18 July 2008

Specialization: Not always a dead end


Ruellia sp. - probably hummingbird specialized.
Photo by Tim Waters.
ResearchBlogging.org One of the big questions in evolutionary biology is about reversibility. That is, once an organism evolves down a path of adaptation to a particular climate or biological community, how easy is it for natural selection to make a U-turn and go back to a less specialized state? Many evolutionary changes are probably irreversible - an idea that was classically expressed in "Dollo's Law": "An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors." But many evolutionary changes may not be irreversible - and it's not always easy to predict which ones those are.


Ruellia brittoniana - probably bee-pollinated.
Photo by petrichor.
A new study in this month's issue of Evolution aims to answer this question [$-a] for a group of flowers in the genus Ruellia. The authors, Tripp and Manos, use a phylogeny to reconstruct the evolutionary history of pollination syndromes, groups of floral traits like color, nectar tube length, scent, &c, that are associated with pollination by different groups of animals.

For instance, bright red flowers with longish, narrow nectar tubes, not much scent, and large volumes of dilute nectar (like the Ruellia species in the upper figure), are almost always pollinated by hummingbirds; blue flowers with short nectar tubes, stronger scent, and small volumes of concentrated nectar (like Ruellia brittoniana in the lower figure) tend to be pollinated by bees or other insects. Other Ruellia species are pollinated by hawkmoths (white flowers, very long nectar tubes) or bats (yellow flowers, short nectar tubes, lots of dilute nectar, strong scent). Generally, syndromes associated with a single, small group of pollinators (hummingbirds, hawkmoths, or bats) are considered "specialized", while syndromes associated with many more different pollinators (bees and insects) are not.

With a phylogeny of the genus Ruellia, Tripp and Manos use the pollination syndromes of currently existing Ruellia species to estimate what pollination syndromes their ancestors may have had. Then they determine how common transitions between pollination syndromes have been in the history of Ruellia, and whether any pollination syndromes are "dead ends" - that is, whether Ruellia species that evolve to specialize on, say, hummingbird pollination are "stuck" that way.

Surprisingly, Tripp and Manos found that some specialized pollination syndromes are dead ends, but one, the hummingbird syndrome, isn't. Hawkmoth- and bat-pollinated species tended to have evolved from ancestors with the bee/insect syndrome, and they seem to be "stuck" once they get there. But in several cases, hummingbird-specialized ancestors have given rise to bee/insect-pollinated species. This has never been seen before in other, similar groups of plants. Hummingbirds are generally thought to be more efficient pollinators than bees, so while it makes sense for flowers to evolve from using bees to using birds, it's not clear how natural selection would work in the opposite direction.

Reference
Tripp, E.A., Manos, P.S. (2008). Is Floral Specialization an Evolutionary Dead-End? Pollination System Transitions in Ruellia (Acanthaceae). Evolution, 62(7), 1712-1737. DOI: 10.1111/j.1558-5646.2008.00398.x

2 comments:

  1. Hi again. This article made me think of a question I've been pondering for awhile. How does evolution explain dinosaurs? I know I probably learned this in 9th grade, but I've forgotten. Specifically what I'm curious about is since dinosaurs became extinct how did the evolutionary process continue in the few species that were left multiply to the millions of animals and plants that exist today?

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  2. Krista -
    That's a good and interesting question. If I understand correctly, you're wondering how diversity "re-generates" after natural selection, or a big, random extinction event, prunes it away.

    The answer is, simply, mutation. There are really two parts to evolution as Darwin described it: mutation, then natural selection. Mutation makes offspring different from their parents - sometimes in useful ways, sometimes in unhelpful ways, mostly in ways that don't make any difference at all. But if the organisms are variable, then they may have variable reproductive success - and then natural selection can get to work. If variable traits make a difference in how many children you have, and if they are passed on from parents to children (if they're "heritable"), then traits that increase the number of children you can have will eventually beat out traits that don't.

    Without mutation, natural selection would just winnow out everything but the very best individuals, and then it would stop, because everybody would be identical, and there would be no difference in their reproductive success. But because mutations accumulate over time, living things can evolve diversity even after a huge asteroid strike wipes many of them out.

    Hope that makes sense!

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