UD and Darwin’s Finches

Over at UD Pav has totally misunderstood a paper by the Grants on the Evolution of Character Displacement in Darwin’s Finches. Having just reviewed a book by the Grants I was looking forward to ripping into some creationist mumbo jumbo. Alas, it is not to be.


At least not in the form I had originally imagined. Pav was originally arguing that the Grants were interpreting their data wrong. It all has someting to do with beak size changes and National geographic using the word “instant” or something:

Now to my prediction: my prediction is that changes in the morphology of species is driven mostly, if not completely, by environmental ‘triggers’.

At this point I was quite excited. I was thinking about all the experiments the Grants did on determining how much energy it required to crack open the different types of seeds found in the Galopagos, on the most efficient types of beaks for opening the different types of seeds (based on how much force each type of beak can exert) among others.
Pav concludes:

The more reasonable interpretation of the Grants data is this: there is some protein(s) found in T. cistoides that cause beak size (and other (6) correlated characteristics, see Table 2) to increase, and that in the absence of these protein(s) beak size will diminish. The most likely method of this change is, I believe, through a changed developmental pattern in the next generation. (One possibility is that RNA is involved here, and that somehow the presence of protein(s) that come(s) from the ‘large’ seeds is able to transmit the fact of its presence, or absence, to the subsequent generation).

In order to get to this conclusion Pav makes a number of inaccurate statements about the Grant article, which I will get to in a minute. There is, however, one glaring, mind numbingly obvious error in this conclusion. In order to understand it we will need to look at this quote by Pav:

Notice the difference between the observed feedings of G. fortis between 1977 and 2004. While both were small, the number of feedings on T. cistoides for G. fortis dropped from one sixth for 1977-1989, to one twelfth for 2004–half as much! But that’s not all of the story. In the paper, the Grants indicate that when examing these lowered ‘feedings’, whereas in normal years an average ‘feeding’ on T. cistoides was 9 to 23 mericarps, in 2004 it was never on more than 2 mericarps. That’s a factor of somewhere between 5 and 12. Taken altogether, then, this means that the amount of T. cistoides consumed by G. fortis fell in 2004 to somewhere between one tenth, and one twenty-fourth, of its normal consumption.

This is one hundred percent dead wrong! This part of the quote:

In the paper, the Grants indicate that when examing these lowered ‘feedings’, whereas in normal years an average ‘feeding’ on T. cistoides was 9 to 23 mericarps, in 2004 it was never on more than 2 mericarps.

Actually refers to Geospiza magnirostris. So what we have is G. fortis eating few to no T. cistoides and G. magnirostris eating few to no T. cistoides. The beak of G. fortis did indeed get smaller. The Grant’s don’t mention this in their paper, upu will have to read their newest bok to find the info, but the beaks of G. magnirostris grew larger during this same period – which is a direct contradiction of Pav’s hypothesis. If Pav was correct the beak size of both species should have been reduced. So, you can see I don’t have much more to write about, or do I? I pointed out Pav made an error in saying the above quote referred to G. fortis, what other errors or distortions can we find?
Well in a later comment Pav makes several errors. For example:

In line with the expectation that in a drought year the large G. fortis would do better than the smaller one, we see a rise in mean beak size in 2004 over that of 2003; that is, after the first year of the drought.

This is incorrect. Beak size in G. fortis dropped from 2003 to 2004. Pav also takes issue with the question of whether or not breeding occurred during the drought years. The Grants mention in their book that:

Hardly any rain fell in 2003 and 2004…There was no breeding in either year, little or no renewal of their food (seed) supply, and the numbers of both species declined drastically.

Finally, Pav asks:

The shift happened in ONE year? What kind of population genetics are at play here?

The kind of genetics at play is high selection on highly heritable traits. I would suggest that Pav do the math but given that he thinks:

The size of the confidence interval and the standard deviation are directly related. Variance is simply the standard deviation squared.

I’m not sure he would be capable…

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3 Responses

  1. I’m glad you brought this topic up actually, irrespective of what the IDists think, because quite a while ago vaguely similar questions did run around my head. I was essentially trying to figure out in my own mind if Darwin’s finches really were an example of natural selection in action, or simply a case for phenotypic plasticity. Basically, since the evolutionary changes in beak size arise from shifts in expression of Bmp4, is there actually any need to invoke natural selection at all? Could you not just chalk these changes down to plasticity?

  2. I think PaV’s problem stems from the ID predictions thread where he started suggesting that Lamarckian inheritance is possible. He then somehow missed the possibility that the Grant & Grant paper could be about differential survival. I guess he decided that the part of the paper that discusses selection differentials wasn’t important.
    SteveF – the Grant and Grant paper shows that there was strong selection, and also mention their previous work showing high heritability. They then show that there is actually a response to selection, i.e. the offspring also have smaller beaks.
    Bob

  3. Thanks for the reply Bob. I mainly got to thinking about this because of the Grant’s 2002 paper “Unpredictable Evolution in a 30-Year Study of Darwin’s Finches”. The speed of change struck me as extraordinary and I thought maybe simple plastic response could be a more parsimonious explanation than natural selection.
    Partly this thought was inspired by reading a couple of papers by Loso on Anolis lizards. Losos et al, investigate the observation that species of Anolis lizards that use broad substrates have long legs, whereas those that use narrow surface have only short legs. Previously this had been assumed to be the result of selection, but the authors demonstrate that phenotypic plasticity plays a major role (though this can then presumably be acted on by selection) and so evolution and plasticity had actually been confused:
    Losos, J.B. et al. (2000) Evolutionary implications of phenotypic plasticity in the hindlimb of the lizard Anolis sagrei. International Journal of Organic Evolution, 54, 301-305.
    So I thought maybe there could have been similar confusion in the case of finches. Quickly scanning Grant and Grant (2006) leads me to wonder (and I’ve still got a lot to learn about evolution) how the paper actually “shows that there was strong selection”? I don’t know much about how these kinds of experiments are designed, but does measuring traits (as the Grants do here) really distinguish between plasticity and selection? I only scanned the paper so maybe I need to re-read properly later.
    It seems to me that the key point is the response to selection that you mention – the offspring also having smaller beaks. This would be hard to explain as a plastic response.

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