Quality, not quantity, leads to speciation

In 1859, Charles Darwin published a book that changed how biologists understand the living world. Although his book was titled “On the Origin of Species by Means of Natural Selection”, it never actually explained the process of speciation. Although an elegant and thorough explanation of the mechanisms underlying evolution, he extrapolates this to say “species have changed, and are still slowly changing by the preservation and accumulation of successive slight favorable variations”. In other words, natural selection produces change, and the accumulation of enough changes produces a new species. A century and a half later, this is still accepted wisdom.

For most Biologists (and scientists in general), accepted wisdom is not enough. Ernst Mayr put forth the notion of allopatric speciation – that physical isolation can allow genetic change to occur separately in two populations, eventually leading to reproductive isolation and thus to speciation. Steven Gould and Niles Eldridge produced the idea of punctuated equilibrium – that the history of life consists of long periods of stasis (ie species remain relatively unchanged) punctuated by rapid bursts of species diversification.

Although many viewed speciation and macroevolution as the rapid accumulation of many individual variations, effectively speeding up the process, others (Goldschmidt, Gould) proposed that these events occurred through single (or a small number of) events that produced significant morphological or physiological change. The technical term for this is saltation (meaning a jump), though Goldschmidt’s term “hopeful monster” is also used – not always kindly.

In the early 80’s, Homeobox genes were discovered. These are genes that regulate body pattern, and it was discovered that the duplication, relocation, and modification of these genes is responsible for significant changes in body plan. Suddenly there were genes that could produce “hopeful monsters” without significant genetic change. And yet, the gradualistic model of speciation through accumulation of many small changes has persisted, primarily because there has been no evidence that a single or small number of changes can produce enough change to create a new species.

Recently, Dr. Mark Pagel at the University of Reading decided to put to the test. He reasoned that if speciation is dependent on the accumulation of a number of genetic changes, that would show up statistically as a normal distribution in a survey of the number of genetic differences between species in the family trees of different groups.

What he found was that the distribution did not follow a normal curve, but an exponential (poisson) distribution instead. This is the distribution one finds with truly random distributions, such as the frequency of lightning strikes. The differences between these two random distributions are subtle but significant. The most important difference is that a normal distribution has a mode – a peak value around which the values are distributed. A poisson (exponential) distribution does not – any value is equally likely.

The implication for evolution is that there is no typical, usual, or expected number of mutations required for a population to become a separate species from its parent population. In other words, We can’t say that a new species arises after about 20 (or 60 or 2000) mutations. It may take any number of mutations to form a new species – 50, 100, 1000. Or just one.

At first this seems to fly in the face of the “accepted wisdom” that there is some sort of threshold for speciation. But given that mutation is random, any number of mutations could occur that produce little or no effect, or that do produce an effect but in genes that are unimportant. For speciation to occur, a change must arise that leads to a reproductive barrier. What this study tells us is that a sufficiently significant change is as likely to occur from a single mutation as from 10, 30, or 100.

So while the accepted wisdom – or at least the default assumption – is that speciation arises as the result of an accumulation of minor changes, it now looks more like a single significant change is responsible for a speciation event. The implication being that the quality of mutation, not quantity, is the deciding factor for speciation.

Although in retrospect this makes perfect sense, it does provide additional context for models of evolutionary change – such as punctuated equilibrium and Goldschmidt’s “hopeful monsters” – that were ridiculed when first proposed. Which just goes to show that Hamlet was right: There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.

http://www.nature.com/nature/journal/v463/n7279/full/nature08630.html

http://www.newscientist.com/article/mg20527511.400-accidental-origins-where-species-come-from.html?full=true

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