Category Archives: science

Back to basics: The “Big Four”

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ResearchBlogging.orgThe nice thing about a field season away from all regular internet access is that it gives you a real sabbatical of a sort—a chance to reassess plans and set new goals. One of the new goals I set myself this last field season was to introduce a new kind of topic here at Denim and Tweed.

Most of my writing about science at D&T focuses on recently published discoveries in evolution and ecology. It’s fun writing, and it coincides neatly with my regular journal reading, and I intend to keep doing it. But I’ve discovered that when I want to put new work in context, I often need to discuss fundamental concepts of evolutionary biology that aren’t necessarily common knowledge, such as genetic drift or sexual selection. However, I rarely have room to explain these concepts in depth within a blog post devoted to something else.

So maybe the solution is to devote some posts to explaining these “basics.” I’m going to start with a series of posts on the “Big Four” processes of population genetics. These are the four processes that account, in one way or another, for every change in the frequency of genes within natural populations. In other words, the Big Four account for much of evolution itself. They are:

  • Natural selection, changes in gene frequencies due to fitness advantages, or disadvantages, associated with different genes.
  • Mutation, the source of new forms of genes;
  • Genetic drift, or changes in gene frequencies that arise from the way probability works in finite populations; and
  • Migration, or changes in gene frequencies due to the movement of organisms from site to site.

Lay readers may be surprised both by what we know, and what we don’t, about how these four processes operate in nature. Natural selection is relatively easy to measure, and apparently ubiquitous [PDF] in natural populations—but we don’t know how often the resulting short-term changes impact evolution over millions of years. Mutation, the source of variation on which natural selection acts, seems to vary widely among living things. Genetic drift means that a trait can come to dominate a population even if it has no fitness effect—or sometimes a deleterious one. Finally, migration across variable landscapes can interact with selection, drift, and mutation [$a] to completely alter their effects.

I’ll devote one post each to selection, mutation, drift, and migration, discussing classic findings as well as more recent scientific discoveries about each. They’ll arrive as my usual mid-week science posts for the next four weeks, and I’ll update this post with links to the others as they go online—so if this looks worth following, you can either bookmark this post, or subscribe to D&T’s RSS Feed.

Natural selection, mutation, genetic drift, and migration act together to shape the evolution of natural populations. Photo by jby.

References

Drake JW, Charlesworth B, Charlesworth D, & Crow JF (1998). Rates of spontaneous mutation. Genetics, 148 (4), 1667-86 PMID: 9560386

Kingsolver, J., Hoekstra, H., Hoekstra, J., Berrigan, D., Vignieri, S., Hill, C., Hoang, A., Gibert, P., & Beerli, P. (2001). The strength of phenotypic selection in natural populations. The American Naturalist, 157 (3), 245-61 DOI: 10.1086/319193

Slatkin, M. (1987). Gene flow and the geographic structure of natural populations. Science, 236 (4803), 787-92 DOI: 10.1126/science.3576198

Wright S (1931). Evolution in Mendelian populations. Genetics, 16 (2), 97-159 PMID: 17246615

Science online, back on track edition

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Every little bit helps. Photo by jby.

Here’s what caught my eye when I finally picked up the old RSS feeds this week.

  • I will not call this a “fish story.” While overfishing is (un)naturally selecting most species for smaller body size, tournament marlins (which are only fished for sport) have gotten bigger over the last fifty years. (Southern Fried Science
  • Not a neutral question. The number of species in a community may determine whether the makeup of that community is more due to chance, or the competitive ability of its members. (The EEB & flow)
  • You can always do better than nothing. Just a single tree in the middle of an agricultural field can boost the diversity of birds and bats found in the area. (Conservation Maven)
  • Well, it is an earthworm. Scientists at my own University of Idaho have captured specimens of the Palouse Giant Earthworm, which hasn’t been seen since 2005. It turns out to be somewhat less than giant. (NPR)
  • Maybe they’re allergic? Elephants warn each other away from bees. (The Thoughtful Animal)
  • Future work to be funded by Best Buy. Octopuses aren’t fooled by video images at old-fashioned resolution, but respond to HDTV images as though they were real. (Observations of a Nerd)

And finally, via The Other 95%, a praying mantis encounters a hummingbird.

While I was out

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Things that happened while I was in the middle of the Nevada desert harassing Joshua trees:


Yeah, it was worth it. Photo by jby.

Well, I’m back

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So another field season came to a close Friday, and after a 1,000-mile drive from Nevada, I’m in Salem for the week, tying up loose ends and getting ready for the Evo-WIBO 2010 meeting next weekend. I’ll probably write up something in greater detail—and announce D&T’s first special series of posts—in the next few days, but for now, here’s my final set of field season photos.

It’s that time of year …

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Joshua trees are about to bloom. Which means I’m off to the desert until mid-April first to tour Joshua Tree National Park with my parents for a week, then to spend a month or more at a field site in central Nevada, extending studies of co-divergence in Joshua tree and its pollinator moths.

All of which is to say, posting to D&T is about to drop to near-zero for the foreseeable future. I’ll take lots of photos, and put them online when I get to an Internet connection, but really that’s all I can promise. After all, what good is fieldwork if not as an Internet detox?


Photo by jby.

Science online, virgin birth chromosomes edition

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Now this is a radical feminist. Photo by J.N. Stuart.
  • Males replaced by an extra round of DNA replication: Female whiptail lizards can lay fertile eggs without the help of a male because they start egg formation with extra copies of their chromosomes. (Not Exactly Rocket Science)
  • Pathogens. It’s always pathogens: In his inaugural article as a member of the National Academy of Sciences, Robert Rickleffs proposes that coevoluiton with pathogens explains most of the diversity of life on Earth. (Coevolvers)
  • Evolutionary conservation biology? To conserve the diversity of life, we need to know how it evolved in the first place, and how it might evolve in the future. (The EEB & flow)
  • Plant vs. plant: The spread of one invasive plant can be checked by creating barriers of native plants. (Conservation Maven)
  • You mean it’s not just to make winter that much more miserable? Flu cases may peak in winter months because drier air transmits the flu virus more effectively. (Influenza A (H1N1) Blog)
  • Unintended consequences: Fifty years of selecting foxes at a fur farm for their tameness also changed the shape of their ears and tails. (The Thoughtful Animal)
  • The virus only has to get lucky once: Even as we find new ways to attack HIV, the virus keeps mutating; which is why the “cocktails” of drugs taken by HIV patients must target many different viral proteins. (The Daily Monthly)
  • We all made a big mistake in coming down from the trees in the first place: Mammals that live most of their lives up in the trees tend to live longer than similar-sized mammals that live on the dangerous, dangerous ground. (New Scientist, Gene Expression)

Finalist!

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Research Blogging Awards 2010 FinalistThe finalists for the ResearchBlogging 2010 Awards have just been announced, and D&T is now a finalist in the category of “Best Blog — Biology.” The other finalists in that category include some extremely strong entrants, ranging from group blogs (Southern Fried Science) to single-authored ones (Observations of a Nerd) and writing for general audiences (Mystery Rays from Outer Space) as well as scientists (The EEB & flow). In company like this, it’s an honor, as they say, just to be nominated.

Voting for the winners in each category will be open to ResearchBlogging.org members starting 4 March.

Invasive species runs out of evolutionary “steam” as it invades

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ResearchBlogging.orgFor invasive plants, flowering time is a trait that may often be under selection during colonization—when a plant flowers determines its climatic tolerances, its vulnerability to herbivores, and its compatibility with the local pollinator community. In a study just released online at Proceedings of the Royal Society B, Colautti and coauthors examined the evolution of this trait in a plant that has swept across eastern North America since its introduction from Europe: purple loosestrife, and found that it may be reaching the evolutionary limits of its invasive-ness.


Purple loosestrife, Lythrum salicaria, may be running out of evolutionary steam as it invades more northerly climes. Photo by Steve_C.

Loosestrife, like many organisms, faces a trade-off in establishing a time to reproduce, between early flowering and accumulating resources for seed production. Early flowering means producing fewer seeds, or provisioning them less thoroughly—but as loosestrife colonizes more and more northerly climes, it will be under selection to flower earlier in compensation for shorter and shorter growing seasons.

But natural selection can only do so much. In order for natural selection to operate on a population, individuals in the population must vary in some trait that affects how many offspring they produce—if all individuals have the same trait value, or the same number of offspring, they’ll all have equal chances to contribute to the next generation, which will probably look pretty much like the parental generation. Furthermore, species colonizing new territory may actually lose variation, either because new popualtions may be founded by just a few individuals, or because of the action of natural selection itself. Finally, there may be a point at which plants simply cannot flower any earlier, because they must reach a certain developmental point before reproducing.

Add these up for an invasive species moving north, and you might expect that the most recently-arrived (and most northerly) populations would flower earlier, and have less variation in flowering time, than more southerly populations. Using theoretical and experimental approaches, Colautti et al. show that exactly this process is occurring in purple loosestrife. They first built a mathematical model of natural selection acting on flowering time, which behaved as I’ve outlined above. They followed this by raising loosestrife seeds from northern and southern populations together in experimental sites located at different latitudes, and found that, even raised in southern conditions, seeds from northern sites grew into smaller, less productive plants. Raised in greenhouse conditions, seeds from southern populations produced plants with a much wider range of flowering times than seeds from northern populations. Together, these suggest that loosestrife has evolved earlier flowering times at northern sites—and may be running out of variation, the raw material for natural selection, as it moves north.

Invasive species often evolve in response to their new habitats, and force native species to evolve in response to their arrival. As they colonize Australia, for instance, cane toads (soon to be a major motion picture) have evolved longer legs [PDF] so as to win the race for unoccupied breeding ponds; and exerted selection on native black snakes to tolerate the toads’ defensive toxins and to attack the toads less frequently. Better models of how newly introduced species respond to and exert natural selection may help conservation biologists anticipate the results of biological invasions.

References

Colautti, R., Eckert, C., & Barrett, S. (2010). Evolutionary constraints on adaptive evolution during range expansion in an invasive plant. Proc. R. Soc. B DOI: 10.1098/rspb.2009.2231

Phillips, B., Brown, G., Webb, J., & Shine, R. (2006). Invasion and the evolution of speed in toads. Nature, 439 (7078) DOI: 10.1038/439803a

Phillips, B., & Shine, R. (2006). An invasive species induces rapid adaptive change in a native predator: cane toads and black snakes in Australia Proc. R. Soc. B, 273 (1593), 1545-50 DOI: 10.1098/rspb.2006.3479

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification Trends in Ecology & Evolution, 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017