What changes, exactly?

Evolution is a biological process where the heritable characteristics of a population change from one generation to the next. That wording matters. A population means a group of organisms of the same species living in the same area at the same time and able, in principle, to interbreed. A heritable characteristic is a trait whose variation can pass from parents to offspring through genetic information.

So evolution does not just mean “change”. A child learning a language, a tree growing lopsided in a windy place, or an athlete developing stronger muscles has changed during a lifetime, but none of these counts as evolution unless the underlying inherited information in the population changes.

Acquired characteristics are traits an organism develops during its lifetime because of environment, use, learning or injury, rather than because of inherited genetic differences. This is the key distinction between Darwinian evolution and Lamarckism. Lamarckism proposed that useful acquired traits could be inherited; Darwinian evolution depends on heritable variation being passed on and changing in frequency in populations.

A useful test is simple: if the trait disappears when offspring are raised in a different environment, it was probably acquired rather than inherited.

Evolution as a scientific theory

A scientific theory is a broad explanatory framework that accounts for many observations and makes testable predictions. Calling evolution by natural selection a theory does not mean “a guess”. It means the explanation has unusually wide explanatory power: it predicts and explains patterns such as antibiotic resistance, pesticide resistance, sequence similarity, homologous structures and convergent evolution.

The nature of science matters here. We cannot formally prove a general theory true by checking every possible organism in every possible past and future case. Strong theories instead survive repeated testing, fit evidence from independent sources, and keep making successful predictions. In that sense, evolution by natural selection is a pragmatic truth: a scientific claim treated as true because it works reliably in explaining and predicting observations. Strong evidence in biology usually looks like this too: not one striking example, but independent lines of evidence pointing to the same explanation.

Sequence data as evidence of common ancestry

A base sequence is the order of nucleotide bases in a DNA or RNA molecule that stores genetic information. An amino acid sequence is the order of amino acids in a protein, determined by the genetic code during protein synthesis. Since inherited traits depend on genetic information, evolution should leave traces in DNA, RNA and proteins — and it does.

Closely related species usually show fewer sequence differences in the same gene or protein than species that are more distantly related. That matters because the evidence is not just about appearance. If two species share many details in a DNA sequence, the simplest explanation is that they inherited much of that sequence from a recent common ancestor. As lineages stay separate for longer, mutations build up independently, so sequence differences tend to increase.

Sequence comparisons are therefore useful when building cladograms, which are branching diagrams that represent hypotheses about evolutionary relationships. A cladogram based on sequences often matches one based on anatomy or morphology. When independent sources of evidence agree, sequence data becomes especially strong evidence in biology.

DNA, RNA and protein evidence

DNA sequence comparisons are used for most cellular organisms. RNA sequence comparisons are especially useful in some viruses and when comparing conserved molecules such as ribosomal RNA. Protein sequences provide another line of evidence because proteins are the products of genes; if the gene sequence diverges, the amino acid sequence may also diverge.

Gene families can be particularly revealing. A gene family is a group of related genes that arose by duplication of an ancestral gene and later diverged in sequence and function. Hox genes, for example, occur across many animal groups and help control body patterning. Their similarities are much easier to explain by common ancestry followed by duplication and modification than by each lineage independently inventing highly similar developmental genes from scratch.

Artificial selection shows that heritable traits can change quickly

Selective breeding is a human-directed process where individuals with desired heritable traits are chosen as parents for the next generation. Artificial selection is selection imposed by humans, not by environmental survival and reproduction.

Domesticated animals and crop plants show very directly that populations can change a lot in a relatively short time. Many dog breeds look and behave very differently from each other, and from wolves. Dairy cattle, egg-laying chickens, sheep, horses and cats all show traits that humans have favoured for particular uses. The same pattern appears in crop plants: varieties of wheat, rice, maize, cotton or roses can differ markedly from their wild relatives and from other varieties of the same crop.

The key point is not that human choice works exactly like natural selection. It’s that repeated selection of heritable variation can shift the characteristics of a population quickly. If humans can produce large changes in domesticated forms over thousands, hundreds or even fewer generations, then natural selection, acting over much longer timescales, can reasonably produce large evolutionary changes in wild populations.

Selective breeding gives evidence that evolutionary change is possible, observable and sometimes fast. It also shows that evolution does not require an individual organism to transform during its life; it requires differential reproduction of heritable variants over generations.

Homology: same inherited plan, different uses

Homologous structures are anatomical features in different species that come from the same evolutionary origin, even when they now do different jobs. They support evolution because they reveal a common underlying body plan that has been modified in different lineages.

The classic example is the pentadactyl limb, a vertebrate limb pattern with five digits, or a modified version of that arrangement. In forelimbs, the basic pattern is one proximal bone, two distal bones, wrist bones, and digits. In mammals, for example, this matches the humerus; radius and ulna; carpals; metacarpals and phalanges. In hindlimbs, the equivalent pattern is femur; tibia and fibula; tarsals; metatarsals and phalanges.

From the outside, a human arm, bat wing, whale flipper and horse forelimb look very different because they are used for manipulating, flying, swimming or running. Even so, the same bones sit in the same relative positions. Evolution explains this through common ancestry: an ancestral tetrapod had a limb with this basic arrangement, and its descendant lineages inherited it, then modified it under different selection pressures.

Homologous structures show both unity and diversity neatly. Unity comes from the shared inherited plan. Diversity comes from the different functions produced by modifying that plan.

Analogy: same job, different origin

Analogous structures are features in different species that do the same job, or a similar one, but come from different evolutionary origins. Convergent evolution is an evolutionary process where unrelated or distantly related lineages evolve similar traits because they face similar selection pressures.

Bird wings and insect wings make the idea easy to see. Both are used for flight. A bird wing, however, is a modified vertebrate forelimb with bones, muscles and feathers, while an insect wing is an outgrowth of the exoskeleton. Similar function; different evolutionary origin.

The camera-type eye of humans and octopuses is another good example. Both form focused images using a lens and retina-like light-sensitive tissue, but the detailed arrangement is not the same. In the vertebrate eye, nerve fibres run in front of the retina, and there is a blind spot where the optic nerve leaves the eye. The octopus eye avoids that particular arrangement. Similar visual demands led to similar optical solutions.

Convergent evolution explains why life can sometimes look repetitive without needing recent common ancestry for every similarity. Homologous structures point to common ancestry followed by diversification; analogous structures point to similar selection pressures acting on different starting materials. Together, they answer one of the big questions of this topic: evolution explains both commonality and diversity.

New species arise by branching, not by gradual improvement within one line

Speciation is the evolutionary process where one pre-existing species splits into two or more species. New species have appeared only by this route: new branches grow from older branches.

A species may change gradually over time and still remain one species. For example, a population might change in body size, colour or behaviour, but if it still forms a single interbreeding lineage, that is evolution, not speciation. Speciation needs a split: two populations become separate enough that they no longer make up one shared breeding population.

That is why evolution is shown as branching trees, not ladders. A lineage can split again and again, producing many related species. Speciation increases the total number of species on Earth. Extinction is the permanent loss of all members of a species, and it decreases the total number of species.

Don’t think of speciation as one species “turning into” another while the old one simply disappears. In the usual branching model, an ancestral species gives rise to separate descendant lineages. One or both may later persist, change further, split again or become extinct.

Two ingredients: isolation and different selection

Reproductive isolation happens when populations do not successfully interbreed, so genes are not exchanged between them. A gene pool is the total set of alleles present in a population. Gene flow is the movement of alleles between populations through interbreeding.

For speciation to happen, gene flow has to be reduced or stopped. If two populations keep interbreeding freely, their alleles keep mixing, so differences tend to be spread across the species rather than kept separate. Isolation lets each population’s gene pool follow its own evolutionary path.

Differential selection is natural selection that acts differently on separate populations because their environments or ecological pressures differ. After populations become isolated, different climates, predators, food sources, competitors or diseases can favour different traits. Over generations, the populations diverge.

Geographical isolation and the Congo River example

Geographical isolation is reproductive isolation caused by a physical separation, such as a river, mountain range, glacier, desert or ocean channel. It often leads to speciation because it prevents mating and can expose populations to different selection pressures.

Bonobos and common chimpanzees make a clear example. Both belong to the genus Pan, but the Congo River separates their present ranges. Neither species is considered able to cross this deep river easily. One plausible scenario is that an ancestral chimpanzee-like population was split when some individuals became isolated on one side of the river. Once separated, the populations experienced different selection pressures and diverged in body size, social behaviour and other traits. In time, they became distinct species.

The point is precise: isolation alone is not enough if the populations remain genetically and ecologically similar, and selection alone is not enough if gene flow keeps mixing the populations. Speciation is most likely when reproductive isolation and differential selection act together.

Same outcome, different geography

Allopatric speciation is speciation in which populations become separate species while living in different geographical areas. It usually begins with geographical isolation: a barrier splits populations and stops interbreeding.

Sympatric speciation is speciation in which populations become separate species while living in the same geographical area. It’s less straightforward, because no physical barrier divides the population; reproductive isolation has to arise some other way.

Comparison of allopatric and sympatric speciation mechanisms.

The vocabulary matters less than the shared outcome. Both require reproductive isolation and divergence. The difference is the route to isolation. In allopatric speciation, geography does much of the early separating. In sympatric speciation, barriers to reproduction develop within the same area.

Geographic, behavioural and temporal isolation

Reproductive isolation can be geographic, behavioural or temporal. Geographic isolation means separation by physical space or barriers. Behavioural isolation is reproductive isolation caused by differences in courtship, mating signals or mate choice. Temporal isolation is reproductive isolation caused by breeding at different times, such as different seasons, months or times of day.

Behavioural isolation can happen when individuals choose mates with particular songs, colours, pheromones or courtship movements. Temporal isolation can happen when two groups are fertile or active as adults at different times, so they rarely get the chance to mate. In both cases, gene flow falls, and divergence can follow.

Sympatric speciation is harder to demonstrate than allopatric speciation. Closely related species now living in the same area may have diverged elsewhere first, then later come back into contact. That’s a useful caution: in biology, strong evidence often means ruling out plausible alternative explanations.

One ancestor, many niches

Adaptive radiation is an evolutionary pattern where multiple closely related species evolve from a common ancestor and become adapted to different ecological niches. An ecological niche is the role of a species in its ecosystem, including how it obtains resources, where it lives and how it interacts with other organisms.

Adaptive radiation tends to happen where niches are vacant or underused. A founder population may arrive, or a lineage may gain access to new resources. Over time, different populations adapt to different ways of life, and speciation produces a cluster of related species.

This raises biodiversity, the variety of life at genetic, species and ecosystem levels. It can also let closely related species coexist, because they are not all using the same resources in the same way. If one species feeds mainly on hard seeds, another on insects under bark and another on nectar, direct competition is reduced.

Galápagos finches are the classroom classic: related species have different beak forms suited to different food sources. Still, the key idea is not just “different beaks”. It is diversification from common ancestry into different niches, allowing several related species to live in the same broader ecosystem without competing so intensely that only one persists.

Why species boundaries can stay intact

Hybridization is reproduction between genetically different individuals or populations; here, it refers to mating between different species. An interspecific hybrid is an offspring produced by parents from two different species. An allele is a variant form of a gene at a particular locus.

If interspecific hybrids were common and fertile, alleles could move freely between species. The species boundary might then weaken or disappear. Barriers to hybridization and hybrid sterility help stop alleles from mixing between species.

Barriers before and after fertilization

A prezygotic barrier is a reproductive barrier that prevents fertilization from producing a zygote. Courtship behaviour is a common example in animals. Many animals use species-specific songs, displays, dances, pheromones or calls. When the signal is wrong, mating does not happen. Courtship, then, is not just decoration; it acts as a biological filter for recognizing a suitable mate of the same species.

A postzygotic barrier is a reproductive barrier that acts after fertilization, reducing survival or fertility of hybrid offspring. Hybrid sterility is one major example. A mule, produced from a horse and a donkey, is usually sterile. Because the parental species have different chromosome numbers and genetic differences, meiosis in the hybrid is disrupted. The mule may be strong and healthy, but it usually cannot pass alleles on to another generation.

These barriers matter for biodiversity. If they are weak and hybrids are fertile, alleles can mix between species. In some cases, this can reverse speciation or threaten a rare species through genetic swamping by a more common relative.