How does natural selection lead to adaptation and the evolution of new species?
The process of natural selection, the types of adaptation, how natural selection leads to evolution and speciation, and the evidence for evolution.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on natural selection and adaptation, covering the process of natural selection, anatomical, physiological and behavioural adaptations, evolution and speciation, and the evidence for evolution.
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What this dot point is asking
Edexcel wants you to describe the process of natural selection, classify adaptations, explain how natural selection leads to evolution and speciation, and outline the evidence for evolution. Applying the natural-selection sequence to real examples such as antibiotic resistance is one of the most common exam questions.
The process of natural selection
The steps are: genetic variation exists (from mutation and meiosis); there is competition for limited resources; the best-adapted individuals survive and reproduce; they pass on the advantageous alleles; the allele frequency in the population changes.
Types of adaptation
Evolution and speciation
Over many generations, natural selection changes the allele frequencies of a population: this is evolution. Speciation occurs when two populations become reproductively isolated (for example by a geographical barrier), so their gene pools diverge until they can no longer interbreed to produce fertile offspring.
Evidence for evolution
Evidence comes from several independent sources that agree:
- Fossil record: shows a sequence of changing forms over geological time and transitional fossils.
- Comparative anatomy: homologous structures (such as the pentadactyl limb of mammals, birds and amphibians) share a common plan, suggesting descent from a common ancestor with divergent evolution.
- Molecular evidence (the strongest): the more similar the DNA base sequences or amino acid sequences of a protein (such as cytochrome c or haemoglobin) between two species, the more recently they shared a common ancestor. This can be used as a molecular clock to estimate when lineages split.
Examples in context
Example 1. Peppered moth. Before industrialisation, pale peppered moths were camouflaged on lichen-covered trees and dark forms were eaten by birds. Industrial soot darkened the trees, so dark moths were now camouflaged and pale moths conspicuous. The dark allele increased in frequency in polluted areas, a measurable case of natural selection driven by a changed environment. When clean-air laws reduced soot, pale forms recovered, showing selection tracks the environment.
Example 2. Darwin finches. On the Galapagos islands, finch populations isolated on different islands faced different food sources. Selection favoured different beak shapes (large crushing beaks for hard seeds, fine beaks for insects). Reproductive isolation between islands let the populations diverge into separate species, a classic example of allopatric speciation driven by natural selection.
Try this
Q1. Explain how natural selection can lead to an increase in the frequency of an advantageous allele. [3 marks]
- Cue. Individuals with the allele survive and reproduce more, pass it on, so it becomes more common over generations.
Q2. State the type of evidence for evolution that compares DNA base sequences. [1 mark]
- Cue. Molecular (or biochemical) evidence.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20186 marksExplain how the overuse of antibiotics has led to the evolution of antibiotic-resistant strains of bacteria such as MRSA.Show worked answer →
A 6-mark explain question rewarding the full natural-selection sequence applied to bacteria.
Within a bacterial population there is genetic variation due to random mutation, so some bacteria carry an allele giving antibiotic resistance. When antibiotics are used, they kill the non-resistant bacteria, but the resistant ones survive (selection pressure). The survivors reproduce rapidly by binary fission, passing on the resistance allele, so the frequency of the resistant allele increases in the population. Over time most of the population is resistant, forming a resistant strain such as MRSA. Overuse increases the selection pressure and the chance of resistant strains becoming common. Resistance genes can also spread between bacteria on plasmids.
Markers reward: variation from mutation, resistance allele present before exposure, antibiotic is the selection pressure, resistant survive and reproduce, allele frequency increases, resistant strain forms, plasmid transfer as an extra point.
Edexcel 20214 marksDescribe how a new species could form when a population is divided by a geographical barrier (allopatric speciation).Show worked answer →
Markers want isolation, divergence and reproductive isolation.
A geographical barrier (such as a river, mountain range or sea) physically separates a population into two, preventing interbreeding so the gene pools no longer mix. Each population experiences different environmental conditions and selection pressures, and different mutations arise in each. Natural selection and genetic drift change the allele frequencies differently in each population, so they diverge. Eventually the two populations are so genetically different that even if reunited they can no longer interbreed to produce fertile offspring, so they are separate species.
Award marks for: barrier prevents gene flow; different selection pressures or mutations; gene pools diverge; reproductive isolation so cannot produce fertile offspring.
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)