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Experiments have revealed how single mutations can have huge effects that drive evolution. But small steps pave the way, finds Tanguy Chouard.
Since the origin of evolutionary science, biologists have insisted that adaptation is an achingly slow process. 'Natura non facit saltum' (nature does not take leaps) was a favourite incantation of Charles Darwin. As the combined power of genetic mutation and natural selection became better appreciated in the 1930s and 1940s, theorists solidified a gradualist doctrine: adaptation must rely on innumerable genetic changes, each with effects so small that any attempt to catch them experimentally was considered futile.
Suggestions to the contrary were met with ridicule: geneticist Richard Goldschmidt, in 1940, envisioned subtle developmental mechanisms producing great leaps of adaptation, but his use of the phrase "hopeful monsters" was misrepresented as extreme saltationism (perfection in one jump), and equated with belief in miracles. But through fish in the murky depths of a British Columbia lake and through bacteria in the flasks of a Michigan lab, the monsters have returned. Experimental evidence has shown that individual genetic changes can have vast effects on an organism without dooming it to the evolutionary rubbish heap.
Single-gene changes that confer a large adaptive value do happen: they are not rare, they are not doomed and, when competing with small-effect mutations, they tend to win. But small-effect mutations still matter - a lot. They provide essential fine-tuning and sometimes pave the way for explosive evolution to follow. As the molecular details unfold, theory badly needs to catch up.
"This is a very exciting age," says Joe Thornton, who studies protein evolution at the University of Oregon in Eugene: new molecular approaches are bringing mechanistic understanding to the field of 'evo-devo' (evolutionary developmental biology), ushering in what Thornton calls "the functional synthesis"1. The shift even promises to bridge microevolution and macroevolution, suggesting how, for example, genetic changes - large and small - might eventually lead to a new species.
Marine gladiators
The promise of such insights was what drew developmental biologist David Kingsley from Stanford University in California and thenpostdoc Katie Peichel, to the three-spined stickleback (Gasterosteus aculeatus). To move beyond evo-devo studies that compared gene expression differences between vertebrate species, they decided to identify the genetic changes that...