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In the tree of life, we often envision evolution working like a patient gardener, pruning species that don't quite fit, bit by bit. But that's not how extinction works in practice. Throughout our planet's history, mass extinction has occurred five times — most recently 65 million years ago, when the dinosaurs finally died out — taking out vast amounts of life all at once, usually due to a catastrophic and sudden climatic change.

But it turns out that even during the relatively peaceful eras between global calamities, during what is known as background extinction, whole families of species can disappear, pushed out of existence together. And it's not random. According to a new study published in the August 7 issue of Science, vulnerability to extinction runs in families, meaning that some groups of species have a higher likelihood of becoming extinct than others. "It turns out that some branches of the tree of life are more extinction-prone than others," says Kaustuv Roy, a biology professor at the University of California, San Diego. "Those traits aren't just a part of extinctions that human beings cause, but a general feature of extinction itself."

Roy and his colleagues at the Smithsonian Institution and the University of Chicago examined 200 million years of history of marine clams, oysters and mussels; they picked the simple bivalves because they have a long and detailed fossil record. Going back to the Jurassic period, researchers analyzed when each genus — a taxonomic category just above species — disappeared, and whether relatives vanished at the same time. On average they found that closely related groups of clams went extinct together at a rate that was more often than expected by blind chance — generally those groups of species were confined to a fairly small geographic area. "Extinctions tend to be clustered, which means the effects tend to be worse than what you might expect from random," says Roy. "That's true for mass extinctions as well — they end up culling the most vulnerable lineages, leaving the more resistant ones."

This matters because if extinction were truly random, we'd have a much richer evolutionary history, because at least some representatives of all living things would make it to the present. But because extinction tends to be clumped around certain lineages, when extinction occurs we lose whole groups of species. "The long-term consequences are therefore much worse for biodiversity," says Roy.

The Science study will provide needed ammunition for modern-day conservationists as well. We're in the middle of what some scientists have begun to call the sixth great extinction event, this one caused almost entirely by human beings. Human expansion, hunting, deforestation and ultimately climate change are eliminating species at a rate up to 1,000 times higher than the evolutionary norm. Species like the Yangtze River dolphin and the golden toad have disappeared, while a range of animals — from the Sumatran tiger to the silky Sifaka lemur of Madagascar are on the brink.

Since extinction tends to target groups of vulnerable species, conservationists would be smart to identify and focus their efforts on the most susceptible families. That means species that have a narrow geographic spread — always a risk factor for extinction, in case something happens to their habitat — and, interestingly, large body size, which also tends to be associated with extinction. "It's a quick and dirty way to get a better picture of which species are likely to be most impacted," says Roy. "Then you can go in and mark your priorities." Extinction may be a part of life, but as the dominant species, we have the ability to influence it — for better or for worse.

• Bryan Walsh
• A new study finds that the risk for extinction isn't random — it tends to run in evolutionary families, wiping out groups of species at a time