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October 24, 2008

An evolutionary sampler

This week, I offer short summaries of several papers (and links to abstracts) rather than my usual longer analysis of one or two papers. Let me know if you see a nice blog summary of any of these, and I will add a link.

Note that this week's crop of evolution papers, from just three leading peer-reviewed journals (Nature, PNAS, and Proceedings of the Royal Society), equals the total number of peer-reviewed journal articles claimed by "intelligent design" (in the International Journal of Fuzzy Systems, etc.) over the entire history of this failed idea.

Reproductive skew in female common marmosets: what can proximate mechanisms tell us about ultimate causes?
Marmosets are monkeys; "proximate"="how does it work?" whereas "ultimate"="why did it evolve?" Subordinate females rarely ovulate and are less sexual because of past natural selection, imposed by infanticide against the offspring of subordinate females by dominant females.

Migration strategy and divergent sexual selection on bird song
Two components of sexual selection, female choice and male competition, can be hard to separate. Male blackcap birds sing differently to females, who prefer complex songs, and to other males, who are more intimidated by more repetitive songs. Comparing songs among populations suggests that female choice is more important in migratory populations and male competition is more important in nonmigratory populations. The latter should be particularly relevant to African swallows.

Hippocampal volumes and neuron numbers increase along a gradient of environmental harshness: a large-scale comparison
Chickadees living in colder climates had brain structure consistent with natural selection for ability to remember the location of cached food. [But could it be individual acclimation to cold climate or a maternal-environment effect rather than genetic evolution of populations?]

Selection and gene flow on a diminishing cline of melanic peppered moths
A favorite example of recent evolution was apparent in patterns over space as well as time and still is. Explaining the current pattern requires considering migration as well as natural selection.

An amino acid polymorphism in the couch potato gene forms the basis for climatic adaptation in Drosophila melanogaster
They found a gene for diapause (similar to hibernation) in fruit-flies, which shows the geographic pattern expected with natural selection imposed by winter weather.

Infrared radiation from hot cones on cool conifers attracts seed-feeding insects
Insects that feed on pine cones use infrared radiation to find them. [Trees may produce cones in batches too large for all the insects to eat, but maybe they should just chill.]

DNA variation and symbiotic associations in phenotypically diverse sea urchin Strongylocentrotus intermedius
Two visually-distinct forms of sea urchin aren't genetically different enough to be species, but they consistently harbor different symbiotic bacteria. Maybe adaptation to these different symbionts will eventually lead to speciation.

A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers
Who're you calling "bizarre?" Another fossil that should help us understand the evolution of birds from dinosaurs.
For more information, see the discussion by Carl Zimmer or this one by PZ Meyers.

October 18, 2008

Cooperative fish, cheating ants

Cooperation is widespread in nature, despite theoretical predictions that "cheating" mutants could displace cooperators over just a few generations of evolution. We don't apply human moral standards to other species, of course, but define cheating as contributing less, while benefiting from activities of others. The evolutionary persistence of cooperation is usually attributed to reciprocity (trading resources or services) or to kin selection: cooperation among relatives, such as parental care, can persist even without reciprocity. Fish that clean parasites from other fish are a standard example of reciprocity -- they get to eat the parasites -- whereas nonreproductive worker ants are a standard example of kin selection. I will briefly discuss one recent paper on each of these.

Redouan Bshary and coauthors report in Nature that "Pairs of cooperating cleaner fish provide better service quality than singletons." Cleaner fish often prefer to eat client mucus (yum!) than client parasites, but clients don't like this and tend to leave. When a male and female cleaner work together, the client fish may leave if either of them takes a bite of mucus. Females, in particular, were less likely to do this when cleaning with their male partner rather than alone. The authors also did an experiment to see whether cleaner fish would eat a less-preferred food (fish flakes, perhaps analogous to client parasites) if eating their more-preferred food (prawns, perhaps analogous to client mucus) resulted in the food plate being taken away. They did, especially the females. This may have been because the male often chased her if she ate a prawn, costing them both the rest of their meal. Overall, pairs appear to provide better service to clients, because they are better-behaved together than alone, especially the female.

Shigeto Dobata and coauthors reported on "Cheater genotypes in the parthenogenetic ant Pristomyrmex punctatas" in Proceedings of the Royal Society. Social insects, such as ants and bees, usually have reproductive queens and nonreproductive workers. Worker genes are transmitted to the next generation by the queen, who is typically the workers' mother and therefore shares most of their genes. Pristomyrmex punctatas is different. An individual ant may reproduce (usually when young) and also work. Some individuals are more like queens, however. These are larger, reproduce more, and do little or no work for the colony. If these nonworking ants were close relatives of the workers, this behavior could perhaps be maintained by kin selection. It could be an example of division of labor for mutual benefit, a less-extreme version of the more-familiar worker/queen division. So Dobata and coauthors analyzed the DNA of hundreds of ants to see how they were related. They found that these nonworking ants were much less closely related to workers than queens usually are. Most of the ants (working or not) reproduced parthenogenetically, essentially cloning themselves without sex. Working hard while unrelated individuals profit from your work doesn't usually work out over the long run, but these nonworking ants have been seen in the field for over 25 years. Is this an evolutionary dead end? A similar situation with Cape honey bees, whose colonies are parasitized by unrelated "pseudoqueens" usually leads to colony extinction. I look forward to reading more about this interesting ant species.

October 10, 2008

Experimental evolution of predation and sexual attractiveness

Fossils and DNA-sequence comparisons among species are like fingerprints and other clues found at crime scenes. We can often draw reliable conclusions about past events (evolutionary or criminal) from such physical evidence, but some people prefer eyewitness accounts. So evolutionary biologists are increasingly doing experiments that let us see evolution in action. Evolution is a change over generations, so the short generation times of microbes make them especially useful for experimental evolution.

Two recent examples, both published in Proceedings of the Royal Society, are "Experimental evolution of a microbial predator's ability to find prey", by Kristina Hillesland, Greg Velicer, and Richard Lenski, and "Experimental evolution of a sexually selected display in yeast", by David Rogers and Duncan Grieg.

Predation

Mxococcus xanthus bacteria hunt in packs, surrounding other bacteria and releasing enzymes that break them open, then taking up the nutrients released. Hillesand and colleagues hypothesized that if their prey were farther apart, M. xanthus populations might evolve traits that would let them cover more ground, looking for prey. To test this hypothesis, they grew M. xanthus for a year (many generations) on culture plates with either closely or widely spaced patches of prey bacteria. Sure enough, evolving in an environment with widely spaced prey bacteria led to greater improvement in the ability to find more prey patches per day.

"Optimal foraging" theory predicts tradeoffs between the ability to find and "process" prey. That didn't seem to be the case in these experiments, although they only measured how fast the evolved predators moved through prey patches, not (say) their rate of nutrient uptake there per hour. There did seem to be a tradeoff between hunting speed and spore production. Like hunting, spore production is a cooperative activity in M. xanthus. To confirm this tradeoff, they would need to let populations evolve in an environment that selects for greater spore production, to see whether this reduces hunting speed as a side-effect.

Sexual selection

Some individuals are more attractive to potential mates than others, so genes for attractiveness are assumed to spread by "sexual selection." The peacock's tail is the best-known example, but we don't know the genetic basis of traits that make peacocks attractive to peahens. Also, bird generations are long enough to make multigeneration evolution experiments difficult. Yeasts (single-cell fungi, thought to play a major role in sexual attraction among humans by producing ethanol) have much shorter generation times, but present their own challenges.

Sexual attraction among yeast sex cells -- there are two mating types, a and alpha, analogous to sperm or eggs, except that they are similar in size -- is based on chemical signals, similar to the role of odors in many animals. The authors reasoned that producing more signal should increase the chance of finding a mate, at least when competition for mates is strong. So they competed alphas differing only in how much signal they produce. (Weak signalers were genetically engineered to produce less signal.) They created strong or weak mate competition among alphas by adjusting the ratio of alphas to a's. It may help to think of the alphas as males and a's as females, and the two environments as colleges differing in male:female ratio.

When "males" outnumbered "females" (strong competition among males for mates) the frequency of males that were strong signalers increased over generations (to 85% from an initial value of 1%), because they were much more likely to attract one of the few "females." There was much less increase in the frequency of strong signalers when "females" outnumbered "males." Not a surprising result, perhaps, but this is apparently the first time sexual selection has been shown to increase the frequency of a particular gene responsible for attractiveness.

Earlier, I contrasted physical evidence with eyewitness accounts. Actually, eyewitnesses are often less reliable than fingerprints or DNA evidence in proving whether a particular person was at a crime scene. Experimental evolution of microbes is easy enough, however, that similar experiments will probably be repeated by others, giving us the equivalent of multiple independent eyewitness accounts documenting the same evolutionary process.

October 4, 2008

Book review: Uneasy relations

Selective forces in the environment change, and we, or any other organisms, respond to those forces, not to some long-range design or some supposed future condition. If we – oh, heck, you know all that. -- "Skeleton Detective" Gideon Oliver in Aaron Elkins' Uneasy Relations
If you like mysteries and haven't discovered Aaron Elkins yet, his latest novel is a reasonable place to start. Enjoy it yourself, then lend it to someone who might benefit from the passing references to evolution. The "you" who knows all that is Gideon's park-ranger wife, Julie. One or the other of them is always getting invited to a meeting somewhere interesting, with the other usually tagging along. His technical expertise and her clear thinking come in handy when, inevitably, someone gets murdered. This time, the meeting is in Gibraltar and revolves around Neanderthal remains found there. If you want to compare fact with fiction, head on over to John Hawkes' weblog and then read the book.