Adaptive radiation
Published on 26 October 2026 at 07:31 GMT
By Editorial Team Global Society & AI
Adaptive radiation sounds like something from a physics laboratory. It is one of the quietest stories in biology – the story of how one becomes many – and it begins with a single bird.
In 1981, a large cactus finch arrived on Daphne Major, a small island in the Galápagos. It had crossed more than 100 kilometres from the island of Española. The team of Peter and Rosemary Grant, who had been studying the island's finches since 1973, noticed the newcomer: he was bigger than the local birds, and he sang an unfamiliar song.
He mated with a female of the resident medium ground finch. His sons learned his song, and that song did not attract the island's other females. Within two generations, the "Big Bird" lineage was breeding only among itself. The researchers followed it for six generations, and in 2017 they described in the journal Science what they had witnessed: the rise of a new species from a hybrid, observed as it happened.
How can there suddenly be so many species of one kind? The answer biology gives is less sudden than it looks. It starts small.
How one becomes many
No population is uniform. Some individuals are a little larger, some have stronger beaks, some cope better with drought. When groups are separated – by the sea between islands, or simply because they live on different food – each is sifted by its own conditions, and small differences grow. At some point the groups stop interbreeding; on Daphne Major, a song was enough. When one ancestor branches out in this way into many forms that fill different niches, biologists speak of adaptive radiation.
Darwin's finches are the classic case: from a single common ancestor, 18 species have arisen within no more than about two million years. The cichlid fish of Lake Victoria in East Africa went faster still. The lake dried out between roughly 20,000 and 16,000 years ago and filled again about 15,000 years ago. In the time since, more than 500 cichlid species have emerged there – a blink in the history of life.
Seen from far away, it looks like an explosion. Seen up close, it is a tree that keeps branching. But every tree has roots. If one can become many so readily – where did the first one come from?
And before that?
In 1953, Stanley Miller filled a flask with water, methane, ammonia and hydrogen and sent electric sparks through it to imitate lightning. After a week, it contained amino acids, the building blocks of proteins. Simple gases and energy had been enough to make some of life's raw material.
How chemistry turned into biology is still an open question. A widely favoured hypothesis, the "RNA world", proposes that early life relied on RNA, a molecule that can both store information and speed up chemical reactions. One of its strongest clues sits in every cell today: the catalytic core of the ribosome, the machinery that builds proteins, is made of RNA. Still, it remains a hypothesis.
Researchers trace everything alive today, from bacteria to redwood trees to humans, back to a single common ancestor known as LUCA. A study published in 2024 estimates that it lived around 4.2 billion years ago – and that it was probably not alone. Other early organisms simply left no descendants.
Why does life not simply start again today? Charles Darwin offered an answer in 1871. In a letter to the botanist Joseph Dalton Hooker, he imagined life beginning in a "warm little pond" and added that at the present day such matter would be "instantly devoured, or absorbed" – by the living things already there. Once life exists, it occupies the ground. And for a very long time, occupying the ground was all it did.
How many became one
For more than two and a half billion years, life on Earth consisted only of microbes. The next great steps came through cooperation.
Mitochondria, small structures inside our cells, were once free-living bacteria. In 1967, Lynn Margulis argued that they had been taken up by another cell and had stayed; today the idea is widely accepted. Later, cells joined to form multicellular organisms, solitary insects formed colonies, and humans formed societies held together by language. The evolutionary biologists John Maynard Smith and Eörs Szathmáry called these steps the "major transitions" of evolution – and pointed out that there is no theoretical reason why life should grow more complex at all.
Martin Nowak of Harvard University summed up the common thread in 2006: "Cooperation is needed for evolution to construct new levels of organization."
There is a catch, and it matters for what follows. In these transitions, the partners often paid with their independence. The smaller units became parts of a larger whole and frequently cannot reproduce without it. Mitochondria have lost too many genes to live freely again.
Cooperation built the levels from the cell upward. Nothing guaranteed the next one. So how likely was any of this – a planet of microbes turning into a planet with someone on it to ask the question? One physicist asked it over lunch.
Where is everybody?
In the summer of 1950, at Los Alamos, the physicist Enrico Fermi made a quick estimate. The galaxy, it suggested, should be full of civilisations. So where were they? In one remembered version, he simply asked: "Where is everybody?" Many answers have been proposed since. None is settled.
Our own planet offers one sobering hint. Life appeared early, but for most of its history it stayed microscopic; multicellular life is only a little over a billion years old. Nothing in evolution guaranteed that complex life, let alone intelligence, would ever arrive.
Chance played its part, too. About 66 million years ago, an asteroid struck what is now the Yucatán Peninsula in Mexico. Roughly three quarters of all species died out, among them all dinosaurs except the birds. In the aftermath, mammal diversity exploded: in less than 20 million years, new species evolved to live in the air, in the oceans and in the treetops. It was, in effect, another adaptive radiation – and the beginning of the age of mammals, to which we belong.
Back at the lake
That is as far out as this journey goes. From here it turns back – to Earth, to the lake, to us.
Diversity grows slowly and can vanish fast. In 1954, the Nile perch, a large predator, was introduced into Lake Victoria. Together with pollution and other pressures, it contributed to the loss of around 200 of the lake's unique cichlid species within about half a century. What had grown over thousands of years was gone within a human lifetime.
And us?
The science ends here. What follows is our reading of it.
This is where people differ from mitochondria. Mitochondria did not choose. People can. Experiments show that human altruism is a powerful force and unique in the animal world. We cooperate not only because we must – we can decide to.
What such decisions can achieve has been studied. Elinor Ostrom, who received the Nobel Prize in Economic Sciences in 2009, showed in field studies that local communities sharing pastures, fisheries and forests can develop their own rules and use these resources sustainably – without a central authority and without privatisation.
For the global society, the long journey from finch to cell to society carries a double lesson. Life became rich by branching into many forms, and it climbed to new levels by joining forms together. The global society needs both – and it must not pay the price the mitochondria paid. This is also what the Global Consent says, eleven commitments of the global society set down in January 2026. One of them is to act through cooperation: "Sustainable solutions emerge from cooperation, shared responsibility, and mutual respect. We reject conquest and domination as principles of progress." Another is to protect the differences of societies from "domination, exploitation, and forced uniformity".
The opposite would be a social monoculture: one crop where life wants many. History knows its clearest forms. Totalitarian states and racist movements have wanted one people, one creed, one way of life – and have needed force to get it. Some even claimed evolution as their warrant, as if nature rewarded the domination of the strong. Nature's record says otherwise: its richness came from branching, its great leaps from cooperation. A monoculture is efficient for a season and fragile for ever; one blight, and the field is gone. And the pull towards uniformity is not confined to dictators. It appears wherever one model is declared the only right one – in markets, in bureaucracies, in movements with good intentions, and in ourselves.
Cooperation that keeps its members different is harder than uniformity, and harder than going it alone. But unity can be chosen; uniformity has to be forced. Only the first kind lasts without force.
And you?
The journey that began with one bird ends with one person. If nothing in evolution was fixed in advance, the future is not fixed either. That is no guarantee. It is room to act.
The great transitions of life began with small units doing something together. Ostrom's commons began with neighbours agreeing on rules. The Global Consent asks the same of everyone: "We take responsibility where we stand." That does not require anything heroic. It may be a meeting you call, a neighbour you finally speak to, a group you decide to join.
In 1981, nobody on Daphne Major could have known that one stray bird with an unfamiliar song would start something new. Most beginnings look like that.
Written by a human author, edited with AI assistance.
Further information
Uppsala University — The origin of a new species of finches observed (2017), on the Big Bird lineage (Lamichhaney et al., Science 2017). https://www.uu.se/en/news/2017/2017-11-24-the-origin-of-a-new-species-of-finches-observed
The Linnean Society of London — Icons of Evolution: Why are there so many Darwin's finches? (2024). https://www.linnean.org/news/2024/10/23/icons-of-evolution-why-are-there-so-many-darwins-finches
Knowable Magazine — One fish, two fish, 3,000 fish: the evolution of East Africa's cichlids (2024). https://knowablemagazine.org/content/article/living-world/2024/evolution-of-cichlid-fishes-east-africa
Eawag — Cichlids: watching speciation in real time (2018). https://eawag.ch/en/info/portal/news/news-detail/cichlids-watching-speciation-in-real-time
IUCN Global Invasive Species Database — Lates niloticus (Nile perch) in Lake Victoria. https://www.iucngisd.org/gisd/species.php?sc=89
The Scientist — Miller-Urey amino acids, circa 1953 (2009). https://www.the-scientist.com/miller-urey-amino-acids-circa-1953-44523
Chemistry World — How RNA reveals clues to life's origins on Earth. https://www.chemistryworld.com/features/how-rna-reveals-clues-to-lifes-origins-on-earth/4022833.article
University of Bath — Insight into one of life's earliest ancestors (LUCA), Nature Ecology & Evolution (2024). https://www.bath.ac.uk/announcements/insight-into-one-of-lifes-earliest-ancestors-revealed-in-new-study/
Darwin Correspondence Project — Letter from Charles Darwin to J. D. Hooker, 1 February 1871 ('warm little pond'). https://www.darwinproject.ac.uk/letter/DCP-LETT-7471.xml
Embryo Project Encyclopedia (Arizona State University) — 'On the Origin of Mitosing Cells' (1967), by Lynn Sagan (Margulis). https://embryo.asu.edu/pages/origin-mitosing-cells-1967-lynn-sagan
Nature — Szathmáry and Maynard Smith, The major evolutionary transitions (1995). https://www.nature.com/articles/374227a0
Science — Martin A. Nowak, Five rules for the evolution of cooperation (2006). https://doi.org/10.1126/science.1133755
BBC Sky at Night Magazine — Where are all the aliens? The Fermi Paradox (2020). https://www.skyatnightmagazine.com/space-science/fermi-paradox-search-extra-terrestrial-intelligence
American Museum of Natural History — Impact: The End of the Age of Dinosaurs (2025). https://www.amnh.org/about/press-center/impact-opens
Nature — Ernst Fehr and Urs Fischbacher, The nature of human altruism (2003). https://doi.org/10.1038/nature02043
The Nobel Prize — Elinor Ostrom, Prize in Economic Sciences 2009. https://www.nobelprize.org/prizes/economic-sciences/2009/ostrom/facts/
Encyclopaedia Britannica — Social Darwinism. https://www.britannica.com/topic/social-Darwinism
New England Journal of Medicine / United States Holocaust Memorial Museum — Susan Bachrach, In the Name of Public Health: Nazi Racial Hygiene (2004). https://www.ushmm.org/m/pdfs/07192004-nazi-racial-hygiene-bachrach.pdf
Wikimedia Commons — Darwin's finches by John Gould (1845), public domain. https://commons.wikimedia.org/wiki/File:Darwin%27s_finches_by_Gould.jpg
