AP Syllabus focus:
‘Adaptations are genetic variations that provide organisms with advantages in particular environments.’
Environmental change can rapidly shift which traits help organisms survive and reproduce. This page explains how genetic variation produces adaptations, how natural selection acts on that variation, and why “advantage” depends on environmental context.
Core idea: adaptations come from heritable variation
Natural selection can only act on heritable differences among individuals. When environments change, the traits that increase survival and reproduction may also change, altering which alleles become more common over generations.
Genetic variation: Differences in DNA sequences (alleles) among individuals in a population.
Genetic variation matters because individuals with different alleles can produce different phenotypes (observable traits), and some phenotypes leave more offspring than others under specific conditions.
Practice Questions
FAQ
They may use relatedness-based methods and statistical models to partition phenotypic variance.
Approaches include:
Pedigree analyses (known parent–offspring relationships)
Genomic relatedness matrices using SNP data
Comparing resemblance among relatives while accounting for shared environment
Strong selection can still fail if key limits apply:
Insufficient genetic variation for the needed trait
Small population size causing inbreeding and loss of adaptive alleles
Environmental change occurring faster than generation time
A reaction norm describes how one genotype’s phenotype changes across environments.
Selection can act on:
The trait value in a given environment
The shape/slope of the reaction norm (plasticity) if plasticity is heritable
If an allele increases fitness in one condition but reduces it in another, multiple alleles can persist.
Mechanisms include:
Spatially varying selection (different habitats)
Temporally varying selection (year-to-year climate shifts)
Costs of tolerance (e.g., slower growth when stress is absent)
Signals may include:
Rapid allele frequency shifts at particular loci
Reduced genetic diversity near a selected allele (selective sweep)
Higher differentiation ($F_{ST}$) at specific genomic regions compared with the rest of the genome
