Natural Selection Study Pack
Kibin's free study pack on Natural Selection includes a 7-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Natural Selection Study Guide
Unpack the core mechanisms driving evolutionary change, from directional, stabilizing, and disruptive selection to heterozygote advantage and sexual selection.
Key Takeaways
- •Natural selection acts on heritable phenotypic variation within a population, causing alleles that improve reproductive success to increase in frequency over generations.
- •Fitness is defined in terms of reproductive output, not physical strength — an organism is "fit" if it survives long enough to pass more alleles to the next generation than its competitors.
- •The three modes of natural selection — directional, stabilizing, and disruptive — each shift allele frequencies in distinct ways and produce different effects on the distribution of traits in a population.
- •Heterozygote advantage is a special case in which the heterozygous genotype has higher fitness than either homozygote, allowing two alleles to persist in a population indefinitely — a phenomenon demonstrated by the sickle cell allele in malaria-endemic regions.
- •Sexual selection, a subset of natural selection, can drive the evolution of traits that reduce individual survival odds but increase mating success, explaining features such as elaborate plumage or large antlers.
- •Natural selection does not produce perfect organisms — it is constrained by available genetic variation, developmental trade-offs, and historical contingency.
The Raw Material: Variation, Heritability, and Differential Reproduction
Natural selection cannot operate on a population that is genetically identical; it requires heritable differences among individuals that translate into differences in survival and reproduction.
Sources of Heritable Phenotypic Variation
- •Mutation is the ultimate source of new alleles — changes in DNA sequence that alter protein structure, gene expression, or regulatory regions.
- •Sexual recombination during meiosis shuffles existing alleles into novel combinations, greatly expanding the phenotypic diversity natural selection can act upon.
- •Only variation encoded in DNA can be inherited; traits acquired through learning or environmental exposure alone (e.g., muscle mass gained through exercise) are not passed to offspring.
Differential Reproductive Success as the Engine of Selection
- •Individuals whose heritable traits allow them to survive longer, find more mates, or produce more viable offspring leave a disproportionate share of alleles in the next generation.
- •Over successive generations, alleles associated with higher reproductive success increase in frequency — this cumulative change in allele frequency is measurable evolution.
- •Selection acts on the phenotype but changes the genotype pool; a beneficial allele spreads because the body it builds outcompetes alternatives, not because the allele itself is 'trying' to replicate.
Fitness: A Precise Biological Meaning
Biologists use the term fitness in a narrow, quantitative sense that is easily misunderstood when borrowed from everyday language.
Defining Biological Fitness
- •Fitness measures an individual's (or genotype's) relative contribution of offspring to the next generation compared to other individuals in the same population under the same conditions.
- •A large, powerful organism that dies before reproducing has fitness of zero; a smaller, weaker organism that raises many surviving offspring may have the highest fitness in the population.
- •Fitness is always relative to the current environment — the same genotype can be highly fit in one habitat and nearly lethal in another.
Fitness Trade-offs and Constraints
- •Organisms face competing demands on limited energy and resources, so improving one fitness-related trait often reduces another — a phenomenon called a trade-off.
- •Classic examples include the trade-off between current reproduction and parental survival (semelparity versus iteroparity) and between immune investment and growth rate.
- •These trade-offs explain why natural selection does not simply maximize every trait simultaneously and why 'optimal' organisms are never actually observed.
Three Modes of Natural Selection and Their Effects on Trait Distributions
Depending on which phenotypes in a population have the highest fitness, natural selection pushes the distribution of traits in three fundamentally different directions.
Directional Selection
- •Directional selection favors individuals at one extreme of the phenotypic distribution, causing the population mean to shift toward that extreme over time.
- •The classic example is the increase in average beak depth in Galápagos finches (Geospiza fortis) during drought years when only large, hard seeds remained — larger-beaked individuals survived preferentially.
- •Directional selection reduces variation as the disfavored extreme is eliminated from the gene pool.
Stabilizing Selection
- •Stabilizing selection favors intermediate phenotypes and eliminates both extremes, narrowing the distribution around the existing mean without shifting it.
- •Human birth weight is a widely cited example: infants much below average weight face survival challenges, while infants much above average face delivery complications — both extremes suffer reduced fitness.
- •This mode maintains existing adaptations and is thought to be the most common form of selection acting on populations most of the time.
Disruptive Selection
- •Disruptive selection (also called diversifying selection) favors both extremes while penalizing intermediate phenotypes, potentially splitting a single population into two distinct phenotypic clusters.
- •In the African seedcracker finch (Pyrenestes ostrinus), small-beaked individuals efficiently crack soft seeds and large-beaked individuals crack hard seeds, but intermediate beaks handle neither well.
- •Sustained disruptive selection can be a precursor to speciation if the two favored phenotypic groups also become reproductively isolated.
Allele Frequency Dynamics: From Selection Coefficients to Balancing Selection
Population geneticists track natural selection by measuring how allele frequencies change across generations, using mathematical tools to connect phenotypic fitness differences to genotype pool shifts.
Selection Coefficients and Relative Fitness Values
- •The selection coefficient (s) quantifies how strongly selection acts against a genotype; a genotype with s = 0 is neutral, while s = 1 means that genotype is completely lethal before reproduction.
- •Relative fitness (w) is calculated by setting the most fit genotype to 1.0 and expressing all others as fractions of that maximum — e.g., w = 0.7 means a genotype leaves 70% as many offspring as the best competitor.
- •Even small fitness differences (s = 0.01) produce significant allele frequency changes over hundreds of generations, which is why natural selection can reshape populations across evolutionary timescales.
Heterozygote Advantage and Balancing Selection
- •Balancing selection describes any scenario in which selection actively maintains multiple alleles in a population rather than driving one to fixation.
- •Heterozygote advantage (overdominance) occurs when the heterozygous genotype (Aa) has higher fitness than either homozygote (AA or aa), as seen with the HbA/HbS genotype in malaria-endemic regions — heterozygotes resist malaria better than HbA/HbA individuals while avoiding the severe anemia of HbS/HbS individuals.
- •Because both homozygotes are penalized relative to the heterozygote, neither allele can be completely eliminated, producing a stable equilibrium allele frequency called a balanced polymorphism.
Frequency-Dependent Selection
- •In negative frequency-dependent selection, a phenotype's fitness increases as it becomes rarer — a situation common in predator-prey dynamics and immune evasion, where an uncommon variant is less recognized.
- •This mechanism also maintains multiple alleles in a population because any allele that becomes common is immediately penalized, preventing fixation.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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A large, powerful organism dies before reproducing. What is its biological fitness?
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Biological Fitness
Explain what biologists mean by 'fitness' in your own words. How does this differ from the everyday use of the word, and why is fitness always described as relative to an environment?
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