Introduction to evolution

In plain words

• Evolution is the process of change in living things over many generations. Biologists who study how this happens are doing evolutionary biology.¶3
• Populations change through genetic changes, which are changes to genes. Genes are the hereditary material that parents pass to their young. A mutation is a random change in a gene, and it can come from damage or copying mistakes in DNA.¶3¶5
• Shuffling of existing genes during sexual reproduction also creates new mixes of traits in offspring.¶5
• Offspring differ slightly from their parents in random ways. If a difference helps, that offspring is more likely to survive and reproduce. Over generations, helpful traits become more common.¶5
• These small differences add up over time, changing the population. This process is why there are so many different kinds of life.¶5
• Evolution explains how early life developed into today's complex ecosystems. It does not explain how life first began; that question is covered by abiogenesis, a separate topic.¶4

Think of it like this: Think of a garden after a long dry spell. Plants with roots that reach deeper water are more likely to survive and drop seeds, so over many seasons the garden fills with deep-rooted plants.

In plain words

• The modern idea of evolution began with Charles Darwin's 1859 book On the Origin of Species. Gregor Mendel's plant work, done between 1856 and 1863, later explained how traits are passed on.¶6
• Natural selection is the idea that individuals differ. Those with traits that help them survive and reproduce leave more offspring, so those traits become more common over generations.¶20
• Darwin saw that orchids have special parts that attract insects. The insects carry pollen between flowers. He argued these parts were built from the same basic structures as other flower parts, changed over time by natural selection.¶21
• Alfred Russel Wallace sent Darwin a letter describing a very similar theory, and the two published together. Both saw life as a family tree that branches from common ancestors. Darwin called this process descent with modification.¶22
• The idea that all life shares a common ancestor is supported by more than 99 percent of scientists. Some religious groups object to it.¶23
• The phrase survival of the fittest came from Herbert Spencer in 1864. It is a poor description of natural selection, because natural selection is not only about survival, and the fittest do not always survive.¶24
• Evolution is a key idea in many fields, such as medicine, psychology, agriculture and forensics. This is why it matters beyond biology alone.¶6

What this shows: This is a portrait of Charles Darwin sitting in a dark jacket and blue vest, with his hands resting in his lap. It appears in the article as a picture of the naturalist who proposed the theory of evolution by natural selection.

What this shows: This is a colourful botanical plate showing a bunch of orchid flowers in pinks, whites, yellows and purples. It illustrates Darwin's observation that orchids have complex features for pollination that grow from basic floral parts.

Think of it like this: Natural selection works a bit like a school team picking players. The players whose skills fit the game best get chosen more often, so those skills spread over time.

In plain words

• Darwin saw that living things differ from one another, and that natural selection can act on those differences. He could not explain where the differences came from.¶27
• Darwin wrongly thought traits gained through use, or lost through disuse, could be passed on to offspring. This idea is called Lamarckism. August Weismann's experiments in the 1880s suggested it was not true, and it fell out of favour.¶27
• Gregor Mendel's experiments with pea plants showed that inheritance works by mixing hereditary information from two parents. He called these pieces factors. They later became known as genes, the basic units of heredity.¶28
• Genes are made of DNA, a long molecule. Its sequence of building blocks carries information, much as letters carry meaning in words. Together the genes work like an instruction manual for building and running an organism.¶29
• New variation comes from mutations, which are changes to DNA. It also comes from chromosomes (packages that carry DNA) being reshuffled during sexual reproduction. Gene flow, which happens when populations interbreed, can bring in genes that were not there before.¶29
• Mutations are random, but natural selection is not. The environment decides which changes succeed, so evolution has no goal of making life more advanced. Organisms end up suited to their surroundings, and rapid environmental change often causes extinctions.¶30¶31

Think of it like this: Like shuffling a deck of cards: each child gets a random half of the cards from each parent, so no two hands are exactly alike.

In plain words

• Genetic drift is a change in how common gene versions (alleles) are in a population. It happens by chance, because only a random sample of alleles passes to the next generation.¶32
• Drift can remove an allele from a population, which lowers variety. It cannot add new alleles. Drift affects small populations more than large ones.¶32¶33
• The Hardy-Weinberg principle describes an idealised large population with no selection, mutations, or moving in or out. In that case, allele frequencies stay the same over generations. Real populations never fully meet these conditions, so their frequencies change.¶35¶36¶37
• A population bottleneck is a sudden, large drop in numbers caused by outside forces. Chance decides who survives, and variety is lost. The northern elephant seal fell to about 30 animals from hunting. It recovered to about 100,000, but its low variety still raises the risk of disease and genetic disorders.¶40¶41
• The founder effect happens when a small group splits off to form a new population. The new group's gene mix may differ from the original. The Amish example shows this: two founders carried a rare recessive allele, so the related condition is far more common among the Amish today.interpretation¶43¶44
• The modern evolutionary synthesis defines evolution as change in allele frequencies. The causes are genetic drift, gene flow between groups, and natural selection. Natural selection is considered the most important cause.¶45

What this shows: This diagram shows three circles of coloured balls representing a population. The first circle has many balls of several colours, and later circles show fewer colours after a bottleneck. Frequency tables below give the share of each allele in the initial population and in the 10th and 20th generations, showing that some alleles are lost by chance.

What this shows: This diagram shows a large circle with many red and blue dots, with an arrow pointing to three smaller circles containing only a few dots each. The smaller groups have different colour mixes from the large group. It illustrates the founder effect, where a small group that splits off has different allele frequencies from the parent population.

Think of it like this: Drift is like pulling marbles from a bag. If you draw only a few, some colours may disappear by chance, and you cannot get new colours that were never in the bag.

In plain words

• Scientists find evidence for evolution in three places: fossils, body structures shared by different species, and similarities in DNA.¶49
• Fossils show how life changed over long periods. Some fossils are transitional species that link older and newer life forms, such as Archaeopteryx, which had both reptile and bird features.¶50¶52
• Homologous structures are body parts that share a similar basic build but do different jobs. Human, cat, whale and bat forelimbs all have similar bones. Evolution explains this as a shared ancestor whose limbs changed over time to suit different tasks.¶57¶56
• Similar looks do not always mean close relation. Convergent evolution happens when unrelated animals in similar conditions develop similar features, as sharks and dolphins did for swimming. The bird and bat wings show this too.¶59¶58
• Vestigial structures are leftover body parts with little or no use, such as the small bones in whales that seem to be remnants of leg bones. Humans have some too, including the appendix and wisdom teeth.¶63
• Closely related organisms have very similar DNA, and more distant relatives have more differences. Humans and chimpanzees share about 96 percent of their DNA, so they are more closely related to each other than either is to gorillas.¶67

What this shows: This is a close-up photo of a bat in flight against a dark background, with its wings spread and its face and ears visible. The caption notes that a bat is a mammal whose forearm bones have been adapted for flight. It supports the article's point about homologous forelimbs.

What this shows: This photo shows a hawk with its wings spread wide against a blue sky, with its head turned upward. The caption notes that bird and bat wings are examples of convergent evolution. It shows how unrelated animals can develop similar features for flight.

What this shows: This is a drawing of four finch heads with different beak shapes, labelled as Geospiza magnirostris, Geospiza parvula, Geospiza fortis and Certhidea olivacea from the Galápagos Archipelago. The beaks vary in size and shape, reflecting how they are adapted to different food sources. It shows an example of adaptive radiation.

What this shows: This is a labelled 3D model of a DNA double helix, with coloured strands twisting around each other. Labels show the width of about 2 nanometers, one full turn of about 10 base pairs spanning 3.4 nanometers, and the major and minor grooves. It illustrates the molecule that carries genetic information in living things.

Think of it like this: A bat's wing and a whale's flipper are like two tools made from the same set of spare parts. Each tool is shaped for a different job, but the parts show they were built from the same plan.

In plain words

• Artificial selection is human-controlled breeding. People choose which plants or animals breed, so they decide which genes are passed on.¶71
• Dogs show this clearly. A Chihuahua and a Great Dane look very different because of selective breeding. All dogs came from wolves tamed by humans less than 15,000 years ago.¶71
• Humans pick the traits they like, while nature picks traits that help a species survive where it lives. Both start from random mutations, which are chance changes in DNA.¶73
• Coevolution is when two species shape each other's evolution. A type of ant and an acacia plant are an example. The ant guards the plant and clears away competing seeds. The plant grows swollen thorns for the ant to live in and flower parts for it to eat.¶75
• The ant and the acacia do not choose to help each other. Small changes that helped both became more common over many generations.¶75
• A new species forms through speciation, which is the splitting of one population into two separate species. A common definition says members of a species can breed and produce fertile offspring. This definition does not work well for bacteria, which reproduce without mating.¶77¶78
• In allopatric speciation, a population is split by a barrier such as a mountain range. The groups then change separately. If they later meet and can no longer interbreed, they count as different species.¶78
• Speciation is usually very slow. It has still been seen in real life. Five new cichlid fish species came from one ancestor that was cut off fewer than 5,000 years ago in Lake Nagubago.¶79

What this shows: A close-up of a bright blue cichlid fish with a dark tail and reddish-orange underside, swimming near the sandy bottom. It shows one example of the many cichlid species that vary greatly in body form. The article uses cichlids to show how evolution can produce new species.

Think of it like this: Think of two groups of friends who stop meeting. Each group picks up its own habits, until they no longer understand each other.

In plain words

• Scientists still argue about how fast evolution happens and what natural selection acts on. The theory is widely accepted, but it is not fixed.¶80
• Darwin and his contemporaries saw evolution as slow and gradual. Big differences between species were built up from many small changes over long periods.¶81
• Punctuated equilibrium is the idea that evolution moves in bursts of faster change, lasting about 50,000 to 100,000 years, separated by long periods of little change. Niles Eldredge and Stephen Jay Gould proposed it in the 1970s.¶82
• This idea fits the fossil record, which often shows a new species appearing suddenly and then staying the same for a long time. It does not contradict Darwin's ideas.¶82
• The unit of selection is the thing that natural selection acts on most directly. The usual answer is the organism, meaning the individual living thing.¶84
• Richard Dawkins argues that we can also view evolution from the gene's point of view. In his view, genes are the copies that last, and bodies are their survival machines.¶84¶85
• Stephen Jay Gould suggested that selection can work at several levels at once, not only at the level of the organism or the gene.¶86
• So the debate is not only about speed. It also asks whether the individual, the gene, or several levels together are the main target of selection.¶80¶84¶86

What this shows: A smiling older man with grey hair and glasses, wearing a brown jacket and a name badge, sits in front of a screen. He is Richard Dawkins, an evolutionary biologist. The article mentions him as one who argues that the gene, rather than the organism, is a common unit of selection.

Think of it like this: Think of a river that flows slowly for ages, then suddenly carves a new channel during a flood. Some scientists picture evolution the same way: quiet periods broken by bursts of change.