Natural selection describes how individuals within a population that are better suited to their environment tend to survive longer and produce more offspring than those less adapted. This leads to the gradual accumulation of favorable traits, shaping species to fit their surroundings.
Genetic Variation
Differences among individuals within a population provide the raw material for natural selection to act upon. Without this inherent diversity, there would be no differing traits for selection to favor or select against, making evolutionary change impossible.
Genetic variation arises through two main processes: mutations and genetic recombination. Mutations are random changes in the DNA sequence, which can introduce entirely new alleles into a population. While many mutations are neutral or even harmful, some can be beneficial, offering an advantage to an organism in its environment.
Genetic recombination occurs during sexual reproduction, shuffling existing genes into new combinations. This ensures offspring receive unique trait combinations, contributing significantly to the overall variation observed within a population. The continuous generation of new genetic combinations through these processes ensures a steady supply of diverse traits upon which natural selection can operate.
Environmental Selection Pressures
The environment plays an active role in natural selection, acting as a filter that determines which traits are advantageous. These “environmental selection pressures” favor individuals with characteristics that enhance their survival and reproductive success. Organisms with traits that align well with these pressures are more likely to thrive and pass those traits to their offspring.
Various factors can exert environmental pressure. Competition for limited resources means that individuals better equipped to acquire these resources will have an advantage. Similarly, the presence of predators favors prey species with traits that help them evade capture, like camouflage, speed, or defensive mechanisms.
Climate and weather patterns also impose selection pressures. Organisms adapted to extreme temperatures, varying rainfall, or specific seasonal changes are more likely to survive and reproduce. Disease resistance is another pressure, as individuals with genetic predispositions to resist common pathogens are more likely to survive outbreaks. Sexual selection, where traits increase an individual’s success in attracting mates, also drives the prevalence of certain characteristics.
Heritability of Traits
For natural selection to lead to evolutionary change, the advantageous traits must be heritable. This means the traits that provide a survival or reproductive advantage must be passed down from parents to their offspring through genetic inheritance. If a beneficial trait is not genetically determined, it cannot become more common in future generations.
Heritability is a measure of how much of the variation in a trait is due to genetic factors. If a trait has high heritability, it means that genetic factors play a substantial role in its expression, making it more likely to respond to selective pressures. This genetic transmission allows favorable adaptations to accumulate over time.
Population Size and Gene Flow
Population size and the extent of gene flow influence the effectiveness and speed of natural selection. In smaller populations, random events, known as genetic drift, can have a more pronounced impact on gene frequencies. This randomness can sometimes override or interact with the effects of natural selection, potentially leading to the loss of beneficial traits or the increase of less favorable ones.
Conversely, in larger populations, the effects of natural selection are more consistent. Large populations maintain greater genetic diversity, providing more raw material for selection. This allows beneficial adaptations to spread more effectively, making natural selection a more dominant force in shaping evolution.
Gene flow introduces new genetic variations between populations. It enriches the gene pool, providing additional raw material for natural selection. However, extensive gene flow can dilute existing adaptations if it introduces traits that are not well-suited to the local environment, potentially hindering local adaptation.