Natural Selection: Introduction
Authors: Reznick David , Travis Joseph 
Published: October 2021

All organisms produce more offspring during their lifetimes than are required to replace themselves, which means that all organisms should have growing populations. Most organisms instead have relatively stable populations, so that, of all those born, only some survive to reproduce and those that reproduce vary in how successful they are. If the features of organisms that give them greater lifetime reproductive success are heritable, meaning that they are transmitted from parents to offspring, then in each successive generation a greater proportion of the population will share these attributes. This process, called natural selection, explains how organisms adapt to their physical and biotic environment. We also think it is the primary mechanism that causes cladogenesis, or the diversification of one ancestral species into multiple descendant species.

Key Concepts

Artificial selection shows us how selection works and illustrates the capacity of organisms to evolve.

Evolution by natural selection is inevitable because organisms vary in traits that are transmitted from parent to offspring, and some of those traits influence reproductive success, or the number of offspring that survive to produce offspring of their own.

The traits favoured by natural selection are ‘context specific’, meaning that they are specific to the environment the organism experiences; ‘environment’ includes both the physical and biotic environments, with the latter pertaining to the other organisms the target organism interacts with.

Evolution by natural selection can cause speciation, or the origin of populations that are sufficiently different from all others to be recognized as a distinct entity.

The distinctness of species and higher divisions in the taxonomic hierarchy (genera, families, etc.) is defined by extinction, which eliminates the evidence of the continuous change of organisms over time.

Natural selection affects individual survival and reproductive success, but evolution is perceived at the level of populations and in how populations change over time.

Natural selection can enforce stability if selection favours the average phenotype and eliminates extremes.

Negative frequency‐dependent selection, or selection that favours rare phenotypes, can also promote stability because formerly rare phenotypes lose their advantage as they become more common. This process occurs in many ways and is likely a major force in evolution.

DNA sequences can reveal portions of genes with rates of base pair substitutions that are either too slow (stabilising or purifying selection) or too fast (directional selection) relative to noncoding regions of DNA.

Studies of how molecules function can reveal how different forms of a protein are better suited to different environmental conditions, which is another way to assess how natural selection has moulded the features of organisms.

The rate at which evolution by natural selection can occur and the amount of change that is attainable suggests that natural selection by itself is sufficient to account for the origin of the diversity of life.