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What Does T. cistoides Have To Do With Darwin’s Finches?

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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?

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. BobBob O'H
January 26, 2008
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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.hrun0815
January 25, 2008
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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?PaV
January 25, 2008
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hrun0815 - I guess you're right. It's a question of how accurately the paper, and the system, should be understood. BobBob O'H
January 25, 2008
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hrun0815 - it could simply be that they are less efficient in eating the seeds - they might be messier eaters of the seeds.
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
January 25, 2008
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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. BobBob O'H
January 25, 2008
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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
January 25, 2008
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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?
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).
Also, when you write this:
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.
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. :-) BobBob O'H
January 24, 2008
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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.hrun0815
January 24, 2008
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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
January 24, 2008
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Bob O'H Err, yes. And where in that paragraph does it say that G. fortis with large beaks don’t normally eat small seeds? Nowhere. 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?PaV
January 24, 2008
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Note to my previous post:
Of course, I also agree that in this particular situation having a small beak was an ‘advantage’. They are not mutually exclusive.
Here I mean that having a large beak can be a 'disadvantage' while having a small beak can be an 'advantage'-- and thus not mutually exclusive. It is of course not possible for a bird to have a small and a large beak at the same time.hrun0815
January 24, 2008
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PaV, I understand your summary of the facts. I do not understand why you classify this specifically as an 'advantage for small beaks' rather than a 'disadvantage for large beaks'. 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'. Of course, I also agree that in this particular situation having a small beak was an 'advantage'. They are not mutually exclusive. In fact, I don't see anywhere in the paper that the authors prefer one interpretation over the other. Clearly they think that being small beaked is an advantage, otherwise they wouldn't write that 'the only escape was available to the smallest, most G. fuliginosa–like, members of the G. fortis population [...]'. So in the end, the only disagreement I can tell that you have with the paper is about to what extent the presence of G. magnarostris or the second drought year had an affect on the availability of T. cistoides seeds. Again though, it seems like the author agree with you or otherwise they would not write:
Replicated experiments with suitable organisms are needed to demonstrate definitively the causal role of competition, not only as an ingredient of natural selection of resource-exploiting traits (12) but as a factor in their evolution (33).
As you can see, the authors are well aware that further experiments are required to show causality of the competitor species and not just correlation.hrun0815
January 24, 2008
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Bob: read the second to the last paragraph on page 225. Earlier in the paragraph they said that the most G. fuliginosa-like members of the G. fortis were known to eat these seeds. The G.fuliginosa have long, narrow beaks.
Err, yes. And where in that paragraph does it say that G. fortis with large beaks don't normally eat small seeds? Nowhere. Now, I guess you could claim that G. fortis doesn't eat small seeds, but you would have to explain the data in their reference 15, which shows the opposite. BobBob O'H
January 24, 2008
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My last sentence in the last post should have begun this way: "The better explanation for the dramatically decreased mean beak size of the G. fortis in 2005 would seem to be......"PaV
January 24, 2008
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Bob: read the second to the last paragraph on page 225. Earlier in the paragraph they said that the most G. fuliginosa-like members of the G. fortis were known to eat these seeds. The G.fuliginosa have long, narrow beaks. hrun0815: I say: What is the difference? Can you explain? It's quite simple. The normal source of food for all the G. fortis are the "small", "medium" and "large", with the "small" seed being preferred by all, no matter the beak size. An alternative is the "Opuntia", which was also not plentiful in 2004. With the scarcity of T. cistoides seeds, you had the entire population of G. fortis competing for the "small" and "medium" seeds. One would think that in such a situation, it would be equally likely for a large-beaked G. fortis to survive as a small-beaked G. fortis, which would result in a mean beak size that would be close to that of the previous year. (Notice that they say this on the first page: "In 1977, a drought on Daphne revealed that small seeeds 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.") It's for that reason that the authors suggest that the smallest-beaked G. fortis, with beaks siimilar to G. fuliginosa, had resource to "small" seeds outside the normal "small" seeds that G. fortis feeds on. They go onto mention that "it might be signifcant that two G. fuliginosa individuals were present on the island in 2004, and both survived to 2005." You err in saying that the T. cistoides "enabled the large G. fortis to surive the previous drought." Those "large" seeds didn't enable them to survive; rather, it gave them a selective advantage that year over the small-beaked G. fortis. You'll notice that after the first drought year the mean beak size increased. In the second year, with a presumably great scarcity of seed, the large-beaked finches "didn't have" their selective advantage available to them (since the T. cistoides doesn't reproduce in a drought year, and, of course the presence of G. magnarostris.), wheras the "smallest" of the G. fortis had access to seeds that the other, larger, G. fortis don't feed on. So, by way of summary, in the first drought year the large-beaked G. fortis had a slight advantage, and mean beak size modestly increased. In the second drought year, there was little selective advantage for the large-beaked G. fortis while there was a real selective advantage for the smallest-beaked G. fortis, and so the mean beak size dramatically decreased. The better explanation would seem to be that the "smallest" beaked G. fortis had an advantage, rather than blaming it on competition from the G. magnarostris, which by the way were down to only four females and nine males by the beginning of 2005.PaV
January 24, 2008
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larrynormanfan: We had a thread here on Haldane's Dilemna about a year ago. On that thread, I said two things: I said there was no way that Darwin's finches could change beak sizes so quickly without there being a terrible decimation of the population; and, I said that if that happened, we would hear about it. Both those predictions have proven true.PaV
January 24, 2008
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These are not the same “small” seeds that the large G. fortis normally eat.
And what is your evidence for this? I couldn't find anything in the paper about it. BobBob O'H
January 24, 2008
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With that fact in mind, my point that the better interpretation is that the smallest G. fortis finches have an advantage rather than the large G. fortis having a disadvantage still stands.
PaV, I really don't understand what you are trying to say here. It is clear that you agree that the competition with G. magnarostris and the drought severely depleted T. cistoides seeds, thus taking away the food source that enabled large beaked G. fortis to survive the previous drought. Now we are down to explaining why G. fortis did not die uniformly (with respect to beak size) -- like G. magnarostris -- but why there was a rather strong correlation of mortality to beak size. You say: "Small beaked G. fortis finches had an advantage over large beaked G. fortis." The authors say: "Large beaked G. fortis had a disadvantage over small beaked G. fortis." I say: What is the difference? Can you explain?hrun0815
January 24, 2008
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Bob O'H: Huh? Where’s your evidence that large-beaked G. fortis can’t feed on small seeds? The Grants wrote: "We have no feeding observations to indicate that [G. fortis] survived as a result of feeding on the typical components of the G. fuliginosa diet: the very small seeds of Sesuvium edmonstonei and Tiquilia fisca....Nevertheless, it may be significant....." (btm p 225) These are not the same "small" seeds that the largeG. fortis normally eat. With that fact in mind, my point that the better interpretation is that the smallest G. fortis finches have an advantage rather than the large G. fortis having a disadvantage still stands.PaV
January 24, 2008
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PaV:
On what basis are you saying that the T. cistoides were the main food source for G. fortis?
I don’t think that T. cistoides is the main food source for G. fortis in general, but that it is the main food source for LARGE G. fortis in times of DROUGHT. From the paper:
G. magnirostris is a potential competitor as a result of diet overlap with G. fortis (Table 1), especially in the dry season when food supply is limiting. Large-beaked members of the G. fortis population are capable of this maneuver—indeed, survival in the 1977 drought to a large extent depended on it (13, 16)—but on average they take three times longer than G. magnirostris to gain a seed reward (13, 24). The smallest G. fortis never attempt to crack them (18, 24).
PaV:
As to environmental triggers, why did the finches not breed in drought years? Just a coincidence?
Of course not. Who would suggest something like that? The environmental trigger was most likely that the population of finches was rapidly starving to death. Starvation induced delay or inhibition of ovulation has been shown to occur animals as varied as cockroaches and mice. PaV:
And, as far as ID and microevolution is concerned, most IDers readily accept the results of microevolution. They would disagree, of course, that anything as simple as microveolution could be projected out and serve as an explanation for macroevolution.
At no time in this thread did anybody here bring up microevolution or use it as an explanation of macroevolution. As far as I’m concerned we were discussing a scientific paper on finches that you suggested falsely interprets the data. You suggested an alternative interpretation that turned out to be false. Now we are discussing what you think is a different flaw in the interpretation of the data. Namely that the shift in beak size of the G. fortis population was not due to competition of the large G. fortis with G. magnirostris, but due to the large G. fortis being outcompeted by small G. fortis. However, as to the first half of the argument (not due to competition with G. magnirostris), you yourself write:
So, with two consecutive drought years resulting in very little T. cistoides, and its availability diminished due to the presence of the G. magnarostris, the large-beak G. fortis have to now compete with all of their fellow G. fortis for the available small and medium seed.
So you agree that the presence of G. magnarostris factored into the diminished availability of T. cistoides. As to the second half of the argument (due to the competition by small G. fortis) clearly the authors of the paper agree. If there was no competition by small beaked G. fortis (i.e. if large and small beaked G. fortis could survive equally well on small seeds) then there would be no correlation of mortality with beak size. Thus, taken together, I really don’t understand what your specific disagreement with the conclusions of the authors is.hrun0815
January 24, 2008
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PaV,
Taken together, this means that we’re dealing with the effects of attrition, and not an environmental trigger. I grant this much.
Only that much? Why, that's the whole shootin' match. Time to cry "uncle."larrynormanfan
January 24, 2008
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Van [39], are you suggesting that Nijhout's work is creationist or IDist? Doesn't look that way to me.larrynormanfan
January 24, 2008
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What becomes critical in the second year of the drought is NOT the competition between the large-beaked G. fortis and the G. magnarostris, but the fact that the smallest-beaked G. fortis had an additional food source which the large G. fortis did not have.
Huh? Where's your evidence that large-beaked G. fortis can't feed on small seeds? G. fortis usually feeds on small seeds (and Opuntia in our winter, see ref. 15 in the paper). During the drought in the 1970s, they switched to larger seeds. They couldn't do this in 2005, because G. magnarostris was excluding them.
So, with two consecutive drought years resulting in very little T. cistoides, and its availability diminished due to the presence of the G. magnarostris, the large-beak G. fortis have to now compete with all of their fellow G. fortis for the available small and medium seed. In this environment, the “smallest” G. fortis are the ones who have an advantage because of the avaiability of small seeds. So, the “smallest”-beaked G. fortis will survive better than any others. This would then be the reason that the mean beak size changed the most it had ever been seen to do in the 33 years of measurement.
And that's more or less what Grant and Grant conclude. BobBob O'H
January 23, 2008
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hrun0815: I don’t see how you can argue that large G. fortis where not being disadvantaged by the presence of G. magnarostris, a direct competitor for their main food source already in very low supply due to a drought in the previous year. On what basis are you saying that the T. cistoides were the main food source for G. fortis? As to environmental triggers, why did the finches not breed in drought years? Just a coincidence? And, as far as ID and microevolution is concerned, most IDers readily accept the results of microevolution. They would disagree, of course, that anything as simple as microveolution could be projected out and serve as an explanation for macroevolution. DLH: I think it would be hard to capture in the lab what happened in the wild here. I think whatever you did in the lab might not ever survive charges that the results depend on an artifical environment that was created in the lab.PaV
January 23, 2008
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How about in the lab?DLH
January 23, 2008
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DLH: In the wild, you'd have to wait for a situation in which you had two drought years in a row at a time when the G. magnarostris is not numerous. But I wonder if that will take another 33 years.PaV
January 23, 2008
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PaV Sounds plausible. How would you propose testing these competing parameters?DLH
January 23, 2008
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The better conclusion would be that the smallest G. fortis had a selective advantage in the second year of the drought rather than the larger G. fortis being disadvantaged by the presence of G. magnarostris.
I don't see how you can argue that large G. fortis where not being disadvantaged by the presence of G. magnarostris, a direct competitor for their main food source already in very low supply due to a drought in the previous year. In addition, the authors agree that these results do not prove their conclusions, but they do give strong evidence to support them:
Replicated experiments with suitable organisms are needed to demonstrate definitively the causal role of competition, not only as an ingredient of natural selection of resource-exploiting traits (12) but as a factor in their evolution (33). 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.
So maybe now, after this discussion, you would agree that even from an ID point of view, the suggested experiments are actually not a waste of time and that OTOH looking for proteins in the seeds that affect beak size might not be warranted.hrun0815
January 23, 2008
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Finches reach adult size in six months. The breeding season is early in the Galapagos. That means that at the end of a given calendar year, it's possible to be measuring those finches that were hatched earlier that year. So, do the measurements of beak size in the year 2005 represent the "new" generation mixed with the parental generation, or is it simply the remanants of 2003-2004. That is the question before us. I looked at the supplemental data on methods. They measured the number of adults in the first three months of each year. Presumably, this is also when they made the measurements. They also say that the "2005 generation measured in 2006 was significantly smaller than that in the 2004 sample of the parental generation before selection." Taken together, this means that we're dealing with the effects of attrition, and not an environmental trigger. I grant this much. But I nonetheless think that this data has been misinterpreted. The better conclusion would be that the smallest G. fortis had a selective advantage in the second year of the drought rather than the larger G. fortis being disadvantaged by the presence of G. magnarostris. Here's why: (1) In the drought year 1977, they state that the mortality that year was heaviest amongst the smaller G. fortis. This makes a lot of sense. We're told that only the large-beaked G. fortis can crack open the T. cistoides seed. That means that in a drought year, when the flora is limited, the large-beaked G. fortis can eat the small, the medium and the large seeds, whereas the smaller G. fortis can feed only on the small and the medium. (2) 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. (3) What becomes critical in the second year of the drought is NOT the competition between the large-beaked G. fortis and the G. magnarostris, but the fact that the smallest-beaked G. fortis had an additional food source which the large G. fortis did not have. Why do I say this? Two reasons: (a) T. cistoides, we're told, does not reproduce in a drought year. After two consecutive drought years, very little T. cistoides seed would have been available for feeding in 2004. This is what indeed happened since the G. magnarostris, who feed almost exclusively on the T. cistoides, were almost completely wiped out. But for the large G. fortis this would only represent a loss of an advantage it would normally have over the small G. fortis, as it had had the previous year (again, the mean beak size went up in 2004!); (b) we're told that in the midst of this great competition for food between the large and the small G. fortis "[t]he only escape was [the one] avialable to the smallest, most G. fulginosa-like, members of the G. fortis population, which are known to feed like G. fulginosa on small seeds with little individual energy reward." So, with two consecutive drought years resulting in very little T. cistoides, and its availability diminished due to the presence of the G. magnarostris, the large-beak G. fortis have to now compete with all of their fellow G. fortis for the available small and medium seed. In this environment, the "smallest" G. fortis are the ones who have an advantage because of the avaiability of small seeds. So, the "smallest"-beaked G. fortis will survive better than any others. This would then be the reason that the mean beak size changed the most it had ever been seen to do in the 33 years of measurement.PaV
January 23, 2008
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