Posts Tagged ‘evolution’

Are you a fan of criticizing evolutionary psychology? Grab some popcorn, folks, because there has been some criticizing and counter-criticizing going on that you may find entertaining! In most cases, it’s actually serious, well-reasoned debate, too (as long, I assume, as you don’t read the comments). Things began with Rebecca Watson‘s talk about pop evo psych at a skeptic conference, which was…I won’t say “debunked”, but countered, by evolutionary psychologist Edward Clint. This has sparked some dialogue, primarily on Freethought Blogs, about evolutionary psychology as a whole field and about the media coverage of the science (or, often, “science”) of gender differences. I present the (interesting parts of the) conversation so far in chronological order (perhaps I’m missing some contributions from blogs I don’t regularly read, so additions in the comments are welcome; also please note that I’m interested in collecting links that discuss the science or lack thereof involved, NOT those discussing What Rebecca Watson Really Meant):

Edward Clint’s response to Watson’s talk (the latter is embedded here and at the first Almost Diamonds link below)

Justin Griffith’s take on the above

Stephanie Zvan’s rebuttal of Edward Clint’s post

Tangential to the debate per se, Zvan also documents the gleeful response from the section of the internet that reflexively detests Watson

Zvan’s counterarguments, continued

PZ Myers begins a series critiquing evo psych

Clint’s response to criticism (this, and some other posts linked within, is more about tone and whether people are misinterpreting what other people said/wrote, which I consider not popcorn-worthy because I want to read about science)

Jerry Coyne discusses the field

Part II of Myers’s critique (and apparently more parts are planned)


Myers, part III

Greg Laden’s take

Shall I write up my own contribution here? Perhaps if I run out of popcorn.

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I’m confused. Am I supposed to wear red to attract a mate, or not?

ResearchBlogging.orgThese seemingly contraditory findings (which, as I’ll explain in a moment, aren’t actually contradictory) were amusingly published in the same journal within less than two weeks of each other, so I can’t resist a discussion.

It’s an old canard of pop evolutionary psychology that the colour red denotes sex. It had been hypothesized that red ornamentation—especially lipstick—mimics the colour of receptive female genitalia, and therefore advertised (perhaps falsely) fertility or sexual receptivity. A study which I described several weeks ago laid this particular hypothesis to rest by showing that straight men were less sexually interested in pictures of redder female genitals. But still, the colour red has strong cultural connotations, perhaps with evolutionary significance. The newer study was intended to show that red denotes (female) sexual availability—particularly for casual sex.

This study had three parts. First, the researchers recruited women and asked them to pretend they were creating an online dating profile. Half of these women were asked specifically to imagine that they were creating this profile to find casual sex partners. They were asked questions about what their profile picture would look like, including whether they would wear jewelry, and what colour they would wear. Interestingly, they were given only four options: green, blue, black, and red. Women in the “casual sex” group were indeed more likely to say they’d wear red, but by only a small margin (it was just barely statistically significant, at p=.047).

The next part of the study looked at whether this stated preference existed on real online dating sites. The researchers selected profiles of 500 women who were looking for casual sex and 500 who weren’t (must resist urge to make snide remarks about this methodology!). They had three people classify the predominant clothing colour in these profile pictures (again, only red, black, blue, and green were considered). And, indeed, women who were interested in casual sex were more likely to wear red prominently than those who weren’t*.

The third and final part of this study was similar to the second, except that it compared women on websites specifically dedicated to casual relationships to women on sites that emphasized more long-term relationships. A similar result was found: red was more common on the casual sex-focused website.

Now, what can and can’t we conclude from these results, assuming they’re sound? We can say for sure that women (more specifically, women who fit the online dating demographic) who are looking for casual sexual relationships tend to display red clothing more often in the context of looking for those relationships. We cannot say whether this tendency is learned or instinctive, or whether it has an evolutionary “purpose”, or even whether it has anything to do with fertility (=fitness). The authors of this study do a great job of pointing out these limitations. For example, they note that their findings may not hold for face-to-face interactions or for all personalities.

I want to discuss why these results say little about evolution, though, because this is the sort of study that tends to be spun into an evolutionary psych fairy tale. First, it does not distinguish learned from genetically entrained behaviour (and, of course, there may be a little bit of both genes and memes at play). But if this red=casual sex link has a weak genetic basis, it’s probably not something that arose in our species as a result of natural selection in the traditional sense. Second, there’s an underlying assumption that red=casual sex=increased fitness (i.e. more babies). I have a feeling that the average woman these days is not pursuing casual sex in order to get pregnant. Perhaps this was the case in our evolutionary past, but it’s a pretty big assumption.**

Nevertheless, this study is not bad in terms of making wild claims about evolution. I do have some problems with its methods, though. Only four colour options? (None of which include, say, orange or pink—something closer to red.) And no mention of whether shades of pink, orange, or purple could be classified as red. On top of that, having people score what they thought was the “most prominent” colour in the profile pictures seems like not the best method, even though it was repeatable between scorers. (I’m thinking you could come up with a Photoshop manipulation to determine redness of a selected area of clothing. I think it’s been done with stickleback! (That is, with their red throats, not their clothing.))

What you can take away from this paper is that red is associated in women’s minds with sexuality in certain contexts. This is probably not surprising to anyone, but having data to back up the conventional wisdom is always good. However, it’s a huge stretch to ascribe evolutionary significance to this observation. Whether it’s as far a stretch to use it to choose your lipstick colour, though, is entirely up to you.

This study has also been covered eloquently at Scicurious. (Special bonus points to this set of comments.)

*For both of the online dating site studies, most photos had people wearing black, which is interesting if red is really that important a signal. Also, why were so few people wearing green? It’s clearly the best colour. (But not a real green dress, that’s cruel.)

**Also, and this is a question I could probably answer easily with a bit of Google Scholar-ing but I’m too lazy, what about red-green colour blindness, which occurs in 10% of men?

Elliot, A., & Pazda, A. (2012). Dressed for Sex: Red as a Female Sexual Signal in Humans PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0034607

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I promised more about Wolbachia, and here it is. Quick recap: Wolbachia is a type of bacteria that lives within the cells of many insects and other arthropod species.

Wolbachia has all sorts of possible effects ranging from parasitism to symbiosis. Anopheles mosquitoes, the bugs that carry malaria and thus kill hundreds of thousands of people every year, are naturally uninfected with Wolbachia, but scientists have been investigating ways to introduce Wolbachia into them so as to prevent malaria transmission. One recent study found that, in addition to suppressing malaria transmission by mosquitoes, a certain strain of Wolbachia killed many of the infected mosquitoes, but only after they had had a blood meal.

Now, the pathogen literature is not something I normally read; I stumbled across this research on a science fiction blog. This article I’ve linked to takes a rather cute approach to this study by suggesting that it could be used to kill vampires in the event that they begin to plague the human race. Commenters on that article raised the spectre of unintended side-effects of infecting people with Wolbachia. Well, commenters, here is what you (thought you) wanted to know:

Imagine you’re a Wolbachia cell. You’re inside the cell of an insect, say a mosquito. That mosquito spreads you (and your descendents) to other mosquitoes not by biting them or sneezing on them, but by reproducing: since you’re already inside the mosquito’s cells, you just get packaged into their baby-making cells. Well, not necessarily—if you’re in a male mosquito, you’re screwed, because you just won’t fit into a tiny little mosquito sperm. If you’re in a female mosquito, though, you’re in luck (that’s right, you’re maternally inherited). But you’re a clever Wolbachia and you’re thinking not just of your kids, but of your grandkids (which will all also be your clones because you’re a bacterium!). You don’t want any of your descendents to end up in male mosquitoes. What can you do to prevent this? Let’s brainstorm:

  • kill all the male offspring of the female you’re infecting
  • turn the male offspring into functional females
  • make your host into a parthenogen, i.e. make her reproduce asexually!

Guess what! Wolbachia can do all of the above! SF writers, have at it.

Hughes, G., Koga, R., Xue, P., Fukatsu, T., & Rasgon, J. (2011). Wolbachia Infections Are Virulent and Inhibit the Human Malaria Parasite Plasmodium Falciparum in Anopheles Gambiae PLoS Pathogens, 7 (5) DOI: 10.1371/journal.ppat.1002043

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ResearchBlogging.orgThe search for new species brings scientists to places as near as their lab bench and as distant as the Antarctic. They may be looking for traces of species long extinct or sightings of contemporary rarities. But one new study has found evidence for the existence of an unknown organism by looking at the DNA of another species.

Drosophila quinaria, a type of fruit fly, is well known to scientists. But in the course of studying its DNA, and the prevalence of a parasitic bacterium, researchers found evidence that, sometime in the past, D. quinaria hybridized with another fruit fly species unknown to science.

This evidence comes from a comparison of nuclear and mitonchondrial DNA. Here’s where I need to back up and explain some background genetics:

Mitochondria are the parts of the cell that produce, in an intricate series of chemical reactions, the energy needed for all our biological functions. They are fascinating structures for many reasons, but particularly because they contain their own DNA (usually abbreviated as mtDNA). What you probably think of as the human genome—two copies of each chromosome (apart from the sex chromosomes), one inherited from from each parent—lives in a part of the cell called the nucleus. mtDNA comes in a single copy and is only inherited from the mother, largely because egg cells are so much bigger than sperm and thus contribute all of the non-nuclear component of the cell.

Mitochondrial DNA has another interesting property: it doesn’t recombine. That is, if two mutations arise in the same mitochondrial genome, they’ll stay together in their descendents, while two mutations in the same nuclear genome won’t necessarily both end up in the offspring. This makes it easier to trace the evolutionary history of a mitochondrial genome than of an entire nuclear genome.

Back to the fruit flies: researchers found two main types of mtDNA in Drosophila quinaria. This is not particularly unusual. What was unexpected was just how different the two mtDNA types were. Not only were they as different as one might expect when looking at two different, though related, species, they were just as different from all of D. quinaria‘s closest relatives (data were unavailable for a handful of rare species). Despite this huge gulf, all the flies, regardless of mtDNA type, had similar nuclear DNA, indicating that they are indeed members of the same species.

What could be the cause of this pattern? It’s not unheard of for hybridization—ancient or recent—to allow a species to pick up the mitochondrial genome of another. But the fact that the mystery mtDNA was not particularly closely related to that of any known fruit flies suggests that the species D. quinaria hybridized with is either undiscovered or no longer existent.

Strange, no? It’s about to get stranger. What allowed the mtDNA of another species to creep into D. quinaria‘s genome and stay there? It may well have been a versatile bacterium called Wolbachia. This bug lives inside the cells of many insects and their ilk, with variable and sometimes sinister effects (more on which another day). Since it lives inside cells, but not in the nucleus, it is transmitted across generations just like mitochondria. Some, but not all, populations of D. quinaria host Wolbachia, and one of the populations in this study actually has a mix of infected and uninfected flies. And lo and behold, whether an individual fly has Wolbachia predicts almost perfectly which mtDNA group it belongs to (the single exception could mean that the occasional fly is “cured” of its infection). Wolbachia sometimes helps protect its host from viral infections, so maybe D. quinaria acquired it by hybridization (not at all unusual among flies), along with some mtDNA, and kept it because of these effects.

This fascinating study is probably not grounds for describing a new species, but if the unknown Drosophila is not extinct, we’ll be able to recognize it, and we’ll have an exciting new system for studying hybridization, the effects of Wolbachia, and possibly the genetics of speciation.

DYER, K., BURKE, C., & JAENIKE, J. (2011). Wolbachia-mediated persistence of mtDNA from a potentially extinct species Molecular Ecology DOI: 10.1111/j.1365-294X.2011.05128.x

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It’s high time I explained what exactly I’m doing chest-deep in a pond every morning.

My life’s focus these days is a little fish called the threespine stickleback. They’ve been studied systematically for many years, starting with Niko Tinbergen’s work on their territorial behaviour. Today they are considered an emerging model system: there are extensive, well-established lab resources (including a sequenced genome) for studying them, and research on these fish is used to answer more general questions in biology, particularly evolution.

I am tempted to say that the stickleback is more than just a fish. This is not just me being flippant; they are actually more than one species. The large saltwater stickleback population that stretches around the northern hemisphere is subdivided into clusters that don’t interbreed (i.e. species), and marine stickleback have repeatedly colonized lakes and streams, evolving into new species as they did so. It is these repeated, parallel, and most importantly recent—often they occurred less than 12,000 years ago as the last ice sheets retreated—speciation events, and their accompanying changes in morphology and behaviour, that have made sticklebacks into such an important study organism for evolutionary biology.

It’s two of the lake species that my research at the ponds focusses on. In five watersheds that we know of, the sticklebacks that colonized freshwater became not one but two species. One, the benthic species, lives near the lakebed and is larger; the other, the limnetic species, lives in open water and is much smaller. Since these species are so young, they can hybridize, though in the wild they rarely do. My research focuses on why they don’t; in other words, why they remain separate species.

Sticklebacks are odd fish. The males build nests, for one, and guard their eggs and later their fry. As the water warms in spring, each males stakes out a territory in the littoral zone. His throat turns bright red and his body a bright blue-green. He gathers bits of vegetation, sand, anything he can find, like a bird collecting twigs for its nest, and glues them together with a protein made in his kidneys. The nest has a tunnel through the middle; sometimes he swims through it to shape it. Here, he hopes, a female will lay her eggs.

Many people have noticed that in at least some of the “species pairs”, the benthics hide their nests under dense vegetation while the limnetics nest in plain sight on bare sand, rock, or sunken logs. My experiment this spring and summer aims to test whether this difference in habitat actually affects a female’s choice of mate. In other words, if I put a male of the “right” species on the “wrong” habitat, will a female still mate with him?

This is why I’ve been moving algae around for the past few weeks. I’ve built 1×1 metre pens in the shallow ends of two ponds. Half of them are left open while the other half has been covered with plants. I collected fish from one of the species pair lakes, and they’re being held in aquaria until I’m ready to experiment on them. I’ve now put the first two males into enclosures, and they’ve both built nests. As soon as I have some females that look like they’re ready to lay eggs, I’ll put one in each enclosure and see whether they spawn. If being on the “wrong” habitat decreases the chance that a pair spawns, then this difference in habitat plays a role in reproductive isolation, the lack of hybridization between two related species.

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ResearchBlogging.orgThe study of speciation—the formation of new species—has had a long history in evolutionary biology, but the past few decades have seen leaps in how we think about the process that creates biodiversity. We now know that natural selection is almost always heavily involved in the process, and that new species can form even when there is some ongoing hybridization between the evolving lineages.

One way to approach a recently evolved pair of species is to ask (1) why did they split in the first place and (2) how do they keep separate. (more…)

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The title of this post is a reference to this paper (only the first page is available for free, but only the first two paragraphs are relevent) and to a Stephen Jay Gould essay. Both address the question of whether the group of animals corresponding to a colloquial name is actually an evolutionary entity, a monophyletic group—meaning they are all the descendents of a single common ancestor, and all the descendents of that ancestor are included in the group. As you’ll soon see, the answer in the case of beavers is yes.

The question of where in the family tree of rodents the two extant beaver species should be placed has been an open question for some time. A recent paper in PLoS ONE sheds further light on the answer. By comparing the complete mitochondrial* genomes of both North American (Castor canadensis) and Eurasian (Castor fiber) beavers to those of other rodents, the authors determined that

  • beavers are a monophyletic group within the “mouse-related” rodents
  • of the other species used to build the evolutionary tree, beavers are most closely related to a type of flying squirrel called the scaly-tailed squirrel (genus Anomalurus)
  • the rodents may or may not be monophyletic; the glires (rodents plus lagomorphs, i.e. rabbits and their kin) are

Having complete mitochondrial genomes allowed the authors to do some other interesting things. First, they were able to calculate the substitution rate—how many base pairs of DNA are likely to mutate, by chance, every million years. This rate could be considered a proxy for the rate of evolutionary change in the species as a whole. Conversely, if we know this rate for some species, we can use it to estimate the age of other related species. This study found that in beavers, the substitution rate is much lower than in other rodents, possibly because they’re much longer lived. (Think about it: all rodents might have the same substitution rate per generation, but if beavers live much longer, they’ll have fewer generations in a given time period and thus fewer total substitutions than a shorter lived species. It would be interesting to see if the substitution rate is also low in the capybara, the largest living rodent.)

With substitution rates for many different kinds of rodents, the authors were also able to date the origin of the rodent order (with the assumption that they’re monophyletic). The data, 67 million years ago, fits rather nicely among other published estimates of the age of rodents and conveniently falls right around the great extinction at the end of the Cretaceous—i.e. the end of the dinosaurs. It’s only a correlation, but it’s fun to think of the end of the dinosaurs allowing rodents—the most speciose group of mammals—to take over the world.

Finally, the authors note that the closest living relative of the beavers is still not known. Although they’re most closely related to Anomalurus in this study, there was no complete mitochondrial genome available for the pocket gophers and kangaroo rats, which have also been proposed as close relatives of the beaver. And, of course, the fossil beavers—there are quite a few, including some truly massive species—aren’t represented in this study. Still, it seems likely that they will fall on the same branch of the evolutionary tree of their similar-looking (but much smaller!) modern relatives.

*The mitochondria are parts of cells that generate energy from food. They have their own DNA, separate from the 46 chromosomes that we more commonly call the human genome. The mitochondrial genome has some interesting properties, including the fact that it’s only transmitted matrilineally.
Horn, S., Durka, W., Wolf, R., Ermala, A., Stubbe, A., Stubbe, M., & Hofreiter, M. (2011). Mitochondrial Genomes Reveal Slow Rates of Molecular Evolution and the Timing of Speciation in Beavers (Castor), One of the Largest Rodent Species PLoS ONE, 6 (1) DOI: 10.1371/journal.pone.0014622

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