Evolution and Natural Selection

Natural Selection

Natural selection is the non-random differential survival and reproduction of individuals based on heritable phenotypic variation. Darwin's logic rests on three observations and two inferences: (1) populations produce more offspring than can survive (overproduction), (2) individuals vary in their traits (variation), and (3) some of this variation is heritable. Inference 1: individuals with traits better suited to their environment survive and reproduce at higher rates (differential reproductive success). Inference 2: over generations, the frequency of advantageous alleles increases in the population (descent with modification). The unit of selection is the individual organism, but the unit of evolution is the population. Key requirements: variation must exist, variation must be heritable, and variation must affect fitness (survival and reproductive output). Fitness is relative—it is measured against other genotypes in the same population, not against an absolute standard. Directional selection shifts the population mean toward one extreme (e.g., increasing beak depth during drought). Stabilizing selection favors intermediate phenotypes (e.g., human birth weight—too small or too large both reduce survival). Disruptive selection favors both extremes against the intermediate, and can drive sympatric speciation if assortative mating follows.

Adaptation

An adaptation is a heritable trait that enhances fitness in a specific environment—the product of natural selection over many generations. Adaptations are not purposeful; they arise from random genetic variation filtered by selection. Key distinctions: adaptation (the trait itself) vs. adaptedness (the degree of fit between organism and environment). Exaptations are traits that evolved for one function but were later co-opted for another (e.g., feathers originally evolved for thermoregulation or display, later co-opted for flight). Not every trait is an adaptation—some are byproducts of developmental constraints, genetic drift, or linkage to selected traits (spandrels, per Gould and Lewontin). Convergent evolution produces analogous structures: similar environmental pressures select for similar solutions in distantly related lineages (e.g., wings in birds, bats, and insects—analogous as wings, but the forelimb bones are homologous). Divergent evolution produces homologous structures from a common ancestor modified for different functions (e.g., mammalian forelimbs modified into human arms, whale flippers, bat wings). The distinction between homology (shared ancestry) and analogy (shared function) is foundational to phylogenetic reconstruction.

Genetic Drift

Genetic drift is the random change in allele frequencies due to sampling error in finite populations—it is the stochastic complement to the deterministic force of selection. Drift is strongest in small populations and can fix or eliminate alleles regardless of their fitness effects. Two classic scenarios: the bottleneck effect occurs when a population is drastically reduced in size (e.g., by a catastrophe), and the surviving gene pool may not represent the original population's diversity. The founder effect occurs when a small group colonizes a new habitat; the new population's allele frequencies reflect the founders, not the source population. Both reduce genetic diversity and can increase the frequency of deleterious alleles by chance. Drift is one reason small populations face elevated extinction risk. In the neutral theory of molecular evolution (Kimura), most genetic variation at the molecular level is selectively neutral, and drift—not selection—is the dominant force shaping nucleotide diversity. The MCAT may ask you to identify whether a described scenario reflects drift or selection: if the outcome correlates with fitness (directional, predictable), it is selection; if the outcome is random with respect to fitness and correlated with population size, it is drift.

Gene Flow

Gene flow (migration) is the movement of alleles between populations via dispersal of individuals or gametes (e.g., pollen). Gene flow has two primary effects: it introduces new alleles into a population, increasing genetic diversity, and it reduces genetic differences between populations, homogenizing them over time. Gene flow can counteract both natural selection and genetic drift: if selection favors different alleles in different environments, gene flow between them can prevent local adaptation by introducing maladaptive alleles. Conversely, restricted gene flow is a prerequisite for speciation—when populations are isolated, they diverge independently through selection and drift. The balance between gene flow and selection determines the scale of local adaptation. In human evolution, gene flow between archaic and modern human populations (e.g., Neanderthal introgression into non-African genomes) is now recognized as a significant source of genetic variation. On the MCAT, gene flow often appears in Hardy-Weinberg disruption scenarios: migration violates the no-gene-flow assumption, shifting allele frequencies toward the migrant population's frequencies.

Sexual Selection

Sexual selection is a special case of natural selection acting on an organism's ability to obtain or successfully copulate with a mate. It operates through two mechanisms: intrasexual selection (competition between members of the same sex, usually males, for access to mates—e.g., antlers, body size, ritualized combat) and intersexual selection (mate choice, usually by females, based on traits that signal genetic quality or resource-provisioning ability—e.g., peacock plumage, bowerbird displays, frog calls). Sexually selected traits are often costly and appear maladaptive under natural selection alone (the peacock's tail impairs flight and attracts predators). The handicap principle (Zahavi) explains this paradox: costly traits are honest signals of quality because only high-fitness individuals can afford them. Runaway selection (Fisher) posits a positive feedback loop where female preference for a trait and the trait itself become genetically correlated and exaggerate together until natural selection imposes a cost ceiling. Sexual dimorphism—phenotypic differences between males and females of the same species—is often a signature of sexual selection.

Speciation

Speciation is the evolutionary process by which one lineage splits into two reproductively isolated species. The biological species concept (Mayr) defines species as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups—but this concept does not apply to asexual organisms or fossils. Alternative concepts: morphological species concept (based on structural similarity), ecological species concept (based on ecological niche), and phylogenetic species concept (based on shared, derived characters defining a monophyletic group). Speciation requires reproductive isolation, which can be prezygotic (before fertilization: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation) or postzygotic (after fertilization: hybrid inviability, hybrid sterility, hybrid breakdown). Allopatric speciation occurs when a geographic barrier physically separates populations; it is the most common mode. Sympatric speciation occurs within the same geographic area, often via polyploidy (common in plants) or disruptive selection with assortative mating. Adaptive radiation is the rapid diversification of a single lineage into many species filling different ecological niches (e.g., Darwin's finches, Hawaiian honeycreepers, cichlid fishes).

Phylogenetic Relationships

Phylogenetic trees (cladograms, phylograms) are hypotheses of evolutionary relationships among taxa. Each branching point (node) represents a common ancestor; sister taxa are each other's closest relatives. Trees are built from shared, derived characters (synapomorphies)—traits present in a group and its common ancestor but absent in more distant relatives. Shared ancestral characters (symplesiomorphies) do not inform relationships within the group. Monophyletic groups (clades) include an ancestor and all of its descendants—these are the only valid taxonomic groupings in cladistics. Paraphyletic groups include an ancestor and some, but not all, descendants (e.g., reptiles excluding birds). Polyphyletic groups include taxa from multiple ancestors (e.g., flying animals). Outgroup comparison is used to determine character polarity: the outgroup (a taxon known to have diverged earlier) establishes the ancestral state. Molecular phylogenetics uses DNA, RNA, or protein sequence data; the assumption is that sequence differences accumulate roughly proportionally to time since divergence (the molecular clock hypothesis, with caveats for rate variation). Maximum parsimony chooses the tree requiring the fewest evolutionary changes; maximum likelihood and Bayesian methods incorporate explicit models of sequence evolution. The MCAT expects you to read trees, identify sister taxa, recognize monophyletic groups, and understand that tree topology—not branch order at the tips—conveys relatedness. Rotating branches around a node does not change the tree's meaning.

How it works

Evolution integrates population genetics with ecology. The Hardy-Weinberg principle provides the null model: in an infinitely large, randomly mating population with no mutation, migration, or selection, allele and genotype frequencies remain constant. When observed frequencies deviate from Hardy-Weinberg expectations, one of the assumptions is violated—and identifying which assumption is the MCAT's favorite evolution question type. Natural selection changes allele frequencies predictably in response to environmental pressures; genetic drift changes them randomly, inversely proportional to population size; gene flow homogenizes; mutation introduces novelty. Speciation occurs when gene flow between populations ceases long enough for reproductive isolation to evolve—geographic separation (allopatry) is the most common trigger. Phylogenetic trees are reconstructed from the distribution of shared derived characters, and the principle of parsimony selects the simplest evolutionary narrative. The takeaway: evolution is not one force but the net effect of several forces acting simultaneously on populations. The MCAT expects you to disentangle them.

How it works

Evolution integrates population genetics with ecology. The Hardy-Weinberg principle provides the null model: in an infinitely large, randomly mating population with no mutation, migration, or selection, allele and genotype frequencies remain constant. When observed frequencies deviate from Hardy-Weinberg expectations, one of the assumptions is violated—and identifying which assumption is the MCAT's favorite evolution question type. Natural selection changes allele frequencies predictably in response to environmental pressures; genetic drift changes them randomly, inversely proportional to population size; gene flow homogenizes; mutation introduces novelty. Speciation occurs when gene flow between populations ceases long enough for reproductive isolation to evolve—geographic separation (allopatry) is the most common trigger. Phylogenetic trees are reconstructed from the distribution of shared derived characters, and the principle of parsimony selects the simplest evolutionary narrative. The takeaway: evolution is not one force but the net effect of several forces acting simultaneously on populations. The MCAT expects you to disentangle them.

Comparisons

• B/B (Hardy-Weinberg): p² + 2pq + q² = 1 for genotype frequencies; p + q = 1 for allele frequencies. Identify which assumption is violated in a given scenario—this is a guaranteed MCAT question type.
• B/B (Selection types): Directional, stabilizing, and disruptive selection appear in passage-based questions showing phenotypic distributions before and after an environmental change.
• B/B (Phylogenetic trees): Interpret cladograms—identify most recent common ancestor, sister taxa, monophyletic groups. Rotating branches around a node does not change relationships.
• C/P (Molecular clock): Amino acid/nucleotide substitution rates connect to chemical stability and mutation rate biochemistry.
• P/S (Evolutionary psychology): Sexual selection concepts (mate choice, intrasexual competition) appear in social science passages.
• B/B (Population bottlenecks): Drift and founder effects reduce heterozygosity; correlate with conservation biology passages about endangered species.

Common confusions

• Confusing fitness with strength or health. Fitness is relative reproductive success—an organism that lives briefly but produces many offspring has higher fitness than one that lives long but produces few.
• Thinking natural selection is survival of the fittest in a circular sense. Selection requires heritable variation AND differential reproduction, not just survival.
• Treating genetic drift as selection. If allele frequency changes are random and not tied to fitness, it is drift—especially likely in small populations. The MCAT loves to describe a founder event and ask you to identify it as drift.
• Assuming all traits are adaptations. Some traits are byproducts (spandrels), pleiotropic effects, or fixed by drift. Not everything has an adaptive explanation.
• Confusing homology with analogy. Homologous structures share ancestry (vertebrate forelimbs); analogous structures share function but not ancestry (bird vs. insect wings). This distinction is central to phylogenetic inference.
• Reading a phylogenetic tree by looking at the order of taxa at the tips. Relatedness is determined by the most recent common ancestor (node depth), not by which tip is closest on the page. Rotating branches around a node does not change the tree's meaning.
• Assuming allopatric speciation requires a physical barrier like a mountain or ocean. Any mechanism that prevents gene flow—including behavioral differences or temporal isolation—can initiate speciation.
• Forgetting that Hardy-Weinberg is a null model, not a description of real populations. Deviations from H-W equilibrium are the evidence FOR evolution, not evidence against the model.

Quick review

• Evolution = change in allele frequencies in a population over generations. Four forces: natural selection, genetic drift, gene flow, mutation.
• Natural selection requirements: heritable variation, differential reproduction, trait-fitness correlation. NOT random—predictable by environment.
• Directional selection: shifts mean toward one extreme. Stabilizing: favors intermediate. Disruptive: favors both extremes against intermediate.
• Genetic drift: random allele frequency changes, strongest in small populations. Bottleneck + founder effects are classic drift scenarios.
• Gene flow: movement of alleles between populations. Homogenizes, counteracts local adaptation, introduces new variation.
• Sexual selection: intrasexual (male-male competition) and intersexual (female choice). Handicap principle: costly traits are honest fitness signals.
• Speciation requires reproductive isolation. Prezygotic (before fertilization) and postzygotic (after fertilization) barriers.
• Allopatric speciation = geographic barrier. Sympatric speciation = same area, often polyploidy or disruptive selection with assortative mating.
• Biological species concept: interbreeding natural populations, reproductively isolated. Does not apply to asexual organisms or fossils.
• Phylogenetic trees: nodes = common ancestors. Closer nodes = more recent common ancestor = closer relationship. Monophyletic = ancestor + ALL descendants.
• Hardy-Weinberg: p² + 2pq + q² = 1. Assumptions: no mutation, no migration, no selection, random mating, infinite population size. Violation = evidence of evolution.
• Homology = shared ancestry (vertebrate forelimbs). Analogy = shared function, convergent evolution (bird vs. bat wings).