Natural selection is a fundamental mechanism of evolution, where organisms better suited to their environment tend to survive and reproduce more successfully. This process involves differential survival and reproduction of individuals based on their observable traits, known as phenotypes. Over successive generations, advantageous traits become more prevalent, leading to gradual changes and adaptations. Natural selection continuously shapes life forms as environments shift and new challenges arise.
Directional Selection Explained
Directional selection occurs when individuals at one extreme of a phenotypic range possess higher fitness, shifting the population’s average trait value. This favors phenotypes adapted to new or changing environmental conditions. Consequently, the frequency of alleles associated with the favored extreme trait increases over generations.
An example is the beak size of Darwin’s finches. During drought, small, soft seeds become scarce, leaving only larger, tougher seeds. Finches with larger, stronger beaks crack these, gaining a survival advantage. This leads to increased reproduction and average beak size over time.
Antibiotic resistance in bacteria is another instance. When exposed to an antibiotic, bacteria with pre-existing mutations that confer resistance survive and multiply, while susceptible bacteria are eliminated. This selective pressure shifts the population towards a higher proportion of antibiotic-resistant strains.
Stabilizing Selection Explained
Stabilizing selection favors intermediate variants within a population, acting against individuals at both extremes of the phenotypic range. This reduces phenotypic variation, maintaining the status quo for a particular trait. It is common because many traits do not undergo drastic changes over extended periods.
An example in humans is birth weight. Infants with very low birth weights face heat regulation and disease susceptibility challenges, while very high weights cause delivery difficulties. Babies in an intermediate range have higher survival rates, concentrating births around an optimal average and reducing extreme sizes.
Similarly, in birds, clutch size often demonstrates stabilizing selection. Laying too few eggs might not produce enough surviving offspring, while too many strain parental resources, leading to malnourished chicks and lower survival rates. An intermediate clutch size maximizes healthy offspring survival.
Disruptive Selection Explained
Disruptive selection, also known as diversifying selection, favors individuals at both extremes of the phenotypic range over intermediate variants. This increases genetic variance, leading to a population diverging into distinct phenotypic groups. It may also precede new species formation.
An example is finch populations with specialized food resources. If an environment has abundant very small and very large seeds, but few medium ones, finches with very small or very large beaks thrive. Small-beaked finches efficiently consume small seeds, while large-beaked finches crack large ones.
Medium-beaked finches struggle with either, having lower survival and reproductive success. Over time, this pressure leads to a bimodal distribution of beak sizes, with fewer intermediate individuals. This suggests a population split as individuals adapt to distinct ecological niches.