UNIT 08

Evolution & Populations

// Unit of Evolution

Populations are the unit of evolution, not individuals. The Hardy-Weinberg calculator below makes the math observable — and shows when a population isn't at equilibrium, which is most of the time.

08.1 Causes of Natural Selection

For natural selection to operate, four conditions must hold. Natural selection is one mechanism of evolution — not its synonym.

Condition 01

Variation

Individuals within the species differ in their traits.

Condition 02

Overproduction

More offspring are born than the environment can support.

Condition 03

Heritability

Advantageous traits can be passed down to offspring genetically.

Condition 04

Selective Pressure

Environmental factors make some traits more advantageous than others.

08.2 Mechanisms of Evolution

Five forces drive allele frequencies to change over time:

Mutations — the raw material of variation.
Gene flow — migration mixes populations.
Sexual selection — mate choice shapes traits.
Genetic drift — chance shifts allele frequencies (random).
Natural selection — pressure favors certain traits (non-random).

Adaptive radiation: one species diversifies to fill many niches.

Convergent evolution: unrelated species independently evolve similar traits — usually because they face similar environments.

Three Modes of Selection

Observe how natural selection shifts the population curve (phenotype frequency) over time.

Extreme Trait A Average Trait Extreme Trait B

Directional: A trait is wanted more (or less) — the curve drifts in one direction.

08.3 Population Genetics

Mutations — permanent DNA changes that may produce new heritable traits.
Genetic drift — allele frequencies shift by chance, not selection.
Bottleneck effect — a sudden population crash leaves survivors' alleles dominant.
Founder effect — a small breakaway group's allele frequencies define the new population.
Gene flow — migration between populations changes their genetics.

08.4 Hardy-Weinberg Equilibrium

A population is in Hardy-Weinberg equilibrium when none of the five evolutionary forces is acting. Deviation from these proportions is evidence of evolution at work.

No mutations No gene flow Random mating Large pop. No selection

Allele Frequencies

p + q = 1

Genotype Frequencies

p² + 2pq + q² = 1

Equilibrium Calculator

Adjust the dominant allele frequency (p). The recessive allele (q) and the expected genotype frequencies will calculate automatically.

Dominant Allele (p) 0.60

Recessive Allele (q) 0.40

p²

2pq

q²

0.36

p² (Homozygous Dominant)

0.48

2pq (Heterozygous)

0.16

q² (Homozygous Recessive)

08.5 Reproductive Isolation

Mechanisms that prevent species from interbreeding and producing viable, fertile offspring.

Pre-zygotic

Blocks fertilization from occurring.

Geographic — physical barrier
Ecological — habitat difference
Temporal — different mating times
Behavioral — different courtship
Mechanical — different body structures
Gametic — fertilization fails

Post-zygotic

Occurs after fertilization.

Reduced hybrid viability — genes don't work well together; offspring weak.
Reduced hybrid fertility — offspring survive but cannot produce gametes (sterile).
Hybrid breakdown — first generation is fine, second generation is sterile or weak.

08.6 Human Effects

The biggest human force on biodiversity is artificial selection: only the organisms with traits we like are allowed to breed and survive — variety quietly drains.

Add habitat alteration and direct pressure on certain species, and you have the biological signature of the present. — Anthropocene Impact