Variation and Evolution: Natural Selection

Natural Selection

Natural selection is a key mechanism of evolution, proposed by Charles Darwin in his theory of evolution by natural selection.
This process leads to organisms becoming better adapted to their environment over generations.
The basic principles of natural selection include variation, inheritance, selection, and time.
Variation within a species is due to differences in DNA, achieved either through mutation or sexual reproduction combining parental genes.
Inheritance means that these variations can be passed down from parent to offspring.
Natural selection relies on competition for resources like food, mates and shelter. This competition is the selection pressure.
The individuals who are most successful in this competition are the fittest - they have the traits that best equip them to survive and reproduce. This is known as survival of the fittest.
Over time, these advantageous traits become more common in the population, as the fittest individuals are most likely to reproduce and pass on their genes to the next generation.

Directional and Stabilising Selection

Both directional and stabilising selection are types of natural selection.
Directional selection occurs when environmental changes favour traits at one extreme of the population distribution. This leads to a shift in the population mean towards this extreme.
An example of directional selection is the increase in average beak size in a population of finches after a drought causes small-beaked finches to die out.
Stabilising selection occurs when environmental conditions are stable, so traits near the average of the population distribution are most favoured, leading to a decrease in variation in the population.
An example of stabilising selection is human birth weight, with babies of average weight more likely to survive and reproduce than babies at the extremes of the weight range.

Selection and Resistance

Natural selection can lead to the development of resistance, as seen in bacteria developing antibiotic resistance.
Genetic variation in a bacteria population might include a few individuals with a mutation that makes them resistant to a certain antibiotic.
When the population is exposed to this antibiotic, only the resistant bacteria survive and reproduce, leading to a population of mainly resistant bacteria.
This is why it’s important to finish a full course of antibiotics and not overuse them, to prevent the development of antibiotic-resistant superbugs.