[UCSB SUMMER ‘26] EEMB 2 - Evolution: Week 2 Set

What is natural selection?

From existing heritable variation, natural selection increases frequencies of genotypes that confer greater reproductive success.

What is evolutionary or Darwinian fitness?

The contribution an individual makes to the next generation's gene pool relative to other individuals.

What is relative fitness?

The reproductive contribution of a genotype compared with alternative genotypes at the same locus.

How is relative fitness calculated?

Divide each genotype's reproductive output by the highest reproductive output; the highest is set to 1.0.

What are the relative fitnesses for genotypes producing 12, 9, and 3 offspring?

1.0, 0.75, and 0.25.

Why is survival alone not enough to define high fitness?

An organism must survive and successfully reproduce; fitness is measured through genetic contribution to future generations.

What happens under selection for a dominant allele against a recessive allele?

The dominant allele rises, but the recessive allele can persist at very low frequency because it becomes hidden in heterozygotes.

Why is a rare recessive allele difficult for selection to eliminate?

Most copies are in heterozygotes where the recessive phenotype is not expressed.

What happens under selection for a recessive allele against a dominant allele?

The recessive allele can increase, initially slowly when rare because few copies occur in homozygotes; once common enough, selection becomes more effective.

What is balancing selection via heterozygote advantage?

The heterozygote has the highest fitness, so selection maintains both alleles in the population.

What is heterozygote advantage?

A condition in which heterozygotes have greater relative fitness than either homozygote.

What is a balanced polymorphism?

Maintenance of two or more alleles at appreciable frequencies over time.

What is a classic example of heterozygote advantage?

Sickle-cell allele: HbA/HbS heterozygotes can have a fitness advantage in malaria-prone regions, helping maintain both alleles.

What is frequency-dependent selection?

The fitness of a phenotype/genotype changes depending on how common or rare it is.

How can negative frequency-dependent selection maintain variation?

Rare forms gain a fitness advantage, preventing any one form from fixing.

What are the three major modes of selection on polygenic traits?

Directional, stabilizing, and disruptive selection.

What is directional selection?

Selection favors one phenotypic extreme, shifting the population mean toward that extreme.

What is stabilizing selection?

Selection favors intermediate phenotypes and acts against both extremes, reducing variation around the mean.

What is disruptive selection?

Selection favors both extremes over intermediate phenotypes, potentially increasing variation and contributing to divergence.

Which selection mode shifts the mean toward one extreme?

Directional selection.

Which selection mode usually reduces variance without changing the mean much?

Stabilizing selection.

Which selection mode can generate a bimodal phenotype distribution?

Disruptive selection.

Why can't natural selection engineer perfect organisms?

Selection works with existing variation and historical constraints, environments change, trade-offs occur, and chance processes also affect evolution.

What is coevolution?

Reciprocal evolutionary change between interacting species, where each imposes selection on the other.

What is the Red Queen hypothesis?

Interacting species must continually evolve just to maintain relative fitness as their antagonists or partners also evolve.

What is a major advantage of sexual reproduction?

Recombination creates new allele combinations and can help purge deleterious mutations.

What is a major cost of sexual reproduction?

It requires finding mates and can reduce the direct genetic contribution of an individual compared with cloning.

What is Muller's Ratchet?

In asexual lineages, deleterious mutations can accumulate irreversibly because recombination cannot recreate mutation-free genotypes once lost.

Why can sex slow Muller's Ratchet?

Recombination can produce offspring with fewer deleterious mutations than either parental chromosome set.

What is isogamy?

Gametes are similar in size and form.

What is anisogamy?

Gametes differ in size, typically many small sperm and fewer large eggs.

How can anisogamy create different reproductive strategies?

Because large eggs are costly and small sperm are cheap, selection can favor different mating investments and competition patterns in the sexes.

What is sexual selection?

Selection arising from differences in mating success.

What is sexual dimorphism?

Systematic phenotypic differences between males and females of the same species.

How can sexual selection generate sexual dimorphism?

Traits that increase mating success in one sex can become exaggerated even if they impose survival costs.

What was the central question in Endler's Trinidad guppy experiments?

How predation pressure and mate choice/sexual selection jointly shape male coloration.

What general pattern did Endler's guppy work demonstrate?

Predation can select against conspicuous coloration, while female choice can favor conspicuous males; trait evolution reflects this trade-off.

What is artificial selection?

Human-directed selective breeding for desired traits.

How is artificial selection evidence for evolutionary change?

It shows that heritable variation can respond rapidly to consistent selection across generations.

What is genetic divergence?

Accumulation of genetic differences between populations, producing increasingly different gene pools.

What three processes can create genetic differences between separated populations?

Mutation, natural selection, and genetic drift.

What is gene flow?

Physical movement of alleles into or out of populations through migration of organisms or gametes.

How does gene flow affect divergence?

It homogenizes populations and generally impedes genetic divergence.

How can gene flow slow local adaptation?

By introducing alleles that are not suited for the local environment.

What is genetic drift?

Random change in allele frequencies across generations due to chance sampling.

Does genetic drift favor alleles because they increase fitness?

No, drift is independent of fitness.

What are the two long-term allele outcomes promoted by drift?

Loss (0% frequency) or fixation (100% frequency).

How does genetic drift affect genetic variation?

It tends to reduce genetic variation and increase homozygosity.

Why is drift stronger in small populations?

Sampling error is larger when fewer individuals contribute alleles to the next generation.

What is sampling error in population genetics?

Random deviation of allele frequencies from expected values because only a finite sample of gametes/individuals reproduces.

Can drift fix deleterious alleles?

Yes, especially in small populations, because drift is based on chance rather than fitness.

What is mutational meltdown?

Accumulation/fixation of many deleterious mutations that progressively lowers population fitness.

Why can small populations be vulnerable to mutational meltdown?

Drift is strong enough to fix harmful alleles and loss of variation reduces the ability of selection to respond.

What is the bottleneck effect?

A severe reduction in population size that changes allele frequencies and reduces genetic diversity.

Why does a bottleneck magnify drift?

The survivors are a small, potentially unrepresentative sample of the original gene pool.

What are examples of bottlenecks mentioned in lecture?

Northern elephant seals, sea otters, cheetahs, plus disturbances such as disease, hunting, habitat loss, or volcanic events.

What is the founder effect?

Genetic change when a few individuals establish a new population whose allele frequencies do not represent the source population.

How does founder effect differ from bottleneck effect?

Founder effect begins a new population from a small migrant sample; bottleneck is a sharp reduction within an existing population.

Why can founder populations diverge rapidly?

Small size strengthens drift, gene flow may be low, and selection may differ in the new environment.

What is an extinction vortex?

A positive-feedback cycle in which small population size causes inbreeding/drift and loss of variation, reducing fitness and causing further population decline.

What is effective population size, Ne?

The size of an idealized breeding population that would experience the same amount of genetic drift as the real population.

Why is Ne often smaller than census population size?

Not all individuals reproduce equally; sex ratio, reproductive variance, and population fluctuations reduce the number effectively contributing genes.

What equation is used for Ne when numbers of breeding males and females differ?

Ne = 4NmNf / (Nm + Nf).

What happens to Ne when the sex ratio becomes strongly skewed?

Ne decreases, strengthening drift even if the census population is large.

What is reproductive isolation?

Any structural, functional, or behavioral difference resulting from genetic divergence that reduces gene exchange between populations.

Why is reproductive isolation crucial for speciation?

It prevents diverging populations from coalescing back into one gene pool.

What is a prezygotic isolating mechanism?

A barrier that prevents mating or fertilization before a zygote forms.

What is a postzygotic isolating mechanism?

A barrier that acts after fertilization, reducing hybrid survival or reproduction.

What is ecological isolation?

Populations use different habitats or resources and therefore rarely encounter one another for mating.

What is geographic isolation?

Physical separation prevents or greatly reduces gene flow.

What is temporal isolation?

Populations reproduce at different times of day, seasons, or years.

What is ethological isolation?

Behavioral differences, especially courtship signals, prevent mating.

What is mechanical isolation?

Morphological incompatibility prevents successful mating or pollen transfer.

What is gametic isolation/wastage?

Gametes meet or are transferred but cannot successfully fuse or fertilize.

What is zygotic wastage?

Fertilization occurs, but the hybrid zygote/offspring has reduced survival or reproductive success.

What is hybrid inviability?

Hybrid offspring fail to develop properly or have low survival.

What is hybrid sterility?

Hybrid offspring survive but cannot produce functional gametes or offspring.

What is a hybrid zone?

A geographic area where genetically distinct populations meet and produce hybrids.

What are three possible long-term outcomes of a hybrid zone?

Reinforcement of isolation, fusion of the populations, or stable persistence of a hybrid zone.

What is reinforcement?

Selection strengthens reproductive barriers because hybrids have reduced fitness.

What is fusion in a hybrid zone?

Gene flow overwhelms divergence and the populations merge back into one.

What is hybrid-zone stability?

Hybrids continue to be produced over time without complete fusion or complete reproductive isolation.

How do gene flow and drift differ in their effect on divergence?

Gene flow generally decreases divergence; drift generally increases divergence, especially in small isolated populations.

What happens to divergence when two isolated populations receive frequent migrants?

It should slow because gene flow homogenizes allele frequencies.

What process occurs when a tiny isolated population loses an advantageous allele by chance?

Genetic drift.

What process is immediately amplified when a hurricane leaves 15 random survivors from 10,000 individuals?

Genetic drift through a bottleneck effect.

Is a bottleneck itself natural selection?

Not necessarily; the key evolutionary effect emphasized is that the surviving gene pool can be an unrepresentative sample, amplifying drift.

Can gene flow introduce alleles that are new to a population without creating new alleles globally?

Yes, gene flow moves existing alleles; mutation creates new alleles.