Evolutionary Biology and Diversity of Life
Evolution
Items test the mechanisms that change allele frequencies, the evidence for common descent, speciation, and how organisms are classified and related.
Mechanisms of evolution
- Natural selection acts on heritable phenotypic variation that affects fitness; it is not goal-directed and does not create new variation.
- Mutation is the ultimate source of new alleles. Gene flow and genetic drift (bottleneck, founder effect) also change frequencies; drift is strongest in small populations and is random with respect to fitness.
- Directional, stabilizing, and disruptive selection shift, narrow, or split a trait distribution.
- Hardy–Weinberg equilibrium (p² + 2pq + q² = 1) holds only with no mutation, no migration, random mating, infinite population size, and no selection - a null model, not a description of most real populations.
- Speciation: allopatric speciation can follow geographic isolation; sympatric speciation can occur via polyploidy or habitat/behavioral divergence. Prezygotic barriers (temporal, habitat, behavioral, mechanical, gametic) prevent fertilization; postzygotic barriers (hybrid inviability, hybrid sterility) act afterward.
Evidence and classification
Homologous structures share ancestry (forelimb bones); analogous structures share function only (bird and insect wings) and indicate convergence. Fossils, biogeography, embryology, and molecular sequence comparisons support common descent. Cladistics groups organisms by shared derived characters (synapomorphies); monophyletic groups contain an ancestor and all its descendants. A common three-domain teaching model distinguishes Bacteria, Archaea and Eukarya; mitochondria and chloroplasts descend from endosymbiotic bacteria.
Worked example
In a supplied two-allele autosomal model with complete recessivity and penetrance, 16% show the recessive phenotype. Assuming Hardy-Weinberg equilibrium, aa = q² = 0.16, so q = 0.4 and p = 1 − 0.4 = 0.6. Heterozygote frequency is 2pq = 2(0.6)(0.4) = 0.48, and homozygous dominants are p² = 0.36; the three sum to 1.00. Note the reasoning step: the recessive phenotype frequency equals q² only when the trait is recessive and the population is in equilibrium.
Traps
Equating dominant with frequent; assuming a population at Hardy–Weinberg is evolving; confusing allele frequency with genotype frequency; calling drift "selection"; treating analogous structures as evidence of close relatedness; applying reproductive isolation as a universal species definition even to asexual organisms, rather than recognizing the limits of the biological species concept; using a phenotype frequency as q without squaring it.