Topic 5: Mircoevolution

Microevolution

Changes in the frequency of an allele within a population

Macroevolution

The over-arching history of life; changes in lineages

Over time, microevolutionary change can lead to macroevolution

Genotype frequencies

Note that by definition, frequencies must sum to 1 (or 100%)

Allele Frequencies

Hardy Weinberg model

In the absence of evolutionary processes, the allele frequencies in a population do not change from generation to generation

a NULL MODEL

Assumes that there is no natural selection, genetic drift, gene flow, or mutation.

(no migration, large population, no selection)

Directional selection

Changes the average trait value

An extreme phenotype provides a fitness benefit so the average phenotype in the population moves towards that extreme

Stabilizing selection

Reduces the amount of variation in a trait

Phenotypes in the middle of the range provide a fitness benefit, so extreme phenotypes become less common.

Disruptive selection

Increases the amount of variation in a trait Phenotypes at both extremes provide a fitness benefit, so intermediate phenotypes become less common

Balancing selection

Genetic variation maintained No single phenotype is favoured at all times, so a diversity of phenotypes is maintained in the population

Genetic Drift

When random chance changes allele frequencies

• All populations experience random events that can shift allele frequencies

• These random events do not lead to adaptation

• The effect of genetic drift is greater in small populations where any random event affects a larger proportion of the population

Founder Effect

A new population is formed (“founded”) with different allele frequencies

Random immigration of individuals means the new population has different allele frequencies.

Genetic Bottleneck

A sudden decrease in population size reduces allele number

individuals are removed at random from the population; severe bottlenecks may lead to local extinction.

Gene Flow

When individuals (or gametes) migrate between populations

Gene flow increases the similarity in allele frequency between populations that exchange genes.

Gene flow can increase or decrease the fitness of a population

Mutation

Produces new alleles: the ultimate source of all genetic variation

Mutation rates are so low that they rarely cause evolution within populations EXCEPT in combination with selection or drift

Not all mutations add alleles to the gene pool

Point mutation

a change in a single base pair in DNA

Chromosome-level mutation

a change in the number of composition of chromosomes

Somatic mutations

mutations within the body (soma) of an organism—as they are in non-reproductive cells, these are not passed on to offspring and do not change allele frequencies