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.
Over the past several years there has been a growing trend of parents that are terrified of vaccinating their kids citing reasons such as the debunked link to autism or that it just isn’t “natural.” A healthcare blog run by several infectious disease doctors called Controversies in Hospital Infection Prevention has run frequent stories reporting on the declining vaccination rates as well as problems that ensue because of that, most recently about the whooping cough epidemic in Washington and wondering why Jenny McCarthy has so much influence on national views on vaccinations.
Linking microevolutionary processes to macroevolutionary patterns
I have always been interested coevolutionary interactions, particularly host-parasite interactions. I have often wondered if the local patterns of interaction between host and parasite (e.g. local adaptation) can scale up and lead to patterns of host specificity. Having a thorough understanding of these selective forces may help us better understand the conditions for disease emergence and perhaps disease virulence evolution.
Population genetics is concerned with the processes that generate evolutionary change within species and populations. A major question in evolutionary biology is whether these same processes ultimately generate patterns of diversity at higher organizational levels. While interactions between species such as hosts and their parasites (or plants and pollinators, herbivores and plants) have long been implicated as a means of generating patterns of diversification (Ehrlich and Raven, 1964,Thompson, 1994, 2005), the process by which microevolutionary forces generate macroevolutionary patterns is not well understood for coevolutionary systems.
Highly specific interactions between pairs of species can result in population level patterns. Both theoretical and empirical studies show that genetic specificity combined with specific gene flow patterns lead to parasites tracking of local host populations (Dybdahl and Storfer, 2003,Gandon et al., 1996,Gandon and Michalakis, 2002,Kaltz and Shykoff, 1998). In a mutualism between plant and pollinator, the seeming match between the length of a flower corolla and bill may be the result of strong selective pressure. Although we have many good examples of the processes that work at the population level, we have little evidence as to how those processes generate patterns of diversity among interacting species (Thompson, 2005). At the macroevolutionary scale, the processes leading to the observed patterns of host specificity have remained unclear.
The authors of a recent perspective piece in Evolution have addressed this exact issue by asking:
“Can microevolutionary adaptive processes acting at the within-species level explain macroevolutionary patterns across host and pathogen taxa?” (Antonovics et al., 2013)
What processes can explain why most pathogens cannot infect all encountered hosts ?
Evolutionary change by means of Natural Selection needs a couple of things in order to happen: heritability and variation in fitness. That is, offspring need to resemble their parents at least a little (heritability) and individuals need to differ in their survival and offspring production (fitness). We’ll worry about heritability in another post, but variation is something that seems like it might be hard to maintain. Some forms of Natural Selection will reduce variation as more fit individuals become frequent and all the different kinds of less fit individuals are eliminated from the population. However, there is a force, common in nature, which may maintain variation, parasites.
Interactions between hosts and parasites can generate strong selective pressures on each player, especially if your life depends on infecting a host. Often, biologists make an analogy to an arms race where players are developing bigger and better defenses or weapons. Antagonistic interactions may also generate negative frequency dependence where a rare host type is favored because the parasites are adapted to a common type. You can learn more by checking out CJ’s post on the Red Queen Hypothesis or Jeremy’s post on a different coevolutionary puzzle. A key component for maintaining variation via negative frequency dependent selection is specificity. There must variation in the interaction among different host genotypes and parasite genotypes. This is sometimes referred to as a GxG interaction. If parasites can infect all the hosts, there is no specificity. Specificity allows different hosts to be favored over time depending on the composition of the parasite population.
Theoreticians love to use different models of interactions between hosts and parasites, but without empirical evidence, there seems little point. In a recent paper by Rouchet and Vorburger (2012), the authors looked for evidence of just the kind of genetic specificity would result in the maintenance of genetic variation.
It is a truth universally acknowledged in evolutionary biology, that one species interacting with another species, must be having some effect on that other species’ evolution.
Actually, that’s not really true. Biologists generally agree that predators, prey, parasites, and competitors can exert natural selection on the other species they encounter, but we’re still not sure how much those interactions matter over millions of years of evolutionary history.
On the one hand, groups of species that are engaged in tight coevolutionary relationships are also very diverse, which could mean that coevolution causes diversity. But it could be that the other way around: diversity could create coevolutionary specificity, if larger groups of closely-related species are forced into narower interactions to avoid competing with each other.
Part of the problem is that it’s hard to study a species evolving over time without interacting with any other species—how can we identify the effect of coevolution if we can’t see what happens in its absence? If only we could force some critters to evolve with and without other critters, and compare the results after many generations …
Oh, wait. That is totally possible. And the results have just been published.
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.
Conventional wisdom suggests that pathogens and parasites are more rapidly evolving because of various reasons such as short generation time or stronger selection. Yet somehow, they have not completely won the battle against the host. Recently, a theoretical paper on coevolution in Nature caught my eye (Gilman et al., 2012). Here the authors address this paradox: “How do victim species survive and even thrive in the face of a continuous onslaught of more rapidly evolving enemies?”
Instead of treating a coevolutionary interaction between two species as the interaction of only two traits, the authors investigate the nature of an interaction among a suite of traits in each species. It’s not hard to think of a host having a fortress of defenses against attack from a parasite with an arsenal loaded with many weapons.
A paper by Morris and colleagues (2012) has generated some stir among biologists. The authors are proposing the Black Queen hypothesis to explain genomic reductions among free living interacting microbes. Rather than rehash arguments that have been made more eloquently, I’d like to just point out some informative ones
Quick summary over at the New Scientist
In depth critique by Robert T. Gonzalez
Tommy Leung also reminded me of a great review paper by Sachs et al (2011) over at TREE that is highly relevant to this debate.
A reed warbler feeds a cuckoo chick
Brood parasitism, the reproductive strategy of choice for cuckoos and cowbirds, sounds like a lazy approach to parenting: lay your eggs in another bird’s nest, and let the unwilling adoptive parents take the trouble to raise your chicks. But contracting out parental care like this comes with many of its own complications. Chicks raised by parents of a different species have to eliminate competition from their adoptive nestmates, and may grow up a bit confused; reluctant host birds may need to be told, and reminded, that raising cuckoo chicks is an offer they can’t refuse.
But before crossing all those hurdles, a brood parasite’s first task is to lay eggs in the nest of a host who won’t immediately recognize and reject them. The strong natural selection imposed by host rejection has led cuckoos to evolve “host races” that lay eggs whose color and spotting pattern matched to those of their preferred host species. This kind of broad-scale pattern could arise without much direct effort by female cuckoos—those who lay eggs in the nest of the best matching host species would simply be the ones most likely to have chicks that survive to the next generation. But is it possible that cuckoos do take an active role in matching up to their hosts, seeking out host nests containing eggs that look like their own?
The answer, according to a series of studies over the last several years, is yes—probably.