Natural Selection

evolution

Change in the heritable characteristics (allele frequencies) of a population over successive generations. Populations evolve; individuals do not.

natural selection

The mechanism of evolution in which individuals with heritable traits better suited to the environment survive and reproduce at higher rates, increasing the frequency of those traits over time. Requires variation, heritability, overproduction of offspring, and differential reproductive success.

Charles Darwin

English naturalist who, with Alfred Russel Wallace, proposed natural selection as the mechanism of evolution. Published On the Origin of Species (1859), drawing heavily on observations from his voyage aboard the HMS Beagle, especially the Galápagos finches.

Jean-Baptiste de Lamarck

Earlier naturalist who proposed that species change over time, but by an incorrect mechanism: inheritance of acquired characteristics and "use and disuse" (a giraffe stretches its neck, then passes the longer neck to offspring). Traits acquired during a lifetime are not heritable. Credit him for the idea that species aren't fixed; reject his mechanism.

paleontology

The study of fossils and ancient life. Supplies the fossil record: transitional forms, dated strata showing change over time, and evidence of extinct lineages.

biogeography

The study of the geographic distribution of species. Distribution patterns match continental drift and isolation

flora

The plant life of a particular region or time period.

fauna

The animal life of a particular region or time period. (Flora and fauna together are the biogeographic data used to compare regions.)

embryology

The study of embryonic development. Shared developmental stages across distantly related organisms

morphological homologies

Anatomical similarities between species that result from descent from a shared ancestor. The broad category of structural evidence for common ancestry.

homologous structures

Structures with the same underlying anatomy and shared ancestral origin but different functions. Example: the tetrapod forelimb

analogous structures

Structures with similar function but different anatomy and no shared ancestry, arising because unrelated lineages faced similar selective pressures. Example: bird wings vs. insect wings; dolphin vs. shark body shape. Evidence of convergent evolution.

molecular biology

At the evidence level: comparison of DNA, RNA, and protein sequences across species. Considered the strongest evidence for evolution. All life shares the genetic code, ribosomes, and ATP; degree of sequence similarity tracks how recently two species shared an ancestor.

continuing evolution

Evolution is ongoing and directly observable, not just a historical event. Examples: antibiotic resistance in bacteria, pesticide resistance in insects, viral evolution.

common ancestor

A species from which two or more later lineages descended. Shared traits among descendants are inherited from it.

phylogenetic tree (cladogram)

A branching diagram representing a hypothesis about evolutionary relationships. Nodes represent common ancestors; branch points represent divergence. Relatedness is read by most recent common ancestor, not by how close two tips appear on the page.

out-group

The taxon in a cladogram that is least closely related to all the others. It's included to root the tree and establish which character states are ancestral versus derived.

genetic variability

The extent of genetic differences among individuals in a population. It is the raw material selection acts on; populations with high variability are more likely to survive environmental change.

peppered moths

Biston betularia, the textbook case of directional selection. Before industrialization, light moths were camouflaged on lichen-covered trees; soot-darkened bark during the Industrial Revolution favored dark (melanic) moths, whose frequency rose sharply. Frequencies reversed after pollution controls.

environmental pressure (selective pressure)

Any environmental factor

random mutation

A change in DNA sequence occurring by chance. It is the only source of entirely new alleles. Mutations are random with respect to fitness: the environment does not cause useful mutations, it only selects among those that already exist.

adaptation

A heritable trait that increases an organism's fitness in its environment, produced by natural selection over generations. Not something an individual develops in its lifetime.

evolutionary fitness

An organism's reproductive success

sexual selection

Selection based on mating success rather than survival. Intrasexual: competition within one sex (antlers, male combat). Intersexual: mate choice by the other sex (peacock tails, birdsong). Produces sexual dimorphism, sometimes at a cost to survival.

genetic drift

Random, chance-driven change in allele frequencies between generations. Non-adaptive: it can eliminate beneficial alleles or fix harmful ones. Effects are strongest in small populations, where sampling error is proportionally larger.

bottleneck (founder effect)

Two related forms of drift that sharply reduce genetic diversity:

Bottleneck effect

a catastrophe (disease, hunting, natural disaster) drastically shrinks a population; survivors carry a random, reduced sample of the original alleles. Ex: cheetahs, northern elephant seals.

Founder effect

a small group leaves and colonizes a new area, carrying an unrepresentative allele sample.

gene flow

Movement of alleles between populations via migration or the transfer of gametes (e.g., pollen). Makes populations genetically more similar and can introduce new alleles. Stopping gene flow is a prerequisite for speciation.

directional selection

One extreme phenotype is favored; the trait distribution shifts toward that extreme. Ex: antibiotic resistance, peppered moths.

stabilizing selection

Intermediate phenotypes are favored and both extremes are selected against; variation decreases. Ex: human birth weight.

disruptive (diversifying) selection

Both extremes are favored and intermediates are selected against; the distribution becomes bimodal. Can be an early step toward speciation. Ex: bill size in African seedcracker finches.

artificial selection

Humans, rather than the natural environment, choose which individuals reproduce. Demonstrates how quickly selection can produce large morphological change. Ex: all dog breeds from wolves; broccoli, kale, cabbage, cauliflower, and Brussels sprouts all bred from wild mustard, Brassica oleracea.

species

Under the biological species concept, a group whose members can interbreed in nature to produce fertile offspring. (Limitations: doesn't apply to asexual organisms, fossils, or ring species.)

reproductively isolated

Unable to successfully interbreed and produce fertile offspring with another population, due to prezygotic or postzygotic barriers. This is the defining criterion separating two species.

divergent evolution

Related lineages sharing a common ancestor become increasingly different over time as they adapt to different environments. Produces homologous structures. Ex: Darwin's finches.

punctuated equilibrium

A model in which species remain largely unchanged (stasis) for long periods, interrupted by short bursts of rapid change, often associated with speciation events. Accounts for the abrupt appearances and gaps in the fossil record.

gradualism

The contrasting model: evolutionary change accumulates slowly and steadily over long spans of time, with continuous intermediate forms. Both models are supported by different parts of the fossil record.

adaptive radiation

Rapid diversification of one ancestral species into many species occupying different ecological niches. Triggered by mass extinction, colonization of a new habitat, or a key evolutionary innovation. Ex: Darwin's finches; mammals after the K-Pg extinction; Hawaiian silverswords.

pre-zygotic barriers

Reproductive isolating mechanisms that prevent fertilization from occurring at all:

Habitat isolation

populations occupy different microhabitats

Temporal isolation

breed at different times of day or year

Behavioral isolation

incompatible courtship rituals or signals

Mechanical isolation

incompatible reproductive structures

Gametic isolation

sperm and egg cannot fuse

post-zygotic barriers

Mechanisms acting after a hybrid zygote forms:

Reduced hybrid viability

hybrid fails to develop or survive

Reduced hybrid fertility

hybrid survives but is sterile (mule, from horse × donkey)

Hybrid breakdown

first-generation hybrids are fertile, but later generations are feeble or sterile

convergent evolution

Unrelated lineages independently evolve similar traits because they face similar selective pressures. Produces analogous structures. Ex: wings in birds, bats, and insects; streamlined bodies in sharks and dolphins. Can mislead phylogenetic trees built on morphology alone.

speciation

The evolutionary process by which one species splits into two or more reproductively isolated species.

allopatric speciation

Speciation with geographic separation. A physical barrier (river, mountain range, canyon) divides a population, gene flow stops, and drift plus different selective pressures drive divergence until reproductive isolation exists. The most common mode. (allo- = other, patria = homeland.)

sympatric speciation

Speciation without geographic separation, occurring within the same area. Mechanisms include polyploidy, habitat differentiation (apple maggot flies shifting host plants), and sexual selection (cichlid mate preference).

polyploidy

Possession of more than two complete chromosome sets, usually from errors in meiosis. Instantly reproductively isolates the polyploid from the diploid parent population, producing sympatric speciation in a single generation. Very common in plants

Hardy-Weinberg law

States that allele and genotype frequencies remain constant across generations in a population that is not evolving, described by:

Alexander Oparin and J. B. S. Haldane

Working independently in the 1920s, they proposed the "primordial soup" hypothesis: early Earth had a reducing atmosphere with no free oxygen, and energy sources such as lightning and UV radiation drove the abiotic synthesis of organic molecules, which accumulated in the early oceans.

Stanley Miller and Harold Urey

Ran the 1953 experiment that tested the Oparin–Haldane hypothesis. They circulated water vapor, methane, ammonia, and hydrogen through a closed apparatus while applying electrical discharge to simulate lightning, and produced amino acids and other organic monomers

RNA-world hypothesis

Proposes that RNA, not DNA, was the first genetic material. RNA can both store hereditary information and catalyze chemical reactions (ribozymes), so it could serve as gene and enzyme simultaneously. DNA