Topic 7: Natural Selection

Evolution is the unifying theme of biology. Unit 7 zooms out from individual organisms to entire populations and asks how their genetic makeup changes over generations. The engine of adaptive change is natural selection: when heritable traits differ among individuals and some variants leave more offspring than others, allele frequencies shift over time. This unit pairs the mechanisms of selection with a quantitative tool — the Hardy-Weinberg equilibrium — that lets you detect whether a population is evolving, plus the multiple independent lines of evidence that document common ancestry and the branching history of life.

Populations and Variation

Evolution acts on populations (groups of interbreeding individuals of the same species), not on individuals. Individuals do not evolve; allele frequencies within a population do. The raw material for evolution is heritable variation.

Sources of genetic variation include:

• Mutation — the ultimate source of new alleles; random changes in DNA sequence.
• Sexual reproduction — independent assortment, crossing over, and random fertilization reshuffle existing alleles into new combinations.

Without phenotypic variation that has a genetic basis, selection has nothing to act on. Variation that is purely environmental (not heritable) cannot drive evolution.

Natural Selection

Darwin and Wallace proposed natural selection as the mechanism of adaptive evolution. The logic rests on four observations:

1. Individuals in a population show variation in traits.
2. Some of that variation is heritable (passed to offspring).
3. Organisms produce more offspring than the environment can support, creating a struggle for existence.
4. Individuals with traits better suited to the environment survive and reproduce more — differential reproductive success.

The result: advantageous heritable traits become more common over generations, producing adaptation. The environment is the selective agent; it does not create variation but selects among existing variants. Selection is therefore not random, even though mutation (the source of variation) is.

Fitness and Modes of Selection

Fitness is an organism's reproductive success — its relative contribution of offspring to the next generation. The 'fittest' individual is not necessarily the strongest or fastest; it is the one that leaves the most surviving, reproducing offspring in a given environment.

Selection on a continuous trait can take three forms:

• Directional selection — favors one extreme; the trait mean shifts (e.g., antibiotic resistance increasing).
• Stabilizing selection — favors intermediate phenotypes; reduces variation (e.g., human birth weight).
• Disruptive selection — favors both extremes over the middle; can split a population.

Selective pressures (predation, disease, climate, resource competition) shift over time and space, so what is adaptive in one environment may be neutral or harmful in another.

Artificial Selection

In artificial selection, humans — not the natural environment — choose which individuals reproduce based on desired traits. This is selective breeding. Examples include the diverse breeds of dogs derived from wolves, the many vegetables bred from wild mustard (broccoli, cabbage, kale, Brussels sprouts), and high-yield crops and livestock.

Artificial selection demonstrates that selection can produce rapid, dramatic phenotypic change in relatively few generations, supporting the plausibility of natural selection. The key difference is the selective agent: human preference versus environmental pressures. Both rely on the same underlying requirement — heritable variation.

Other Mechanisms of Evolution

Natural selection is one of several processes that change allele frequencies:

• Genetic drift — random change in allele frequencies due to chance; strongest in small populations. The bottleneck effect (a population crash) and founder effect (a few individuals start a new population) are forms of drift that reduce genetic diversity.
• Gene flow (migration) — movement of alleles between populations; tends to make populations more genetically similar and introduces new alleles.
• Mutation — introduces new alleles, the only source of entirely new genetic variation.
• Nonrandom mating — mate choice based on phenotype (e.g., sexual selection) alters genotype frequencies.

Unlike natural selection, genetic drift and gene flow are not adaptive — they do not necessarily improve fit to the environment.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg (H-W) principle is a null model: it describes a population that is not evolving, so allele and genotype frequencies stay constant generation to generation. Deviation from H-W predictions is evidence that evolution is occurring.

The two equations:

• p + q = 1 — allele frequencies, where p = frequency of the dominant allele and q = frequency of the recessive allele.
• p² + 2pq + q² = 1 — genotype frequencies, where p² = homozygous dominant, 2pq = heterozygous, and q² = homozygous recessive.

A typical problem starts from the recessive phenotype: q² gives the frequency of homozygous recessive individuals, so q = √(q²), then p = 1 − q. Plug back in to get carrier (2pq) and homozygous dominant (p²) frequencies.

Conditions for Hardy-Weinberg

Equilibrium holds only if all five conditions are met — conditions that essentially require the absence of every evolutionary mechanism:

1. No mutation — allele set does not change.
2. No natural selection — all genotypes have equal fitness.
3. No gene flow — no migration in or out.
4. Random mating — no mate preference.
5. Large population size — no genetic drift.

Real populations rarely meet all five, so the model is a baseline for comparison. If observed genotype frequencies match the p² + 2pq + q² predictions, the population is in equilibrium for that gene; a significant mismatch signals that one or more forces are acting.

Evidence of Evolution

Multiple independent lines of evidence converge on common descent:

• Fossil record — shows change over geologic time and transitional forms; dated by radiometric methods.
• Biogeography — geographic distribution of species (e.g., island endemics resembling nearby mainland species).
• Comparative anatomy — homologous structures (same origin, different function, e.g., vertebrate forelimbs) indicate shared ancestry; vestigial structures are reduced remnants of ancestral features.
• Comparative embryology — similar developmental stages among related groups.
• Molecular evidence — shared DNA, RNA, and protein sequences; the near-universal genetic code; conserved core metabolic pathways. The more similar the sequences, the more recent the common ancestor.

Analogous structures (similar function, different origin, e.g., bird and insect wings) arise from convergent evolution and are NOT evidence of close relatedness.

Common Ancestry, Phylogeny, and Cladograms

All life shares a common ancestor, evidenced by universal features: DNA/RNA as genetic material, a shared genetic code, ribosomes, ATP, and conserved metabolic pathways.

A phylogenetic tree (or cladogram) is a branching diagram of hypothesized evolutionary relationships. Key reading rules:

• Nodes represent common ancestors; each branch point marks a divergence.
• Shared derived characters (synapomorphies), often marked along branches, define groups (clades).
• Relatedness is determined by the most recent common ancestor, not by how close tips appear or the left-to-right tip order.
• A clade includes an ancestor and ALL its descendants (monophyletic).

Trees are hypotheses revised as new molecular and morphological data emerge. Out-groups help root the tree and establish ancestral states.

Speciation and Extinction

A species (biological species concept) is a group of organisms that can interbreed and produce fertile offspring. Speciation is the formation of new species, requiring reproductive isolation that halts gene flow:

• Allopatric speciation — a geographic barrier physically separates populations, which then diverge.
• Sympatric speciation — reproductive isolation arises without geographic separation (e.g., polyploidy in plants, habitat or behavioral divergence).

Isolating mechanisms are prezygotic (prevent mating or fertilization: temporal, habitat, behavioral, mechanical, gametic) or postzygotic (reduce hybrid viability/fertility). Rates vary: gradualism (slow, steady change) versus punctuated equilibrium (long stasis broken by rapid change).

Extinction removes species permanently. Background extinction is ongoing; mass extinctions eliminate many lineages quickly and open niches that drive adaptive radiation of survivors. Extinction rates rise when environmental change outpaces adaptation.

The Origin of Life

Hypotheses about how life began on early Earth include:

• Abiotic synthesis of organic molecules — simple organics (amino acids, nucleotides) formed from inorganic precursors under early-Earth conditions; the Miller-Urey experiment showed amino acids can form from a simulated early atmosphere with energy input.
• Polymerization — small monomers joined into polymers, possibly on mineral or clay surfaces.
• RNA world hypothesis — RNA may have been the first self-replicating, catalytic molecule (acting as both gene and enzyme via ribozymes) before DNA and proteins took over those roles.
• Protocells — membrane-bound droplets that concentrated molecules and could grow and divide, a precursor to true cells.

These are scientific hypotheses supported by laboratory evidence and geochemistry; the deep similarity of all living cells is consistent with a single origin of life.