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April 12, 2008

Fear of flying -- in plants

“Every one is familiar with the difference between the ray and central florets of, for instance, the daisy… But with respect to the [two types of] seeds, it seems impossible that their differences in shape…can be in any way beneficial”—Charles Darwin

The theory of evolution is famously linked to the Galapagos Islands, but this week’s paper “Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta,” published in Proceedings of the National Academy of Science, studied much smaller “islands.” In an urban environment dominated by concrete, patches of soil around sidewalk trees (below left) are among the few places where plants can grow.
CrespisBlog.jpg
Photo credits: Gilles Przetak and Eric Imbert.

Members of the daisy or sunflower family (Asteraceae) often produce two types of seeds (above right) on the same disk-shaped composite flower head. Seeds from the center of the disk are light in weight and plumed, so they are easily dispersed by wind. Those from the outer edge of the disk are heavier and not plumed, so they tend to fall near the mother plant. Although Darwin apparently failed to see the benefit of having two types of seeds, this kind of diversity acts as a form of bet-hedging. Wind dispersal of seeds over a wide area decreases the chances that all of a plant’s offspring will be killed.

Then why not disperse all of the seeds? Because, given that the mother plant managed to reproduce -- many plants don't -- conditions near the mother plant may be better than where most wind-blown seeds might land. This was particularly true in the study discussed here. Earlier, Jonathan Silvertown pointed out, in an essay titled “When plants play the field," that the ratio of the two seed types changes in beneficial ways with changes in flower head diameter. The area of a disk increases four-fold as the circumference doubles, giving proportionally more of the wind-dispersed central seeds. So the plant will always drop some seeds in the same place that it managed to reproduce. But if favorable conditions lead to larger flower heads, more seeds will be dispersed by wind over a larger area, where they can compete with other plant's seedlings rather than with each other.

So, without any genetic change, this disk-size dependence adjusts the ratio of dispersing to nondispersing seeds to match current conditions. But what if conditions consistently favor more or less seed dispersal? Can this ratio also evolve, with a genetic change over generations?

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April 07, 2008

Evolutionary trees

This week's paper is "Rapid evolution towards heavy metal resistance by mountain birch around two subarctic copper–nickel smelters", published in the Journal of Evolutionary BIology by J.K. Eranen.

Evolution is a change over generations, so evolution is typically faster (more change per year) in species with short generation times. Signficant evolutionary change in bacterial populations, therefore, can take only a day or two, under ideal conditions. Long-lived species like humans and trees evolve, too, but it takes much longer. So, for example, are trees likely to evolve fast enough to survive climate change?

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December 07, 2007

The ghost of infections past, present, and future

Summary: A 39-year record of host-parasite interaction, recovered from sediment layers in a pond, is consistent with rapid coevolution.
Link: Host-parasite /`Red Queen/' dynamics archived in pond sediment

As I've discussed previously, archival samples often prove useful for answering questions that weren't being asked when the samples were collected. But what if nobody collected and preserved the samples you need for your research? Maybe you can find a "natural archive" that has what you need.

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September 11, 2007

Evolving enzymes in the lab

This week's paper is another example of how nonrandom selection from among random variants can solve problems so difficult that we are unable to "design" a solution. As in an earlier post, the selection process was automated, not requiring the human judgement used in breeding crops or dogs.

"Selection and evolution of enzymes from a partially randomized non-catalytic scaffold" was written by Burckhard Seelig and Jack Szostak, both of Boston, and published in Nature (448:828). Their goal was to evolve an enzyme to link two RNA bases together in a particular way, a reaction not found in nature.

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May 29, 2007

Coevolution and gene flow

Two species coevolve when changes in either lead to changes in the other. This includes “arms races” between species that compete with each other, but also interactions that benefit both species. “Gene flow” is the movement of genes from one population into another, of the same or related species. For example, some genes in modern cows seem to have come from mating with wild aurochs, before they went extinct. Gene flow often provides new genes; some may be useful to the recipient population. For example, pollen from transgenic sugar beets could transfer herbicide resistance (along with other crop genes) to related weed beets. More often, genes that were useful in the source environment may be harmful to the recipient population. Natural selection will tend to eliminate these, unless gene flow rates are too high. For example, if plants growing on toxic soil around an old mine are outnumbered by neighbors on nontoxic soil nearby, gene flow may swamp natural selection, preventing evolution of tolerance to toxic soil.

This week I’ll discuss a review article on coevolution and then an experimental paper showing how gene flow can affect coevolution. The review is “Variable evolution” by Elizabeth Pennisi, published in the May 4 issue of Science. It discusses coevolution of wild parsnip with the webworms that eat them and coevolution of pine trees with birds and squirrels, among other topics.

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May 06, 2007

How disturbed are most cheaters, really?

Yesterday, my wife asked, "why are there so many theoretical papers in evolutionary biology?" I suggested one reason may be that evolutionary theory is better developed, in the sense of making accurate predictions, than theory in much of biology. This week's paper, comparing results from an evolution experiment to predictions of a mathematical model, is a good example.

The paper is about the evolution of cooperation. This is a hot topic and also my own area of research. Humans enforce cooperation, to varying extents. For example, we often punish cheaters, those who try to benefit from cooperative activities of others without contributing anything themselves. Human cheaters are mostly pretty stupid -- don't even think about plagiarizing this blog for a term paper! -- but what about cheaters with no brains at all?

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