Evolutionary Biology and Diversity of Life
Domain 4 - Evolutionary Biology and Diversity of Life
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 (geographic isolation) is most common; 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. Life is organized into Bacteria, Archaea, and Eukarya; mitochondria and chloroplasts descend from endosymbiotic bacteria.
Worked example
In a population, 16% show the recessive phenotype. Assuming Hardy–Weinberg, 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; forgetting that reproductive isolation, not mere difference, defines a species; using a phenotype frequency as q without squaring it.