Ecology and Evolution: Mechanisms, Models, and Evidence
Core concepts
Understand evolutionary mechanisms-mutation, selection, drift, gene flow, nonrandom mating-and the conditions for Hardy-Weinberg equilibrium. Know species concepts and speciation modes (allopatric, sympatric, polyploidy), reproductive isolating barriers, and phylogenetic reasoning (synapomorphy vs homoplasy, parsimony, molecular clocks). In ecology, cover population growth models (exponential vs logistic, carrying capacity), life-history tradeoffs (r/K continuum), species interactions (competition, predation, mutualism, parasitism), community structure (keystone species, trophic cascades, succession), and ecosystem-level flows of energy and matter.
Traps
Questions often conflate acclimation (plastic, within a lifetime) with adaptation (heritable, across generations). Natural selection acts on phenotypes and changes allele frequencies within populations; it does not create variation on demand. In population models, the logistic growth rate is highest at intermediate population size, not at carrying capacity. In phylogenetics, shared derived characters define clades while shared ancestral characters do not-a frequent distractor.
Worked rule of thumb
For any evolution scenario, run the variation-inheritance-differential-fitness checklist: is there heritable variation, and does it predict reproductive success? If any element is missing, selection cannot drive the described change, and drift, gene flow, or plasticity must be considered instead. This checklist also flags items that improperly describe selection as goal-directed.
Study strategy
Practice reading population and phylogenetic data critically, and always ask what evidence would falsify the proposed mechanism.