Natural selection

Natural selection describes how individuals with certain observable traits are more likely to survive and reproduce, passing those traits on to the next generation. This process changes the heritable features of a population or species over time and is a key mechanism of evolution. Charles Darwin popularized the term, setting it apart from artificial selection—which is deliberate—because natural selection is not intentional.

Darwin saw natural selection as a principle arising from three processes: inheritance (the passing and development of heritable material from parent to offspring), variation (partly caused by an organism’s own actions), and the struggle for existence (which includes both competition and cooperation, or “mutual aid,” especially among social plants and animals). Variation in traits—both genetic and physical—exists in all populations. Some traits make survival and reproduction more likely, so they tend to be inherited more often. If the environment favoring these traits stays stable, they become more common. When new traits are favored due to changes in a specific niche, microevolution occurs; when broader environmental changes drive this, macroevolution follows. Occasionally, radically different traits can lead to new species.

Several factors determine which traits are passed down. Some traits are selected because they help organisms adapt to their surroundings. Others are favored because mating partners actively prefer them—this is sexual selection. Female bodies also tend to select traits that impose the lowest cost to reproductive health, known as fecundity selection.

Natural selection is a cornerstone of modern biology. Darwin and Alfred Russel Wallace jointly presented the concept in 1858, and Darwin expanded it in his 1859 book *On the Origin of Species*. He compared natural selection to artificial selection, where humans systematically breed animals and plants for desired traits. The idea originally developed without a valid theory of heredity; modern genetics came later. The combination of traditional Darwinian evolution with classical genetics formed the modern synthesis in the mid-20th century.

In the 21st century, new evidence has led evolutionary biologists to question the gene-centered view of evolution, producing extended evolutionary syntheses that place organisms back at the center of natural selection theory. Meanwhile, molecular genetics has spurred evolutionary developmental biology, which compares developmental processes across species to understand how they evolved. Although genotypes can change slowly through random genetic drift, natural selection remains the primary explanation for adaptive evolution.

**Historical development**

**Pre-Darwinian theories**

Ancient philosophers like Empedocles and the Roman poet Lucretius suggested that nature produces a vast variety of creatures randomly, and only those that can feed themselves and reproduce persist. Aristotle criticized Empedocles’ idea that organisms arise purely from incidental causes like heat and cold, arguing instead for natural teleology—that form exists for a purpose, citing the regularity of heredity in species. However, Aristotle did accept in his biology that new types of animals, or “monstrosities,” could appear very rarely. In Darwin’s 1872 edition of *On the Origin of Species*, Aristotle is quoted considering whether different body parts (like teeth) might arise accidentally, with only the useful forms surviving: “So what hinders the different parts [of the body] from having this merely accidental relation in nature? … Wheresoever, therefore, all things together … happened like as if they were made for the sake of something, these were preserved … and whatsoever things were not thus constituted, perished, and still perish.” Yet Aristotle rejected this possibility in the next paragraph, clarifying that he was discussing embryonic development, not the origin of species, noting that natural things “either invariably or normally come about in a given way,” unlike chance events.