Abominable is not the sort of word that most people may associate with flowers, but for Darwin, it was a perfect fit. He saw life on Earth today as the result of millions of years of victories and defeats in the evolutionary arena. Flowering plants, by that reasoning, were among the greatest champions of all. There are some 250,000 known species of flowering plants, and the total is probably double that. The closest living relatives of flowering plants (pine trees, firs, gingkos, and the likecollectively known as gymnosperms) make up a grand total of just over 800. These numbers are all the more remarkable when you consider that flowering plants emerged around 140 million years ago, and gymnosperms had already been on Earth since at least 360 million years ago. In over twice the time, gymnosperms have produced less than 1% of the diversity found in flowering plants. What was the secret of flowers? Darwin wrote to his friend the botanist Joseph Hooker that this question was an abominable mystery.
Darwin assumed that there must be one secret to be found, and since then most botanists have agreed. Perhaps it was their ability to be pollinated by bees and other insects, or perhaps the way animals that ate their fruit could disperse seeds in their droppings. In order to test hypotheses like these, scientists need to figure out how all the species in question are related, to see exactly when the explosion of diversity took place over the course of their evolution. But when youre talking about 250,000 species, thats no easy task.
In recent years, different teams of scientists have compared the genes of different species of flowering plants. Researchers from Florida and England developed mathematical methods to combine 46 of these trees into a supertree. While this tree does not actually have a twig for every species of flower, its 379 branches include every family of species. This is only a step towards the full tree of flowering plants, but its a huge onejust ten years ago scientists thought such a goal was impossible to meet. The researchers were also able to plot the tree against time, estimating the dates at which major new groups of flowers evolved. The scientists present their tree today in the Proceedings of the National Academy of Sciences.
Supertree now in hand, the scientists then tried to solve Darwins abominable mystery. Did flowers explode in diversity at one particular point in their history? Or was there one particular strategy for survival used by some flowers that made them more diverse than other flowers?
None of the above, it appears. Early flowers did undergo a small burst of diversity, but there have been many bursts since then, and they dont seem to follow any one rule. There is no one secret to the success of flowers, but perhaps many small ones waiting to be discovered. Its possible for example that in some cases flowering plants thrived particularly well as the planet has gradually cooled. (Gymnosperms may, by constrast, be locked into a biology better suited to the much warmer climates 200 million years ago.) Darwins abominable mystery has split into many small mysteries, like a pool of mercury breaking into an elusive flock of droplets. But when scientists finally create a species-level tree of flowering plants (an achievement that may take decades), they may finally pinpoint some of the solutions.
I dunno, seems pretty straightforward to me. Small size, fast growth, sexual reproduction: tada, massive diversity. You've got a combination of factors which allow flowers to explore new characteristics pretty quickly and throughly with as little effort as possible. Sexual reproduction increases the effectiveness of natural selection, by creating genetic variation without the downside of harmful mutations (that's the efficiciency part). And the small size and fast growth decreases the cycle time between generations being able to chart new territory diversity wise. The rest is just a matter of the winds and chance blowing flowers into new habitats and new challenges and forcing that diversity engine to do its magic. Am I missing some obvious reason why these things alone shouldn't be enough?
It's a fair hypothesis (except for "small size" and "fast growth"--flowering plants include oak trees and other species that are not exactly small or fast growing). But the big problem is that you're proposing that these features enhance diversity for all flowering plants. If that were true, then you'd expect equal rates of diversification throughout the 140 million year history of this group. And that's not what the supertree reveals, as you can see from the paper. Diversity speeds up from time to time on different parts of the supertree, with no clear feature common to all the bursts. In the end, flowering trees have become incredibly diverse, but for no simple reason.