That “mystery of mysteries”: What makes a species?

ResearchBlogging.orgIn a special issue of Philosophical Transactions of the Royal Society on speciation, James Mallet argues that the Biological Species Concept is at odds with Charles Darwin’s original ideas about what a species is – and that current research supports Darwin [$-a].

When The Origin of Species was first published, biologists mostly thought species were easy to recognize – they looked different from each other, and they couldn’t successfully interbreed with each other. This view was a problem for Darwin’s ideas about gradual evolution by natural selection, since gradual divergence shouldn’t give rise to nice, discrete species. In fact, as Darwin argued, different groups of organisms exhibit a whole spectrum of reproductive isolation, from complete interfertility to total isolation – and the degree of isolation is not easy to predict based on how similar organisms look. In Darwin’s description, species are just labels that humans put on clusters of similar-looking organisms.

By the mid-Twentieth Century, evolutionary biologists favored what is commonly called the Biological Species Concept (BSC), defining species as non-interbreeding populations of living things. Research on speciation has accordingly focused on the ways that evolution creates reproductive isolation between populations. Mallet argues that this amounts to an abandonment of Darwin’s insights, and that by focusing on isolating mechanisms, biologists have returned to viewing species as distinct, “real” entities, missing much of the evolutionary process as a result.

I’m not sure I believe the distinction that Mallet makes between Darwin’s description of species and the BSC; they seem to me more different in their emphasis than in their fundamentals. Darwin was interested in demonstrating that species arise gradually, as accidents of adaptation to different environments – and, as Mallet says, he was trying to overcome the then-predominant view that species were real, discrete entities instead of the names that humans assign to clusters of similar organisms. Research motivated by the BSC generally takes this view as well, but it’s interested in the processes that create such clusters, and can prevent them from merging into nearby clusters by interbreeding.

Two types of Joshua tree
Photo by jby.

Research on the evolution of isolating mechanisms necessarily focuses on cases where isolation is incomplete, somewhere between complete speciation and free interbreeding. A prime example is my lab’s research on the two pollinator-associated types of Joshua tree, Yucca brevifolia. It’s not clear that the two types are reproductively isolated – preliminary genetic data suggests they’re not [PDF] – even though they’re pollinated by different moth species, and classified as separate subspecies, the taller Y. brevifolia brevifolia and the short, bushy Y. brevifolia jaegeriana. They may be on the way to becoming different species, but they’re not there yet. Two other examples out of the endless forms available: marine snails that choose mates by their slime trails, and wildflowers that would interbreed if only they could survive each other’s habitat.

As Mallet concludes in the more empirical part of his review, this is what we see across the diversity of life: a continuum of reproductive isolation between populations, not a granular world of neatly divided, obviously different species. Rather than over-simplifying this reality, the Biological Species Concept gives us a framework through which to understand it.


Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. First ed. London: John Murray. Full text on Google Books.

Mallet, J. (2008). Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation Phil. Trans. R. Soc. B, 363 (1506), 2971-86 DOI: 10.1098/rstb.2008.0081

Smith, C., W. Godsoe, S. Tank, J. Yoder, & O. Pellmyr (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

The heart of the debate

Richard Grant channels Stephen Jay Gould in a robust defense of the non-overlap between the magisteria of science and religion. His talking points are that scientists don’t understand science (or rather, what science doesn’t provide):

The thing is, people often make the mistake of assuming that the faithful invent a religion because they need to explain something—usually the natural world. And while it’s true that religions have and do spring up for this reason, it is not why people are christians.

And (echoing Slacktivist) the religious don’t understand theology:

Creationism is used as a proof, as evidence for the existence of (a) God. … So if you tie your faith to a ‘proof’ you actually end up trying to prove that your proof is true, rather than seeking out ‘truth’. Which is the cleft stick Creationists find themselves in.

Via the Daily Dish.

The ever-expanding science blogosphere

Just before I left for the field, I happened to see a familiar-looking article title in the feed, associated with a familiar-sounding new blog. Turns out, it was familiar for good reason: Coevolvers is the newly-launched blog for the Palouse Coevolution Study Group, a journal club of UI and WSU scientists who study the ecology and evolution of species interactions.

Photo from Coevolvers.

I’ve been involved since I started grad school here, and was lucky enough to be able to contribute (a very little bit) to the group’s 2007 review on studying geographic mosaics of coevolving species, which is freely available online. The blog will never be able to capture the club’s, um, robust give-and-take interactions, but it’s a great way to see what we’re reading and what we think about it.

The trouble with studying desert plants

Some years, they don’t bloom. I’m just back from a week and a half of attempted fieldwork in Nevada, with a hiatus to Southern California for a lecture to a Desert Institute class. Very few Joshua trees were in flower, so the trip was kind of a bust. But it was still good to get out into the desert. The weather was only really cold a couple nights, and almost too warm in Palm Springs. When I drove back into Moscow this afternoon, it was snowing.

In which a pun fights evil

Via the Daily Dish: Flier distributed at a counter protest against Westboro Baptist Church, the congregation that has built a theology around the hateful, un-Christian catchphrase “God hates fags.”

Photo by froboy.

(A more readable, freely copy-able, version is provided here.) Given that Jesus makes no reference whatsoever to homosexuality, there is in fact a stronger case to be made that God hates figs, in a purely dueling proof-text kind of way. (I’ll see your Pauline epistle and raise you two Gospels!) It’s fully in the spirit of Westboro’s abuse of scripture that the flier pulls text from two different accounts of Jesus rebuking the fig tree.

See also: God hates shrimp.

Giant’s Shoulders #9 now online

The ninth edition of the Giant’s Shoulders, a monthly blog carnival celebrating classic research, is now online at The Evilutionary Biologist. Topics range way beyond my own foci, from Einstein’s other groundbreaking equation to a really cool nineteenth century attempt to define death in purely materialistic terms. Check it out!

The environmental impacts of war

ResearchBlogging.orgLast year Bioscience published a review article proposing a new discipline in conservation ecology: warfare ecology [PDF]. It’s now making the rounds in the science blogosphere, with good coverage at Conservation Blog and Deep Sea News, where I first happened upon it – and it deserves all the attention it can get.

In the U.S., at any rate, war and preparation for war tend to get priority over everything – especially tree-hugging environmental concerns. Exhibit A is last year’s Supreme Court decision that the Navy’s need to practice with sonar trumps the damage sonar can do to whale populations, to the extent that the Navy could not be required to do an environmental impact assessment before beginning the exercise. War is treated as an emergency, and who worries about environmental impacts during emergencies?

Yet environmental damage caused in the course of war has direct impact on the human aftermath of conflict. Refugees provided with nowhere else to go will often set up camp in protected lands. Materials used in warfare – Agent Orange defoliant used in Southeast Asia, depleted uranium in Iraq – can continue to kill people long after the fighting ends. On the other hand, the review’s authors, Machlis and Hanson, point out that demilitarized zones and military training grounds often serve as (perhaps overly-well protected) accidental preserves.

This is a subject I’ve thought about quite a bit before – way back in my undergraduate days, I won a Mennonite Central Committee oratorical contest with a speech that connected peace theology to environmental concerns. That speech now looks to me like slightly embarrassing juvenalia, but the central idea still holds, and it’s great to see that working ecologists are thinking along similar lines. By laying out a framework for thinking about the environmental impacts of war, Machlis and Hanson’s paper can hopefully help push governments to consider the longer-term environmental, economic, and social consequences of ecological decisions made in the course of preparing for and prosecuting war.


G. Machlis, & T. Hanson (2008). Warfare ecology BioScience, 58 (8), 729-36 DOI: 10.1641/B580809

Pollinator isolation and divergence in floral shape

ResearchBlogging.orgThis post was written for The Giant’s Shoulders, a monthly blog carnival focusing on classic research.

Since Darwin, evolutionary biologists have thought that interactions between species cause diversification. However, it wasn’t until the second half of the Twentieth Century that scientists began to draw a connection between species interactions and speciation. One of the earliest of these studies was Verne Grant’s 1949 discovery of cleverly indirect evidence that pollinator isolation shapes the evolution of flowers [$-a].

A bee at work. (Flickr: jby)

Pollinator isolation is reproductive isolation created when animal pollinators don’t transfer pollen between plants of two different species. This could be because of pollinator behavior – say, because pollinator species tend to prefer a single plant. Or it could be because of the mechanics of pollen presentation by a flower, with each plant species applying pollen to a different part of a pollinator’s body so that foreign pollen is less likely to come into contact with the female floral parts. In either case, flowers are the key to the isolation – either to guide pollinators to their preferred target, or to make sure that the wrong pollen isn’t delivered.

Grant reasoned that pollinator isolation should have a real effect on how plant species are classified. Pollinator-isolated species probably have very different flowers; taxonomists, who look for characteristics that easily differentiate between related organisms, might therefore be more likely to use floral characteristics to tell pollinator-isolated plant species apart. To test this, Grant collected published classifications of plants pollinated by specialized animals (birds, bees, and long-tongued flies) and plants pollinated either by non-specialized animals, by water, or by wind.

Figure 1 from Grant (1959), showing the effect of pollinator isolation.

The result is presented in the tidy graph seen here. In plants pollinated by birds (A) and bees or long-tongued flies (CD), a much larger of the characteristics used by taxonomists to identify species were floral traits, compared to plants with non-specialized pollinators (E) or wind- and water-pollinated plants (FG). To follow up this result, Grant took systematic observations of pollinators’ movements through an experimental garden planted with three subspecies of Gilia capitata, each of which had differently colored flowers. The bees seemed to forage mainly among plants with similar flowers, and when Grant raised seeds from the experimental plants in the greenhouse, he found that there were fewer hybrids between subspecies than would be expected from random pollinator movement.

Generally, today, we wouldn’t assume that species classifications are an unbiased proxy for biological diversity – to some degree, they’re human constructs. But the basic idea that Grant develops, that speciation is an accidental consequence of plants’ interactions with pollinators, is still very important to how we understand the history of life. Together, flowering plants and insects make up the majority of the diversity of life on Earth, and it seems reasonable to think that this may be because the two groups interact so intimately.

More than fifty years after Grant’s study, pollinator isolation is a well-established mechanism for speciation. And the principle that Grant proposed, that increased divergence in floral traits is a sign of pollinator isolation, is still very useful. My lab, for instance, recently found that two forms of Joshua trees pollinated by different moth species are more different in certain floral dimensions than in non-floral traits [PDF]. That’s only the first step in what promises to be a long program of research (including my dissertation), seeking answers to some of the same questions that motivated Grant’s study.


W. Godsoe, J.B. Yoder, C.I. Smith, & O. Pellmyr (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism. The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

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