. Greater variety in the gene pool usually leads to better survival in changing environments.

The Hardy-Weinberg Principle

Describes a population in genetic equilibrium (no evolution).

Conditions for Hardy-Weinberg equilibrium:

- Large population size

- No migration.

- No mutations.

- Random mating.

- No natural selection.

If these conditions are met, allele and genotype frequencies remain constant over time.

Applications of the Hardy-Weinberg Equation

It is used to:

- Predict the proportion of carriers for genetic conditions.

- Determine if a population is undergoing microevolution (changes in allele frequency).

- Calculate actual numbers of individuals with certain genotypes (requires knowing population size, N):p²N + 2pqN + q²N = N

Microevolution

Occurs if allele frequencies change over time.

Genetic Equilibrium

Occurs if allele frequencies remain constant.

Natural Selection

A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits.

Gene Flow

Movement of alleles into or out of a population due to the migration of individuals to or from the population. It increases genetic diversity in the receiving population.

Genetic Diversity

The degree of genetic variation within a species or population.

Mutation

A change that occurs in the DNA of an individual. Randomly occurring. Can be inheritable or non-inheritable.

Gene Mutation

A mutation that impacts a single gene (a portion of the DNA).

Chromosomal Mutation

A mutation that impacts the structure of a chromosome or the number of chromosomes.

Back Mutations

Mutations that reverse the effect of former mutations.

Resistance to HIV

Caused by a rare mutation in a gene that codes for a protein receptor on the surface of white blood cells. In people without the mutation, HIV can use the protein receptor to enter the white blood cells. People who are homozygous for the mutation lack the functioning receptor and are therefore resistant to HIV. Selective advantage.

Norway Rat

Rats who had a mutation for Warfarin (a blood thinner) before it was used. They survived, reproduced, and passed on the trait. By the 1960s, many resistant rats existed in Europe.

Non-Random Mating

Occurs when individuals choose mates based on traits like appearance or behaviour. It can increase certain allele frequencies and reduce genetic diversity.

Inbreeding

Mating between closely related individuals, increasing homozygosity, which can lead to inbreeding depression (decreased fitness because of expression of harmful recessive alleles).

Inbreeding Depression

When individuals with similar genotypes - typically relatives - breed with each other and produce offspring that have an impaired ability to survive and reproduce.

Genetic Drift

Changes to allele frequency due to random choice (chance event) Eg. founder effect, bottleneck effect, and gene flow.

Founder Effect

When a new population is started by a few individuals, it may have reduced genetic diversity and different allele frequencies than the original population. Rare alleles become common eg. the Amish population has an increased amount of Ellis Van Creveld syndrome.

Bottleneck Effect

A sharp reduction in population size (due to disease, disaster, hunting) leads to a loss of genetic drifts. Survivors carry only a subset of original alleles, increasing effects of genetic drift.

Adaptation

A heritable trait that improves an organism's fitness and increases over generations.

Sexual Selection

A form of natural selection where traits that improve mating success (like bright feathers) become more common.

Heterozygote Advantage

When individuals with one normal and one mutated allele have a survival benefit such as carriers of cystic fibrosis resisting cholera.

Artificial Selection

Changes to allele frequency due to the selection of certain traits (alleles) by humans over other traits.