The Four Pillars of Natural Selection

Before selection can happen, there must be differences to choose from. Genetic Variation refers to the diversity in gene frequencies within a population. These differences arise from mutations (random changes in DNA) and sexual reproduction (shuffling of genes). Without variation, a population is like a team where every player has the exact same skill—if a challenge arises that no one can handle, the whole team fails. In biology, if a population is identical and a new disease hits, the entire species could go extinct. Variation ensures that some individuals might have a 'toolkit' better suited for survival.

Most species produce far more offspring than the environment can support. This is called Overproduction. For example, a sea turtle may lay 100 eggs, but only a few will reach adulthood. This leads to Competition, or the 'struggle for existence.' Resources like food, water, and nesting space are limited. This creates a natural filter: only the individuals best adapted to find resources or avoid danger will survive. We can represent the population limit as the carrying capacity ($K$), where the number of births equals the number of deaths.

The final pillars are Differential Survival and Reproduction. It’s not enough to just survive; an organism must reproduce to pass on its traits. In biology, Fitness is defined by reproductive success. If a fast rabbit survives but never has kits, its 'fast' genes die with it. Over generations, the frequency of advantageous traits increases. If $p$ represents the frequency of a 'fast' trait, and that trait helps survival, we expect $p$ to increase in the next generation: $p_{next} > p_{current}$.