How do selection, drift and reproductive isolation change allele frequencies and produce new species?
Individuals within a population of a species may show a wide range of variation in phenotype. This variation may be the result of genetic factors, environmental factors or a combination of both. Differential reproductive success and its effect on the allele frequency within a gene pool. Directional selection, for example antibiotic resistance in bacteria, and stabilising selection, for example human birth weights. The role of geographic isolation (allopatric speciation) and reproductive isolation (sympatric speciation) in the production of new species, and the importance of genetic drift in causing changes in allele frequency in small populations, including the founder effect.
A focused answer to the AQA 3.7 dot point on evolution and speciation. Covers differential reproductive success, directional and stabilising selection, genetic drift and the founder effect, and contrasts allopatric (geographic) with sympatric (reproductive) speciation.
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What this dot point is asking
AQA wants you to explain how differential reproductive success changes allele frequencies, distinguish directional from stabilising selection with named examples, describe genetic drift and the founder effect, and contrast allopatric (geographic) with sympatric (reproductive) speciation.
Variation and differential reproductive success
Individuals in a population vary in phenotype because of genetic factors (mutation, meiosis, random fertilisation), environmental factors, or both. Where this variation affects survival and reproduction, individuals with advantageous alleles leave more offspring. This is differential reproductive success, and it changes the allele frequency in the gene pool over generations: the favoured alleles become more common. This is natural selection, and it is the mechanism of evolution.
Types of selection
Directional selection favours one extreme of a range of phenotypes, shifting the population mean in that direction. It acts when the environment changes. Example: antibiotic resistance in bacteria, where the resistance allele is favoured when antibiotics are present, so mean resistance rises.
Stabilising selection favours the intermediate phenotype and selects against both extremes, reducing variation and keeping the mean constant. It acts in a stable environment. Example: human birth weight, where very small and very large babies have historically had lower survival, so the population mean stays near the optimum.
Genetic drift and the founder effect
Genetic drift is a change in allele frequency due to chance, not selection. It matters most in small populations, where the random sampling of which individuals reproduce can swing allele frequencies sharply, and alleles can be lost or fixed by chance alone. In large populations chance effects average out.
The founder effect is genetic drift caused when a small group founds a new population (for example, a few birds colonising an island). The founders carry only a small, non-representative sample of the original gene pool, so the new population has reduced genetic diversity and allele frequencies that differ from the source population. This can predispose the new population to inherited conditions that were rare in the original.
Speciation
Speciation is the formation of a new species, which occurs when two populations become reproductively isolated so that their gene pools can no longer mix. Once gene flow stops, mutation, selection and drift act independently on each gene pool until they diverge enough to be different species.
Allopatric speciation is driven by geographic isolation. A physical barrier (a river, mountain range, sea) separates two populations. The separated gene pools experience different selection pressures and undergo independent mutation and drift, diverging until they can no longer interbreed even if reunited.
Sympatric speciation occurs without geographic separation: a reproductive (biological) isolating mechanism arises within a single area. Examples include seasonal isolation (breeding at different times), behavioural isolation (different courtship behaviours), or polyploidy in plants producing instantly incompatible chromosome numbers.
Try this
Q1. Distinguish between directional and stabilising selection, giving one named example of each. [4 marks]
- Cue. Directional shifts the mean toward one extreme (antibiotic resistance); stabilising favours the mean and selects against extremes, reducing variation (human birth weight).
Q2. Explain why genetic drift has a greater effect on small populations than large ones, and define the founder effect. [3 marks]
- Cue. In small populations chance sampling of reproducing individuals causes large random swings in allele frequency; the founder effect is drift when a small, non-representative group founds a new population with reduced diversity.
Q3. Compare allopatric and sympatric speciation. [3 marks]
- Cue. Both require reproductive isolation stopping gene flow; allopatric is caused by a geographic barrier separating populations, sympatric occurs in the same area through reproductive isolating mechanisms (seasonal, behavioural, polyploidy).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2017 AQA5 marksExplain how natural selection has led to antibiotic resistance becoming common in a population of bacteria.Show worked answer →
A full-mark answer links variation, selection pressure, differential survival, reproduction and allele frequency change.
- Variation. Random mutation in the bacterial population produces a few cells with an allele giving antibiotic resistance.
- Selection pressure. When the antibiotic is applied, it kills non-resistant bacteria.
- Differential reproductive success. Resistant bacteria survive and reproduce; non-resistant ones do not.
- Inheritance. The resistance allele is passed to offspring (and can spread by horizontal gene transfer via plasmids).
- Allele frequency change. Over generations the frequency of the resistance allele increases, so resistance becomes common in the population.
This is directional selection. Markers reward naming mutation as the source of variation, the antibiotic as the selection pressure, and the increase in allele frequency over generations.
Related dot points
- Genotype is the genetic constitution of an organism. Phenotype is the expression of this genetic constitution and its interaction with the environment. Most phenotypes are affected by more than one gene. The genotype, phenotype and ratio of offspring can be predicted for monohybrid and dihybrid crosses involving dominant, recessive, codominant and multiple alleles, sex-linkage, autosomal linkage and epistasis. The chi-squared (X2) test can be used to test the significance of the difference between observed and expected results.
A focused answer to the AQA 3.7 dot point on inheritance. Works through monohybrid and dihybrid crosses, multiple alleles, codominance, sex-linkage, autosomal linkage and epistasis, then shows how to run and interpret a chi-squared test on offspring ratios.
- A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. The individuals in a population of a species may show a wide range of variation in phenotype. This is the result of genetic and environmental factors. A gene pool is all the alleles of all the genes in a population. The frequency of an allele in a population is the proportion of organisms carrying that allele. The Hardy-Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from one generation to the next, given that no mutation, migration, selection or genetic drift occurs and that there is random mating in a large population. Since allele frequencies do change, the conditions required to maintain a Hardy-Weinberg equilibrium are rarely met. Students should be able to use the Hardy-Weinberg principle (p + q = 1 and p2 + 2pq + q2 = 1) to calculate allele, genotype and phenotype frequencies in populations and changes in these frequencies.
A focused answer to the AQA 3.7 dot point on populations and Hardy-Weinberg. Defines populations, gene pools and allele frequency, states the five conditions for equilibrium, and works through allele, genotype and phenotype frequency calculations with the p + q = 1 and p2 + 2pq + q2 = 1 equations.
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A focused answer to the AQA 3.7 dot point on populations in ecosystems. Defines ecosystem, community, population size and carrying capacity, explains abiotic and biotic limiting factors and predator-prey cycles, and details estimating populations with quadrats, transects and mark-release-recapture (with its assumptions).
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A focused answer to the AQA 3.7 dot point on succession and conservation. Explains primary and secondary succession from pioneer species to climax community, how each stage changes the abiotic environment and diversity, and how conservation manages succession with reference to conflicting evidence.