Textbook Chapter Readings

What does “descent with modification” mean?

Descendants inherit traits from ancestors, but populations change over generations.

How does descent with modification explain similarities among species?

Related species inherited shared characteristics from common ancestors.

What is natural selection?

Individuals with certain heritable traits produce more surviving offspring, making those traits more common over generations.

What three conditions are necessary for natural selection?

Variation among individuals, inheritance of that variation, and differences in reproductive success associated with the variation.

What is biological fitness?

An individual's reproductive contribution to future generations.

Does being the strongest always mean having the highest fitness?

No. Fitness depends on reproductive success in a particular environment.

What is artificial selection?

Humans choose which organisms reproduce based on desired heritable traits.

How does artificial selection differ from natural selection?

Humans determine which traits are favored in artificial selection; environmental conditions influence which traits are favored in natural selection.

What do artificial selection and natural selection have in common?

Both act on heritable variation and can change populations across generations.

Why is variation important for natural selection?

Individuals must differ in heritable traits for selection to favor some variants over others.

Does natural selection create new alleles?

No. Mutation creates new alleles; selection acts on existing variation.

Do individuals or populations evolve?

Populations evolve as allele frequencies change across generations.

Is an adaptation guaranteed to be beneficial in every environment?

No. A trait's advantage depends on the environment. What is genetic variation?

What is an allele frequency?

The proportion of all copies of a gene in a population represented by a particular allele.

What is microevolution?

A change in allele frequencies within a population across generations.

What is the original source of new alleles?

Mutation.

How does sexual reproduction generate new combinations of alleles?

Through crossing over, independent assortment, and random fertilization.

Does recombination usually create new alleles?

No. It rearranges existing alleles into new combinations.

What is genetic drift?

Random changes in allele frequencies caused by chance.

In which populations does genetic drift usually have the greatest effect?

Small populations.

Does genetic drift necessarily improve adaptation?

No. Alleles can increase or disappear regardless of whether they are beneficial.

How can genetic drift affect genetic variation within a population?

It can reduce variation by causing alleles to be lost or fixed.

What does it mean for an allele to become fixed?

Its frequency reaches 100% in a population.

What is the founder effect?

Genetic drift that occurs when a few individuals establish a new population.

Why can a founder population differ genetically from its original population?

The founders carry only a chance sample of the original population's alleles.

What is a population bottleneck?

A sharp reduction in population size that can cause genetic drift and loss of genetic variation.

How do the founder effect and bottleneck effect differ?

The founder effect involves a small group starting a population; a bottleneck involves an existing population becoming much smaller.

Does population recovery after a bottleneck automatically restore lost alleles?

No. Increasing population size does not automatically restore lost genetic variation.

What is gene flow?

The transfer of alleles between populations through reproduction.

Does migration always produce gene flow?

No. Migrants must contribute alleles to the receiving population through reproduction.

How can gene flow affect variation within a population?

It can increase variation by introducing alleles.

How does gene flow usually affect differences between populations?

It makes their allele frequencies more similar.

How does natural selection differ from genetic drift?

Selection involves nonrandom differences in reproductive success associated with traits; drift involves chance changes.

Which evolutionary mechanism consistently produces adaptation?

Natural selection.

What is directional selection?

Selection that favors one extreme of a trait.

How does directional selection affect a trait's distribution?

It shifts the population toward the favored extreme.

What is an example of directional selection?

A population shifting toward larger beaks when large, hard seeds become the main food source.

What is stabilizing selection?

Selection that favors intermediate phenotypes over both extremes.

How does stabilizing selection affect a trait's distribution?

It reduces variation around the intermediate phenotype, often keeping the average similar.

What is an example of stabilizing selection?

Intermediate birth weights having higher survival than very low or very high birth weights.

What is disruptive selection?

Selection that favors both extreme phenotypes over intermediate phenotypes.

How does disruptive selection affect a trait's distribution?

It favors two extremes and can produce two peaks in the distribution.

What is an example of disruptive selection?

Small and large beaks being favored when small and large seeds are available but medium seeds are scarce.

How can you distinguish the three selection patterns?

Directional favors one extreme; stabilizing favors the middle; disruptive favors both extremes.

What is sexual selection?

Selection caused by differences in success at obtaining mates or fertilizing eggs.

How can sexual selection favor a trait that reduces survival?

The trait's mating benefits can outweigh its survival costs.

What is intrasexual selection?

Competition within the same sex for reproductive opportunities.

What is intersexual selection?

Mate choice based on traits of potential partners.

What is nonrandom mating?

Mating based on traits or relatedness rather than chance.

How does inbreeding affect genotype frequencies?

It increases homozygosity and decreases heterozygosity.

Does nonrandom mating alone necessarily change allele frequencies?

No. It can change genotype frequencies without changing allele frequencies.

How can mutation affect genetic variation?

It introduces new alleles and can increase variation.

Does natural selection always eliminate genetic variation?

No. Its effect depends on which traits and alleles are favored.

How can you distinguish selection from drift in a scenario?

Ask whether a heritable trait affects reproductive success or whether allele frequencies change through chance.