Evolution

Evolution is the organizing framework of biology. For the MCAT it is a set of testable mechanisms with precise definitions, one quantitative tool (Hardy-Weinberg), and a handful of classic named examples. This guide builds the logic first, then the details the exam tests.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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The Core Logic: Why Populations Change Over Time

Must know

Within any population, individuals vary; some variation is heritable; individuals with certain heritable traits leave more offspring. Across generations, traits that improve reproductive success become more common. That is natural selection in one paragraph.

Evolution is formally a change in allele frequencies in a population over time. The individual does not evolve; the population (gene pool) evolves — a distinction the MCAT tests.

Hardy-Weinberg Equilibrium: The Null Hypothesis of Evolution

Must know

The Hardy-Weinberg principle is the baseline of a non-evolving population: allele and genotype frequencies stay constant across generations if five conditions hold — no mutation, no gene flow, no selection, random mating, and large population size (no drift).

For one locus with two alleles, let $p$ = frequency of $A$ and $q$ = frequency of $a$:

$p + q = 1$

$p^2 + 2pq + q^2 = 1$

where $p^2$ = $AA$, $2pq$ = $Aa$ (heterozygotes), $q^2$ = $aa$.

The MCAT usually starts you from the recessive homozygote, the only genotype identifiable from phenotype. If 1% show the recessive trait, $q^2 = 0.01$, so $q = 0.1$, $p = 0.9$, and carriers $= 2pq = 0.18$ (18%).

The five conditions, when violated, are the five mechanisms of evolution: mutation, gene flow, natural selection, non-random mating, and genetic drift, respectively. This pattern recurs throughout the guide.

Quick check: In a Hardy-Weinberg population, the frequency of a recessive disease allele $a$ is 0.04. What fraction are carriers ($Aa$)?

Answer: $q = 0.04$, $p = 0.96$. Carriers $= 2pq = 2(0.96)(0.04) = 0.0768$, ~7.7% — far more common than affected individuals ($q^2 = 0.0016$).

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Natural Selection

Mechanism and Requirements

Must know

Natural selection requires three ingredients: variation, heritability of that variation, and differential reproductive success tied to it. Variation must exist before selection acts — selection filters variation, it does not create it (mutation and recombination are the sources).

Passage-level

The peppered moth in industrial England is the classic example: soot darkened tree bark, light moths became conspicuous, and the dark allele rose to high frequency — directional selection observed in real time.

Types of Selection

Must know

Know the three modes:

Mode	Favors	Effect on variation
Directional	One extreme	Decreases variation; shifts mean
Stabilizing	Intermediate	Decreases variation; maintains mean
Disruptive	Both extremes	Increases variation; may lead to speciation

Sexual selection favors traits that improve mating success rather than survival, either intrasexual (competition, e.g. antlers) or intersexual (mate choice, e.g. peacock tails). It can drive traits that decrease survival if the mating advantage outweighs the cost — fitness is total reproductive output, not survival alone.

Quick check: Antibiotic resistance is which type of selection, and does selection create or select for the resistant mutation?

Answer: Directional selection. The resistant mutation pre-exists at low frequency; the antibiotic favors it. Selection changes the frequency of a pre-existing variant; it does not create it.

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Fitness

Defining Fitness

Must know

Fitness is reproductive success relative to other genotypes — the relative contribution of a genotype to the next generation's gene pool. A large, healthy, long-lived organism that leaves zero offspring has fitness zero. The MCAT tests this against the everyday meaning of "fit."

Relative fitness normalizes to the most successful genotype (set to 1). The selection coefficient $s$ measures the fitness reduction: $w = 1 - s$. A lethal recessive has $s = 1$, so $w = 0$.

Inclusive Fitness and Kin Selection

Know the logic

Inclusive fitness counts an individual's own offspring plus relatives' offspring, weighted by relatedness. Helping relatives reproduce at a personal cost is kin selection, governed by Hamilton's rule:

$r \cdot B > C$

where $r$ = relatedness (0.5 for siblings), $B$ = benefit to recipient, $C$ = cost to actor. This explains why animals sacrifice more readily for close relatives.

Quick check: $AA$ produces 10 offspring, $Aa$ produces 8, $aa$ produces 0. Relative fitnesses?

Answer: $AA$ is most fit ($w = 1$). $Aa = 8/10 = 0.8$; $aa = 0/10 = 0$. Selection coefficient against $aa$ is $s = 1$.

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Selection by Differential Reproduction and Evolutionary Success

Must know

"Survival of the fittest" is misleading because differential survival matters only insofar as it leads to differential reproduction. An individual that survives but reproduces less contributes less to the gene pool.

Evolutionary success is an increase in the percentage representation of a genotype/allele in the next generation's gene pool — frequency change, not individual health or longevity.

Quick check: A mutation makes carriers live 20% longer but reduces reproduction by 10%. Spread or decline?

Answer: Decline. Longevity enhances fitness only if it yields more offspring. Reproductive output falls, so the allele's representation in the next generation falls.

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Concepts of Natural Selection and Group Selection

Must know

Know the contrast. Modern evolutionary biology holds that selection acts primarily at the level of the individual organism / allele: an allele spreads if it boosts its carrier's reproductive success, even at a cost to the group.

Group selection is the contested hypothesis that selection acts on groups, favoring traits that benefit the group at individual cost. Most biologists view it as rare and weak: group-beneficial traits (e.g. a bird's alarm call) are usually better explained by kin selection or reciprocal altruism. Know group selection as a concept, but apply individual/kin-level thinking first.

Quick check: Organisms share food equally even with unrelated individuals, lowering each one's intake. Best explained by individual, kin, or group selection?

Answer: If recipients are unrelated, kin selection cannot explain it. Group selection is one possibility, but reciprocal altruism at the individual level is another and is generally preferred. Recognize group selection as distinct, but default to individual-level explanations.

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Polymorphism

Must know

Polymorphism is the existence of two or more distinct variants (morphs) in a population at frequencies too high for mutation alone. The key question: why is the rarer allele maintained?

Balanced Polymorphism

Must know

Balanced polymorphism = selection actively maintains multiple alleles at stable frequencies. The key mechanism is heterozygote advantage (overdominance): the heterozygote is fitter than either homozygote. The canonical example is sickle cell:

• $HbA/HbA$: normal, fully susceptible to malaria
• $HbA/HbS$: sickle trait, partial malaria protection — highest fitness in malaria-endemic regions
• $HbS/HbS$: sickle cell disease, severely reduced fitness

Because the heterozygote is most fit, both alleles persist, which is why $HbS$ stays common in malaria-endemic populations.

Optional

Other maintaining mechanisms: frequency-dependent selection (rare morphs favored because rare) and environmental heterogeneity (different alleles favored in different microhabitats).

Quick check: In a malaria-free environment over many generations, what happens to $HbS$ frequency?

Answer: It declines. With no malaria, the heterozygote has no advantage, so selection acts unidirectionally against $HbS$ (the $HbS/HbS$ disease state). The balanced polymorphism is destabilized.

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Adaptation and Specialization

Must know

An adaptation is a heritable trait that increases fitness and arose or is maintained by natural selection. Not every trait is an adaptation — some are linked to adaptive traits, some are developmental byproducts, some are neutral.

Know the distinction. Convergent evolution: unrelated lineages independently evolve similar traits under similar pressures, producing analogous structures (similar function, different origin — e.g. dolphin and shark body shape). Contrast with homologous structures: shared ancestry regardless of current function (human arm, bat wing, whale flipper from one ancestral forelimb).

Specialization is the narrowing of a niche; specialists outcompete generalists in their niche but are vulnerable to change. Darwin's finches diversified from one ancestor into species with beaks suited to different foods — adaptive radiation.

Quick check: Two unrelated fish in different oceans both have bioluminescent lures. Homologous or analogous, and what process?

Answer: Analogous, via convergent evolution — similar function arose independently under similar pressure, not from a shared ancestral structure.

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Coevolution

Must know

Coevolution is reciprocal selection between two interacting species, so a change in one drives a counter-change in the other. Patterns: predator–prey / host–parasite arms races (e.g. toxic newts vs. resistant garter snakes) and mutualisms (flowers and pollinators). The signature is reciprocity: each species is both agent and target of the other's selection.

Symbiosis and the Origin of Organelles (Endosymbiotic Theory)

Must know

The endosymbiotic theory: mitochondria and chloroplasts arose when a host cell engulfed a free-living aerobic bacterium (mitochondria) or cyanobacterium (chloroplast) that became a permanent endosymbiont. High-yield evidence:

• Own circular DNA and 70S (bacterial) ribosomes
• Double membrane
• Replicate by binary fission, independently of the cell cycle

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Speciation

Must know

Speciation is the origin of new, reproductively isolated species from a common ancestor. The biological species concept defines a species as populations that interbreed and produce fertile offspring but are reproductively isolated from other such groups. Reproductive isolation is the key.

Allopatric vs. Sympatric Speciation

Must know

Allopatric speciation: a population is split by a geographic barrier; the subpopulations diverge until reproductively isolated. This is the most common mode.

Sympatric speciation: divergence within one area, no physical barrier. Rare in animals but common in plants via polyploidy — genome duplication that instantly prevents fertile crossing with the diploid parents (many crops are polyploids).

Reproductive Isolating Barriers

Know the logic

(Prezygotic prevents mating/fertilization; postzygotic reduces hybrid viability/fertility.) Don't memorize the full table:

Type	Category
Habitat, temporal, behavioral, mechanical, gametic	Prezygotic
Hybrid inviability, hybrid sterility (e.g. mule), hybrid breakdown	Postzygotic

Quick check: Two populations separated by a glacier for 50,000 years can now mate but produce sterile offspring. Separate species?

Answer: Yes. The biological species concept requires fertile offspring. Sterile hybrids = postzygotic isolation (hybrid sterility), so they are reproductively isolated and qualify as separate species.

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Inbreeding

Must know

Inbreeding is mating between close relatives; its consequence is increased homozygosity, because relatives share alleles by common descent.

Optional

The inbreeding coefficient ($F$) is the probability that an individual's two alleles at a locus are identical by descent ($F = 0$ random mating, $F = 1$ fully inbred).

The main consequence is inbreeding depression: reduced fitness from expression of deleterious recessives normally hidden in heterozygotes. This is why small, isolated populations accumulate genetic disorders.

Key point: inbreeding does not change allele frequencies by itself — only genotype frequencies (more homozygotes, fewer heterozygotes). It violates random mating but doesn't directly cause evolution; it exposes recessives to selection, which then changes allele frequencies.

Quick check: A zookeeper continues breeding an already-inbred, low-diversity cheetah population. What is the concern?

Answer: Inbreeding depression — further homozygosity raises the chance that deleterious recessives are expressed, increasing immune, reproductive, and developmental defects as $F$ rises.

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Outbreeding

Must know

Outbreeding (outcrossing) is mating between unrelated individuals — the opposite of inbreeding: it increases heterozygosity.

The fitness benefit is hybrid vigor (heterosis): outbred offspring often show enhanced fitness, mainly by masking deleterious recessives. Exploited in agriculture (hybrid corn).

Optional

Outbreeding depression can occur when crossing highly divergent populations disrupts co-adapted gene complexes — rare but real.

Quick check: Crossing two inbred corn lines gives a larger, more vigorous $F_1$. Mechanism, and what happens in $F_2$?

Answer: Heterosis — the $F_1$ is heterozygous at many loci, masking each line's deleterious recessives. In $F_2$, recombination re-exposes recessives, so average vigor declines.

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Bottlenecks and Genetic Drift

Genetic Drift

Must know

Genetic drift is random change in allele frequencies from chance sampling in a finite population. Unlike selection, it has no direction and can fix beneficial, neutral, or deleterious alleles. Its strength is inversely proportional to population size — negligible in large populations, powerful in small ones. Long-term, drift causes fixation (one allele reaches 100%), reducing diversity.

The Bottleneck Effect

Must know

A bottleneck is a drastic, catastrophic reduction in population size. Survivors are a small random (non-representative) sample, so allele frequencies shift by chance and genetic diversity drops (rare alleles lost). The cheetah is the classic example (so genetically uniform that skin grafts between unrelated individuals aren't rejected).

The Founder Effect

Must know

The founder effect is a bottleneck via colonization: a small group establishes a new population carrying only a subset of the original diversity, so rare alleles can reach high frequency by chance. Example: Ellis-van Creveld syndrome at ~5% in the Old Order Amish.

Both are genetic drift, distinguished only by context (catastrophe vs. colonization).

Quick check: After a volcano kills 99% of a lizard population, an allele rises from 2% to 35% and a 30% allele vanishes. Selection, drift, or both?

Answer: Most consistent with genetic drift (bottleneck). Selection rarely eliminates a 30%-frequency allele or sweeps a rare one to 35% by chance. To confirm, test whether the risen allele confers a fitness advantage; if not, drift is the best explanation.

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Gene Flow (Migration)

Must know

Gene flow is transfer of alleles between populations via migrating individuals or gametes (e.g. pollen). It is a homogenizing force: it reduces differences between populations while increasing variation within the receiving population (can introduce new alleles). Because it pulls diverging populations back together, gene flow opposes speciation — which is why allopatric speciation needs a physical barrier to cut it off.

Quick check: Two isolated lizard populations begin exchanging migrants after a land bridge forms. Effect on the genetic difference between them?

Answer: It decreases. Gene flow homogenizes allele frequencies, counteracting prior divergence.

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Evolutionary Time as Measured by Gradual Random Changes in the Genome

Neutral Theory and Molecular Clocks

Know the logic

Many mutations are neutral — synonymous (silent) changes and many non-coding changes don't affect phenotype, so their fate is set by drift, not selection. The neutral theory holds that most molecular variation is neutral. Implication: neutral substitutions accumulate at a roughly constant rate, so the number of molecular differences between two species is proportional to time since their common ancestor — the molecular clock.

Optional

Fast-evolving sequences (mtDNA) date recent divergences; slow ones (rRNA, cytochrome c) date deep ones.

Gradualism vs. Punctuated Equilibrium

Must know

Know the contrast.

• Phyletic gradualism: slow, continuous change (Darwin's model).
• Punctuated equilibrium: long stasis broken by rapid bursts of change, often at speciation.

The fossil record often shows stasis interrupted by rapid transitions. Both models occur in nature.

Lines of Evidence for Evolution

Must know

Know these categories:

• Fossil record: transitional forms, temporal ordering.
• Homology: homologous structures (tetrapod forelimb) = shared ancestry.
• Vestigial structures: reduced remnants (appendix, whale hindlimb bones).
• Biogeography: geographic distribution reflects descent (island faunas).
• Molecular/genetic similarity: shared sequences and the near-universal genetic code; degree of similarity tracks divergence time.

Genome Comparisons to Reconstruct History

Passage-level

Phylogenetic trees display evolutionary relationships. You should interpret a simple tree — identify which organisms are most closely related, and that the most recent common ancestor is the node where lineages diverge. You do not need the tree-building algorithms.

Quick check: For a slowly evolving gene: A–B differ at 2 positions, A–C at 20, A–D at 18. Which pair diverged most recently, which longest ago?

Answer: A and B most recently (fewest differences); A and C longest ago (most differences). Relative times are readable directly; absolute dates require the substitution rate.

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Common Confusions & Tricks

Evolution acts on populations, not individuals. A single organism doesn't evolve in its lifetime. If a passage asks "what evolved?" the answer is the population or gene pool.

Fitness ≠ strength or health. A strong, healthy organism that fails to reproduce has fitness zero.

Adaptations are not goal-directed. Avoid teleology ("bacteria developed resistance in order to survive") — think pre-existing variation plus differential survival.

Genetic drift ≠ natural selection. Drift is random and strongest in small populations; selection is directional and works even in large ones. Beneficial alleles can be lost by drift.

Bottleneck vs. founder effect: both are drift and reduce diversity. Bottleneck = catastrophic reduction of an existing population; founder = a small subset leaves to start a new one.

Inbreeding changes genotype, not allele, frequencies (by itself): only $AA:Aa:aa$ proportions shift. Allele frequencies change only if selection then removes the exposed recessives.

Balanced polymorphism is maintained by selection, not drift. A stable intermediate frequency points to heterozygote advantage (like $HbS$); drift tends to fix one allele.

Analogous ≠ homologous: analogous = same function, different origin (convergent); homologous = shared origin, possibly different function (divergent). Dolphin fin and shark fin are analogous; dolphin flipper and your arm are homologous.

Hardy-Weinberg starting point: start with $q^2$ (the recessive homozygote, the only phenotype-identifiable genotype), then $q$, then $p$, then $2pq$.

Group selection is the minority view. For a group-beneficial behavior, expect individual or kin selection as the answer.

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Key Takeaways

• Evolution = change in allele frequencies over time; individuals don't evolve.
• Hardy-Weinberg ($p + q = 1$; $p^2 + 2pq + q^2 = 1$) describes a non-evolving population; its five violated conditions are the five mechanisms: mutation, gene flow, selection, non-random mating, drift.
• Natural selection needs heritable variation + differential reproduction. Modes: directional, stabilizing, disruptive, sexual.
• Fitness = relative reproductive success; a long-lived non-reproducer has zero fitness.
• Evolutionary success = increased allele/genotype representation in the next generation.
• Group selection vs. individual/kin selection (Hamilton's rule: $rB > C$); individual/kin is dominant.
• Polymorphism maintained by heterozygote advantage (sickle cell/malaria), frequency-dependent selection, or environmental heterogeneity.
• Adaptation: convergent evolution → analogous structures; divergent → homologous.
• Speciation: allopatric (barrier) vs. sympatric (polyploidy); requires reproductive isolation, prezygotic or postzygotic.
• Inbreeding increases homozygosity → inbreeding depression; no allele-frequency change by itself. Outbreeding increases heterozygosity → hybrid vigor (heterosis).
• Bottleneck (catastrophe) and founder effect (colonization) are both genetic drift → reduced diversity.
• Evidence: fossils, homology, vestigial structures, biogeography, molecular similarity.
• Molecular clocks: neutral mutations accumulate at ~constant rates; more differences = longer divergence.
• Punctuated equilibrium (stasis + bursts) vs. gradualism.
• Named examples: peppered moth (directional), sickle cell (heterozygote advantage), Darwin's finches (adaptive radiation), cheetah (bottleneck), Ellis-van Creveld in Amish (founder effect).