By Matthew Cobb
Some religious folk accept that micro-evolution can be observed – shifts in allele frequency due to natural selection – but argue vociferously that no one has ever seen one species evolve out of another. We know that one reason for this apparent lack of evidence for speciation is due to the time-scales involved. Most speciation of animals takes thousands of years, we think, and we have not yet been able to accumulate enough data.
That doesn’t mean that the creationist argument is right, of course. Just as we know what the life-cycle of a star is without ever having seen their billion-year history unfold before our eyes, so too we know all the essential steps in speciation, because we can observe the various intermediate stages right now, and, in the case of organisms, we can see the intermediate forms in the fossil record.
However, biologists should not cede an inch of ground to the creationists, no matter what brand of sophisticated creationism™ they may propound. As Jerry makes clear in Why Evolution Is True, we can observe speciation directly in front of us, in the shape of allopolyploid evolution in plants. This occurs when a plant doubles up its number of chromosomes due to some error during the formation of the male and female gametes. The result is an organism that cannot sexually reproduce with others of its species. Because plants can reproduce by selfing, this new plant can reproduce with itself and may go on to produce a new species.
The latest example of this well-known phenomenon has just been described in the pages of PhytoKeys, an open access biodiversity journal. Mario Vallejo-Marín of the University of Stirling in Scotland, describes Mimulus peregrinus, a new British species that has recently appeared, apparently by chromosome duplication in a sterile hybrid:
Mimulus plants were introduced into the UK in the 19th century. The two main established species are M. guttatus and M. luteus. M. gutattus has 14 pairs of chromosomes, while M. luteus would appear to have undergone chromosome duplication – it has 30 or 31 pairs. These two species can cross, but their hybrids, which are triploid (ie they have three copies of each chromosome, rather than two) are sterile.
Vallejo-Marín reports that the new species, M. peregrinus has six copies of each chromosome (you can identify the chromosomes by their shape and distinctive banding, and count the amount of DNA in the plant, which shows up as a threefold higher than M. guttatus) – and apparently developed after a chromosome duplication event in a triploid hybrid. These six copies can now pair up quite normally during the creation of the haploid gametes, producing gametes that each have 46 chromosomes (as against 14 in M. guttatus). The new plant, which is known only from the banks of Shortcleuch Waters, Leadhills, South Lanarkshire, has yellow flowers and rather spikey leaves.
It is clearly different from the two other Mimulus species that are already present here, and from their triploid sterile hybrid. These new plants are inter-fertile, and isolated from other closely related species. They form a new species, one that has popped up on the banks of this stream in Scotland:
There is nothing particularly amazing or new about this process – it has been known for decades – but it is striking. Mimulus peregrinus is yet another nail in the creationist coffin, and yet another example of why evolution is true.