Thursday, October 3, 2013

Speciation in plants

On his Sandwalk blog, biochemist Larry Moran recently posted about the many different definitions of the term evolution. Very interesting topic but I see no reason to duplicate his efforts on this much less frequently visited blog. What got me thinking instead was a somewhat incoherent creationist who immediately jumped into the comment thread to declare
Evolution is not a fact or a theory in explaining changes in populations beyond reproducing types at any one point in history. Show where a population changed so that it could no longer reproduce with its parent population??
So he denies the possibility of speciation. Would be laughable in face of all the evidence we have, but the problem is of course that somebody like that will never be satisfied with the kind of evidence a biologist can actually produce. Speciation rarely happens over a human lifespan (although sometimes it can, see below), and so the retort will always be, "were you there?", or to let the creationist speak,
Of coarse extrapolating from these is not scientific evidence for a history of biology. Even if true history.
That is a fairly myopic stance, entirely comparable to doubting that the Roman Empire existed because nobody who is currently alive has seen it and historical documents and archeological artifacts do not constitute direct evidence. I think Richard Dawkins used the same analogy in a recent book, actually.

No, the standard of evidence for processes that take hundreds of thousands of years to happen cannot possibly be "have you seen it with your own eyes?" But just as with the Roman empire or with a murder trial lacking direct witnesses, we can infer events beyond reasonable doubt. The nested structure of biological diversity is our friend here - we can produce phylogenetic trees to find out which species are most closely related, and sometimes we find two species that are genetically so extremely close together that we can infer what may have happened to make them different species, perhaps as little time ago as before the last ice age. In other cases, we find groups in the incipient stages of speciation, somewhere uncomfortably between being able to interbreed and not being able to interbreed.

Anyway, this got me thinking. The evolution/creationism controversy is usually fought over animals, whereas plants are usually cruelly neglected. So what is the state of evidence in plants? I am not talking about a formal review here, but merely a short list of spectacular cases, well understood mechanisms of speciation, and some well researched examples. Here goes.

Polyploid speciaton

This is the easiest answer to "have you seen it" because yes, in some cases we did. In contrast to us, plants (and some animals also) can undergo a duplication of the genome, from the normal diploid stage of having two sets of chromosomes to a tetraploid stage of having four. This is generally a random occurrence but can also be induced for breeding purposes by treating plant cells with colchicine.

The result is that the diploids and the tetraploids will have severe difficultly interbreeding because they would produce triploid offspring, i.e. their children would have three sets of chromosomes. And three sets do not make happy meiotic pairings when said offspring tries to produce its own spores (or eggs, pollen, sperm, as it may be). In other words, because triploids are highly sterile, a tetraploid gains instant genetic isolation from its diploid progenitor.

Another cool thing about polyploid speciation is that it can not only happen within one species but also facilitate hybrid speciation. If two species that cannot normally interbreed form a sterile hybrid, its sterility is often due to the fact that the two chromosome sets inherited by the hybrid are too different to form pairs in meiosis. But if the hybrid becomes polyploid... bingo! Suddenly there are four sets, two times two, and they can once more find partners for meiosis. A new species is born!

Estimates of the frequency of polyploid speciation are as high as 15% for flowering plants and 31% for ferns. What is more, we have very good evidence for the formation of new polyploid species in very recent times. Perhaps among the most spectacular examples are the daisy species Senecio cambrensis and S. eboracensis, which can be assumed to have arisen only in the 20th century, as well as Tragopogon mirus and T. miscellus, which are only slightly older.

For an example closer to everyday experience, consider the true mints. The well known spearmint (Mentha spicata) is a tetraploid hybridogenic species that arose in human cultivation, and thus in historical times, from applemint (M. suaveolens) and horsemint (M. longifolia).

Pollinator shifts

One fairly easy process facilitating speciation in flowering plants that is unavailable to other organisms is a shift in the pollination syndrome. Imagine a plant species that is pollinated by bees, and some isolated small population evolves to become more attractive to a different pollinator, perhaps through something as simple as a point mutation in an enzyme with a role in producing floral scent or flower color. Although some rare gene flow might still happen, it would now be severely restricted, allowing the population to evolve into a separate species by accumulating more differences.

In deceptive orchids such as the genus Ophrys this is presumably an important mechanism. The plants rely on cheating male insects into copulating with the flowers. Because they attract the male insects by mimicking the pheromones of the females, a slight mutation in the scent producing pathway will immediately make a flower attractive to a different insect species, leading to an instant pollinator shift.

Alternatively, a few seeds may end up where the type of insect that is usually pollinating a species does not happen to exist. In this scenario, the plants will find themselves under severe selection pressure to become attractive to a different pollinator, and will thus, over time, evolve to be more attractive to one that is available, perhaps flies or butterflies. When they meet their progenitor population again a few generations later, the different pollinators may keep them isolated enough to finish the speciation process.

Apart from the aforementioned deceptive orchids, the columbines (Aquilegia) and monkeyflowers (Mimulus) are classic model systems for the study of speciation through pollinator shifts.

Strong local selection, avoidance of genes from elsewhere

The idea here is that one population of a wide ranging species invades a new habitat with very different selection pressures, perhaps higher drought stress. Luckily, the local plants soon evolve an adaptation conferring drought tolerance that allows them to flourish, but sadly the large progenitor population constantly "pollutes" them with its non-drought-tolerant genes. Because offspring from crosses between the well-adapted locals and the progenitor population are less fit than pure locals, there is now a strong selective pressure to exclude the pollen from the progenitor population. This can drive the locals into evolving an isolating mechanism that makes them a new species.

I am less well informed about these scenarios but I know that the sunflower genus Helianthus provides a good and currently intensively studied case in North America.

Just your bog standard isolation by distance

And then of course there is good old allopatric speciation, meaning that two populations of the same species become isolated for long enough to evolve into different species. Seriously, the evidence for that having happened fills entire libraries, although in contrast to some of the cases above it is generally the kind of evidence that the dogmatic can insulate themselves against with the myopic "have you seen it" retort.

The answer, in turn, can only be something on the lines of what Darwin himself did in the Origin, where he constantly explains some facts of character distribution or biogeography and then rhetorically asks the reader, does this make more sense under the assumption of special creation or under the assumption of common descent? And just like there is no good explanation under special creation why otherwise very similar species fill different niches on islands, the patterns of distribution, genetics and morphology that we can observe would convince every reasonable observer that recent speciation events are the best explanation. Just to wave in the general direction of one recent publication: Androsace (Primulaceae).

And then, of course, there are island floras, where not only did one seed produce a new species after becoming geographically isolated from the mainland progenitor but it actually produced numerous new species in a quick adaptive radiation. Some of the most amazing examples come once more from the daisies which have already featured strongly in this post. The Hawaiian silver-swords as well as the Maccaronesian shrub-marguerites and sow-thistles are all genetically less and ecologically more diverse than the mainland lineages, which only makes sense under the assumption that they underwent an adaptive radiation into several new species after being dispersed onto their respective islands.

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