What Does T. cistoides Have To Do With Darwin’s Finches?
| January 21, 2008 | Posted by PaV under Biology, Darwinism, Evolution, Intelligent Design |
Because of a prediction, a very strong prediction, I made on another thread, I’ve had reason to look into just what has been happening to Darwin’s finches way off on the Galapagos Islands.
Here is a paper published last year in Science Magazine by the Grants, experts in Darwin’s finches. I looked at their paper, looked at their data, and have come to the conclusion that what I predicted as the ultimate explanation to changed beak sizes is the more reasonable interpretation of the data they present.
But before we even get to the data, here’s a remark from a National Geographic website review of the article that supports my basic position:
“ Researchers from New Jersey’s Princeton University have observed a species of finch in Ecuador’s Galápagos Islands that evolved to have a smaller beak within a mere two decades.
Surprisingly, most of the shift happened within just one generation, the scientists say.”
The shift happened in ONE year? What kind of population genetics are at play here?
Well, to the data:
The most important information that we get from the article (only 3 pages in pdf) are contained in Table 1 and Figure 2.
The Grants’ paper is concerned with how Geospiza fortis and Geospiza magnarostris compete. Their conclusion is that during the drought years of 2003-2004, when the population numbers of both species fell drastically, that the “competition” from G. magnarostris, due to the numbers of both species being almost the same for the first time since G. magnarostris came over from the mainland, caused “character displacement” (beak size change) of G. fortis to a smaller size.
Table 1 gives the number of observed feedings of the three main seed types (small, medium, large) by each of the species. Figure 2 gives the beak size mean of G. fortis over the last 33 years, beginning in 1973-74.
The Grant’s give four ‘lines of support’ for their conclusion. Their fourth ‘line of support’, I believe, becomes the very reason for re-interpreting their results. The fourth ‘line of support’ is that in the drought year 1977, when G. magnarostris were few in numbers, and hence, not able to compete with G. fortis, the beak size of G. fortis actually increased so as to be able to take advantage of the Tribulus cistoides seeds (the large ones), whereas in 2004, with a similar drought taking place, but, however, with G. magnarostris now able to compete with G. fortis, the beak size of G. fortis decreased.
Let’s first notice all of Figure 2. For most of the 33 years that it records beak size, the beak size hardly fluctuates from its mean; there are only two noticeable/significant exemptions: the two drought periods, when, in BOTH instances, beak size changed almost ‘instantaneously’. The title of the National Geographic review had “instant” in its title.
Now to my prediction: my prediction is that changes in the morphology of species is driven mostly, if not completely, by environmental ‘triggers’. Table 1 now becomes important. 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.
Let’s also notice that while both G. fortis and G. magnarostris were decimated, G. magnarostris did not change its beak size to a smaller one. If we look at ‘feeding’ observations for G. magnarostris we see that while they ate a larger proportion of the medium sized seeds than normal, nonetheless, their main intake continued to be T. cistoides.
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).
This thesis, though controversial perhaps, renders what we see—over a 33 year history—sensible, because: (1) It explains why G. magnarostris, despite being decimated by the drought, still does not change—since it is still principally ‘feeding’ on T. cistoides; (2) It explains why G. fortis changes beak size in ONE generation (“Instant” evolution); and (3) most importantly, it explains why in 1977, at a time when G. magnarostirs was very small in numbers, G. fortis INCREASED its beak size in ONE generation; that is, since there were no G. magnarostris to compete with, the G. fortis had all the T. cistoides to themselves. In the next generation beak size “popped-up” in size to the same degree that it decreased in size during the 2004 drought when their consumption of T. cistoides dropped drastically.
As a follow-up to this study, here is what the Grant’s propose: “Our findings should prove useful in designing realistic experiments, by identifying ecological context (high densities at the start of an environmental stress) and by estimating the magnitude of natural selection.”
Here is what I consider to be another important point in all of this. From an ID perspective, this kind of an experiment is a complete waste of time. What would be valuable, OTOH, is an experiment wherein native Galapagos seeds are fed to controlled populations of G. magnarostris and G. fortis while observing changes to beak size (and other traits that are correlated). You see, ID really is “science”!
Finally, let’s remember that Kettlewell’s experiment and the Galapagos Finches are the Modern Synthesis’ great claims to fame. Well, I think they got it completely wrong. What do you think?
70 Responses to What Does T. cistoides Have To Do With Darwin’s Finches?
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homerun 815: (Is that how many Bonds has?)
Having a small beak or a large beak is mutually exclusive. Clearly from your writing I understand that you agree that in this particular case it was a liability for G. fortis to have a large beak: The large T. cistoides were depleted and their beaks did not allow them to switch to an alternative food source available to the G. fortis with a small beak.
Thus, a large beak is strongly correlated with G. fortis mortality. To me, that seems to be a ‘disadvantage’.
Not exactly: First, what I’m saying is that because of the presence of the G. magnarostris the selective advantage of having a large beak is lost for the G. fortis. Second, in the absence of this selective advantage, all the G. fortis must scramble for available resources—with none having any advantage over any of the others, save this: those with smaller bodies can obviously live on a smaller diet than a larger finch. Third, as the authors suggest, a selective advantage arises for those G. fortis with the very smallest beaks since they can take advantage of some very small seeds that aren’t part of the normal diet of the G. fortis. It would seem, then, that the better conclusion would be that change in mean beak size is due to the selective advantage of the “smallest”, G. fuliginosa-like members of the G. fortis that survived the first drought year rather than positing a disadvantage to the large G. fortis.
PaV, the authors are not concerned if the change in beak size is due to a selective disadvantage of the large beaked birds or a selective advantage of the small beaked birds. The result is the same: a strong correlation of mortality and beak size.
But it seems that you have come full circle and now agree with the main conclusions of the authors: their thesis was that the presence of a competitor species that competes for a food source drove ‘character displacement’ in G. fortis.
As you said (in First), due to the presence of G. magnarostris the large beaked G. fortis lost their selective advantage over others.
You say (in Second), that all G. fortis must scramble for available resources where smaller bodied birds have a slight advantage, which the authors also found in the weak (but significant) correlation of mortality with body size.
You point out (in Third), just as did the authors, that in addition to the large beaked birds losing their selective advantage, they also did not have the ability of the small beaked birds to survive. This is of course necessary for the mortality rate to correlate with beak size.
So the authors are in full agreement with all of your three points (or the other way around).
Your only point of contention is terming something a selective disadvantage for one group or selective advantage for the other. And I really don’t see in the paper that the authors necessarily disagree with you there either.
Wow. I feel rather blessed. This is one of the very few times that an online discussion actually yielded a constructive result.
By quoting the paper:
Also, when you write this:
It helps if you have some evidence that birds with larger beaks can’t eat smaller seeds. You still haven’t given any evidence that larger-beaked G. fortis can’t eat smaller seeds.
I agree with hrun0815, you’re making progress. But you’re still not quite there.
Bob
Hi Bob, what do you think explains the strong correlation of mortality and beak size? Why did the larger beaked G. fortis die at a higher rate than the smaller beaked ones if not for food sources available to the small beaked ones that the large beaked ones can not exploit.
Looking at table 2, beak size seems to be the strongest determinant of survival, more so than body size. And we know that the majority of birds died of starvation.
Just trying to clear things up so we can all agree on the interpretation of the results of this study by the Grant’s.
hrun0815 – it could simply be that they are less efficient in eating the seeds – they might be messier eaters of the seeds.
Of course, this is speculation, but I haven’t seen any evidence that the larger-beaked birds can’t eat small seeds, and it’s something I’m sure would have been remarked upon if they did have a different diet. The text points towards them being able to eat smaller seeds, because it talks about the species feeding on smaller seeds and switching to the larger ones during drought.
Bob
So then the disagreement between you and PaV is a matter of degrees? PaV suggests that large beaked fortis are generally unable to eat the smallest of the seeds whereas you say that they are able to do so, just not as efficiently as the small beaked fortis.
I would suggest that who is right does not really have any bearing on the conclusions put forth in the paper, right? The key is that large beaked fortis have a selective disadvantage (or the small beaked fortis have a selective advantage) after the T. cistoides seeds were depleted by drought and G. magnarostris.
hrun0815 – I guess you’re right. It’s a question of how accurately the paper, and the system, should be understood.
Bob
Bob O’H:
PaV:Err, then if the large-beaked G. fortis and smallest-beak fortis had access to the same seed sources, then how do you explain the change in mean beak size?
Bob O’H answers:
By quoting the paper:
“In 1977, a drought on Daphne revealed that small seeds are preferred when they are abundant, but when they are scarce, finches turn increasingly to large and hard seeds that only the large-beaked members of the population can crack (13, 15). Most finches died that year, and mortality was heaviest among those with small beaks (13, 16, 17).”
The quote you are using gives the reason that the beak size “increased” in 2003, and in 1977 (dramatically). But it doesn’t tell us anything about 2004. The ONLY thing it tells us about 2004 is that the large G. fortis DIDN’T have an advantage in 2004 because of the scarcity of T. cistoides. So, now all the birds are on the same playing field. Notice that in 1985, the G. fortis fed on the T. cistoides just as much as they did in 1977; and yet their mean beak size fell the next year. So, the T. cistoides is an advantage only in a drought year. We saw that in 2003. Now the T. cistoides is depleted. The advantage is gone. So, the large G. fortis DON’T have an advantage; we all agree; but that, alone, cannot explain why the beak size fell so dramatically in 2005.
Of the four “lines of evidence” the authors give for the dramatic fall in 2005, the first two simply establish that, unlike 1977, in 2004 G. magnarostris presented competition for the large G. fortis; and the fourth simply points out that whereas in 1977 mean beak size went up, in 2004 it went down. But this simply means that the explanation for the 1977 shift obviously cannot account for the 2004 shift. So, they still haven’t explained anything. That leaves the third “line of evidence”. This “line” indicates that there was severe starvation, that Opuntia did not flower and was unavailable, which left the “only escape”, and that “only escape” was feeding on the tiniest seeds (or a small seed that the vast majority of G. fortis does not feed on); it is only the “smallest” G. fortis, the most G. fuliginosa-like, that feed on this seed. Voila. The explanation: the smallest G. fortis had a selective advantage in 2004, and mean beak size FELL in 2005. The largest G. fortis had an advantage in 1977, and the mean beak size GREW in 1978. Q.E.D. What more is needed?
So PaV, do you still think that Grant and Grant mis-interpreted any of their results?
I was wondering if maybe you wanted to amend the original post to include your changed views. People might get a false impression if they only read the original post without wading through the over 60 comments.
PaV – I was specifically addressing your claim that large-beaked G. fortis didn’t eat small seeds. Yes, it’s a small point, but I have still to see any evidence from you to back up your claim.
But, I think hrun0815 is right that you now seem to have understood the main point of the paper, and I think it might be good to add a note on the post.
Bob