Apparently the blog is working again! Huzzah!
A recent paper in Nature Genetics shows that mutations in two genes tranform an annual plant (Arabidopsis thaliana) that normally grows a few centimeters, fruits, and senesces in a matter of weeks into a perennial – long-lived, large, bushy, and even woody! Their findings are remarkable for a couple reasons. First, crop breeders have been attempting for a long time to make perennial varieties of annual plants because they require less fertilizer and reduce soil erosion. From an evolutionary perspective, these data suggest (not for the first time) that major ecological and morphological transitions can occur with relatively small genetic changes. I emphasize that this is merely a suggestion because laboratory mutants are not necessarily good guides for what happens in nature. For example, if a major mutation confers ‘woodiness’ but also causes a plant to be unattractive to pollinators, then any evolutionary paths involving that mutation cannot be driven by natural selection. In contrast, this situation is not necessarily a barrier to breeders, which is why many suspect that derived mutations in crops may not resemble those in natural populations.
Annual and "Perennial" Arabidopsis
Studies like this prompt an unresolved problem in evolutionary genetics: does adaptation generally occur with few genetic changes of large effect or does it take many cumulative small changes? A rarely discussed facet of this debate, as the title suggests, has to do with probability more than it does biology. I’ll use a couple thought experiments to make my point.
Experiment I:
A bunch of populations face a new selective pressure. Suppose there are two mutually exclusive genetic “routes” to the same “destination.” If route A is closer (requires fewer mutations) than route B from the starting point, then all else being equal, there will be more A populations than B populations. In the limit, if B is infinitely long, then we will only see A.
Experiment II:
As before, a bunch of populations face a new selective pressure. This time, there are two routes as before, A and B. However, they are not equal. A is closer to the starting point than B, but B is more fit, such that in competition between A and B, B will always win. Now there is a trade-off. A populations arise more rapidly, but are less permanent as they are replaced by B’s.
Returning to the original research paper. Should we expect that when we sample natural populations that many have taken shorter, but perhaps less fit, routes to a new adaptive peak? Or should we expect that all the more fit populations, once they have arisen, to have outcompeted the less fit ones? I’m not really sure. For one thing, there are many other complications I have omitted which bear upon the question. Finally, I am reminded of Sir Ronald Fisher’s observation that:
Natural selection is a mechanism for generating an exceedingly high degree of improbability.
In the present context, it suggests that the power of natural selection is such that even very improbable (= very long) evolutionary routes will be most frequently observed.
