Evolution happens because populations of living organisms change over generations. The immediate causes are straightforward: individuals differ from one another, some of those differences are inherited, and individuals with certain inherited traits may leave more offspring than others. Over many generations, those differences can become more or less common in the population.
That basic process explains much of the diversity of life, from bacteria that become resistant to antibiotics to the differences among species of birds, mammals, plants, and insects.
Evolution does not happen because organisms consciously need to change. Instead, evolutionary change emerges from processes that alter the genetic makeup of populations over time.
Evolution happens to populations, not individual organisms
An individual organism does not evolve during its lifetime in the biological sense. A person can become stronger through exercise, for example, but that change is not evolution. Evolution refers to a change in the inherited characteristics of a population across generations.
A population is a group of individuals of the same species living and reproducing in a particular area. Members of a population usually share many genes, but they are not genetically identical.
The inherited differences among individuals provide the raw material on which evolutionary processes act. If the proportion of different genetic variants changes from one generation to the next, the population has evolved.
For example, imagine a population of insects in which some individuals carry genetic variants that make them less affected by a particular pesticide. If the pesticide kills many susceptible insects while more resistant insects survive and reproduce, resistance-associated variants can become increasingly common in later generations.
Nothing in this process requires the insects to “try” to become resistant. The variation existed first; environmental conditions affected which individuals were more likely to survive and reproduce.
Where does the variation needed for evolution come from?
Evolution requires heritable variation. One major source of new genetic variation is mutation, a change in DNA.
Mutations occur through changes in genetic material. Some have no noticeable effect, some are harmful, and some can provide an advantage in particular circumstances. A mutation does not arise because an organism needs a certain trait. Its effects are evaluated, so to speak, by what happens to organisms carrying it under the conditions in which they live.
Another important source of variation is genetic recombination. During sexual reproduction, offspring receive combinations of genetic material from their parents. Processes associated with the formation of eggs and sperm reshuffle genetic variants, producing combinations that may not have existed in either parent.
Mutations introduce new genetic variants, while recombination creates new combinations of existing variants. Together, they contribute substantially to the genetic variation on which evolution can act.
Natural selection changes which traits become common
The best-known evolutionary mechanism is natural selection. It occurs when inherited differences among individuals affect their chances of surviving or reproducing.
For natural selection to produce evolutionary change, three conditions are especially important:
- Individuals in a population vary in their traits.
- At least some of that variation is heritable.
- Different inherited traits are associated with differences in reproductive success.
Suppose a population of plants contains variation in when individuals flower. If the local environment consistently favors plants that flower earlier, and flowering time is partly inherited, earlier-flowering plants may leave more offspring. Over generations, genetic variants associated with earlier flowering can become more common.
Natural selection therefore does not necessarily make organisms stronger, faster, larger, or more complex. It favors inherited characteristics that increase reproductive success in a particular environment. A trait that is advantageous in one setting may be neutral or harmful in another.
This is why evolution has no predetermined direction. Environments change, and different environments impose different pressures.
Evolution can also happen without natural selection
Natural selection is important, but it is not the only process that changes populations.
Genetic drift occurs when genetic variants become more or less common simply because of random differences in reproduction. Drift has a particularly strong effect in small populations. A genetic variant can become common, or disappear entirely, even if it has no effect on survival or reproduction.
This randomness can matter when a population is reduced to a small number of individuals or when a small group establishes a new population elsewhere. In such situations, the genetic composition of the resulting population may differ substantially from that of the original population just by chance.
Gene flow also changes populations. It occurs when individuals or their reproductive cells move between populations and introduce genetic variants into a population where they were previously rare or absent. Migration can therefore make populations more genetically similar to one another.
Together, mutation, natural selection, genetic drift, and gene flow help explain how the genetic composition of populations changes over time.
Why reproduction is at the center of evolution
Evolutionary change depends on differences in reproductive success because genes are passed from one generation to the next.
An inherited trait can affect evolution even if it does not directly improve survival. What matters is whether it influences the number of viable offspring an individual ultimately contributes to the next generation.
This also explains why natural selection can produce traits that seem costly in other respects. A trait may require energy or carry a disadvantage but still spread if it provides a sufficiently large reproductive benefit under particular conditions.
Sexual selection is one important example. Individuals may differ in traits that influence their ability to attract mates or compete for mating opportunities. If those differences have a genetic basis, traits associated with greater mating success can become more common over generations.
The environment does not create useful traits on demand
A common misunderstanding is that organisms develop characteristics because their environment requires them. Evolution works differently.
Environmental conditions influence which existing inherited differences are more successful. They do not direct mutations toward whatever traits an organism happens to need.
If a population encounters a new environmental challenge, individuals may already differ in ways that affect their response. New mutations may also arise, but mutations occur without regard to whether they would be useful. Natural selection can then change the frequency of variants according to their effects in that environment.
This distinction is especially important when thinking about adaptation. Adaptation is an inherited characteristic that increases an organism’s ability to survive or reproduce in a particular environment, as well as the evolutionary process that produces such characteristics.
Adaptations are outcomes of evolutionary processes, not conscious solutions invented by organisms.
Evolution does not always make organisms more complex
Evolution is often described as “progress,” but biological evolution has no universal goal of increasing complexity.
Some organisms have remained relatively simple in structure while being extraordinarily successful. Bacteria, for instance, occupy enormous numbers of environments and have evolved a wide range of biochemical capabilities.
In other cases, evolution can reduce structures that are no longer useful. An inherited feature may become smaller or disappear when maintaining it provides little benefit and other evolutionary processes favor its reduction.
The key question is not whether a trait represents progress. It is whether particular genetic variants become more or less common under the conditions affecting a population.
Why different species evolve in different directions
Populations of the same species can experience different environments. They may face different temperatures, food sources, predators, diseases, competitors, or opportunities for reproduction.
If populations become separated and experience different evolutionary pressures, their genetic compositions can diverge. Random genetic changes, natural selection, and differences in gene flow can all contribute.
Over sufficiently long periods, accumulated differences can become extensive. When populations become reproductively isolated—meaning they can no longer exchange genes effectively enough to remain part of the same evolving gene pool—new species can arise.
This process, known as speciation, is one reason Earth contains such a wide variety of organisms.
Evolution is a change in populations, not a ladder of improvement
Because evolution produces adaptation and biological diversity, it is sometimes imagined as a ladder leading from “primitive” organisms toward increasingly advanced ones. That picture is misleading.
Evolution is better understood as a branching process. Populations split, change, sometimes go extinct, and sometimes give rise to new lineages. Different organisms become suited to different ways of life rather than moving toward a single ideal form.
Humans are not the endpoint of evolution, and modern organisms are not inherently more evolved than organisms that lived long ago. Every living species has an evolutionary history shaped by the environments and populations from which it arose.
Evolution can be observed on relatively short timescales
Although major evolutionary changes can take many generations, evolution does not necessarily require enormous stretches of geological time.
When organisms reproduce quickly and conditions strongly affect reproductive success, genetic changes can spread through populations over comparatively few generations. This is why evolutionary change can be important in areas such as agriculture, disease, and conservation.
The evolution of antibiotic resistance illustrates the principle particularly clearly. Within a population of bacteria, genetic differences can affect susceptibility to an antibiotic. When treatment eliminates susceptible bacteria more effectively than resistant ones, resistant variants may make up a larger fraction of the surviving population and their descendants.
The antibiotic does not cause bacteria to intentionally adapt. Instead, it changes the conditions under which existing genetic variation affects reproductive success.
Chance and selection work together
Evolution is neither completely random nor completely directed.
Mutations arise without anticipating an organism’s future needs, and genetic drift involves random changes in variant frequencies. At the same time, natural selection is a nonrandom process in the sense that inherited traits associated with greater reproductive success tend to become more common under particular conditions.
The relative importance of these processes varies among populations and circumstances. A small population may be strongly affected by genetic drift, while a strong environmental pressure may produce substantial natural selection. Gene flow can introduce variants from elsewhere, and mutation continually contributes new genetic variation.
Evolutionary change is therefore the result of several processes acting on populations over generations.
Why evolution happens
At its core, evolution happens because living populations contain heritable genetic variation and because the frequencies of those genetic variants can change from one generation to the next.
Mutation and recombination provide variation. Natural selection can favor variants associated with greater reproductive success. Genetic drift can alter variant frequencies by chance. Gene flow moves genetic variants between populations. Reproductive isolation can allow populations to diverge until new species emerge.
None of these processes requires life to have a predetermined destination. Evolution is an ongoing consequence of inheritance, variation, reproduction, environmental conditions, and chance. Over generations, those processes can transform populations—and, given enough time, produce the extraordinary diversity of life.
