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Biology Unit 7: Natural selection

Populations change over time because individuals with helpful heritable traits leave more offspring. Many lines of evidence support evolution.

Covers: Evolution by natural selection, evidence for evolution, Hardy-Weinberg, speciation, phylogenetic trees

Study guide

Evolution is change in the genetic makeup of populations over generations. Darwin proposed natural selection, which requires variation among individuals, heritability of that variation, overproduction of offspring, and differential survival and reproduction. Individuals with traits better suited to their environment leave more offspring, so those traits become more common. Fitness means reproductive success. Selection acts on individuals, but populations are what evolve. Other mechanisms of evolution are genetic drift (random changes, strongest in small populations, including bottlenecks and founder effects), gene flow (migration) and mutation.

Evidence for evolution comes from fossils showing change over time and transitional forms; homologous structures, like the forelimbs of humans, whales and bats, that share ancestry; vestigial structures; embryology; molecular similarities in DNA and proteins; and directly observed evolution, such as antibiotic resistance. Analogous structures, like bird and insect wings, show convergent evolution rather than common ancestry.

The Hardy-Weinberg principle describes a population that is not evolving. Allele frequencies satisfy p + q = 1 and genotype frequencies p² + 2pq + q² = 1. It requires a large population, random mating, and no mutation, migration or selection. Comparing real data to these predictions shows whether evolution is occurring.

Speciation happens when populations become reproductively isolated. Allopatric speciation follows geographic separation; sympatric speciation happens in the same area, for example through polyploidy. Prezygotic barriers (habitat, timing, behavior) and postzygotic barriers (sterile hybrids) keep species apart.

Phylogenetic trees show evolutionary relationships. Branch points (nodes) represent common ancestors, and shared derived characters group organisms into clades.

In artificial selection (selective breeding), humans choose which individuals reproduce based on traits they want, such as crop yield or dog size. It works by the same logic as natural selection, but people do the selecting instead of the environment. Directional selection favors one extreme of a trait, so the average shifts. Stabilizing selection favors intermediate values and narrows the range, as with human birth weight. Disruptive selection favors both extremes over the middle and can split a population.

To use Hardy-Weinberg, start with the homozygous recessive frequency: q² equals the fraction showing the recessive trait, so q = √q², p = 1 − q, and the carrier frequency is 2pq. If 16% of a population is recessive, for example, then q = 0.4, p = 0.6 and 48% are carriers. A trait first appeared on the branch just before the node that groups all the species that have it. Two species are most closely related when they share the most recent common ancestor, not when their tips sit next to each other. An outgroup lacks the derived traits and serves as the comparison point.

Key ideas

• Natural selection needs variation, heritability, overproduction and differential survival.
• Populations evolve; individuals don't.
• Evidence: fossils, homologous structures, DNA similarities, embryology, direct observation.
• Hardy-Weinberg: p + q = 1 and p² + 2pq + q² = 1 describe a population that isn't evolving.
• Speciation happens when populations become reproductively isolated.

Formulas and rules

Common mistakes

• Saying individuals evolve. Populations do.
• Mixing up homologous (shared ancestry) with analogous (similar function, different origin) structures.

Words to know

Population

A group of one species in one area; the unit that evolves.

Hardy-Weinberg equation for genotypes

p² + 2pq + q² = 1.

Homologous structures

Similar structures from a common ancestor, like human arms and whale flippers.

Speciation

Formation of new species, driven by reproductive isolation.

Natural selection

Variation, heritability, overproduction and differential survival.

Hardy-Weinberg conditions

Large population, random mating, no mutation, no migration, no selection.

Evolution

Change in a population's genetic makeup over generations.

Fitness

Reproductive success.

Genetic drift

Random changes in allele frequencies, strongest in small populations.

Bottleneck effect

Drift after a sharp drop in population size.

Founder effect

Drift when a small group starts a new population.

Gene flow

Movement of alleles between populations by migration.

Analogous structures

Similar function, different ancestry (bird vs. insect wings): convergent evolution.

Vestigial structure

A reduced structure with little or no function, like the human appendix.

Allopatric speciation

Speciation after geographic separation.

Sympatric speciation

Speciation in the same area, such as by polyploidy.

Prezygotic barrier

Prevents mating or fertilization (habitat, timing, behavior).

Phylogenetic tree

A diagram of evolutionary relationships; nodes are common ancestors.

Clade

An ancestor and all its descendants.

Sample practice questions

19 more questions like these, from level 1 to 5, are in the practice set.