How natural selection causes biological evolution
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Definition of natural selection
Natural selection describes the process by which individuals with traits that increase survival or reproductive success become more common in a population over generations. Cause: variation within a population. Effect: individuals with beneficial traits reproduce more and their alleles rise in frequency. Charles Darwin summarises this process as a recurring 'struggle for existence' that favours profitable variation .
Sources of variation and limiting factors
Genetic variation exists between individuals because of mutation, independent assortment and the mixing of alleles during sexual reproduction. Cause: mutation and genetic recombination. Effect: production of new alleles and novel trait combinations that selection can act upon. Limiting factors for useful variation include low mutation rates, small population size and limited gene flow, which reduce the raw material available for selection .
Selection pressures, adaptation and cause→effect
Selection pressures are environmental factors that change the survival or reproductive success of individuals. Cause: presence of a pressure (for example predators, pathogens, temperature, food supply). Effect: individuals with traits that reduce the negative impact of the pressure survive and leave more offspring. Over generations, the population becomes better adapted to the local environment because alleles for beneficial traits increase in frequency .
Inheritance and cumulative change across generations
Heritable traits pass from parents to offspring through genes on chromosomes. Cause: offspring inherit alleles from parents. Effect: advantageous alleles that increase reproductive success spread through the population. Repetition of differential survival and reproduction causes small allele-frequency changes to accumulate, producing measurable evolutionary change over many generations .
Speciation and reproductive isolation
Speciation occurs when populations become reproductively isolated and accumulate sufficient genetic differences that interbreeding no longer produces fertile offspring. Cause: physical separation or ecological isolation that prevents gene flow. Effect: different selection regimes and random changes magnify variation until two populations diverge into distinct species. Limiting factors for speciation include ongoing gene flow and insufficient time for divergence .
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