How It Works: Natural Selection and Evolution

How It Works: Natural Selection and Evolution

Evolution by natural selection occurs through a sequence of linked events that operate on populations over many generations. Within any population, individuals are not identical — they show variation in their characteristics. This variation arises from differences in genes (caused by mutations and sexual reproduction shuffling alleles).

When resources are limited, individuals compete for survival. Those whose variations give them an advantage in their particular environment — better camouflage, faster running speed, greater disease resistance — are more likely to survive long enough to reproduce. Because the advantageous characteristic is encoded in an allele, surviving individuals pass that allele to their offspring.

Over many generations, the advantageous allele becomes more common in the population while the less useful alleles decrease in frequency. The population as a whole shifts — this is evolution. If populations become geographically separated and experience different selection pressures, they can diverge so much that they can no longer interbreed, forming two distinct species (speciation).

Crucially, no individual organism evolves. Evolution is a change in allele frequencies across an entire population over time.