11 July 2009

Correlation and causation: Why are there so many flowering plants?

ResearchBlogging.orgAmong the flowering plants, groups with flowers adapted to a narrower range of pollinators -- the more specialized ones, like orchids or mints -- tend to contain more species. Why? The classic hypothesis is that coevolution between plants and their pollinators leads to more pollinator-specialized plants, which are then more likely to become reproductively isolated, and eventually form separate species. However, I've just finished reading a review article that suggests an interesting alternative: that angiosperms may not be diverse because they're specialized, but specialize because they're diverse [$-a].

The review's authors, Armbruster and Muchhala, first lay out a list of possible mechanisms connecting diversity and specialization. Three of them have specialization creating diversity, by (1) creating reproductive isolation, (2) enhancing isolation created by other forces, or (3) reducing extinction rates. Finally, there's the possibility that diversity creates specialization, by (4) essentially forcing plants to divvy up the available pollinator community more and more finely.

Collinsia heterophylla, a
member of a plant genus
probably shaped by competition.

Photo by Ken-Ichi.
The first two mechanisms are, as far as I'm concerned, contained within the classic specialization-creates-diversity hypothesis classically advanced by Verne Grant, that increased floral specialization makes it easier to form new species [$-a]. The third is a bit odd -- generally, ecologists think that increased specialization means an increased, not a decreased, risk of extinction [$-a]. It's intuitive that if you rely on fewer pollinator species, you can afford to lose fewer of them, and you have fewer opportunities to colonize new sites; so on the one hand, you're at greater risk of local extinction, and on the other, you have difficulty establishing new populations. However, as Armbruster and Muchhala point out, this process should make more-specialized plant groups less diverse, which is the opposite of what we see.

The fourth hypothesis, that competition for pollinators causes greater to create greater specialization, leads to predictions that nicely differentiate it from the classic hypothesis: that hybridization between related flowering plants should be rare, and that plants should rarely occur in the same community as their closest evolutionary relatives. The first is important because it gives a reason to specialize on one or a few available pollinators -- if a plant can't reproduce with nearby relatives, all the pollen it exchanges with them represents wasted effort, and may actually interfere with pollen transfer from members of its own species. The second is a consequence of that process; plants are most likely to be able to hybridize with their evolutionary sisters, so successful speciation will usually require geographic or ecological isolation.

The authors then evaluate the evidence for these predictions in four plant genera with which they have prior experience: Dalechampia, Collinsia (pictured above), Burmeistera, and Stylidium. For these four groups, they find good support for the diversity-causes-specialization hypothesis -- few natural, or even artificial hybrids, and few co-occurring sister species. To some degree, then, the new hypothesis is an effect of a researcher's favorite study systems influencing their perspective on the broader picture of evolution. Armbruster and Muchhala give the same treatment to orchids, and find that for the most diverse angiosperm family, natural hybrids and co-occuring sister species are not rare. This ambiguity makes the review more interesting -- it overturns the causation commonly inferred from the correlation between diversity and specialization, but it doesn't make the mistake of sweepingly assuming the opposite instead.

Correlation, and causation.


Armbruster, W., & Muchhala, N. (2008). Associations between floral specialization and species diversity: Cause, effect, or correlation? Evolutionary Ecology, 23 (1), 159-79 DOI: 10.1007/s10682-008-9259-z

V. Grant (1949). Pollination systems as isolating mechanisms in angiosperms. Evolution, 3, 82-97

Johnson, S.D., & Steiner, K.E. (2000). Generalization versus specialization in plant pollination systems Trends in Ecology & Evolution, 15 (4), 140-3 DOI: 10.1016/S0169-5347(99)01811-X

Sargent, R. (2004). Floral symmetry affects speciation rates in angiosperms Proc. R. Soc. B, 271 (1539), 603-608 DOI: 10.1098/rspb.2003.2644


  1. Love xkcd. I congratulate your taste.

  2. Interesting! The most diverse vascular plant genus in the Pacific Northwest (and in many other jurisdictions) is Carex (sedge). Carex are wind-pollinated. When asked why Carex can be so diverse in one area, I often point out that many species are microhabitat specialists. I assumed that causation ran from microhabitat specilization to sympatric diversity.

    However, causation may run the other way (a fact suggested by recent research by Andrew Hipp and colleagues). Carex chromosomes have diffuse centromeric activity which can lead to a tolerance of chromosome rearrangements that lead to reproductive isolation. It's plausible that in Carex, chromosome features lead to frequent speciation which lead to specialization as all those species crowd one area.

  3. Cool, Barbara -- I'm going to have to look up Hipp's work.