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Monday, September 17, 2012

Gould on Darwinism and Nonadaptive Change

Some people have trouble understanding the difference between Darwinism and modern evolutionary theory.

In spite of the fact that he has been dead for a decade, Stephen Jay Gould remains the authority on challenges to classical Darwinism and the hardened version of the Modern Synthesis (sometimes referred to as Neo-Darwinism).

If you really want to understand this issue then you have to read The Structure of Evolutionary Theory. One of my criticism of those who would overthrow modern evolutionary theory is that they are often completely ignorant of the work done by Gould and his allies and they end up attacking a strawman version of modern evolutionary theory.

Gould described the essential features of Darwinism in many of his writings. The most important feature is an emphasis on natural selection as the mechanism of evolution. In much of his work Gould emphasizes the roles of contingency, constraints, and non-gradualistic evolution as extensions of Darwinism. However, he doesn't forget direct challenges to Darwinism in the form of nonadaptive mechanisms that don't, under any circumstances, fit within the Darwinian framework.

These are complicated issues and that partially explains why so many people have not been able to follow Gould's reasoning. He doesn't help by using a writing style that requires your full attention. The advantage of that style is that he doesn't dumb down the subject and he covers all the exceptions and qualifications.

Here's Gould explaining why some features could arise as one form of adaptation then shift to serve another adaptive role (functional shift) (page 1246-1247). These features are called exaptations since they did not originally arise as adaptation to their present role. (Think of a defective transposon that becomes a regulatory sequence.)
Nevertheless, also emphasized throughout, ... the basic concept of exaptation remains consistent with orthodox Darwinism (while expanding its purview and adding some structural clarification and sophistication) for an obvious reason: the principle of quirky functional shift does not challenge the control of evolution by natural selection as an adaptational process. Unpredictable shift of function may establish the ground of contingency, and may imply a rule for structural constraints upon phyletic pathways. But this principle does not undermine the functionalist basics of evolutionary change because features so effected remain adaptive throughout: they originate from one function (presumably by natural selection), and then undergo quirky shift to a different utility.

However, the principle of functional shift, ... implies a disarmingly simple and logical extension that does challenge the role of Darwinian mechanics and functionalist control over evolutionary change. Ironically, the very simplicity of the argument has often led to its dismissal as too obvious to hold any theoretical importance—a "feeling" that I shall try to refute in this section, and whose disproof represents an important step in the central logic of this book.

The deeper challenge posed to orthodox Darwinism by the principle of functional shift flows from the implication that, if current utility does not reveal the reasons for hisorical origin, then these initial reasons need not be adaptational or functional at all—for features with current adoptive status may have originated from nonadaptive reasons in an ancestral form. In other words, and in the terminology of table 11-1, when certain aptations rack rank as exaptations rather than adaptations, the coopted source will be identifiable as an ancestral structure with either adaptive origins (for a different function) or nonadaptive origins (for no function at all). ...

The general conclusion may be stated in a simple manner, but I believe that the resulting implications for evolutionary theory are both profound and curiously underappreciated: If many features that operate as adaptations under present regimes of natural selection were exapted from ancestral features with nonadaptive origins—and were not built as adaptations for their current use (or exapted from ancestral features with adaptive origins for different functions)—then we cannot explain all the pathways of evolutionary change under functionalist mechanics of the theory of natural selection. Instead, we must allow that many important (and currently adaptive) traits originated for nonadaptive reasons that cannot be attributed to the direct action of natural selection at all and, moreover, cannot be inferred from the exaptive utility of the trait in living species. Because the subject of evolutionary biology must engage many critical questions about the origins of features, and cannot be confined to the study of current utilities and selective regimes, nonadaptationist themes therefore assume an important role in a full account of life's history and the mechanisms of evolutionary change.
In other words, lots of things can't be explained by Darwinism even if they look adaptive today.


steve oberski said...

Wouldn't the underlying mechanism for "quirky functional shift" or "unpredictable shift of function" be random mutation ?

NickM said...

I've always thought it's a little weird to have a definition of "Darwinism" that would exclude Darwin. E.g. this famous passage, which Gould knew well and cites somewhere in his doorstop (_Structure_).

In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give another instance. Pedunculated cirripedes have two minute folds of skin, called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack. These cirripedes have no branchiae, the whole surface of the body and of the sack, together with the small frena, serving for respiration. The Balanidae or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack, within the well-enclosed shell; but they have, in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunae of the sack and body, and which have been considered by all naturalists to act as branchiae. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiae of the other family; indeed, they graduate into each other. Therefore it need not be doubted that the two little folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly aided in the act of respiration, have been gradually converted by natural selection into branchiae, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack?

Joe Felsenstein said...

The word "Darwinism" is used in a number of senses, including the pejorative one which is the one the Discovery Institute uses: a cult which never questions anything Darwin ever said. There are no known actual members of that cult, but the DI would like people to believe that's what evolutionary biologists are.

The loosest, most inclusive version seems to be this: that Darwin put forward natural selection as the explanation for the high degree of adaptation of organisms. That is still valid: natural selection (at one level or another) is the only force we know capable of explaining why organisms are so much better adapted than they would be if their genomes were just random strings of nucleotides. All the other forces are no more likely to bring about adaptation than they are to decrease it.

Larry, you may not be a Darwinist in the sense that you explain all features of an organism by natural selection, but I bet you are a Darwinist in the sense of agreeing with the above paragraph.

Chinahand said...

Prof. Moran has previously also highlighted the work of Prof Michael Lynch of Indiana University the author of The Origins of Genome Architecture. He has a talk called Complexity Myths discussing his ideas at a National Academy of Sciences Colloquium "In the Light of Evolution: Adaptation and Complex Design".

anon101 said...

Ignoring for a minute that it is not really relevant how Gould defined a word unless it can be demonstrated that this definition is widely accepted among biologists. This quote is really self-defeating. He talks about “orthodox Darwinism “ which implies that there is “unorthodox” or “new Darwinism”.

So within Darwinism there are at least two schools “orthodox Darwinism” and “new Darwinism”. Therefore Larry I absolutely accept that you are a new or unorthodox Darwinist yet you still are a Darwinist.