OEB 53 Lecture 2

What is descent with modification?

Species descend from common ancestors and accumulate changes over generations.

What is homology?

Similarity between organisms that comes from common ancestry.

What is a vestigial trait?

A trait that was adaptive in an ancestor but is no longer useful in the descendant species.

Give an example of a vestigial trait.

The grasp reflex in human newborns.

What is a pseudogene?

A nonfunctional copy of a gene that was functional in an ancestor.

What is an atavism?

An ancestral trait that unexpectedly reappears in a descendant due to mutation or developmental error.

Give an example of an atavism.

A whale developing external hind limbs.

What are intermediate forms?

Organisms or fossils containing a mixture of ancestral and derived traits.

Why aren't intermediate forms “halfway” versions of two modern species?

Modern species descend from a common ancestor; one modern species did not directly turn into the other.

Why is Lucy (Australopithecus afarensis) an intermediate form?

She was bipedal, a derived trait, but had a chimpanzee-sized brain, an ancestral trait.

What does “lousy design” tell us about evolution?

Evolution modifies existing structures rather than designing organisms perfectly from scratch.

What is the panda's “thumb”?

A modified wrist bone rather than a true thumb.

What is biogeography?

The study of geographic distributions of organisms; related species tend to occur near each other because of common ancestry.

What three things make natural selection possible?

Superfecundity, variation, and heritability.

What is superfecundity?

Organisms produce more offspring than can survive.

Why does superfecundity matter?

It creates competition for limited resources.

Why is variation necessary for natural selection?

Individuals must differ for some variants to have greater reproductive success than others.

Why is heritability necessary for natural selection?

Favorable variants must be inherited for them to become more common in future generations.

What is the basic sequence of natural selection?

Variation + competition + heritability → differential reproductive success.

What does natural selection generally operate on?

Individuals.

Does natural selection generally operate “for the good of the species”?

No. Individual selection generally trumps group selection.

Why is “for the good of the species” usually a bad evolutionary explanation?

A selfish individual that reproduces more would have an advantage over individuals sacrificing themselves for the group.

What is the equation for narrow-sense heritability?

h² = R/S

What does S represent?

Selection differential.

How do you calculate S?

Selected parents' mean − original population mean.

What does R represent?

Response to selection.

How do you calculate R?

New generation mean − old generation mean.

If h² = 0, what does that mean?

None of the variation in the trait is attributable to additive genetic variation.

If h² = 1, what does that mean?

All variation in the trait is attributable to additive genetic variation.

Population mean = 25 mph, selected parents = 30 mph, and h² = 0.6. What is S?

5 mph.

Population mean = 25 mph, selected parents = 30 mph, and h² = 0.6. What is R?

3 mph.

Population mean = 25 mph, selected parents = 30 mph, and h² = 0.6. What is offspring?

28 mph.

Is natural selection variational or transformational?

Variational.

What is a variational theory of change?

Different variants already exist, and some are maintained/reproduced more successfully than others.

What is transformational change?

A population changes because the individual members themselves transform.

What did Darwin change about how scientists viewed variation?

Variation became the raw material of evolution rather than meaningless deviation from an ideal type.

What is the difference between selection OF and selection FOR a trait?

Selection of describes what changes; selection for identifies the trait actually causing the fitness advantage.

Can selection indirectly change traits it is not acting on directly?

Yes, through genetic correlations with traits being selected.

Does a trait's current function necessarily explain why it originally evolved?

No.

What is the feather example of historical origin vs. current utility?

Feathers are now important for flight but evolved before flight, probably initially for insulation.

Why would blending inheritance have been a problem for Darwin's theory?

Favorable variants would become diluted and disappear over generations.

What kind of inheritance did Darwin's theory need?

Particulate/non-blending inheritance.

What does particulate inheritance mean?

Alleles remain intact rather than blending together.

When was Mendel's work published?

1866

When was Mendel's work rediscovered?

Around 1900.

What are Mendelian characters?

Discrete, discontinuous traits.

What are biometrical traits?

Continuous, measurable traits that often form bell-shaped distributions.

Who helped reconcile Mendelian genetics with continuous variation?

R. A. Fisher.

What is polygenic inheritance?

A trait is determined by multiple loci, each contributing a small effect.

How can Mendelian genes produce continuous variation?

Many Mendelian loci can contribute small effects to the same trait.

What two things help create continuous, bell-shaped phenotypic variation?

Polygenic inheritance + environmental variation.

How does environmental variation affect polygenic traits?

It blurs genetic categories because the same genotype can produce different phenotypes in different environments.

What is Hardy-Weinberg Equilibrium?

A model predicting genotype frequencies from allele frequencies when specific no-evolution conditions are met.

What do p and q represent?

Frequencies of two alleles in a population.

If there are only two alleles, what does p + q equal?

1

What is the Hardy-Weinberg equation?

p² + 2pq + q² = 1

Under HW, what is the frequency of AA?

p²

Under HW, what is the frequency of Aa?

2pq

Under HW, what is the frequency of aa?

q²

What are the five Hardy-Weinberg assumptions?

Random mating, no selection, no mutation, no migration, and large population size.

Why does HW require a large population?

So sampling error has little effect on allele frequencies.

Why is Hardy-Weinberg called an equilibrium?

Allele and genotype frequencies remain unchanged while HW conditions are met.

How is evolution defined in this lecture?

A change in allele or genotype frequencies within or between generations.

What does Hardy-Weinberg represent in evolutionary genetics?

A no-evolution null hypothesis/model.

What does departure from HW suggest?

Evolutionary processes such as selection may be occurring.

How do you calculate the frequency of A from genotype frequencies?

p = f(AA) + ½f(Aa)

How can you calculate q once you know p?

q = 1 − p

If you don't know whether HW holds, can you calculate p using √f(AA)?

No.

Why can't you use p = √f(AA) before establishing HW?

Because f(AA) = p² only under Hardy-Weinberg expectations.

If AA = 0.10 and Aa = 0.40, what is p?

0.10 + 0.20 = 0.30.

If p = 0.30, what is q?

0.70.

If p = 0.30 and q = 0.70, what are the expected HW frequencies?

AA = 0.09, Aa = 0.42, aa = 0.49.

How do you test whether a population fits HW using genotype frequencies?

Calculate p and q from the observed genotypes → calculate expected p², 2pq, q² → compare expected and observed genotype frequencies.

What is the Modern Synthesis?

The union of Darwinian evolution and Mendelian genetics through population genetics.

When did the Modern Synthesis emerge?

The 1930s.

What are other names for the Modern Synthesis?

New Synthesis and Neo-Darwinism.

What is fitness?

A measure of an organism's adaptation/reproductive success.

Why is fitness relative?

An individual's success depends on how it performs compared with competitors.

What are the three possible fitness effects of a mutation?

Advantageous, deleterious, or neutral.

What does positive selection do?

Increases the frequency of advantageous mutations.

What does negative/purifying selection do?

Reduces the frequency of deleterious mutations.

What happens to neutral mutations?

Selection does not act on them; their fate is largely governed by stochastic processes such as genetic drift.

What is directional selection?

Selection favoring one extreme, shifting the population mean.

What is stabilizing selection?

Selection against both extremes, favoring intermediate phenotypes.

What happens to the mean under stabilizing selection?

It stays roughly the same while extreme phenotypes are removed.

What example of stabilizing selection was given in lecture?

Human birth weight.

What is disruptive selection?

Selection favoring both extremes over intermediate phenotypes.

Which form of selection is described as unusual in the lecture?

Disruptive selection.