Natural Selection Study Pack

Kibin's free study pack on Natural Selection includes a 4-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. 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 21, 2026

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Natural Selection Study Guide

Master the mechanisms driving evolutionary change with this AP Biology study pack on natural selection. Covering heritable variation, fitness, and the three selection patterns — directional, stabilizing, and disruptive — alongside sexual selection and Hardy-Weinberg equilibrium, it clarifies how allele frequencies shift in response to environmental pressure and how natural selection differs from drift, mutation, and gene flow.

Key Takeaways

  • •Natural selection acts on heritable phenotypic variation within a population, favoring individuals whose traits increase reproductive success in a given environment.
  • •Fitness is defined as relative reproductive output, not physical strength — an organism is "fit" only in the context of a specific environment.
  • •Directional, stabilizing, and disruptive selection describe three distinct patterns of how selection pressure shifts allele frequencies over time.
  • •Sexual selection, a specialized form of natural selection, drives the evolution of traits that improve mating success, sometimes at a cost to survival.
  • •Genetic drift, gene flow, and mutation also change allele frequencies, but only natural selection consistently shapes populations in response to environmental pressure.
  • •Hardy-Weinberg equilibrium defines the null condition — no evolution — against which the effects of natural selection and other forces can be measured.

The Preconditions for Natural Selection to Occur

Natural selection is not a random process — it requires three specific conditions to be present in a population simultaneously, and without all three, adaptive evolution cannot proceed.

Heritable Variation Within a Population

  • •Individuals in a population must differ from one another in traits that affect survival or reproduction.
  • •Those differences must be at least partially caused by genetic differences, so that traits can be passed from parent to offspring.
  • •Variation that arises purely from environmental exposure — such as a scar — is not heritable and therefore cannot be acted on by natural selection.

Differential Reproductive Success

  • •Not all individuals survive to reproduce, and among those that do, not all produce the same number of offspring.
  • •When heritable traits are correlated with higher reproductive output, those traits become more common in subsequent generations.
  • •This is the core mechanism: the environment does not create new traits, it filters existing ones.

Population-Level Change Over Generations

  • •Natural selection acts on individuals, but evolution — defined as a change in allele frequencies — happens at the population level.
  • •A single individual cannot evolve; the statistical outcome across many births, deaths, and reproductions in a population is what constitutes evolutionary change.

Fitness and the Environment

The concept of fitness is central to natural selection, but it is frequently misunderstood — fitness is always relative, always context-dependent, and always measured in terms of reproductive contribution to the next generation.

Defining Darwinian Fitness

  • •Fitness refers to an individual's relative reproductive success compared to other individuals in the same population.
  • •A trait that increases fitness in one environment may be neutral or harmful in a different environment, which is why fitness is never an absolute quality.
  • •For example, the sickle-cell allele in hemoglobin reduces fitness in environments without malaria but confers a survival advantage in malaria-endemic regions by reducing the severity of Plasmodium falciparum infection in heterozygotes.

Adaptation as the Product of Selection

  • •An adaptation is a heritable trait that increases an organism's fitness in its current environment.
  • •Adaptations are not designed or goal-directed — they accumulate generation by generation as alleles associated with higher reproductive success increase in frequency.
  • •A trait is only an adaptation if it evolved through natural selection; features that arose by genetic drift or as developmental byproducts of another selected trait are not adaptations in the strict sense.

About this Study Pack

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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