A Joshua tree flower, up close.
A huge diversity of flowering plants rely on animals to carry pollen from one flower to another, ensuring healthy, more genetically diverse offpsring. These animal-pollinated species are in a somewhat unique position, from an evolutionary perspective: they can become reproductively isolated, and to form new species, as a result of evolutionary or ecological change in an entirely different species.
Evolutionary biologists have had good reason to think that pollinators often play a role in the formation of new plant species since at least the middle of the 20th century, when Verne Grant observed that animal-pollinated plant species are more likely to differ in their floral characteristics than plants that move pollen around via wind. More recently, biologists have gone as far as to dissect the genetic basis of traits that determine which pollinator species are attracted to a flower—and thus, which flowers can trade pollen.
However, while it’s very well established that pollinators can maintain isolation between plant populations, we have much less evidence that interactions with pollinators help to create that isolation in the first place. One likely candidate for such pollinator-mediated speciation is Joshua tree, the iconic plant of the Mojave Desert.
Gorteria diffusa. If its spots look like sexy female bee flies to you, you might be a bee fly yourself. Photo by Flickr/thehumofbees.
The South African daisy Gorteria diffusa has a means of attracting pollinators that is either a mean-spirited or brilliant, depending on how much you sympathize with the pollinators in question: its dark-spotted petals fool male bee flies (Megapalpus capensis) into mating with the flower. This is, of course, a fruitless exercise for the bee fly, but not so for the daisy, since the decieved males pick up pollen in the process, which they’ll transfer to another daisy when they’re fooled again.
This is a bit salacious, but this kind of sexual deception isn’t exactly rare among flowering plants. What makes G. diffusa more interesting, to an evolutionary biologist, is that not all populations of the daisy practice this deception. The pattern of G. diffusa‘s petals varies across its range—and not all petal patterns prompt the pollinators to hump the flower. Actually, on all but three of the various forms of G. diffusa, bee flies mostly just come to feed on nectar.
That poses the interesting evolutionary question of why some populations of G. diffusa have evolved to trick their pollinators, when so many others have not. A paper just released online at the journal Evolution attempts to answer that question—but its authors find more new questions than they do concrete answers.
My postdoctoral research is shaping up more and more to be hardcore bioinformatics; apart from some time spent trying to get a dozen species of peanut plants to grow in the greenhouse as part of a somewhat long-shot project I’m working on with an undergraduate research associate, I mostly spend my workday staring at my laptop, writing code. It’s work I enjoy, but it doesn’t often give me an excuse to interact directly with the study organism, much less get outdoors. So, when Chris Smith dropped the hint that he could use an extra pair of hands for fieldwork in the Nevada desert this spring, I didn’t need a lot of persuasion.
Chris is continuing a program of research he started back when he was a postdoc at the University of Idaho, and which I contributed to as part of my doctoral dissertation work. The central question of that research is, can interactions between two species help to create new biological diversity? And the specific species we’ve been looking at all these years are Joshua trees and the moths that pollinate them.
Joshua trees, the spiky icon of the Mojave desert, are exclusively pollinated by yucca moths, which lay their eggs in Joshua tree flowers, and whose larvae eat developing Joshua tree seeds. It’s a very simple, interdependent interaction—the trees only reproduce with the assistance of the moths, and the moths can’t raise larvae without Joshua tree flowers. So it’s particularly interesting that there are two species of these highly specialized moths, and they are found on Joshua trees that look … different. Some Joshua trees are tall and tree-ish, and some Joshua trees are shorter and bushy. Maybe more importantly for the moths, their flowers look different, too.
A euglossine bee collects scent from an orchid
Orchids have some of the most remarkable pollination relationships of all the flowering plants. Their flowers are adapted into wild shapes for placing packets of pollen on precisely the right part of a pollinator’s body, and many species attract pollinators with lures that are somewhat kinkier than simply offering nectar—such as mimicking a female pollinator’s scent and appearance, to dupe males of the species into, er, making intimate contact.
A somewhat less exploitative orchid-pollinator interaction involves offering scent compounds to euglossine bees. Male euglossines collect scents from their environment—things that smell pleasant to humans, as well as things that really don’t—in special structures on their legs. It’s thought that they use the collected scents to attract females. Three large, diverse groups of orchids transport pollen by generating bee-attractive scent compounds, then saddling any bee who comes to collect the scent with a packet of pollen.
From the outside, this looks like a mutually beneficial relationship. The bees get their perfume, the orchids a pollen transporter. Over millions of years, such an interaction should lead bees and orchids to diversify together—when one orchid species splits into two, the bees that collect scent from them might very well speciate with the orchids. A recent paper in Science provides pretty good evidence that, over the long history of euglossines and the orchids that perfume them, the interaction hasn’t worked like that at all.