Gould famously argued that if we “replayed the tape of life” we would get very different outcomes, because the pattern of evolution is unpredictable. In contrast, Conway Morris claims that convergent evolution – the idea that similar conditions produce similar adaptations – is “completely ubiquitous”.
Improbable Destinies focuses on the evidence underlying these opposing positions. However, chatty writing and an unclear structure mean that Losos does not explain the reasons behind Gould’s and Conway Morris’s ideas. Nor does he fully explore how their contrasting world views (Conway Morris is a devout Christian; Gould was a Marxist) influence their thinking.
Losos initially focuses on well-known examples of convergent evolution, such as the tendency of island animals – for instance hippos and mammoths – to become smaller than their continental counterparts. He also describes in some detail a series of experimental studies on lizards and fish that provide support for the centrality of convergent evolution, and thus for Conway Morris’s view.
But in the chapter on Richard Lenski’s ongoing study of bacterial evolution, Losos appears to switch sides. Lenski’s experiment began in 1988 and has, to date, involved nearly 70,000 generations and quadrillions of cells. Initially, the 12 identical lines of bacteria all grew faster and produced larger cells over the generations, so showed convergent evolution. But after around 31,000 generations, one line exhibited a unique adaptation – the ability to feed on citrate. Due to a series of random mutations, this line took a very different evolutionary path from the rest. Lenski’s attempts to encourage other lines to follow suit have failed. “So much for predictability and parallel evolution!” Losos writes.
Losos’s conclusion is that neither Gould nor Conway Morris is right. Faced with similar selection pressures, similar populations will indeed often produce convergent evolutionary outcomes. Even distantly related groups, such as marsupials and placental mammals, may do this – think of the marsupial and placental moles, separated by over 150 million years.
But the process isn’t ubiquitous. Sometimes, stuff happens and evolution goes a little crazy. In New Zealand, there were no terrestrial mammals (bats aside) until humans arrived, but in a striking example of non-convergent evolution, the islands’ birds did not evolve forms resembling mammals elsewhere that have a similar ecological niche and environment.
Alongside the widespread phenomenon of convergent evolution, life produces many unique forms. The human lineage is one such.
But before the reader can conclude that our uniqueness suggests we are the whole point of evolution, Losos plays his trump card: the duck-billed platypus.
This monotreme mammal has hair and a beak, and lays eggs. Like ours, its lineage is unique in the fossil record. Losos concludes that humans are no more the end-point of evolution than is the platypus, with its singular and slightly comical assemblage of characteristics. Not all evolution is convergent, he argues, and uniqueness does not imply destiny. That seems about right.
Matthew Cobb is a zoologist at the University of Manchester, UK.
This article appeared in print under the headline “Playing dice with the animals”.
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