Phenotypic Evolution

What is phenotypic evolution?

changes in observable traits (phenotypes) in populations over time

How did selective breeding and domestication affect humans?

They allowed humans to develop agriculture and settle in communities instead of remaining nomadic

What are quantitative traits?

traits that vary continuously and are controlled by many genes (polygenic traits)

What is quantitative genetics?

The study of how quantitative traits are inherited and evolve.

What does "from genotype to phenotype" mean?

How genetic information produces observable traits

What factors are important in phenotypic evolution?

Fitness, selection, and heritability.

What is teosinte?

The wild ancestor of modern corn

How did corn evolve from teosinte?

Small genetic changes affected traits like branching and cob size

What affects quantitative traits?

Both genes and the environment

What determines how fast quantitative traits evolve?

Selection strength, heritability, and environmental effects

What is genotypic variation?

Variation in DNA sequence, which is discrete because DNA only uses A, T, G, and C.

What are quantitative (phenotypic) traits?

Traits that vary continuously, like height.

Why do phenotypes show continuous variation even though genotypes are discrete?

Because many genes and environmental factors influence the trait

How many genes affect many quantitative traits?

Many quantitative traits are polygenic and can involve hundreds or thousands of genes.

Does natural selection act on genotype or phenotype?

Natural selection acts on phenotype

Can large phenotypic changes occur without new mutations?

Yes, changes in allele frequencies alone can produce large phenotypic changes.

What is a fitness function?

A graph showing how survival or reproductive success changes with a trait.

What are the 3 basic modes of selection on quantitative traits?

1) Directional selection

2) Stabilizing selection

3) Disruptive selection

What is directional selection?

Selection that favors one extreme phenotype and shifts the population mean.

What happens to the population mean during directional selection?

the mean shifts toward the favored trait value

What is stabilizing selection?

Favors individuals near the population mean

What effect does stabilizing selection have on variation?

It reduces variation by selecting against extremes.

What is disruptive selection?

Selection that favors both extremes over intermediate phenotypes

What can disruptive selection lead to?

Increased variation and possibly speciation.

What caused directional selection in Galápagos finches in 1977?

A severe drought changed seed availability.

Which finches survived better during the Galápagos drought?

Finches with larger/deeper beaks survived better

What happened to the average beak size after selection in Galápagos finches?

The mean beak size increased

What is an example of stabilizing selection in humans?

Infant birth weight

Which infants have the highest survival rates?

Infants with average birth weights

Why do very small and very large infants have lower survival?

extreme birth weights are associated with health complications

Why is disruptive selection important evolutionarily?

It increases variation and can split populations into distinct forms.

How can the same fitness function produce different types of selection?

It depends on the trait distribution in the population and environmental conditions.

What is correlational selection?

Selection that favors specific combinations of traits together.

What does correlational selection produce genetically?

linkage disequilibrium (traits/alleles become associated together).

What does the selection gradient measure?

The strength of directional selection on a quantitative trait.

How do biologists estimate the strength of directional selection?

By measuring trait values and fitness for a group of individuals

What is relative fitness?

Relative fitness = mean population fitness / individual fitness

What is plotted to calculate the selection gradient?

Relative fitness vs. trait value.

What is the selection gradient mathematically?

The slope of the regression line (β)

What does a positive selection gradient (β>0) mean?

Directional selection favors larger trait values, increasing the mean trait

What does a negative selection gradient (β<0) mean?

Directional selection favors smaller trait values.

What does a selection gradient of zero (β=0) mean?

No directional selection is acting on the trait

What kinds of fitness measurements are often used instead of lifetime fitness?

Fitness components such as survival or mating success

In what units is the selection gradient measured?

In inverse trait units (e.g., per mm).

What does the guppy example demonstrate about directional selection?

Female guppies prefer males with more orange coloration, causing directional selection for increased orange color

According to studies across natural populations, how common is directional selection on size?

It is common and can sometimes be very strong.

What does evolution by directional selection allow biologists to predict?

How much a trait will evolve due to selection.

What equation predicts evolutionary change from one generation to the next?

The Breeder's Equation.

Δz = h^2 S

In the Breeder's Equation, what does Δzˉ represent?

The evolutionary change in the mean trait value between generations.

In the Breeder's Equation, what does h^2 represent?

Heritability of the trait

In the Breeder's Equation, what does S represent?

the strength of directional selection (selection differential)

What two factors determine the rate of evolution according to the Breeder's Equation?

Heritability h^2

Strength of directional selection S

What does heritability (h^2) measure?

The resemblance between parents and offspring for a trait.

What does h^2 = 0 mean?

No resemblance between parents and offspring.

What does h^2 = 1 mean?

Parents and offspring are identical for the trait

What happens evolutionarily if there is no selection?

No evolutionary change occurs

What effect does directional selection have on offspring mean trait values?

The mean trait value shifts toward the favored trait

What does Vp represent?

Overall phenotypic variance

What does VG represent?

Genetic variance.

What does VE represent?

Environmental variance

What equation describes the relationship between phenotypic, genetic, and environmental variance?

VP=VG+VE

What does the "nature vs nurture" equation show?

Phenotypic variation is caused by both genes and the environment

Give one example of a trait influenced by both genes and environment

Human height — strongly heritable but also influenced by diet

What is partitioning genetic variance?

Breaking total genetic variance into different genetic components

What equation partitions genetic variance?

VG=VA+VD+VI

What does VA represent?

Additive genetic variance

What is additive genetic variance (VA)?

The average effect of substituting one allele for another

What does VD represent?

Dominance variance

What causes dominance variance (VD)?

Interactions between alleles at the same locus

What does VI represent?

Epistatic interaction variance

What causes epistatic variance (VI)?

interactions between alleles at different loci.

Which component of genetic variance contributes directly to evolutionary change?

additive genetic variance (VA)

Why does dominance variance not contribute much to evolutionary change?

Because heterozygotes are not intermediate and dominance effects are not predictably inherited