Tape of life may not always be random

• 16:37 26 January 2015 by Bob Holmes
• For similar stories, visit the Genetics and Evolution Topic Guides

Evolution may have fewer options for adapting to new challenges than you'd think. When terrestrial mammals returned to the ocean to become whales, walruses and manatees, the three lineages sometimes made use of strikingly similar genetic changes.

Natural selection led to the same genetic changes in the walrus, the whale and the manatee (Image: Karen Munro)

Evolutionary biologists have long debated whether rewinding the tape of life and replaying it would give similar results, or whether outcomes depend largely on chance events that push the course of evolution onto radically different tracks.

The two alternatives yield very different views of the history of life on Earth, with some prominent biologists, such as Simon Conway Morris, arguing that human-like, intelligent beings are inevitable products of evolution.

Others, such as palaeontologist Stephen Jay Gould, who popularised the tape of life metaphor, argue that if it were possible to turn back the clock, the history of life would not repeat itself. The world would be unfamiliar, and most likely lack humans.

To test the reproducibility of evolution at the genetic level, an international team took advantage of a natural experiment. Three different groups of terrestrial mammals have at some point in their evolution re-colonised the ocean, giving rise to what we now know as whales, walruses and manatees. Comparing the genetic changes in the three lineages, the researchers reasoned, should reveal whether evolution followed similar or very different paths in each case.

Random idea

They sequenced the genomes of walrus, manatee and two whales – killer whales and bottlenose dolphins. The comparisons showed that many genes changed independently in each lineage, suggesting that randomness did indeed play an important role in their evolution.

But for 15 genes, natural selection led to exactly the same genetic changes occurring in all three lineages. This suggests that for some of the challenges of life in the sea, evolution repeatedly arrived at the same solution – that is, replaying the tape does indeed give much the same result again and again. This is a high-resolution replay of the tape, looking at what would happen to individual lineages, rather than what overall diversity would eventually result, which is what Gould looked at.

The team has not yet shown directly that any of these convergent genetic changes is actually adaptive, though some they found – affecting, for example, the structure of ear bones or metabolism related to deep diving – could plausibly be so.

However, this result may say less about the predictable creativity of evolution than about a paucity of viable options. When the team performed a similar analysis of the genomes of dog, elephant and cow – related mammals that remained on land – they also found a comparable amount of convergence in their mutations, even though those animals share few similarities of lifestyle.

Lack of options

This may imply that the vast majority of mutations are lethal, so that evolution stumbles on the same few viable ones over and over again. "We think it's because there's only so much you can change and still be functional," says Kim Worley, a genome biologist at Baylor College of Medicine in Houston, Texas.

"If you replayed the tape, you'd probably see the same changes again amongst the marine mammals, but if you took a walrus and a camel, you'd still see the same changes, because of these constraints," says Andrew Foote , an evolutionary biologist at the University of Copenhagen.

But David Wake, an evolutionary biologist at the University of California at Berkeley, cautions that the study was essentially a genome-wide fishing expedition to look for interesting patterns. Much more detailed follow-up work will be needed to show whether the team's hypothesis holds up.

"I find it intriguing, but I think the evidentiary basis for it is still pretty weak," says Wake. "But we're just starting out."http://www.newscientist.com/article/dn26857-tape-of-life-may-not-always-be-random.html

Journal reference: Nature Genetics, DOI: 10.1038/ng.3198