OxfordAQA International A Level Biology 3.3.8 Evolution may Lead to Speciation Exam Questions

About OxfordAQA International A Level Biology 3.3.8 Evolution may Lead to Speciation

This topic closes Unit 3 by explaining how populations change over time and how new species arise. You need to define evolution as a change in allele frequencies, explain how predation, disease and competition lead to differential survival and reproduction, and use data to explain natural selection. You must describe directional, disruptive and stabilising selection, and explain how reproductive separation of populations can lead to allopatric or sympatric speciation. It draws together variation, meiosis, species and taxonomy, inheritance and allele frequencies, so it is a good source of synoptic questions. Examiners often ask you to explain a named example of selection step by step.

What you need to know

• Natural selection Variation exists in a population because of mutation and meiosis. Individuals with advantageous alleles are more likely to survive and reproduce, passing those alleles on, so the frequency of advantageous alleles increases over generations.
• Selection pressures Predation, disease and competition for resources cause differential survival and reproduction. The effect is a change in allele frequency, which is evolution.
• Types of selection Directional selection favours one extreme of the range, shifting the mean. Stabilising selection favours the mean and reduces variation, as in human birth mass. Disruptive selection favours both extremes and can lead to two distinct forms.
• Reproductive isolation When populations become reproductively separated, gene flow stops. Different mutations, selection pressures and genetic drift change allele frequencies separately until they can no longer interbreed to produce fertile offspring.
• Allopatric and sympatric speciation Allopatric speciation follows geographical isolation. Sympatric speciation happens in the same area, for example through differences in breeding season, behaviour or body structure that prevent mating.

Common mistakes in 3.3.8 Evolution may Lead to Speciation

• Saying organisms adapt or mutate because they need to. Mutations are random; selection acts on existing variation.
• Writing that individuals evolve. Populations evolve as allele frequencies change over generations.
• Leaving out reproduction. Survival alone is not enough; individuals must reproduce and pass on the alleles.
• Confusing allopatric and sympatric speciation. Allopatric involves geographical separation; sympatric happens without it.

Try three 3.3.8 Evolution may Lead to Speciation exam-style questions