sexual selection

I really can't get over how much work went into the cichlid pigmentation paper that's out in the current Science (Roberts et al. 2009). The paper examines the genetic basis for a "orange blotched (OB)" phenotype. It's a simple kind of genetic question, and it hooks into a story that evolutionary biologists like -- sex conflict in gene expression.

But wow how much work they did to iron out all the details. An example:

Pedigreed families from several species of the genera Labeotropheus and Metriaclima were genotyped for newly identified microsatellite markers on LG5 (46 families, 678 individuals, 349 OB individuals; table S1). Breakpoint analysis localized OB to an interval of less than 1 cM, corresponding to a region from 3.9 to 4.0 Mb on Tetraodon chromosome 11 (Fig. 2A). We then used association mapping in natural populations to pinpoint the causative locus. Single-nucleotide polymorphism (SNP) markers were developed spanning the region, including an intronic SNP marker for each annotated gene within the OB interval. We measured linkage disequilibrium (LD) between OB and SNPs within natural populations of Labeotropheus, Metriaclima, and Tropheops from the northern and southern parts of Lake Malawi (Fig. 2B). Within each population, a peak of LD was found in a region overlapping the interval defined by breakpoint analysis (Fig. 2, C to G). Given the shared pattern of association across study populations, we created a lake-wide mapping panel to increase the effective size of the mapping population. This lake-wide panel contains BB and OB individuals from 36 distinct populations of 12 species segregating OB (table S2). Analysis of marker data across these populations increased the resolution and statistical significance of LD beyond that available from any single population (Fig. 2H) (taxonomic names not italicized, because I'm in a Viennese cafe and too lazy right now, OK?).

This is the kind of stuff that a few years ago took multinational networks of cooperation; now it can be done in the context of a single lab with association mapping assisted by field biology.

It helps a lot that the trait is conspicuous and basically Mendelian. I would guess they had a strong presumption that it would be near the sex-determination locus, since the OB phenotype is very rare in males. They even end up concluding that the rare males who have it are probably products of some alternative sex-determining mechanism. So there were hints -- and the hints went along with the idea of testing the hypothesis of sex conflict.

But you can see the potential here for widespread genomic work on cichlids. There are an endless number of questions, with hundreds of species in the three big African lakes. These are Lake Malawi cichlids in this paper (for the most part), but Victoria cichlids in particular are so young that you can figure most of their phenotypic differences will be genetically simple -- it's like postglacial stickleback evolution, but with hundreds of different switches. And each one is telling a story about the segregation of original genetic variations in rapidly speciating populations, the spread of new alleles under rapid selection, and the tectonics of the adaptive landscape that occur as genetic backgrounds quickly shift.

All this rich field biology, with the bonus that you can keep a bunch of inbred lines in fish tanks to test for Mendelian ratios and other classical genetic observations (as they did in this study).

Although Lake Victoria cichlids haven't been around very long, their generations are a lot shorter than ours. So if you were thinking to compare the cichlid case with human timescales, like since the Last Glacial Maximum, think of what those fishes may have been like 1000 years after they reached the lake. They probably speciated fast, and we (of course) haven't done so at all. Yet in other respects this may be an interesting comparison -- and may be even more so as biologists move beyond the conspicuous pigmentation (and we of course do have many recent pigment variations...) and consider the genetics of habitat specializations and diet adaptations.

References:

Roberts RB, Ser JR, Kocher TD. 2009. Sexual conflict resolved by invasion of a novel sex determiner in Lake Malawi cichlid fishes. Science 326:998-1001. doi:10.1126/science.1174705

Gretchen found this one:

Naughty Neanderthals nixed monogamy

Oh, no. Where is this going?

Emma Nelson of the University of Liverpool and a team of researchers combed through literature on early human-like primates in search of fossils that contained hands with intact index and ring fingers (the second and fourth digits).

Oh, no. Now I can see where this is going. It's the magical anthropometric: the 2D:4D digit ratio!

Though highly contentious, studies indicate that men who receive high levels of androgen before birth are more likely to be stronger, faster, and more sexually competitive. Women who receive high levels of androgen may have similar traits.

Well, it's true -- a number of studies have found 2D:4D correlated with lots of things. My favorite is the one about international athletes:

We have found that low 2D:4D ratio is associated with a high level of attainment across a number of sports and also with high mental rotation scores. Professional football players had lower 2D:4D ratios than controls; 1st team players had lower ratios than youth team members or reserves; international players had lower 2D:4D ratios than those who have not yet represented their Country; and in a one-tailed test, 2D:4D ratio was negatively related to number of international appearances after the affect of Country was removed. We suggest that low 2D:4D ratio in men is a correlate for high ability in many sports, including football (Manning and Taylor 2001).

The problem is that the ratios have a high variability within groups like this. So that the 2D:4D ratio may be correlated with performance, but it's a poor predictor of performance. It's not like you'd want to measure this ratio on one person and then start talking about whether he would be a good football player.

Of course, if you're dealing with fossil hominins, then you're stuck with what you can dig up:

Nelson's work suggests the same holds true for most primates living today, but the team wanted to see how our ancient relatives stacked up. They found two Neanderthals and one Australopithecus afarensis skeleton with the first bones of the index and ring fingers intact — enough detail to do the job.

The digit ratio varies a lot within humans, much more than its correlation with anything. So it would take a very extreme ratio in these two Neandertals to say you had any significant evidence of anything -- we're talking, outside the human range. And since we're not talking about complete hands, but instead parts of hands, I don't think we're going to get there.

Well, what about Australopithecus?

A. afarensis, made famous by the popular "Lucy" skeleton, lived between 4 and 3 million years ago, long before modern humans. Its short ring finger hints that it was faithful to a single mate, but Nelson says that doesn't sit well.

"These were small creatures that probably lived in groups and were being eaten by predators." she said. "How do you keep from mating with different members of the group?"

I'm sure the conclusion of monogamous early hominins would make Owen Lovejoy happy, but it's really just utter sheer speculation, which a quote from Nelson clarifies later in the article. Nelson has a good website, where we can see where her research is going. The description along the same lines, but more reasonable than the "Naughty Neanderthals" article -- the idea is that we already know that the 2D:4D ratio varies within primate species and that it has several interesting social correlates within some species. For example, it correlates with social rank in female macaques. So there's some reason to think that it might vary among species in a way that reflects social systems, which (according to Nelson) they've found.

I just want to remind people of the obvious: 2D:4D may be correlated with mating system in primates, but that doesn't mean it's a good predictor of mating system. Canine dimorphism, which has been studied for a long time in relation to sexual dimorphism and mating competition, isn't even that good a predictor of mating system. And even if the mean were a good predictor among species, doesn't mean that an individual ratio is a good predictor of the species mean.

As fossil hominins go, I wouldn't expect the story to go any further -- there just aren't many hands, so there's never going to be a significantly predictive result.

And as stories go, this one could have been a lot worse. After all, the digit ratio is also correlated with homosexuality in men. I'm surprised the headline wasn't "Fingers Point to Gay Neanderthals".

References:

Manning JT, Taylor RP. 2001. Second to fourth digit ratio and male ability in sport: Implications for sexual selection. Evol Hum Behav 22:61-69. doi:10.1016/S1090-5138(00)00063-5