Category Archives: evolution

Pollinator isolation and divergence in floral shape

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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.

References

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

Milkweed’s bitter arms race against herbivores

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ResearchBlogging.orgPlants are locked in a long twilight struggle with herbivores, particularly insects – sometimes they evolve a new defensive mechanism, “escaping” to diversify into new groups [$-a], but mostly natural selection works with the traits they already have. That means arms races – plants evolving greater concentrations of defense chemicals, and herbivores evolving greater tolerance of those chemicals. In this month’s Evolution, a new study of defensive chemistry evolution in milkweed [$-a] documents exactly this process.


Asclepias viridis, a milkweed
Photo by gravitywave.

The study by Agrawal et al. follows up on earlier work in the same group, which established the evolutionary relationships between the members of the milkweed genus, Asclepias. Milkweeds are named for their defense against insect herbivores, a milky sap full of nasty chemicals – coumaric acids, caffeic acids, cardenolides, and flavonoids. The authors raised a large sample of milkweed species in a controlled environment, then measured the levels of these chemicals in each species. By mapping the chemical profiles onto the previously-developed phylogeny of Asclepias, they could estimate how milkweeds’ chemistry has evolved since the genus first arose.


Aphids on Asclepias
Photo by aroid.

This analysis revealed that milkweeds have gotten nastier over their evolutionary history. But it’s not that clear-cut: the diversity of defensive chemicals present in Asclepias decreased, even as the total production increased – so the plants seemed to be paring down an initial diversity of defenses into a few chemicals that worked especially well. Coumaric and caffeic acids, which are produced from the same biochemical precursors, forced a trade-off so that as one increased, the other decreased. On the other hand, cardenolides and flavonoids, which are both produced in another biochemical pathway, were positively associated.

If this sounds complicated, that’s because it is. As Agrawal and his coauthors point out, we actually don’t have a good sense at what timescale an arms race should manifest – that is, are we talking about plants evolving greater defenses over a few generations, or over millions of years, as this study? Natural selection can appear to be moving a population strongly in one direction for a year or two – and then turn out to be fluctuating all over the place [$-a] if you watch for decades. How year-to-year selection acting on multiple traits translates into the grand trends of evolution – whether the explosive diversification of flowering plants or the emergence of human intelligence – remains one of the big puzzles for those of us who study the living world.

Reference

A.A. Agrawal, J.-P. Salminen, M. Fishbein (2009). Phylogenetic trends in phenolic metabolism of milkweeds (Asclepias): Evidence for escalation. Evolution, 63 (3), 663-73 DOI: 10.1111/j.1558-5646.2008.00573.x

P.R. Ehrlich, P.H. Raven (1964). Butterflies and plants: a study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/pss/2406212

P.R. Grant, B.R. Grant (2002). Unpredictable evolution in a 30-Year study of Darwin’s finches Science, 296 (5568), 707-11 DOI: 10.1126/science.1070315

Want to speciate? Stay home.

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ResearchBlogging.orgI’ve said it before, and I’ll say it again: the formation of new species is almost always an accident. There you are, adapting to changes in your obligate pollinator, or the local environment – and suddenly, you can’t mate with the folks on the other side of the mountain. That’s the lesson I take from an article in last week’s PNAS, which suggests that a diverse group of birds got that way by being homebodies [$-a]


Oriental white-eye
(Zosterops palpebrosus)
Photo by Lip Kee.

The authors (including Jared Diamond, who communicated the paper to PNAS), set out to determine why there are so many species of white-eyes, a group of songbirds distributed across Africa, Southeast Asia, and the southern Pacific. They built a phylogeny for the group, calibrated it to real time using the geological dates of origin for Pacific islands occupied by white-eyes, and then estimated the rate at which the group produced new species. They found, as reported by Wired.com, that the largest group of white-eyes have one of the fastest species-accumulation rates recorded in vertebrates, about 1.6 new species every million years.

That’s a weird result, when you think about it – we’re talking about birds, and widely-distributed birds, here. All things being equal, speciation is facilitated by lack of movement – Appalachian salamanders, for instance, diversified largely because they’re too gimpy to move between stream drainages very often [$-a]. Furthermore, the authors say, white-eyes don’t display a lot of ecological differences that might contribute to isolation. So how did they speciate at a record-setting pace?

The solution? The authors propose that white-eyes are prone to rapid changes in their dispersal ability. As evidence, they cite numerous cases in which white-eyes must have crossed great distances to colonize one island, then failed to make it across much smaller distances to colonize others nearby. Nodding to Diamond’s groundbreaking work on human history and cultural evolution, they compare this to the colonization of Polynesia, in which people stopped traveling long distances as the chance of discovering an uninhabited island decreased.

References

K.H. Kozak, D.W. Weisrock, A. Larson (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon) Proceedings of the Royal Society B: Biological Sciences, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326

R.G. Moyle, C.E. Filardi, C.E. Smith, J. Diamond (2009). Explosive Pleistocene diversification and hemispheric expansion of a “great speciator” Proceedings of the National Academy of Sciences, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105

Isn’t dating Darwinian enough already?

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Sent to me by Roxy Allen, who seems to be out to wave public misperceptions about evolution in my face (on Darwin Day, she pointed me to that depressing, depressing poll result): Darwin Dating is your go-to for eugenic relationship-building, or so it claims.

Sick of dating websites filled with ugly, unattractive, desperate fatsos? We are.

Darwin Dating was created exclusively for beautiful, desirable people. Our strict rules and natural selection process ensures all our members have winning looks. Will you make the cut?

Obvious issues: (1) members are actually self-selected, as long as they send in a reasonably good photo; (2) since when are photos on Internet dating sites honest indicators? (3) do you really want to go looking for a mate amongst attractive/dishonest people who self-selected for a pretend-exclusive fringe dating site? Bio-nerd issue: attractiveness may have little relation to true Darwinian fitness (i.e., ability to successfully raise lots of offspring).

“25 things” infection dynamics

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Slate’s survey of its readers determines that spread of the Facebook meme followed a pattern analogous to emergent disease evolution – a long period of low density, during which various mutants (“16 things,” “17 things,” &c) competed; then a rapid spread when one variant hit the big time.

Darwin’s 200th: What evolution can teach Christianity

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ResearchBlogging.org
Today is the 200th anniversary of Charles Darwin’s birth, and 150 years since he published his groundbreaking book, The Origin of Species. The Origin provided the first widely-accepted explanation for the evolution of life on Earth, and, although Darwin was wrong on some points (if only he had known about genes!), a century and a half of scientific work has shown that he was right about more.

That century and a half has not diffused the perception, especially in the United States and other highly religious countries, that acceptance of a scientific account for the history of life is antithetical to religion. As a Darwinian and a Christian, this is a topic with which I struggle, and about which I’ve written a great deal here. Although I’m not sure that science can coexist with a real belief in the supernatural, I do hold that science is both compatible with the moral questions at the heart of religion and essential to answering them.


Photo by rmcnicholas.

For Darwin’s 200th, then, I’d like to briefly present three examples of evolutionary insights that complement the Christian moral perspective. I focus on Christianity here (and elsewhere in this blog) not because I think it has an exclusive hold on the truth, but because it is the tradition in which I was raised, and the one that shapes my own moral perspective. I think the following points are easily applicable to just about any other moral system, religious or non.

Our evolutionary past shapes us today.

Christianity (and, indeed, most other religions) starts from the fundamental problem of human behavior: We do things that we know are hurtful to those around us, often because we enjoy doing them. As the apostle wrote, “For what I do is not the good I want to do; no, the evil I do not want to do—this I keep on doing.” (Romans 7:19)

The Christian tradition calls this original sin; the evolutionary perspective points to its origin in the remnants of past adaptations. We have two bones in each forearm because we evolved from ancestors with those two bones in their pectoral fins [$-a]; we may be hostile to outsiders because that parochialism helped early humans to form closer-knit societies [$-a]. Far from giving us an excuse to do whatever we feel like, these results can help us figure out how to overcome evolved behaviors that hurt others.

Christ calls us to transcend our past.

Just as it shapes our hurtful impulses, our evolutionary past has a hand in the better angels of our nature. We may care for our children and close relatives, for instance, in part because they carry many of our genes – so helping them helps our own evolutionary fitness [$-a]. Similarly, the need to live peacefully with our immediate neighbors may have shaped deep emotional aversions to murder [PDF].

In the Sermon on the Mount, though, Jesus lays out a moral model that calls us beyond what comes naturally:

“You have heard that it was said to the people long ago, ‘Do not murder,’ … But I tell you that anyone who is angry with his brother will be subject to judgment.” (Matt. 5:21-2)

And:

“You have heard that it was said, ‘Love your neighbor and hate your enemy.’ But I tell you: Love your enemies and pray for those who persecute you … If you love those who love you, what reward will you get? (Matt. 5:43-6)

Evolutionary thinking can help us realize Christ’s call.

When we understand the deep causes of hurtful behavior, we can figure out better how to overcome them. To pick just one example: Jesus proposes a moral solution to the problem of hostility to strangers mentioned above in the parable of the Good Samaritan (Luke 10:25-37) when he redefines the concept of “neighbor” to mean something bigger than “people of the same race/religion.” But how do we overcome deep-seated biases against people who don’t look like us? One new study suggests hacking the mental habits that create those biases in the first place, by making the effort to become familiar with people of other races – Caucasian volunteers trained to better differentiate between African American faces showed reduced evidence of bias against African Americans.

Like the Christian moral model, the evolutionary perspective understands that humans are imperfect – but suggests ways we can do better. This is why it pains me to hear other Christians dismiss evolutionary science out of hand (apart from my nerdy compulsions to correct factual error): Understanding evolution can help us in our ongoing struggle to live together, if only we’re open to the data science provides. The current advances in our understanding of human behavior are only possible because today’s researchers stand on the shoulders of a giant: Charles Darwin.

References

J.-K. Choi, S. Bowles (2007). The coevolution of parochial altruism and war Science, 318 (5850), 636-40 DOI: 10.1126/science.1144237

K. Foster, T. Wenseleers, F. Ratnieks (2006). Kin selection is the key to altruism Trends in Ecology & Evolution, 21 (2), 57-60 DOI: 10.1016/j.tree.2005.11.020

J.D. Greene (2001). An fMRI investigation of emotional engagement in moral judgment Science, 293 (5537), 2105-8 DOI: 10.1126/science.1062872

S. Lebrecht, L.J. Pierce, M.J. Tarr, J.W. Tanaka (2009). Perceptual other-race training reduces implicit racial bias PLoS ONE, 4 (1) DOI: 10.1371/journal.pone.0004215

T. Lewens. (2007). Darwin. New York: Routledge. Amazon.com.

M. Ruse. (2000). Can a Darwinian be a Christian? Cambridge University Press. Amazon.com.

N.H. Shubin, E.B. Daeschler, F.A. Jenkins (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb Nature, 440 (7085), 764-71 DOI: 10.1038/nature04637

The Times annotates the Origin

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As part of its special coverage of Darwin’s 200th, the New York Times has a very well put-together presentation of The Origin of Species, with annotation of key passages by working scientists. The complete text of the first edition is also offered for download [PDF], if you haven’t got it already.

Evolutionary merchandising, just in time for Darwin Day

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I’ve never quite found a t-shirt design that speaks to me as a bio-nerd. Sure, there are tees out there that promote evolution (usually in opposition to creationism), or science in general, and some are pretty awesome. But where’s the evolutionary biology equivalent of the infinite in-jokes for computer programmers, say? I haven’t seen ’em.

Here, then, are my first attempts to fill this minuscule hole in the market, two designs capturing the twin evolutionary forces of natural selection and genetic drift. Design by me, printing by Spreadshirt. I’ve ordered my own copies of the two pictured here, and they’re great – stylish, comfortable American Apparel tees with clean, bright printing. More options are at my new Spreadshirt store Denim & Tees, or you can work up your own in Spreadshirt’s nifty online designer.

Natural selection and speciation, 150 years later

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ResearchBlogging.orgScience kicks off the week of Darwin’s 200th with a special section devoted to the latest on speciation [$-a], the literal origin of species. It includes a new review by Dolph Schluter, discussing the role of natural selection speciation [$-a], which suggests a new way to think about selection creating reproductive isolation.

Schluter contrasts ecological speciation, in which reproductive isolation arises in the course of adaptation to different environments, “mutation-order” speciation – isolation arising by the accumulation of different genetic and morphological changes in the course of adaptation to the same (or the same kind of) environment. That is, natural selection can cause a population to split into two species if different parts of population are “solving” different ecological problems, or if they arrive at different “answers” to the same problem.

The mutation-order scenario makes sense, though it’s new to me. As an example, Schluter cites a recent study in Mimulus in which a mutation of the mitochondrial DNA in one population creates sterile males in hybridization with other populations [$-a]. He proposes that much mutation-order speciation occurs because of conflict between different levels of natural selection, as when “selfish genes” create reproductive incompatibilities in the course of spreading through a host population. This is a departure from what biologists usually consider speciation by natural selection, but Schluter makes an interesting point.

References

A.L. Case, J.H. Willis (2008). Hybrid male sterility in Mimulus (Phrymaceae) is associated with a geographically restricted mitochondrial rearrangement Evolution, 62 (5), 1026-39 DOI: 10.1111/j.1558-5646.2008.00360.x

D. Schluter (2009). Evidence for ecological speciation and its alternative Science, 323 (5915), 737-41 DOI: 10.1126/science.1160006

A. Sugden, C. Ash, B. Hanson, L. Zahn (2009). Happy birthday, Mr. Darwin Science, 323 (5915) DOI: 10.1126/science.323.5915.727