BIOL1020 Chap.11 Natural Selection and Evolutionary Change
Natural Selection and Evolutionary Change
Natural selection changes the composition of populations when heritable variants differ in reproductive success. This chapter organises the required conditions of variation, inheritance, excess offspring and non-random reproduction, then distinguishes selection from random genetic drift. Fitness is treated as relative reproductive contribution in a particular environment, not physical strength or intention.
Population size alters the competition between selection and sampling, and replicated evidence helps decide whether a repeated frequency change is adaptive or random.
What this chapter covers
- 01
Definition of population evolution
- 02
Conditions for natural selection
- 03
Fitness and reproductive success
- 04
Heritable and environmental variation
- 05
Genetic drift and sampling
- 06
Population size and selective efficiency
Worked example · free
Distinguishing selection from drift in replicate populations
- +1The exposed populations show a repeatable association between the allele and higher reproductive success.
- +1Consistent directional change across replicates supports natural selection under the toxin.
- +1The small unexposed populations lack a stated fitness difference and vary unpredictably.
- +1Random sampling has larger proportional effects in small populations, supporting genetic drift there.
Key terms
- Natural Selection
- Non-random change in variant frequencies caused by differences in survival or reproduction linked to heritable traits.
- Fitness
- An individual's relative contribution of surviving offspring to later generations in a particular environment.
- Adaptation
- A heritable feature shaped by selection because it improved fitness in past environments.
- Genetic Drift
- Random allele-frequency change caused by sampling, especially influential in small populations.
- Selection Coefficient
- A measure of the relative fitness effect associated with a variant compared with another variant.
Natural Selection and Evolutionary Change FAQ
What conditions are needed for natural selection?
Individuals must vary, some variation must be heritable, more offspring must be produced than can all reproduce, and survival or reproduction must be non-random with respect to the traits. Without inheritance, a filtered phenotype does not reliably create an evolutionary response.
How is natural selection different from genetic drift?
Selection is directional with respect to fitness, while drift changes frequencies through random sampling. Drift can move an allele up or down regardless of effect and has stronger proportional influence in small populations. The two processes can operate together.
Does evolution happen because organisms need to adapt?
No. Mutation and recombination generate variation without foresight. Individuals carrying some heritable variants may leave more descendants in a given environment, causing population frequencies to change afterward. Natural selection filters existing variation rather than creating a needed trait on demand.
Why is biological fitness context dependent?
Fitness describes reproductive contribution under particular environmental conditions. A variant that improves survival or reproduction in one setting can be neutral or costly in another. Strength, lifespan or health is relevant only when it changes descendant contribution, so the environment must be included in the comparison.
How does genetic drift differ from selection?
Drift changes allele frequencies through chance sampling, with especially large effects in small populations. Selection produces non-random differences in reproductive success associated with heritable variation. Both can change a population, but repeated populations are more likely to show a consistent directional response under the same selective pressure.
What distinguishes bottleneck and founder effects?
A bottleneck sharply reduces an existing population, so survivors may carry an unrepresentative sample of its former alleles. A founder effect begins when a small group establishes a new population. Both are forms of sampling drift and can reduce diversity without favouring the sampled variants.
Why must variation be heritable for selection?
Selection can sort phenotypes within one generation, but an evolutionary response requires descendants to inherit information associated with those differences. Purely environmental variation may affect survival without producing the same trait pattern in offspring. Heritability connects unequal reproductive success to frequency change across generations.
How do replicate populations strengthen an evolutionary explanation?
Repeated populations let the investigator compare consistency under similar conditions. The same directional change paired with reproductive differences supports selection, whereas divergent frequency changes reveal a larger role for chance. Replication cannot remove every alternative, but it tests whether one observed trajectory is a repeatable response or an isolated sampling outcome.
Exam move
Memorise the four conditions for selection as a causal checklist rather than an acronym. Apply them to an unfamiliar trait and identify which observation supports each condition. For drift comparisons, vary population size and ask whether change is repeatable across replicate populations. Keep fitness tied to reproductive contribution, not strength or lifespan alone.
Rewrite purpose statements such as a species evolved a trait in order to survive into population statements about prior variation and unequal descendants. End each answer by naming the population-level frequency change. Include environmental context because a variant's fitness effect can reverse when conditions change.
Compare at least two replicated populations before deciding whether a directional pattern exceeds the change expected from sampling.
Working through Natural Selection and Evolutionary Change in BIOL1020? Sia is AskSia’s AI Biology tutor — ask any BIOL1020 Natural Selection and Evolutionary Change question and get a clear, step-by-step explanation grounded in how BIOL1020 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
