The evolutionary rates of sea urchins are more complex than previously thought, a finding that could apply across the evolutionary tree.
Evolution within groups of organisms was first thought to occur continuously, at a constant rate. Fossil-based analyses soon led to the belief that many groups quickly reach maximum diversity early on in their history, followed by a decline in evolutionary rates as habitat types fill up.
Now, in a detailed analysis of a group of marine invertebrates called echinoids, Dr Melanie Hopkins of the American Museum of Natural History and Museum palaeobiologist Dr Andrew Smith have found a branch of the evolutionary tree that has increased its evolutionary rate over time.
Modern echinoids originated 265 million years ago, just before the Permian-Triassic mass extinction, an event that wiped out around 96% of all marine species. They still exist today as sea urchins and sand dollars.
Despite the abundance of ecological space left behind after the mass extinction, Dr Hopkins and Dr Smith found that echinoids experienced the lowest rates of evolutionary diversification during this early phase. Said Dr Smith of the result:
This slow start is very different from the standard model of high initial rates of diversification followed by a slowing down as ecological space gets filled that we have come to expect.
Bursts of diversity
When they looked in more detail at sub-groups of echinoids through time, they discovered that some that underwent episodes of 'early bursts' in evolution, primarily associated with the adoption of new feeding strategies.
For example, one particular group of echinoids - the sand dollars - evolved a novel method of 'deposit-feeding' that allowed them to filter nutrients from the sand, and this innovation coincided with a marked increase in morphological innovation.
Regular echinoids like the sea urchin (left) have five-fold symmetry and can head in any direction, whereas irregular echinoids, like the sand dollar (right) have two-fold symmetry, with defined 'front' and 'back' ends.
A question of scale
The overall pattern of slowing evolutionary rates punctuated by smaller 'early burst' events within certain subgroups points to the importance of considering scale when assessing the evolutionary history of any group. Said Dr Smith:
Rates of evolution turn out to be quite different when viewed at different scales, and both 'continuous' and 'early burst' patterns of evolution may apply to the same group depending upon how you view them.