What conditions are required for natural selection to occur?
Topic 7.1 Introduction to Natural Selection: explain the conditions required for natural selection and how it leads to changes in a population.
A focused answer to AP Biology Topic 7.1, covering Darwin's reasoning, the conditions for natural selection (variation, heritability, overproduction, differential reproduction), fitness, and how selection changes allele frequencies, with a worked example.
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What this topic is asking
The College Board (Topic 7.1) wants you to explain the conditions required for natural selection and how it leads to change in a population, including Darwin's reasoning and the meaning of fitness.
Darwin's reasoning and the conditions
This is Darwin's logic: from heritable variation plus competition for limited resources comes adaptation.
Fitness and differential reproduction
What natural selection does and does not do
This is why an adaptation is always relative to a particular environment: a trait that is favored in one environment may be a disadvantage in another. When the environment changes, the same trait can shift from advantageous to harmful, which is why no species is ever permanently or perfectly adapted.
Try this
Q1. List the conditions required for natural selection. [3 points]
- Cue. Heritable variation in a trait; overproduction of offspring (competition for resources); differential survival and reproduction based on the trait.
Q2. Explain why natural selection acts on individuals but changes populations. [2 points]
- Cue. Selection determines which individuals reproduce, but the measurable result is a change in the frequency of alleles and traits across the whole population over generations.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). (a) Identify the conditions required for natural selection to occur in a population. (b) A beetle population varies in color, and birds eat the more visible beetles. Explain how natural selection would change the color distribution of the population over many generations.Show worked answer →
A 4-point identify-and-explain FRQ on natural selection.
(a) Identify (2 points): natural selection requires (1 point) heritable variation in a trait among individuals, and (1 point) differential survival and reproduction (more offspring overproduced than can survive, so some variants reproduce more than others).
(b) Explain (2 points): (1 point) the better-camouflaged beetles are eaten less, survive more and reproduce more, passing on the alleles for camouflage; (1 point) over many generations the frequency of the camouflage alleles increases, so the population becomes better camouflaged (its color distribution shifts).
Markers reward listing the conditions and tracing how differential reproduction changes allele frequencies over generations.
AP 2017 (style)1 marksSection I (multiple choice). In evolutionary terms, the fitness of an organism is best defined as its: (A) physical strength. (B) ability to reproduce and pass on its alleles. (C) body size. (D) lifespan.Show worked answer →
The correct answer is (B).
Evolutionary fitness is reproductive success: the relative ability to survive and produce offspring that themselves reproduce, passing on alleles. It is not about physical strength (A), size (C) or lifespan (D) except insofar as these affect reproduction.
Related dot points
- Topic 7.2 Natural Selection: explain how directional, stabilizing and disruptive selection change the distribution of phenotypes in a population.
A focused answer to AP Biology Topic 7.2, covering directional, stabilizing and disruptive selection, sexual selection, and how each changes a phenotype distribution, with a worked interpretation of selection on a trait.
- Topic 7.4 Population Genetics: explain how natural selection, mutation, gene flow, genetic drift and non-random mating change allele frequencies.
A focused answer to AP Biology Topic 7.4, covering the gene pool, allele frequencies, and the five mechanisms of microevolution (selection, mutation, gene flow, genetic drift, non-random mating), including bottleneck and founder effects, with a worked allele-frequency calculation.
- Topic 7.5 Hardy-Weinberg Equilibrium: use the Hardy-Weinberg equations to calculate allele and genotype frequencies and test whether a population is evolving.
A focused answer to AP Biology Topic 7.5, covering the Hardy-Weinberg conditions, the equations p + q = 1 and p squared plus 2pq plus q squared = 1, and how to calculate and interpret allele and genotype frequencies, with worked calculations.
- Topic 7.6 Evidence of Evolution: describe the lines of evidence (fossil, anatomical, molecular, biogeographical) that support evolution.
A focused answer to AP Biology Topic 7.6, covering fossil, anatomical (homologous and vestigial structures), embryological, molecular and biogeographical evidence for evolution, with a worked interpretation of molecular data.
- Topic 5.2 Meiosis and Genetic Diversity: explain how crossing over, independent assortment and random fertilization produce genetic variation.
A focused answer to AP Biology Topic 5.2, covering crossing over, independent assortment and random fertilization as the three sources of genetic variation, with a worked calculation of gamete combinations.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)