|M. lewisii (A), an F1 hybrid (B), M. cardinalis (C), and examples of variation in floral traits found in F2 hybrids (D–L).|
Imagine for a moment a species of flowering plant with a three important genes, each of which has two different alleles. Let's say Gene A which can be A1 or A2, gene B which can be B1 or B2 and gene C which can be C1 or C2. Gene A determining flower shape, gene B determining flower colour and gene C determining how much nectar the flower produces.
Now imagine two different pollinators which transfer pollen from one flower to another while gathering nectar, one having a preference for one shape and colour and the other having a preference for a different colour and more nectar.
It doesn't take a genius to work out what will happen. The flowers with the alleles which attract one pollinator are much more likely to pollinate other flowers with those same alleles and those with the other alleles will be much more likely to pollinate the flowers with these alleles.
In effect, there will be a premating or 'prezygotic' barrier to cross-pollination between plants with different combinations of alleles and selection pressure against permutations of alleles which don't so strongly attract either pollinator. These barriers are the very thing that produce and maintain speciation and cause one species to diverge into two or more daughter species.
Now researchers at the Department of Botany and College of Forest Resources, University of Washington, Seattle, WA, USA have found this exact mechanism to be the cause of speciation to produce two closely-related species of monkeyflower or Mimulus, each of which is pollinated by two different types of pollinator - bees and hummingbirds, respectively.
A paradigm of evolutionary biology is that adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect. Here, we test this hypothesis by investigating the genetic basis of pollinator discrimination in two closely related species of monkeyflowers that differ in their major pollinators. This system provides a unique opportunity to investigate the genetic architecture of adaptation and speciation because floral traits that confer pollinator specificity also contribute to premating reproductive isolation. We asked: (i) What floral traits cause pollinator discrimination among plant species? and (ii) What is the genetic basis of these traits? We examined these questions by using data obtained from a large-scale field experiment where genetic markers were employed to determine the genetic basis of pollinator visitation. Observations of F2 hybrids produced by crossing bee-pollinated Mimulus lewisii with hummingbird-pollinated Mimulus cardinalis revealed that bees preferred large flowers low in anthocyanin and carotenoid pigments, whereas hummingbirds favored nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele that increases nectar production doubled hummingbird visitation. These results suggest that genes of large effect on pollinator preference have contributed to floral evolution and premating reproductive isolation in these monkeyflowers. This work contributes to growing evidence that adaptation and reproductive isolation may often involve major genes.
Because they use different pollinators and so exist as isolated gene pools, even though they can be artificially induced to hybridize, producing a whole array of intermediates, these two species fulfil all the rules for being classified as different species, just like, for example, two closely related finches or butterflies or lizard. This prezygotic barrier to hybridization has produced two distinct species, and with small variations in just a small number of genes. No increase in information; just a change in its meaning.
Now, creationists, for reasons which they never seem to get round to explaining, will often chant their favourite mantra that so-called macroevolution, i.e. evolution leading to different species, is impossible. Perhaps a creationist can explain what exactly is impossible in the mechanism I've just outlines and of which these researchers have found an almost perfect example.
Or perhaps they'll just have to keep chanting their mantra knowing they can't justify it and can't refute science showing it to be wrong.
Thanks to to PZ Myers for bringing this to my attention via his Pharyngula blog.
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