Topic 8 - Mechanisms of Microevolution

Microevolution

Change in frequencies of alleles, or genotypes, in a population across generations

Macroevolution

Pattern of evolution at, or above, the species level (origin of new species)

Mechanisms of Microevolution

1. Mutation
2. Natural Selection
3. Non-Random Mating
4. Gene Flow
5. Genetic Drift

Mutations

(random), Mutations are permanent changes to the DNA
• Produce new alleles
– Can be positive, negative, or neutral
• Random with respect to fitness
– randomly occurring events at low frequency

Natural Selection

(non-random), A form of selection in which individuals with favorable
phenotypes survive to reproduce more than
individuals with different phenotypes

Non-Random Mating

(non-random), A form of selection in which individuals that
possess a specific phenotype are more likely
to mate.

Assortative Mating

(non-random), Individuals more likely to mate with other individuals
that are either similar or dissimilar to them

Positive assortative mating

choose similar mates,

• Pairs share more traits than
expected at random
• Can decrease heterozygosity
• Can lead to disruptive selection

Negative assortative mating

choose dissimilar mates,

• Can increase heterozygosity
• Can lead to balancing selection

Non-Random Mating

(inbreeding),

Mating of two genetically similar relatives
– Increases homozygosity
– May have negative
consequences, due to over recessive alleles

Self-fertilization

(Non-Random Mating)

• Decreases heterozygosity
• Still Sexual Reproduction (meiosis with fertilization)
• Does not necessarily produce offspring identical to parents

Gene Flow [Allele Flow]

(random), The transfer of traits in or out of a population, NOT adaptive, but can affect the rate of adaptive evolution (Selection): increasing or decreasing the rate of change.

Immigration

Traits (alleles) move
into the population from
elsewhere
‒ Often increases trait diversity
in the population

Emigration

Traits (alleles) move out of a population to elsewhere
‒ Often decreases trait diversity

Genetic Drift

(random), any change in allele, or genotype, frequencies in a population that is due to chance (random events)

1. Random differences in survival of individuals
2. Random sampling of gametes during reproduction

Fixation

when there is only one allele for a gene locu

Extinction/Loss

when a previous allele for a gene locus is lost

Bottleneck

One population goes through a reduction is size.

Founder Effect

Small number of individuals start a new population.

Genetic Drift - Bottleneck

• Large ↓ in population size
• Genetic variation lost by
chance