Still evolving

Evolution did not stop

The chain did not end with you. Every generation, some inherited variants become a little more common and others a little rarer. Usually this is too slow to see. Sometimes it is fast enough to measure in a human lifetime.

What “evolving” means here

A population evolves when inherited variants change in how common they are from one generation to the next. Individuals do not evolve during their lives; populations do, over generations. Evolving does not mean becoming better or more advanced, only better suited to the conditions of the moment, and sometimes not even that.

How fast?

Each dot is one of the cases below, placed by roughly how many years the change took. The scale jumps tenfold at each step. Hollow dots are very uncertain. The finches and the elephants changed within about one generation; the human cases took hundreds of generations.

5 Finch beaks after a droughtOne generation
4 Elephants without tusksAbout 25 years
6 Lizards on a new islandAbout 35 years
7 Toads that spread fasterAbout 70 years
8 Clover in the cityDecades to centuries
3 Divers with bigger spleensUnknown; they have lived from the sea for at least 1,000 years
1 Drinking milk as an adultWithin the last 7,500 years
2 Breathing thin airPerhaps about 9,000 years, very uncertain
1 year10 years100 years1,000 years10,000 years

In people In nature

In people

1 Drinking milk as an adult

Where: Europe, East Africa, the Middle East
How long: Within the last 7,500 years

Like other mammals, most people stop making lactase, the enzyme that digests milk sugar, after childhood. In some people it stays switched on. Variants that keep it on arose several times, separately, in herding peoples, and spread after people began keeping cattle and goats. Today roughly a third of adults worldwide can digest milk easily, with large differences between regions.

How we know

Ancient DNA shows that the most common European variant was still rare about 5,000 years ago and became common only later. Different variants do the same job in East Africa and the Middle East.

Careful: Why the variants spread so fast is debated. People drank milk long before the variants were common. One idea is that during famines and epidemics, milk made people without the variant ill at the worst possible time, so carriers survived more often.

Photo: Luo Shaoyang from Beijing, China · CC BY 2.0

2 Breathing thin air

Where: The Tibetan Plateau
How long: Perhaps about 9,000 years, very uncertain

At 4,000 metres, each breath holds about 40% less oxygen than at sea level. Everyone who goes that high makes extra red blood cells, and in some lowlanders this goes too far and thickens the blood. Most Tibetans carry a version of the gene EPAS1 that keeps this reaction in check.

How we know

This version is almost absent in nearby lowland Han Chinese. Its DNA matches that of the Denisovans, an extinct group of humans, so it probably entered the ancestors of Tibetans when the two met, tens of thousands of years ago. One estimate is that it began to spread about 9,000 years ago, but the range is wide.

Careful: People in the Andes adapted to altitude in other ways. Evolution can reach a similar result by different routes.

3 Divers with bigger spleens

Where: The Bajau of Southeast Asia
How long: Unknown; they have lived from the sea for at least 1,000 years

The Bajau have lived from the sea for at least a thousand years, and some dive for hours a day on a single breath at a time. During a dive, the spleen squeezes extra oxygen-rich blood into the body. Bajau have spleens about 50% larger than their land-based neighbours, even those who do not dive.

How we know

Divers and non-divers among the Bajau both had larger spleens, which points to inheritance rather than training. A gene variant linked to spleen size is common among them.

Careful: This is one study of fewer than a hundred people, and it has not yet been repeated.

In nature

4 Elephants without tusks

Where: Gorongosa National Park, Mozambique
How long: About 25 years

During the civil war from 1977 to 1992, about 90% of the park’s elephants were killed, many for their ivory. Females born without tusks were more likely to survive. Among the females that lived through the war, about half had no tusks, against about one in five before. Among their daughters born afterwards, about a third are tuskless.

How we know

Researchers found the trait follows the X chromosome and linked it to genes that shape teeth. They think the same variant is fatal to males, which would explain why tuskless males are almost never seen.

Careful: This change was driven by ivory hunting during the war. Tusks matter to elephants: they use them to dig and to strip bark, so the change has a cost.

5 Finch beaks after a drought

Where: Daphne Major, Galápagos
How long: One generation

In the drought of 1977, small soft seeds ran out on a tiny island. Medium ground finches with deeper beaks could crack the large, hard seeds that were left, and more of them survived. Their offspring grew beaks about 4% deeper, on average, than the generation before.

How we know

Peter and Rosemary Grant measured almost every bird on the island, year after year, for four decades, and could follow parents and offspring.

Careful: The direction reversed in later years, when wet years favoured smaller beaks. Evolution follows conditions; it has no goal.

Try it yourself in the Evolution Lab →

6 Lizards on a new island

Where: Pod Mrčaru, Croatia
How long: About 35 years

In 1971, researchers moved five pairs of Italian wall lizards to a small island with few insects but many plants. When scientists returned in 2004 to 2006, plants made up a third to more than half of what the lizards ate, against a few percent on their home island. They had bigger heads, a stronger bite, and small valves in their gut, not found in other populations of this species, which slow down food so it can be digested.

How we know

The mitochondrial DNA of the island lizards is indistinguishable from that of the population they were taken from.

Careful: Part of the change may come from how each lizard grew up on that diet rather than from changes in genes. Which part is which is not fully known.

Photo: John Robert McPherson · CC BY-SA 4.0

7 Toads that spread faster

Where: Northern Australia
How long: About 70 years

Cane toads were brought to Australia in 1935 to eat beetles in sugar-cane fields, and spread across the country. Toads at the front of the invasion have longer legs and travel farther each night than those in older populations. The front once advanced about 10 kilometres a year; now it moves more than 50.

How we know

Researchers tracked toads with radio tags and measured their legs across the range, from the oldest populations to the newest.

Careful: This happens partly because the fastest toads end up together at the front and breed with each other, not only because faster toads survive better.

8 Clover in the city

Where: 160 cities in 26 countries
How long: Decades to centuries

White clover can make cyanide to deter animals that eat it. In about half of the cities studied, plants in the centre make cyanide less often than those in the surrounding countryside. The same change has happened again and again, separately, in different cities.

How we know

Over 280 researchers collected about 110,000 plants along city-to-country lines and tested each one for cyanide. In 26 cities they also read the genomes of about 2,000 plants.

Careful: Why cities favour clover without cyanide is not the same everywhere. Fewer grazing animals and different temperatures are both possible reasons.

Has medicine stopped human evolution?

No. Every child is born with around 70 new changes in their DNA that neither parent had. These new variants are the raw material; they are not selection in themselves.
Some people still have more children than others, for many reasons, and studies link part of that to inherited variants. Selection has changed, not stopped.
In evolution, “fitness” only means how many children someone has. It says nothing about a person’s health, talents or worth.
Chance also shifts how common variants are, especially in small groups.
What has changed is what helps or harms. Medicine, food and cities are now part of our environment, just as the drought was for the finches.

Your ancestors were never finished products, and neither are you. You are one link in a chain that is still growing.

Evolution did not stop

What “evolving” means here

How fast?