By THE ECONOMIST
Added: Thu, 28 Aug 2008 23:00:00 UTC

Thanks to Ivan Bailey for the link.

http://www.economist.com/science/displaystory.cfm?story_id=12001839

Beetle drive

Dung beetles provide an object lesson in the speed of natural selection

ONE of the lies regularly promulgated by creationist ideologues is that you cannot see evolution in action right now. For microorganisms this is obviously untrue. The evolution of new viral diseases, such as AIDS, is one example. The evolution of antibiotic-resistant bacteria is another. But bacteria and viruses breed fast, so natural selection has time, within the span of a human life, to make a difference. For species with longer generations, examples are less numerous. But they do exist.

A new one has just been published, appropriately, in Evolution. It concerns dung beetles. Harald Parzer and Armin Moczek, of Indiana University, have been studying a species called Onthophagus taurus. Or, rather, it was a species 50 years ago, but it is now heading rapidly towards becoming at least four of them.

Onthophagus taurus lives naturally in southern Europe and the Middle East, but it has booted about a bit and is now found in many other places too. Mr Parzer and Dr Moczek looked at beetles from the east and west of Australia (where it was deliberately introduced to deal with the dung of non-native livestock) and North Carolina, together with an aboriginal population from Italy. Their interest was the trade-off that the males of various populations make between what they delicately describe as the beetles' primary and secondary sexual characteristics—in other words, their penises and their horns. The researchers' hypothesis was that the bigger the horns, the smaller the penis, and vice versa.

Male Onthophagus beetles use their horns to fight over females. Lose a fight and you do not get to mate. On the other hand, if you do get to mate, having big sexual organs is likely to increase the chance that it will be your sperm, rather than another male's, that fertilise the female's eggs. Beetles, like butterflies and moths, have a four-stage life cycle of egg, larva, pupa and adult. Mr Parzer and Dr Moczek hypothesised that given the limited resources available to make an adult beetle (in other words, the flesh of the larva that made the pupa), those parts of the adult focused on reproducing will take a constant chunk. Exactly how that chunk is allocated will depend on the local conditions the adult has to face. The more fighting it is likely to have to do, the more its horns will require, and the less will be left over for its penis.
The horns of a dilemma

As they predicted, the two researchers found that the bigger a beetle's horns, the smaller its penis. More importantly, though, the ratio was different in each of the four populations, but similar within each population. That suggests it is being set by local natural selection in each place. Moreover, Mr Parzer and Dr Moczek also looked at ten other species of Onthophagus to see whether the trade-off applied to them too. It did. The ratio of horn to penis size was different in each species, but consistent within a species.

Given the need for male and female organs to fit together, the researchers suggest that selection of horn size might be the main method of speciation in Onthophagus. Horn size determines penis size. Penis size then dictates vagina size. That stops crossbreeding between groups and provides the reproductive isolation that groups need to evolve into species.

As evidence, they point out that the genus Onthophagus has 2,400 species—more than any other in the animal kingdom. And their work suggests it is just about to get three more, in the shape of the east and west Australian, and North Carolinian populations, if, indeed, these groups are not species already. Since it is known when these populations were introduced, and none is more than half a century old, evolution seems to have worked its wonders well within a human lifetime. Darwin, no doubt, would have been delighted.

TAGGED: BEHAVIOR, BIOLOGY, EVOLUTION