How Does Natural Selection Lead to Evolution and Speciation?
Learn how natural selection changes allele frequencies over generations and how reproductive isolation can turn diverging populations into separate species.
Natural selection leads to evolution when inherited alleles that improve survival and reproductive success become more common in a population over generations. Speciation can follow if populations become reproductively isolated and diverge until they can no longer produce fertile offspring together. Natural selection alone does not automatically create a new species.
From selection to evolution
An allele is a version of a gene. Random mutations can produce new alleles, while meiosis and random fertilisation create new combinations of existing alleles. If an inherited characteristic gives an organism an advantage in its environment, it is more likely to survive and reproduce. Its offspring may inherit the advantageous allele. Over many generations, that allele can increase in frequency. A change in allele frequencies in a population is evolution.
For example, some bacteria may already carry an allele that gives them antibiotic resistance. When an antibiotic is used, susceptible bacteria are less likely to survive, while resistant bacteria survive and reproduce. The resistance allele therefore becomes more common. The antibiotic does not make individual bacteria mutate because they need to survive.
From evolution to speciation
Speciation requires reproductive isolation: gene flow between populations is prevented. In allopatric speciation, a geographical barrier separates populations. Different mutations, selection pressures and genetic drift can cause their allele frequencies to diverge. If they eventually cannot interbreed to produce fertile offspring, they are separate species. Sympatric speciation follows the same broad outcome but reproductive isolation develops without a geographical barrier.
The key exam distinction is that individuals do not evolve. Natural selection acts on differences between individuals; evolution is the resulting change in a population’s allele frequencies. Speciation needs the further step of reproductive isolation.
