Accidental origins: Where species come from
10 March 2010 by Bob Holmes
Magazine issue 2751.
So, at least, says Mark Pagel, an evolutionary biologist at the University of Reading, UK. If his controversial claim proves correct, then the broad canvas of life - the profusion of beetles and rodents, the dearth of primates, and so on - may have less to do with the guiding hand of natural selection and more to do with evolutionary accident-proneness.
Of course, there is no question that natural selection plays a key role in evolution. [SIC ULTRA PLUS] Darwin made a convincing case a century and a half ago in On the Origin of Species, and countless subsequent studies support his ideas [???]. But there is an irony in Darwin's choice of title: his book did not explore what actually triggers the formation of a new species. Others have since grappled with the problem of how one species becomes two, and with the benefit of genetic insight, which Darwin lacked, you might think they would have cracked it. Not so. Speciation still remains one of the biggest mysteries in evolutionary biology.
Even defining terms is not straightforward. Most biologists see a species as a group of organisms that can breed among themselves but not with other groups. There are plenty of exceptions to that definition - as with almost everything in biology - but it works pretty well most of the time. In particular, it focuses attention on an important feature of speciation: for one species to become two, some subset of the original species must become unable to reproduce with its fellows.
How this happens is the real point of contention. By the middle of the 20th century, biologists had worked out that reproductive isolation sometimes occurs after a few organisms are carried to newly formed lakes or far-off islands. Other speciation events seem to result from major changes in chromosomes, which suddenly leave some individuals unable to mate successfully with their neighbours.
It seems unlikely, though, that such drastic changes alone can account for all or even most new species, and that's where natural selection comes in. Species exist as more or less separate populations in different areas, and the idea here is that two populations may gradually drift apart, like old friends who no longer take the time to talk, as each adapts to a different set of local conditions. "I think the unexamined view that most people have of speciation is this gradual accumulation by natural selection of a whole lot of changes, until you get a group of individuals that can no longer mate with their old population," says Pagel.
Until now, no one had found a way to test whether this hunch really does account for the bulk of speciation events, but more than a decade ago Pagel came up with an idea of how to solve this problem. If new species are the sum of a large number of small changes, he reasoned, then this should leave a telltale statistical footprint in their evolutionary lineage.
Whenever a large number of small factors combine to produce an outcome - whether it be a combination of nature and nurture determining an individual's height, economic forces setting stock prices, or the vagaries of weather dictating daily temperatures - a big enough sample of such outcomes tends to produce the familiar bell-shaped curve that statisticians call a normal distribution. For example, people's height varies widely, but most heights are clustered around the middle values. So, if speciation is the result of many small evolutionary changes, Pagel realised, then the time interval between successive speciation events - that is, the length of each branch in an evolutionary tree - should also fit a bell-shaped distribution (see diagram). That insight, straightforward as it was, ran into a roadblock, however: there simply weren't enough good evolutionary trees to get an accurate statistical measure of the branch lengths. So Pagel filed his idea away and got on with other things.
Then, a few years ago, he realised that reliable trees had suddenly become abundant, thanks to cheap and speedy DNA sequencing technology. "For the first time, we have a large tranche of really good phylogenetic trees to test the idea," he says. So he and his colleagues Chris Venditti and Andrew Meade rolled up their sleeves and got stuck in.
The team gleaned more than 130 DNA-based evolutionary trees from the published literature, ranging widely across plants, animals and fungi. After winnowing the list to exclude those of questionable accuracy, they ended up with a list of 101 trees, including various cats, bumblebees, hawks, roses and the like.
To their surprise, neither of these curves fitted the data. The lognormal was best in only 8 per cent of cases, and the normal distribution failed resoundingly, providing the best explanation for not a single evolutionary tree. Instead, Pagel's team found that in 78 per cent of the trees, the best fit for the branch length distribution was another familiar curve, known as the exponential distribution (Nature, DOI: 10.1038/nature08630).
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Read more here/Leia mais aqui: New Scientist
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Phylogenies reveal new interpretation of speciation and the Red Queen
Chris Venditti1, Andrew Meade1 & Mark Pagel1,2
School of Biological Sciences, University of Reading, Reading, Berkshire, RG6 6BX, UK
Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501, USA
Correspondence to: Mark Pagel1,2 Correspondence and requests for materials should be addressed to M.P. (Email: m.pagel@reading.ac.uk).
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Abstract
The Red Queen1 describes a view of nature in which species continually evolve but do not become better adapted. It is one of the more distinctive metaphors of evolutionary biology, but no test of its claim that speciation occurs at a constant rate2 has ever been made against competing models that can predict virtually identical outcomes, nor has any mechanism been proposed that could cause the constant-rate phenomenon. Here we use 101 phylogenies of animal, plant and fungal taxa to test the constant-rate claim against four competing models. Phylogenetic branch lengths record the amount of time or evolutionary change between successive events of speciation. The models predict the distribution of these lengths by specifying how factors combine to bring about speciation, or by describing how rates of speciation vary throughout a tree. We find that the hypotheses that speciation follows the accumulation of many small events that act either multiplicatively or additively found support in 8% and none of the trees, respectively. A further 8% of trees hinted that the probability of speciation changes according to the amount of divergence from the ancestral species, and 6% suggested speciation rates vary among taxa. By comparison, 78% of the trees fit the simplest model in which new species emerge from single events, each rare but individually sufficient to cause speciation. This model predicts a constant rate of speciation, and provides a new interpretation of the Red Queen: the metaphor of species losing a race against a deteriorating environment is replaced by a view linking speciation to rare stochastic events that cause reproductive isolation. Attempts to understand species-radiations3 or why some groups have more or fewer species should look to the size of the catalogue of potential causes of speciation shared by a group of closely related organisms rather than to how those causes combine.
School of Biological Sciences, University of Reading, Reading, Berkshire, RG6 6BX, UK
Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501, USA
Correspondence to: Mark Pagel1,2 Correspondence and requests for materials should be addressed to M.P. (Email: m.pagel@reading.ac.uk).
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Os itálicos são deste blogger cético da onipotência, onisciência e onipresença de quaisquer mecanismos evolutivos, seja a seleção natural e toda a seleção de mecanismo evolutivos de A a Z explicarem a origem e a evolução da diversidade das coisas bióticas,
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Os itálicos são deste blogger cético da onipotência, onisciência e onipresença de quaisquer mecanismos evolutivos, seja a seleção natural e toda a seleção de mecanismo evolutivos de A a Z explicarem a origem e a evolução da diversidade das coisas bióticas,
