Natural selection is a fundamental biological process explaining how life changes over generations. Proposed by Charles Darwin, it describes how organisms better suited to their environment survive and reproduce more. This non-random process, where environmental conditions favor certain traits, drives evolution and leads to species adapting over time.
Variation Within Populations
A foundational element of natural selection is the presence of variation among individuals within any population. Organisms are not exact replicas of one another; they display a range of differences in characteristics such as size, color, behavior, or resistance to disease. These differences, known as genetic variation, are essential for evolution to occur.
Genetic variation primarily arises from two main mechanisms: mutations and genetic recombination. Mutations are changes in the DNA sequence and can introduce entirely new traits into a population. Genetic recombination, which occurs during sexual reproduction, shuffles existing genes into new combinations, further increasing the diversity of traits. Without this inherent variability, there would be no raw material for natural selection to act upon.
Heritability of Traits
For natural selection to influence a population, the observed variations must be heritable. This means the traits can be passed down from parents to their offspring. If a characteristic is not genetically determined, such as a scar from an injury or increased muscle mass from exercise, it cannot be passed on to the next generation and therefore does not contribute to evolutionary change through natural selection.
Genes carry the instructions for these heritable traits, and copies of these genes are transmitted during reproduction. Traits like eye color, hair color, height, and even certain predispositions to health conditions are examples of heritable characteristics. The extent to which a trait is influenced by genetics versus environmental factors is known as its heritability.
Differential Survival and Reproduction
The culmination of natural selection involves the concepts of overproduction and differential success in survival and reproduction. Most organisms produce more offspring than the environment can realistically support, leading to competition for limited resources like food, water, and shelter. This overproduction creates a “struggle for existence” within a population.
Within this competition, individuals possessing advantageous heritable traits are more likely to survive, thrive, and reproduce successfully. For example, a bird with a slightly stronger beak might be better able to crack tough seeds during a drought, increasing its chances of survival and passing on that beak trait. Conversely, individuals with less favorable traits are less likely to survive to reproductive age or produce fewer offspring. This unequal success in survival and reproduction, where certain traits are favored by the environment, results in those beneficial traits becoming more common in the population over successive generations.