Lecture 10: Selection

How does mutation affect genetic variation?

Introduces new alleles; increases variation.

How does gene flow (migration) affect genetic variation?

Can increase or decrease variation locally.

How does genetic drift affect genetic variation?

Reduces variation, may lead to allele fixation.

How does nonrandom mating/sexual selection affect genetic variation?

May alter genotype frequencies; typically not allele frequencies unless mating success varies with genotype (as in sexual selection).

How does natural selection affect genetic variation?

Can increase or decrease variation.

What is natural selection?

The process by which heritable traits that enable some individuals to survive and reproduce better than others become more common in subsequent generations.

What is relative fitness?

The number of surviving offspring that an individual produces compared with others in the population.

Differences in the relative fitness of individuals are the essence of natural selection.

What is directional selection?

When one extreme of a trait’s phenotypic range is favored over the other, causing the population’s average phenotype to shift in that direction over time due to a changing environment or selective pressure. 

What is stabilizing selection?

A type of natural selection where the average, or immediate, phenotype is favored, while extreme variations are selected against.

This process reduces genetic variation within a population, leading to a more stable and uniform population over time, and is often seen in traits like human birth weight, where both very low and very high weights are associated with higher mortality rates.

What is disruptive selection?

A type of natural selection that favors extreme traits at the expense of intermediate traits, leading to increased genetic and phenotypic diversity within a population.

This can occur when environmental changes or resource availability create distinct niches, making both extreme phenotypes advantageous. Over time, disruptive selection can result in the emergence of two or more distinct groups or even new species (speciation).

What is a balanced polymorphism?

Polymorphism = genetic variation.

A balanced polymorphism is one in which two or more phenotypes are maintained in fairly stable proportions over many generations.

What is heterozygote advantage?

Heterozygote advantage occurs when heterozygotes have higher relative fitness than either homozygote.

Many double-recessive genetic diseases have an associated heterozygous advantage.

What are some constraints of natural selection?

Not every trait is an adaptation.

Some traits are by-products.

Evolution works with what’s available.

Structures are often co-opted.

Developmental constraints matter.

Historical constraints shape what’s possible.

Adaptations often involve trade-offs.