Month: April 2009

Evolutionary psychology: the adaptive significance of semen flavor

April 18, 2009 • 5:42 am

I have long been critical of many evolutionary psychologists for their over-the-top stories, but today I am forced — albeit briefly — to join their ranks. I have thought of a hypothesis that shares all the salient traits of the best ideas of evolutionary psychology: it is brilliant, makes evolutionary sense, and is untestable.

It is the conventional wisdom in human sexuality that semen tastes bad. Anyone with minimal sexual experience knows that although many women will perform fellatio on their partners, most bridle at the thought of swallowing the ejaculate. Its flavor is frequently characterized as revoltingly bitter or salty. The “swallow or spit” dilemma faces any woman who performs such an act, and whose partner regards swallowing as a gesture of love.

The universal distastefulness of semen is attested by the many internet sites that give advice about how to improve the taste of one’s ejaculate, for example, here, here, and here.

To get a better scientific handle on this idea, I took a poll, asking a woman friend, Dr. Fawzia Rasheed, to canvass her female acquaintances about their willingness to swallow after the act of fellatio. Twenty-four women were asked this question:

Sperm…would you spit or swallow? In other words, can you abide by or do you hate the taste?

There were sixteen responses, many including pungent asides that I cannot repeat on a family-oriented website. One answer was a non-response (“I should be so lucky”). The other fifteen included eleven “spits” and four “swallows”. But among the latter, two women commented that they did not like the taste: one, in fact, swallowed to get rid of the flavor as quickly as possible. Two others said “swallow” but did not comment on whether they enjoyed it. Therefore, 13/15, or 87% of informative respondents could not abide the taste of semen.

This near-unanimous response to a random poll demands an evolutionary explanation. Why does semen taste so foul? One answer, of course, is that the chemicals necessary to make an ejaculate effective have the side effect of tasting bad. Semen is only about 5% sperm, with the remainder of the fluid consisting of a complex mixture of compounds from the prostate gland and seminal vesicle. These compounds include sugars such as lactose [CORRECTION: fructose; see below ] (to provide energy for the swimming sperm), enzymes, amino acids, zinc, hormones, and various amines to counteract the acidic environment of the vagina (these are said to give sperm its characteristic smell and flavor). Some of these amines have the names putrescine and cadaverine, which give an idea of how they smell. The function of some of the compounds is unknown; they may help overcome female immune defenses or even function in male-male sperm competition when females are multiply inseminated.

But this proximate answer will not satisfy the diligent evolutionary psychologist. After all, natural selection could presumably add some sugars or good-tasting stuff to semen if it were advantageous to do so. Why does it not do so?

A moment’s reflection gives the answer.

Natural selection maintains the repugnant taste of semen so that a man’s sperm will wind up in the appropriate place: the vagina and not the stomach. So long as sperm tastes bad, women will not be tempted to swallow it, but will turn their male partner towards conventional intercourse, which of course is the only act that will produce children. In other words, any male with good-tasting sperm would have fewer offspring than his competitors. A man whose sperm tasted like honey would probably not have any children at all.

I can think of only two ways to test this hypothesis, both of them impractical or impossible:

1. If women gave birth through their stomachs, semen would taste great

2. Those males with genes giving them better-tasting semen will leave fewer offspring than other males.

This theory is offered as a modest proposal, only partly (excuse me) tongue in cheek. It may even be true.

Notes added post facto: Although light-hearted, the post is somewhat serious; it’s the kind of interesting speculation that evolutionists indulge in over a few beers. And everything in the post is true, including the survey of women.

And note to T.R. Gregory: I don’t think this idea is refuted by finding, say, primate species that don’t have oral sex but do have similar compounds in the semen. The whole idea rests on those compounds TASTING BAD to females, and we’d need to know something about the taste reactions of females in these other primates. The evolution, after all, might have been in the female taste receptors rather than in the semen.

Finally, apologies to readers who find the subject distasteful.

WEIT reviewed on Bad Astronomy

April 17, 2009 • 8:25 am

The Discover Magazine blog Bad Astronomy has reviewed WEIT in its latest posting.  Some kind words:

As an astronomer, my familiarity with the details of biological evolution are about on par with that of an interested layman (though being trained scientifically helps with that understanding, adding insight to the process of the scientific endeavor). I’m familiar with the concepts of descent with modification, genetic mutations, natural pressures for adaptations, and the like. I’m less familiar with other aspects, like allele frequencies, how specifically pressures can change adaptations, and what transitional fossils are in the record, but I can probably hold my own against your run-of-the-mill creationist.That’s why I loved the book Why Evolution is True by biologist Jerry Coyne. This is a clear, easy-to-understand work that shows you — with no compromising and no backing down — that evolution has occurred, the evidence is overwhelming, and that no other explanation for what we see around us makes sense.  . . .

Creationists love to try to pick apart evolution, looking at minor details in isolation and saying it doesn’t make sense. But they’re wrong: evolution is a beautiful tapestry, a complex fabric of countless threads woven together into a grand picture of life on Earth. And it all holds together.I strongly recommend this book to anyone with even a passing interest in evolution, or the manufactured controversy of creationism. Coyne’s work is complete and convincing, slamming the door firmly closed on young-Earth creationism. If you have to deal with creationists in your life, this book is something you should keep very handy.

Some interesting discussion in the comments, of which there are surprisingly many.  The Discover blogs must get a good readership!

More Louisiana citizens reject than accept evolution

April 16, 2009 • 2:49 pm

A new poll, cited in an online editorial from a Louisiana website, shows  this:

. . . . .40 percent of the respondents believe evolution is not well-supported by evidence or generally accepted within the scientific community. Only 39 percent of the respondents said they believed evolution is well-supported by evidence. Twenty-one percent said they did not know.

No real surprise here — these statistics are in line with national polls in America.

P. Z. gets a column, and more on what counts as evidence for evolution

April 16, 2009 • 9:54 am

P. Z. Myers, the beloved (and also despiséd) author of the popular science blog Pharyngula, has started producing a column on the Guardian website.  His first column is on asymmetry in animals — in particular the gene nodal, which sets up directional (left-right) asymmetries in animals.  P. Z. points out recent research (reference below) showing that snails, who have directionally coiled shells, lose the directionality when nodal is inactivated.   The asymmetry of the human body is also generated in ways similar to that of snails, and again nodal plays a key role.  The gene is somehow involved in determining the directionality of the way cilia (small hairs) beat in the early embryo, which sets up a concentration gradient that can make an embryo left- or right-handed.

I’ve always been fascinated by directional asymmetries — those traits that occur on a consistent side (right or left) in a species.  These include the side of the body that harbors the narwhal’s tusk. and our own internal organs. Other such traits include the direction in which the ears of an owl are turned, and what side of its body a flounder comes to rest on when it changes into a bottom-dweller.  Directional asymmetries are not rare in animals. But how are they formed? How does a gene “know” it’s on the right or the left?  The finding that the direction of cilia movement can tell a gene which side it’s on goes a long way to solving this question, but still leaves open the final question:  why do cilia spin in a given direction? How do they know whether to go clockwise or counterclockwise?  This may, ultimately, reside in the asymmetry of molecules that make up cilia.

At any rate, P. Z.’s column is good, but his explanation of nodal on Pharyngula is even better.  P. Z. has a real talent for explaining science clearly and engagingly, and too often this is overlooked by the hordes of people who visit his blog for the controversy, atheism, and trenchant attacks on religion.  (One thing I’ve found from writing this blog is that visits are much more numerous when I’m attacking something than when I’m talking about science, a fact that’s a little bit sad.)
But the point I wanted to make relates to an earlier post I made about what counts as evidence for evolution.  P. Z.’s  elegant description of how nodal works was hijacked by the Guardian editors by putting it under the title:

Lopsided gene that proves

humans are distant cousins

of the humble snail

A gene shared by birds, fish, reptiles, people – and snails – reveals the fundamental relatedness of all living creatures

Well, we already knew, of course, that we were distant cousins of the humble snail.   We don’t need nodal to tell us that.  And the observation that the gene has similar functions in humans and snails is not, to me, dispositive evidence that humans and snails are related. After all, creationists could always say, “Well of course the gene does the same thing in humans and snails! That’s just the way the Creator decided to make asymmetries!  It doesn’t say anything about common ancestry.”  As I’ve mentioned before, the fact that related creatures use similar genes to do similar things does not count as strong evidence for evolution as opposed to a creationist/intelligent-design alternative.  We might as well say that snails have a gene producing cytochrome c as part of their metabolic pathway, and proclaim that this “proves that humans are distant cousins of the humble snail.”  We share hundreds of genes with the humble snail.

The choice of what to emphasize in a headline is the editors’, not P. Z.’s. And I suppose anything touting evolution is a good thing for readers.  Still, the Guardian editors should realize that hundreds and hundreds of genes already testify to common ancestry — if you choose to use genic similarity as evidence.  I prefer to look at dead genes that are active in relatives as far stronger evidence for evolution against the creationist alternative.

Anyway, congrats to P. Z. for his new gig and a good inaugural column.

Reference: Grande, C., and N. H. Patel. 2009. Nodal signalling is involved in left–right asymmetry in snails. Nature 457:1008-1011.

Happy Easter — Aussie style

April 15, 2009 • 12:57 pm

I’m back, with lots to say, but lots of catching up to do on the day job. Let me first thank Matthew Cobb for a terrific job of filling in. His students get the benefit of his omnivorous readings in the form of a Z (zoology)-letter he sends out weekly, detailing all sorts of interesting animal stuff.

For today, until I shovel myself out from under, I post something for a belated Happy Easter. In WEIT I describe the convergences between marsupial and placental mammals, resemblances that imply that some niches antedate the animals who have evolved to fill them. Although the Australian bilby looks like a rabbit, it isn’t really herbivorous but omnivorous, although it does burrow. There used to be two species, the greater and the lesser bilby (the word “bilby” is aboriginal), but the lesser appears to be extinct. The greater bilby, Macrotis lagotis, is highly endangered due to habitat loss and predation by, among other species, feral cats; you can read about its precarious status here. Only a few hundred remain in the wild. To save the animal, extensive efforts are underway; these include widespread annual sale of chocolate Easter bilbies, which provide revenues for conservation. (In WEIT I mistakenly say “Each spring, chocolate bilbies fill the shelves of Australian supermakets. . .”, and was roundly taken to task by Aussies who pointed out, rightly, that the Australian Easter occurs in the fall.)

So, belatedly, here are some baby bilbies from down under, and the chocolate replicas that are helping save them:

Chocolate bilbies (buy them here):

easter_bilby_dl_3

NB: Goofed again. I am informed that in Australia the penultimate season is called “autumn,” not fall.

The evolution of sex chromosomes

April 15, 2009 • 1:57 am

by Matthew Cobb

Sex is an odd business. In some animals, like us, sex is determined by which combination of a pair of chromosomes the individual carries. Males are XY, females are XX. In birds (and butterflies, for some reason) things are the other way round – males are ZZ, females are ZW.

Sex determination by specific chromosomes is not the rule, not was it the ancestral state – in both plants and mammals it appears to be a relatively recent invention. Zsex can also be determined by overall chromosome number (eg ants and bees), and in some reptiles, like crocodiles, sex is determined by the temperature at which eggs are incubated. Some species are hermaphrodite, while others can change their sex in response to the social or environmental changes or the action of a parasite.

In those species that do have chromosomally-based sex determination (like us), there’s nothing particularly special about the sex chromosomes – they were originally just like the other chromsomes (“autosomes”). But as time goes on, the chromosome that cannot exchange genetic material (the Y chromosome in humans, or the W chromosome in birds) gradually loses its genes. In humans the Y chromosome used to have over 1,000 genes. Now it just has a few dozen. How does the corresponding X chromosome cope with the declining number of genes in its opposite number?

In the fruitfly species Drosophila miranda, a new X chromosome has recently been formed (the “neo-X”). A recent study by Doris Bachtrog and colleagues from Berkely, published in PLoS Biology, has looked at the genes on the neo-X and compared them with those on the ancestral X chromosome. They found clear signs that the genes on the neo-X had recently been the subject of intense selection, as they adapted to their interaction with the Y chromosome. The authors conclude:

“Thus, newly formed X chromosomes are not passive players in the evolutionary process of sex chromosome differentiation, but respond adaptively to both their sex-biased transmission and to Y chromosome degeneration, possibly through demasculinization of their gene content and the evolution of dosage compensation.”

As well as providing a fascinating example of how genes and chromosomes interact to form individuals, this kind of genetic study poses a massive problem for all those who refuse to accept the facts of evolution. What other explanation is there, but that genes, and populations, evolve over time? The only other interpretation is that these signatures of selection were put in the fly’s DNA by the Creator as a whim, a joke, or a way of testing our faith…

This is my last post as the vacation blogger – Jerry is back on dry land tomorrow. Thanks for the comments, and thanks for reading!

Citation: Bachtrog D, Jensen JD, Zhang Z (2009) Accelerated Adaptive Evolution on a Newly Formed X Chromosome. PLoS Biol 7(4): e1000082. Open Access here.