From the BBC:
Whole "chunks of life" are lost in extinction events, as related species vanish together, say scientists.
A study in the journal Science shows that extinctions tend to "cluster" on evolutionary lineages - wiping out species with a common ancestor.
The finding is based on an examination of past extinctions, but could help current conservation efforts.
Researchers say that this phenomenon can result in the loss of an entire branch of the "tree of life".
The message for modern conservation, say the authors, is that some groups are more vulnerable to extinction than others, and the focus should be on the lineages most at risk.
Lead researcher Kaustuv Roy, a biologist from the University of California, San Diego, focused on marine bivalves - including clams, oysters and mussels. The fossil record for these creatures dates back almost 200 million years.
By tracing this documented timeline of evolution and extinction, the team was able to see the effects of "background extinctions" as well as the mass extinctions, such as the one around 65 million years ago during which the dinosaurs finally died out.
Many species have become extinct during the relatively stable periods between those global calamities.
But even during such quiet periods, the team found that extinctions tended to cluster into evolutionary families - with closely-related species of clams vanishing together more often than would be predicted by chance.
Richard Grenyer, a biologist from Imperial College London, who was not involved in the study, told BBC News that by going "way back into the fossil record" this study provided important evidence of the patterns of extinction.
"Big groups of organisms tend to be similar to one another," he explained. "Look at the large cats for example."
But genetic similarities also mean, said Dr Grenyer, that "a bad effect that affects one of them, will likely affect all of them".
"It's like a casino of extinctions, with the odds rigged against certain groups."...