Mechanisms of Evolution

Why is genetic variation important?

It provides differences that natural selection and other evolutionary processes can act on, helping populations respond to environmental change.

What five mechanisms drive evolution?

Mutation, gene flow, genetic drift, natural selection, and nonrandom mating/sexual selection (course lists may categorize selection differently).

What is the real definition of evolution?

A change in allele frequencies in a population across generations.

What are allele frequencies?

The proportion of all copies of a gene in a population that are a particular allele.

How does genomic sequencing work?

It determines the order of DNA bases in a genome or selected DNA regions, allowing comparison of genetic information.

What is a mutation, how does it happen, and how can it alter allele frequencies?

A mutation is a DNA-sequence change caused by replication errors or mutagens. It creates new alleles and can change frequencies when inherited and affected by selection, drift, or other processes.

Where must a mutation occur to matter evolutionarily in sexual populations?

In a germline cell or a cell lineage that contributes to gametes, so it can be inherited by offspring.

What is a species?

A commonly used definition is a group of natural populations whose members can interbreed and produce fertile offspring; other species concepts are needed for asexual or fossil organisms.

What is gene flow? What must move?

Gene flow is the movement of alleles between populations; genes/alleles must move, not necessarily the individuals themselves.

How can gene flow happen without migration?

Gametes, pollen, seeds, or other reproductive material can move between populations without the whole organism migrating.

Give an example of gene flow.

Pollen from one plant population fertilizes plants in another population, transferring alleles.

What is genetic drift, and what does its magnitude depend on?

Genetic drift is random change in allele frequencies; it has stronger effects in smaller populations.

What are two important forms of genetic drift?

The bottleneck effect and founder effect.

What happens to genetic variation after genetic drift?

Variation often decreases, and alleles can become fixed or lost by chance, especially in small populations.

What is the bottleneck effect?

A population is sharply reduced in size, leaving a random subset of its genetic variation.

What happens to variation after a bottleneck?

Genetic diversity often decreases, and allele frequencies may differ from the original population.

How are northern elephant seals an example of the bottleneck effect?

Intense hunting reduced their numbers dramatically; the surviving population retained relatively low genetic diversity.

What is an example of a bottleneck effect in humans?

Specific examples depend on the population and event discussed in class; severe population reductions can reduce genetic diversity.

What is the founder effect?

A new population is established by a small number of individuals whose alleles may not represent the source population.

What happens to variation after the founder effect?

The new population often has reduced genetic diversity and unusual allele frequencies by chance.

What is a founder-effect example involving Native Americans?

Some genetic patterns in Indigenous American populations reflect founding by a relatively small ancestral population and subsequent demographic history; use the course's specific example.

What is a founder-effect example involving the Irish?

The study guide does not specify the example; check lecture notes for the particular Irish population or allele discussed.

How can a rare allele become more common in the Amish?

A small founding population and subsequent relative reproductive isolation can increase some allele frequencies by genetic drift; a commonly cited example is Ellis-van Creveld syndrome in some Amish communities.

What does it mean when an allele has no variation?

The allele is fixed at that locus in the population: all copies sampled are the same allele.

What is sexual selection?

Selection caused by differences in mating success.

What are two ways sexual selection works?

Intrasexual selection: competition within one sex, such as male deer competing. Intersexual selection: mate choice, such as birds choosing mates with elaborate displays.

What is sexual dimorphism? Give examples.

Differences in appearance between sexes of a species; examples include peacock tail displays, deer antlers, and differences in body size in some species.

What are three ways natural selection changes a trait distribution?

Directional selection favors one extreme; stabilizing selection favors intermediate traits; disruptive selection favors both extremes.

What is directional selection? What does oscillation mean?

One extreme phenotype is favored. If environmental conditions change back and forth, the favored direction can also change.

Give an example of directional selection.

Antibiotic treatment can favor resistant bacteria, increasing resistance in the population.

What is stabilizing selection? Give an example.

Intermediate phenotypes are favored and extremes selected against; human birth weight is a classic example.

What is disruptive selection? Give an example.

Both extremes are favored over intermediates; for example, birds specializing on either small or large seeds may do better than birds with intermediate beaks.

What is frequency-dependent selection?

The fitness of a trait depends on how common or rare it is in the population.

Positive vs. negative frequency-dependent selection?

Positive frequency dependence favors common traits; negative frequency dependence favors rare traits. Example: predators may focus on common prey forms, benefiting rare forms.

Why might unusual fish forms not disappear completely?

If rare forms have an advantage when uncommon, negative frequency-dependent selection can maintain them; other processes can also preserve variation.

How do we know why natural selection happens?

Evidence from heritable variation, differential survival/reproduction, observed population changes, experiments, and ecological relationships supports the mechanism.

What is artificial selection? Give examples.

Human-directed breeding for desired traits; examples include dog breeds, crop varieties, and livestock.

What is the Hardy-Weinberg equation?

For two alleles, p + q = 1 and p² + 2pq + q² = 1.

How does Hardy-Weinberg work?

It predicts genotype frequencies from allele frequencies under specified assumptions; p² is homozygous for one allele, 2pq is heterozygous, and q² is homozygous for the other.

What can Hardy-Weinberg be used for?

To estimate genotype frequencies and assess whether a population's genetic data depart from expectations under the model.

What are Hardy-Weinberg's limitations?

Its assumptions are idealized; real populations may have selection, mutation, migration, drift, nonrandom mating, or population structure.

What five assumptions does Hardy-Weinberg require?

Very large population, random mating, no mutation, no migration/gene flow, and no natural selection.

Can evolution produce a perfect species? Why not?

No. Environments change, and every adaptation involves trade-offs; evolution works with existing variation and historical constraints rather than designing perfection.

What is diploidy, and how can it preserve variation?

Diploidy means having two chromosome sets. A recessive allele can be hidden in heterozygotes, allowing it to persist even when not expressed.

How are sticklebacks used as an evolution example?

Freshwater and marine stickleback populations show evolved differences, such as armor plates, associated with different environments and genetic variation.

What is heterozygote advantage? Give an example.

Heterozygotes have higher fitness than either homozygote in a particular environment; e.g., sickle-cell trait can provide some protection against severe malaria in malaria-endemic regions.