Natural Selection and Evolution
This topic explains how genetic differences within a species' population provide the raw material for evolution. It details the mechanism of natural selection, where environmental pressures favour certain traits, leading to changes in the population's characteristics over time.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Genetic variation is widespread within any population of a species. This variation arises from processes like mutation and is the essential starting point for evolution.
- Natural selection is the process by which this variation is acted upon. An environmental pressure (e.g., a predator, a disease, a change in climate) causes individuals with certain inherited traits (phenotypes) to have a survival and reproductive advantage.
- Individuals with advantageous traits are more likely to survive, reproduce, and pass on their genes. Over generations, the frequency of these advantageous alleles increases in the population's gene pool.
- Evolution is the outcome of this process: a gradual change in the inherited characteristics of a population over time. If populations become sufficiently different, this can eventually lead to the formation of a new species (speciation).
- A key, rapid example is antibiotic resistance in bacteria. The antibiotic acts as a selection pressure, killing susceptible bacteria. Any bacteria with pre-existing resistance survive, multiply, and pass on the resistance gene, making the resistant strain common.
Definitions
- Evolution
- The change in the heritable characteristics of a biological population over successive generations.
- Natural Selection
- The primary mechanism of evolution, where organisms with phenotypes better suited to their environment are more likely to survive, reproduce, and pass on their genes.
- Phenotype
- The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
- Population
- A group of organisms of the same species living in a particular geographical area.
- Species
- A group of organisms that can interbreed in nature to produce viable, fertile offspring.
Worked example
A population of dark-furred rock mice lives on an outcrop of dark volcanic rock. A nearby volcano erupts, depositing a thick layer of light-coloured ash over the landscape. Describe the likely evolutionary outcome for the mouse population over many generations and explain the mechanism.
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Step 1:
Identify variation and selection pressure.
The initial mouse population has genetic variation for fur colour, with most being dark but a few being light-furred due to random mutation.
The new selection pressure is predation (e.g., by hawks) on a light-coloured background.
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Step 2:
Describe differential survival.
On the light ash, dark-furred mice are easily spotted by predators and are killed at a higher rate.
Light-furred mice are better camouflaged and have a higher survival rate.
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Step 3:
Describe differential reproduction.
The surviving light-furred mice are more likely to reach reproductive age and pass on their alleles for light fur to their offspring.
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Step 4:
Conclude the long-term change.
Over many generations, the frequency of the light-fur allele will increase in the population's gene pool, and the population's average fur colour will shift from dark to light.
Answer: The population of mice will evolve to have predominantly light-coloured fur through the process of natural selection, as this trait provides a survival advantage against predators in the new environment.
Common mistakes
- ×Mistaking natural selection for evolution. Natural selection is the *process*, while evolution is the *outcome* of that process over generations.
- ×Forgetting that variation must be heritable (genetic). An individual might change during its lifetime due to the environment (e.g., build more muscle), but this acquired characteristic is not passed on to offspring and does not drive evolution.
- ×Thinking that variation arises *in response* to a selection pressure. The genetic variation (e.g., for antibiotic resistance) must pre-exist in the population by chance before the pressure is applied.
- ×Believing that individual organisms evolve. Populations evolve; individuals are selected. An individual mouse cannot change its fur colour in its lifetime to adapt.
No-calculator tips
- ✓Always follow the logical sequence for natural selection questions: Variation → Pressure → Selection (differential survival/reproduction) → Inheritance → Evolution (change in frequency).
- ✓Think in terms of probabilities and proportions, not certainties. A trait being 'fitter' just means it gives a higher *chance* of survival and reproduction, not a guarantee.
- ✓When presented with a scenario, your first step should be to identify the specific environmental pressure and the specific trait it is acting upon.