I read both of the two articles today concerning the putative “disaptive” nature of Hemoglobin (Hb) and Myoglobin (Mb) loss.

First I should state that I have no problem whatsoever with the concept of specific molecular events resulting in disaptations. I would be more surprised if such events were never found considering the somewhat chaotic nature of…nature. Again, this fits in with my point above that evolution is usually messy.

Now, I have to say that I have some real problems with the logic in the primary article that postulates that Hb loss was a detrimental change. Let me make it clear that these are only my opinions from a couple of readthroughs of the literature concerned – and I have no problem conceding that I might be totally wrong – so feel free to argue.

1) The first problem is what I see as a general mistake the authors make throughout the paper. They continuously use comparisons between modern white-blooded icefishes and modern red-blooded icefishes to try to make conclusions about the relationship between the fish that originally lost Hb and its relatives that did not.

For example, they show that icefish of today expend more energy in distributing oxygen than modern red-blooded relatives.

But I think it is a leap to take this finding and extrapolate that it must have been bad for those first fish to lose Hb, under what may have been significantly different conditions.

This may have been the case, but their evidence is far from proving it.

2) The authors use only the energy consumption of the cardiovascular system and metabolism as the defining metric for adaptation. No doubt this is a major factor in an individuals success, but it is not hard to imagine that the energy demands might have been outweighed by other effects of Hb loss. I don’t think our understanding of all the physiological effects of such a gene loss (including secondary effects and how they might have affected reproductive success) is anywhere near complete enough to conclude that Hb loss was necessarily detrimental.

3) They make a big deal of arguing that nitric oxide (NO) effects, cardiac enlargement, capillary dilation and density increase are all due to the fish making up for the loss of Mb (and Hb), which they also claim was deleterious. Yet they also show that Mb should work fine at cold temperatures (i.e. Mb didn’t just erode due to uselessness) AND that Mb loss occurred independently multiple times in various icefish lineages.

Sure, you could argue that what little we understand of Mb and it’s connection to NO production and cardiovascular physiology argues that Mb loss was a disadvantage.

However, Occam’s razor (combined with what we already understand about natural selection) leads me to lean toward an explanation that Mb loss gave these fish some adaptive advantage in the frigid Southern Ocean to leave behind more pffspring. Why else would multiple lineages just happen to lose Mb (each via different mutations), AND result in each of those mutations spreading through that particular species population?

Basically, I think their ideas and data are intriguing – but it’s a far cry from what it would take to prove that the original mutational events were disadvantageous in those ancient species living under different physiological, ecological, and environmental conditions.

if i might toss a comment into this thread that follows-up on zen’s comments, i’d have to agree that in my mind adaptation is concerned with demonstrated survival value or reproductive benefit… given that, it becomes a tougher sell for me to see loss of red blood cells in icefish as a spandrel (merely an architectural byproduct of fish evolution in deep, cold habitats)… that being said, i’ve also not read the citations zen provided so can’t comment further…

i would like to point out, however, that i think steve gould would have gotten a kick out of the use of “disaptation”… as zen points out from the citation of the seminal gould/lewontin paper, steve was always cautious and critical of hyper-adaptationist thinking…

i consider myself fortunate enough to have been a grad student in several of his macroevolution seminars, and i can speak from experience that whenever anyone would utter “pre-adaptation” or even “exaptation” steve would perform his characteristic heavy squinting accompanied by vigorous shaking of his head in disgust… this from the man who coined “exaptation” with elizabeth vrba…

i think “disaptation” would have given him conniptions too…