Natural selection explains how life on Earth has changed and diversified. It is a mechanism where organisms better suited to their environment survive and produce more offspring, leading to gradual changes in species and contributing to biodiversity. This process shapes the characteristics of populations across generations.
Variation Within Populations
Variation among individuals within a population is essential for natural selection. Organisms within any species are not identical; they exhibit differences in traits like size, color, speed, or disease resistance. This biological variation provides the raw material for natural selection to act.
These differences primarily arise from random mutations, which are changes in an organism’s genetic material. Genetic recombination during sexual reproduction also shuffles existing genes into new combinations, increasing trait diversity. Without this variability, environmental pressures could not favor or disfavor traits, preventing evolutionary change through natural selection.
Heritability of Traits
For natural selection to lead to lasting evolutionary change, variations within a population must be heritable. This means advantageous traits must be passed down from parents to offspring. Heritable traits are encoded in an organism’s genes, segments of DNA, and are transmitted across generations during reproduction.
Traits acquired during an individual’s lifetime, such as a scar or increased muscle mass, are not encoded in DNA and cannot be inherited. Only variations rooted in an organism’s genetic makeup contribute to evolution by natural selection. The ability of offspring to resemble their parents allows advantageous characteristics to become more common in a population over time.
Overproduction of Offspring
Most species produce more offspring than the environment can support. This results in a “struggle for existence” where individuals compete for limited resources. Resources like food, water, shelter, and mates are finite, creating pressure on populations.
This overproduction ensures that not all offspring survive to reproductive age. For example, a single salmon can lay millions of eggs, but only a small fraction develop into adult fish. Even slow-breeding animals like elephants, if left unchecked, would quickly exceed environmental carrying capacity. This intense competition and high mortality rate sets the stage for selection, as only a subset of individuals succeed in the struggle for survival.
Differential Survival and Reproduction
Building upon previous components, differential survival and reproduction represent the core outcome of natural selection. Individuals with heritable traits better suited to their environment are more likely to survive, thrive, and reach reproductive age. This increased likelihood means they pass on advantageous traits to more offspring.
Evolutionary “fitness” refers to an organism’s reproductive success, or its ability to contribute genes to the next generation. Organisms with traits that enhance their survival or reproductive output, such as camouflage, disease resistance, or efficient foraging, are considered more “fit” in that environment. Over many generations, this differential success increases the frequency of beneficial traits within the population. This process ultimately results in species adapting to their environments and drives evolutionary change.