Monday, February 16, 2009

Variation and Natural Selection

Here's a photograph of many varieties of hybrid radish. It's from the Botany Photo of the Day posted on Darwin's birthday by Nhu Nguyen to illustrate speciation in action.

Please visit the UBC website to see the entire posting. I'd like to comment on one particular statement. Nhu Nguyen writes,
This is a weedy species that grows in coastal (and some central) areas of California. According to research by Norman Ellstrand's group at UC Riverside, this species is evolving in a quantifiable manner. It is a hybrid between Raphanus sativus, the common radish, and Raphanus raphanistrum.

Curiously, the same hybrid occurs elsewhere in similar climates such as that of South Africa, but something special about ecosytems in California allowed it to proliferate. It is now different enough from either of its parents that Ellstrand's group is considering describing this as a new species. This has occurred within the timespan that the two parents were brought together by humans in California."

There are many color variations of this evolving species. It is exactly through this variation that the process of natural selection works. If allowed to go its own way, some of these color morphs may persist, others may perish, all depending on the selective forces present where they occur. Eventually, each of these via time and selection could become a species of its own. California thus would be the center of diversity for a new group of Raphanus species.
What is the evidence that natural selection is acting one these variants? I doubt that there's any evidence at all.

While it's true that evolution may result in many of these variants becoming separate species, there's no reason to suppose that there are "selective forces" working on different colored flowers. It could just as easily happen that one or more colored variations could become fixed in a new species by random genetic drift.

I'm not sure what the problem is here. Is it just sloppy language on the part of some botanists? Do they use the words "natural selection" as a synonym for "evolution" without thinking about it? Or, are they confirmed adaptationists who actually believe that all visible phenotypes must be subject to selection?

There seem to be a large number of scientists who think that all speciation events are driven by natural selection. This was (mostly) what Darwin thought but I was previously under the impression that this had changed in the 20th century to recognize that random genetic drift plays an important role in speciation.


  1. Larry

    While I understand and appreciate your point, the opposite can and does happen. That is, drift is assumed when natural selection is in fact operating. A famous example is Sewall Wright's studies on variation in Linanthus flower color. He thought drift was responsible for the differences in the frequency of blue and white flowers in different populations. Decades later Schemske and Bierzychudek (Evolution 55(7):1269-1282. 2001) showed instead that natural selection favors traits linked to flower color in different environments. So, you are right that we should not just assume our favorite evolutionary force is acting but it is not selection that is always assumed.

  2. and in this case there is experimental evidence that natural selection does act on flower color variation in radishes (Irwin et al. Ecology, 84(7), 2003, pp. 1733–1743)

  3. I agree wit larry. Species hybridization as a mechanism for the origin of new species is NOT natural selection.
    AND, species hybridization has beeen seriously underestimated as a cause of speciation, despite been pretty wel documented in both plants AND animals. Specially alopolyploidy

  4. Speciation needs sexual selection to be fully understand. Overcome the Status Quo and its Natural Selection exclusivity! See how here:

  5. Rui Monteiro says,

    Speciation needs sexual selection to be fully understand.

    Really? How does that work in mushrooms? What about speciation in diatoms or bacteria? Is there a lot of sexual selection going on there?