How do the large-scale patterns we observe in evolution arise? A paper in the journal Evolution argues that many of them are a type of statistical bias caused by our unavoidably recent viewpoint looking back into the past.
For example, the animals appear in the fossil record about 550 million years ago, in an enormous burst of diversification called the “Cambrian Explosion.” Many groups of organisms appear to originate like this, but later on in their evolutionary history, their rates of diversification and morphological change seem to slow down. Graham Budd and Richard Mann make the provocative argument that patterns like this may be largely illusory.
Biologists and palaeontologists use statistical models called “birth-death models” to model how random events of speciation and extinction give rise to patterns of diversity. Just as one can roll a dice five times and get five sixes or none, the outcomes of these random models are very variable. These statistical fluctuations are particularly important at the origin of a group, when there are only a few species. It turns out that the only groups that survive this early period are those that happen to diversify quickly – all the others go extinct. As is it exactly those groups that go on to be the large successful groups we see living today, and that fill most of the fossil record, it follows that they are likely to show this rapid pattern of diversification at their origin – but only because they are a biased subset of all groups. Later in their history, when such groups are diverse, statistical fluctuations have much less effect, and therefore their rate of evolution appears to slow down to the background average.
As a result, the patterns we discover by analyzing such groups are not general features of evolution as a whole, but rather represent a remarkable bias that emerges by only studying groups we already know were successful. (1)
A raging bull coming towards you.
Life and death.
So seemingly similar.
Blood dripping on the ground.
The bull is gone now.
A cold breeze.
Worm crawling on the ground.
Close your eyes.
Looking at the clouds.
So scarily visible…