Bio 130 NATURAL SELECTION, GENETIC DRIFT, AND QUANTITATIVE GENETICS

What is a phenotype?

Any measurable trait of an organism (e.g. height, body size, color, speed)

What is a genotype?

The DNA sequence at a specific region of the genome

Can phenotypes be influenced by the environment?

Yes

What is an example of a continuous trait?

Human height

What is an example of a discrete trait?

Flower color (in some species)

What is the modern definition of biological evolution?

A change in genotype frequencies within a population over time

How does the historical definition of evolution differ from the modern one?

Historical: change in phenotypic traits
Modern: change in genotype frequencies

Why does increased human height over the last 150 years not necessarily indicate evolution?

It may be due to environmental changes (nutrition healthcare), not genetic changes

At what level does evolution occur?
. Populations

not individuals

Why don’t individuals evolve?

Their genotype does not change (except by mutation)

What are the two major mechanisms of evolution?

Natural selection and genetic drift

Who first described natural selection?

Charles Darwin and Alfred Russell Wallace

What are adaptations?

Traits that increase survival or reproduction

Does natural selection have a goal or purpose?

No

What are Darwin’s four postulates of natural selection?

1. Phenotypic variation exists

2. Some variation is genetic

3. Individuals vary in fitness

4. Phenotype–fitness relationship is non-random

What happens if all four postulates are met?

Evolution by natural selection occurs automatically

What are the two sources of phenotypic variation?

Environmental variation and genetic variation

Do environmentally induced traits cause evolution?

No

What is the ultimate source of genetic variation?

Mutation

Are mutations random with respect to fitness?

Yes

Which mutations matter for evolution?

Mutations in germ cells (eggs or sperm)

Why don’t somatic mutations contribute to evolution?

They are not passed to offspring

How does sexual reproduction increase genetic variation?

1. Meiosis reshuffles alleles

2. Gametes from different parents combine

What is biological fitness?

Reproductive success relative to others in the population

What determines fitness?

Survival

Can an individual with zero offspring have fitness?

Yes

When does natural selection occur?

When phenotype and fitness covary non-randomly

What happens if phenotype and fitness are unrelated?

No natural selection occurs

What is directional selection?

Selection favoring one extreme of a trait distribution

What happens to the mean under directional selection?

The mean shifts

What is stabilizing selection?

Selection favoring intermediate phenotypes

What happens to variance under stabilizing selection?

Variance decreases

What is disruptive selection?

Selection favoring both extremes

What happens to variance under disruptive selection?

Variance increases (can create bimodal distribution)

What is quantitative genetics?

Study of continuous traits

What kind of traits does quantitative genetics focus on?

Continuous traits (e.g.

What is the breeder’s equation?

R = h² × S

What does R represent?

Response to selection (change in trait mean)

What does h² represent?

Heritability

What does S represent?

Selection differential

What is heritability?

Proportion of phenotypic variation due to genetics

What are the possible values of h²?

Between 0 and 1

What does h² = 0 mean?

Trait variation is entirely environmental

What does h² = 1 mean?

Trait variation is entirely genetic

How is heritability commonly estimated?

Parent-offspring regression

What does the slope of the regression line equal?

h²

What is the selection differential?

Difference between mean trait of breeders and population mean

How is S calculated?

S = (mean of breeders) − (mean of population)

What does a positive S mean?

Selection favors higher trait values

What happens if h² = 0 even when selection is strong?

No evolutionary response

When is R equal to S?

When h² = 1

What species did Peter and Rosemary Grant study?

Medium ground finch (Geospiza fortis)

What trait did they study?

Beak depth

Why was beak depth important?

Determines ability to crack large seeds

What happened during the 1977 drought?

Birds with deeper beaks survived and reproduced

What was the evolutionary result?

Increased mean beak depth in the next generation

Why do bacteria evolve antibiotic resistance?

Antibiotics create strong natural selection

What are MDR bacteria?

Multidrug-resistant bacteria

Why doesn’t antibiotic treatment work on viruses?

Antibiotics do not affect viruses

Why is multidrug therapy used for HIV?

Makes resistance evolution less likely

What is genetic drift?

Evolution caused by random changes in genotype frequencies

In which populations is drift strongest?

Small populations

What happens to genetic diversity under drift?

It decreases over time

Is genetic drift predictable?

No

What is the founder effect?

Drift caused by small founding populations

Why do island populations have low genetic diversity?

Strong genetic drift during founding

How does human genetic diversity change with distance from Africa?

It decreases

Is natural selection random?

No

Is genetic drift random?

Yes

Which mechanism produces adaptation?

Natural selection

Can drift and selection occur together?

Yes