Real Bio 1070 unit 2

What is evolution?

Changes in allele frequencies in a population from one generation to the next

What is an allele?

Different versions of a gene (e.g., big F and little f)

What is genotype?

The alleles an individual has (e.g., FF, Ff, ff)

What is phenotype?

Observable traits; determined by genotype + environment

What is fitness?

Number of offspring produced; reproductive success

What is an adaptation?

Heritable trait that arose via natural selection and increases survival/reproduction

What is fixation?

When allele frequency reaches 100% (only allele present)

What are the four mechanisms of evolution?

1. Natural selection, 2. Genetic drift, 3. Mutation, 4. Gene flow

What is natural selection?

Differential survival/reproduction based on heritable traits

What is genetic drift?

Random changes in allele frequencies due to chance (sampling error)

What is mutation?

Changes in DNA; ultimate source of all new variation

What is gene flow?

Movement of alleles between populations through migration

What are Darwin's three postulates?

1. Variation exists, 2. Variation is heritable, 3. Differential survival/reproduction based on variation

What does selection act on?

Acts on phenotypes; evolution occurs through genotype changes

What is directional selection?

Shifts trait distribution in one direction (e.g., finch beaks getting deeper)

What is stabilizing selection?

Selects against extremes; favors intermediate values; reduces variation

What is disruptive selection?

Selects against average; favors extremes; can lead to speciation

Finch study main results?

1976 drought: 751→90 finches; beak depth increased 10%; reversed when drought ended

What is founder effect?

Small group establishes new population; reduced variation; frequencies determined by chance

What is bottleneck?

Population crashes; dramatic loss of variation; survival random

Elephant seal example?

Bottleneck to ~20 individuals; now 225,000 but still low variation (3.19 vs 9.8 alleles/locus)

Pingelap Atoll example?

Typhoon→20 survivors; now 10% have colorblindness (vs 0.003% normal) due to drift

Typical mutation rate?

~3 × 10⁻⁹ per genome per generation

Large populations

More total mutations arise ('mutation supply')

Small populations

Fewer mutations, more likely lost by drift

Short-term mutation impact

Single mutation: negligible effect on frequencies

Long-term mutation impact

Ultimate source of ALL genetic variation; every allele originated from mutation

Mutation and selection relationship

Mutations provide raw material; selection shapes it into adaptations

Antibiotic resistance

Mutations occurred randomly everywhere; selection favored resistant mutants where antibiotics present

Gene flow

Makes populations more genetically similar

Is gene flow always beneficial?

Can be beneficial, neutral, or detrimental (depends on how adapted incoming alleles are)

Frog example

Yellow frogs migrate to mainland; new genotypes appear (A1A3, A2A3); A3 frequency increases

Hawks eating yellow frogs

Mainland fitness decreases; island fitness unchanged (if unidirectional flow)

Hardy-Weinberg

Null model predicting genotype frequencies if NO evolution occurring

Five Hardy-Weinberg assumptions

1. Infinite population, 2. No mutations, 3. No migration, 4. No selection, 5. Random mating

Why are Hardy-Weinberg assumptions unrealistic?

Intentionally unrealistic to create null model; all real populations violate these

Hardy-Weinberg equilibrium

Observed frequencies match expected; no detectable evolution at that locus

If observed ≠ expected

Evolution occurring; but H-W cannot identify which mechanism

Hardy-Weinberg notation

p = frequency of allele 1; q = frequency of allele 2; p + q = 1

Hardy-Weinberg equation

p² + 2pq + q² = 1

What does p² represent?

Expected frequency of homozygote 1 (e.g., 4R4R)

What does 2pq represent?

Expected frequency of heterozygote (e.g., 4R7R)

What does q² represent?

Expected frequency of homozygote 2 (e.g., 7R7R)

Why 2pq not just pq?

Two ways to get heterozygote: p from parent 1 × q from parent 2, OR q from parent 1 × p from parent 2

Calculate allele frequency p from genotypes

p = (2 × homozygotes for allele 1 + heterozygotes) / (2 × total individuals)

Why multiply by 2 in denominator?

Each diploid individual has 2 alleles

If you know p, how find q?

q = 1 - p

Calculate expected homozygote frequency

Square the allele frequency (p² or q²)

Calculate expected heterozygote frequency

2pq (2 times p times q)

Example: p=0.821, find p²

(0.821)² = 0.674

Example: p=0.821, q=0.179, find 2pq

2(0.821)(0.179) = 0.294

What if observed 7R7R is 0.067 but expected is 0.032?

Evolution occurring (more than double); possible causes: selection, drift, or assortative mating

Can Hardy-Weinberg identify which mechanism?

No - only shows IF evolution occurring, not WHICH mechanism

Which population closest to Hardy-Weinberg?

Neutral mutation in non-coding region in large population (minimal drift, no selection)

How does population size affect drift?

Small: strong drift, huge impact; Large: weak drift, minimal impact

How does population size affect selection?

Small: drift can override selection; Large: selection more effective

How does population size affect mutations?

Small: fewer mutations, more lost by drift; Large: more mutations arise, larger mutation supply

Why are small populations vulnerable?

Lose variation quickly → cannot adapt → extinction risk

Ultimate source of variation

Mutation (only creates truly new alleles)

Mutation

Only creates truly new alleles.

What maintains variation?

Large population size; gene flow.

What reduces variation?

Drift, bottlenecks, founder effects, small population size.

Why is variation critical?

Necessary for adaptation; selection needs variation to act on.

Can selection create variation?

No - only acts on existing variation; mutation creates new variation.

What is observational study?

Collecting data without manipulation; documents natural occurrence.

Observational study benefits?

Baseline information; ecological validity; cost-effective; necessary starting point.

Observational study limitations?

Cannot determine causality; correlation ≠ causation; snapshot in time.

What is experimental study?

Manipulate one variable while controlling others; test cause-and-effect.

Experimental study benefits?

Determine causality; control confounding variables; replicable.

Experimental study limitations?

Can be impossible (ethical/technological); reduced ecological validity; may oversimplify.

What is mathematical model?

Equations capturing essential aspects of process; simplified representation.

What is computer simulation?

Complex models integrating multiple processes; bundle of mathematical models.

When to use each study type?

Observation: patterns, baseline; Experiment: test causation; Models: predictions, impossible scenarios.

"Organisms evolved because they needed to adapt"?

WRONG - Evolution not need-based; selection acts on existing variation; no forward-thinking.

"Mutations occur more when needed"?

WRONG - Mutation rate constant regardless of need or environment.

"Individuals evolve"?

WRONG - Populations evolve over generations; individuals don't change genetically.

"Evolution always improves organisms"?

WRONG - Evolution is change, not improvement; drift can increase harmful alleles.

"Drift only affects small populations"?

WRONG - Affects all populations; just stronger impact in small ones.

"H-W describes real populations"?

WRONG - H-W is null model with unrealistic assumptions; tool to detect evolution.

How to explain natural selection?

"Individuals with trait X more likely to survive/reproduce, passing alleles to offspring, changing frequencies" NOT "needed to adapt".

Always check in H-W problems?

p + q = 1; p² + 2pq + q² = 1; show all work.

How identify which mechanism acting?

Selection: trait affects fitness; Drift: small population or random event; Mutation: new allele appears; Gene flow: migration.

Key calculation skills?

Calculate allele frequencies from genotype data; Calculate expected frequencies using H-W; Check sums equal 1.

Most important H-W concept?

Null model for NO evolution; deviation shows evolution occurring; cannot identify which mechanism alone.

Most important selection concept?

Requires variation, heritability, differential fitness; not forward-thinking; explains adaptation.

Most important drift concept?

Random; occurs in all populations; stronger in small populations; always reduces variation.

Most important mutation concept?

Random with respect to fitness; ultimate variation source; rate constant; most neutral.

Most important gene flow concept?

Moves alleles between populations; makes populations similar; variable fitness effects.

Most important population size concept?

Determines drift strength, selection effectiveness, mutation supply, and adaptive potential.