I am currently sitting in on a graduate philosophy of biology seminar and the theme of this semester’s seminar is evo-devo and we recently discussed the concept of modularity. I’m also sitting in on a history of biology course and we have talked a little about the early 19th century French scientist, Georges Cuvier. While attending the seminar, I was delighted to make a historical link between the two! (And oddly enough, one of the works we read in the seminar was a chapter from a book on modularity co-authored by Gunther Wagner which opens with the same link I had made.)
In my posts about the Spencer-Weismann debates, one of Spencer’s anti-Darwinian arguments I focused on can be called the “correlation of parts.” It was an idea articulated earlier by Cuvier, who said (as quoted by Wikipedia),
Today comparative anatomy has reached such a point of perfection that, after inspecting a single bone, one can often determine the class, and sometimes even the genus of the animal to which it belonged, above all if that bone belonged to the head or the limbs. … This is because the number, direction, and shape of the bones that compose each part of an animal’s body are always in a necessary relation to all the other parts, in such a way that – up to a point – one can infer the whole from any one of them and vice versa.
This article from the Academy of Natural Sciences quotes another passage from Cuvier:
Every organized being forms a whole, a unique and closed system, in which all the parts correspond mutually, and contribute to the same definitive action by a reciprocal reaction. None of its parts can change without the others changing too; and consequently each of them, taken separately, indicates and gives all the others.
The parts of an animal are so thoroughly integrated, that changing a single part – without proportionately modifying everything else involved as well – would produce a non-working animal. The correlation of parts prevented Cuvier from accepting Lamarckian evolution, or transmutation, in principle.
The correlation of parts later became an argument for Lamarckian evolution against Darwinian evolution,* as exemplified by Herbert Spencer. (I wonder if there has been any study on this shift in the principle’s use?) He believed, like Cuvier, that frequently a feature of an organism is tightly linked to a multitude of other parts and it would be very unlikely for all these parts to simultaneously vary to the appropriate magnitudes and directions required by neo-Darwinian evolution that prohibits the inheritance of acquired characteristics.
For example, an elk’s antlers are supported by thick skulls and strong back and neck muscles. If an elk were to gain an extra point to the rack through continued growth, increasing its weight, its muscles would be strained to keep the head up and the animal wouldn’t survive. However, in Lamarckian evolution, the muscles that the buck strengthens over its lifetime are passed on to its offspring. Over generations, the muscles would continue to strengthen to the point where extra growth in the rack could be supported. Thus the correlation of parts showed that pure Darwinian evolution couldn’t work in elk antlers, but Lamarckism could – according to Spencer, anyway.**
So, historically, according to some, evolution (or specifically, Darwinian evolution) couldn’t work because of the correlation of parts. I think the concept of modularity easily resolves this.
A module can be loosely defined as a semi-autonomous individual part or process. (I have written about modularity before in a previous post.) An example of modular part is the assortment of lobster appendages: they act and move mostly separate from the rest of their body. A modular process is exemplified by the development of the appendages themselves: each appendage develops indepently of the others, and these appendages are specified and created by modular genetic networks and pathways. Additionally, these modular structures, processes, and genetic networks can evolve freely from the rest of the organism-at-large.
Modularity answers Cuvier’s and Spencer’s objections to evolution. Yes, an organism may be well-integrated, but it is also built of semi-autonomous modules. A part or process of an organism can vary without the rest of the organism falling apart! Spencer argued that if Lamarckian inheritance, which sidesteps the correlation of parts, weren’t true, then evolution couldn’t happen, but perhaps he was half-right: modularity (which also sidesteps the correlation of parts), not Lamarckian inheritance, is what allows evolution to happen.
* I am obviously omitting Darwin’s views on the topic, but given what I read in Gould’s The Structure of Evolutionary Theory, his views seem a bit muddy and complicated. Perhaps I will investigate Darwin’s views on the correlation of parts at a later date, but here I am focusing on correlation of parts vs. modularity.
** I don’t understand why Spencer excludes the possibility of later fluctuations or mutations that could arise after the weight increase in the antlers. He also doesn’t mention how the skull would thicken as a result of Lamarckian processes. I don’t think Spencer’s argument works at all given his framework, but it still illustrates the use of the principle of correlation of parts as an argument against Darwinian evolution.
Kestrels and Cerevisiae 
Three points that merit consideration in addition to the modularity argument:
1) The basic premise is wrong.
Parts don’t have to be correlated, particularly if change is incremental. Adding a couple of feet to an elk’s antlers would doubtless cause him problems. Adding a millimetre to them wouldn’t even be noticeable.
2) The logic is wrong.
As you point out, even where parts do have to be correlated – a change in A necessitates a change in B – then nothing says the changes have to be contemporaneous. An elk develops longer antlers that cause him some trouble – not enough to be lethal, but enough to reduce his number of descendants. However, some of those descendants acquire another change that allows them to support the antlers.
3) The mechanisms are wrong.
The key here is plasticity. Yes, larger antlers need larger bone structure and musculature to support them. If the developmental program for bone and muscle says “Grow as big as you need to be”, then antlers can change in size without needing any additional mutations in the bone and muscle. Instead of looking at the blacksmith and saying “His offspring will evolve bigger muscles”, they should have looked at him and said “Wow, humans are able to grow bigger muscles if they need to”.
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2) Or perhaps the elk’s increased musculature develops through mutation or maturation. Then antlers grows until they signals “big enough”. They may well grow to accommodate varying strength since they are shed yearly and would need to adapt to aging.
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Our numbering system’s gone a bit screwy here :-)
I’d call yours 3b – it’s another example of plasticity. Either the antlers alter (and the muscles follow because their program is plastic), or the muscles alter (and the antlers follow because their program is plastic).
The order would be easy enough to test in a series of experiments of varying degrees of unethicality.
1) Hang weights on an elk’s antlers: will he develop larger muscles?
2) Cut off part of an elk’s antlers: will the muscles atrophy?
3) Induce hyperdevelopment of an elk’s muscles (perhaps via steroids?): do the antlers grow to match?
4) Weaken an elk’s muscles (surgically?): do the antlers shrink?
Everyday experience with exercise indicates that (1) and (2) at least are true. Muscles are plastic, and change in size all the flipping time – a fact so obvious that it’s perhaps overlooked for that very reason. More interesting would be the skeleton/antler interaction.
You also make a very good point about aging – the antlers/skeleton/musculature change in size and shape constantly as the animal grows, yet no new mutations are needed to keep them synchronised. That tells you straight away that the body (a) is plastic and (b) has feedback mechanisms maintaining the integration of its various parts.
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