AP Biology: Evolution and Natural Selection

Variation in a trait The variation is heritable More offspring are produced than can survive Some variants survive and reproduce more than others

Published in On the Origin of Species (1859). Alfred Russel Wallace reached the same idea independently.

An individual's relative reproductive success: its contribution of genes to the next generation compared with others.

A population. Individuals do not evolve; allele frequencies in a population change over generations.

p + q = 1 p^2 + 2pq + q^2 = 1

No mutation Random mating No natural selection Very large population (no genetic drift) No gene flow (migration)

0.48 (48%). q² = 0.16, so q = 0.4 and p = 0.6; 2pq = 2 × 0.6 × 0.4 = 0.48.

Random change in allele frequencies from chance events, strongest in small populations.

Genetic drift after a sharp drop in population size leaves survivors whose gene pool does not represent the original, e.g. northern elephant seals.

Genetic drift when a few individuals colonize a new area, so rare alleles can become common, e.g. Ellis–van Creveld syndrome among the Old Order Amish.

Movement of alleles between populations through migration; makes populations more alike.

Directional: favors one extreme. Stabilizing: favors the average (human birth weight). Disruptive: favors both extremes over the middle.

Selection for traits that improve mating success, such as peacock tails; often produces sexual dimorphism.

Structures shared because of common ancestry even if their functions differ (human arm, bat wing, whale flipper).

Structures with similar functions that evolved independently, not from a common ancestor (bird wing vs. insect wing).

Reduced remnants of structures that were functional in ancestors (whale pelvic bones, the human tailbone).

Unrelated species evolve similar traits because they face similar environments (streamlined bodies of sharks and dolphins).

Fossil record Comparative anatomy (homologous and vestigial structures) Molecular similarities in DNA and proteins Embryology Biogeography Direct observation (antibiotic and pesticide resistance)

Resistant bacteria already exist because of random mutation. The antibiotic kills susceptible cells, so resistant ones survive and reproduce, raising the frequency of resistance.

No. Mutations arise randomly; selection can only act on variation that already exists.

A species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring.

Allopatric: a geographic barrier separates populations. Sympatric: new species arise in the same area, e.g. by polyploidy in plants or by habitat or sexual isolation.

Habitat, temporal, behavioral, mechanical and gametic isolation. They prevent mating or fertilization.

Reduced hybrid viability, reduced hybrid fertility (a mule is sterile) and hybrid breakdown.

Rapid evolution of many species from one ancestor into diverse ecological niches, e.g. Darwin's finches in the Galápagos.

Gradualism: slow, steady change over time. Punctuated equilibrium: long periods of little change broken by brief bursts of rapid change.

The most recent common ancestor of the lineages that split there.

Find the pair that shares the most recent common ancestor (the closest node), not the pair drawn next to each other.

An ancestor and all of its descendants.

Organic molecules such as amino acids can form from simple inorganic gases plus an energy source (electric sparks) under conditions meant to model early Earth.

The idea that early life used RNA both to store genetic information and to catalyze reactions (ribozymes) before DNA and proteins took over.