The horseshoe crab and the barnacle: induction vs deduction in evolution

Charles Darwin had incredible patience.  After his many-year, global voyage on the HMS Beagle, he nestled in at Down House, where he was somehow able to stay calm and study mere barnacles to an endless extent (and to write 4--four--books on these little creatures).  Who else would have had the obsessive patience (or independent wealth and time on one's hands) to do such a thing?

      From Darwin's books on barnacles (web image capture)

Darwin's meticulous work and its context in his life and thinking are very well described in Rebecca Stott's compelling 2003 book, Darwin and the Barnacle, which I highly recommend, as well as the discussion of these topics in Desmond and Moore's 1991 Darwin biography, The Life of a Tormented Evolutionist.  These are easier, for seeing the points I will describe here, than plowing through Darwin's detailed own tomes (which, I openly confess, I have only browsed).  His years of meticulous barnacle study raised many questions in Darwin's mind, about how species acquire their variation, and his pondering this eventually led to his recognition of 'evolution' as the answer, which he published only years later, in 1859, in his Origin of Species.

Darwin was, if anything, a careful and cautious person, and not much given to self-promotion.  His works are laden with appropriate caveats including, one might surmise, careful defenses lest he be found to have made interpretive or theoretical mistakes.  Yet he dared make generalizations of the broadest kind.  It was his genius to see, in the overwhelming variation in nature, the material for understanding how natural processes, rather than creation events, led to the formation of new species.  This was implicitly true of his struggle to understand the wide variation within and among species of barnacles, variation that enabled evolution, as he later came to see. Yet the same variation provided a subtle trap:  it allowed escape from accusations of undocumented theorizing, but was so generic that in a sense it made his version of a theory of evolution almost unfalsifiable in principle.

But, in a subtle way, Mr Darwin, like all geniuses, was also a product of his time.  I think he took an implicitly Newtonian, deterministic view of natural selection.  As he said, selection could detect the 'smallest grain in the balance' [scale] of differences among organisms, that is, could evaluate and screen the tiniest amount of variation.  He had, I think, only a rudimentary sense of probability; while he often used the word 'chance' in the Origin, it was in a very casual sense, and I think that he did not really think of chance or luck (what we call genetic drift) as important in evolution.  This I would assert is widely persistent, if largely implicit, today.

One important aspect of barnacles to which Darwin paid extensive attention was their sexual diversity.  In particular, many species were hermaphroditic.  Indeed, in some species he found small, rudimentary males literally embedded for life within the body of the female.  Other species were more sexually dichotomous.  These patterns caught Darwin's attention.  In particular, he viewed this transect in evolutionary time (our present day) as more than just a catalog of today, but also as a cross-section of tomorrow.  He clearly thought that what we saw today among barnacle species represented the path that other species had taken towards becoming the fully sexually dichotomous (independent males and females) in some species today: the intermediates were on their way to these subsequent stages.

This is a deterministic view of selection and evolution: "an hermaphrodite species must pass into a bisexual species by insensibly small stages" from single organisms having both male and female sex organs to the dichotomous state of separate males and females (Desmond and Moore: 356-7).

But what does 'must pass' mean here?  Yes, Darwin could array his specimens to show these various types of sexual dimorphism, but what would justify thinking of them as progressive 'stages'?  What latent assumption is being made?  It is to think of the different lifestyles as stages along a path leading to some final inevitable endpoint.

If this doesn't raise all sorts of questions in your mind, why not?  Why, for example, are there any intermediate barnacle species here today?  Over the eons of evolutionary time why haven't all of them long ago reached their final, presumably ideal and stable state?  What justifies the idea that the species with 'intermediate' sexuality in Darwin's collections are not just doing fine, on their way to no other particular end?  Is something wrong with their reproduction?  If so, how did they get here in the first place?  Why are there so many barnacle species today with their various reproductive strategies (states)?

Darwin's view was implicitly of the deterministic nature of selection--heading towards a goal which today's species show in their various progressive stages.  His implicit view can be related to another, current controversy about evolution.

Rewinding the tape
There has for many recent decades been an argument about the degree of directedness or, one might say, predictability in evolution.  If evolution is the selection among randomly generated mutational variants for those whose survival and reproduction are locally, at a given time favored, then wouldn't each such favored path be unique, none really replicable or predictable?

Not so, some biologists have argued!  Their view is essentially that environments are what they are, and will systematically--and thus predictably--favor certain kinds of adaptation.  There is, one might quip, only one way to make a cake in a particular environment.  Different mutations may arise, but only those that lead to cake-making will persist.  Thus, if we could 'rewind the tape' of evolution and go back to way back when, and start again, we would end up with the same sorts of adaptations that we see with the single play of the tape of life that we actually have. There would, so to speak, always be horseshoe crabs, even if we started over.  Yes, yes, some details might differ, but nothing important (depending, of course, on how carefully you look--see my 'Plus ça ne change pas', Evol. Anthropol, 2013, a point others have made, too).


Others argue that evolution is so rooted in local chance and contingency, that there would be no way to predict the details of what would evolve, could we start over at some point.  Yes, there would be creatures in each local niche, and there would be similarities to the extent that what we would see today would have to have been built from what genetic options were there yesterday, but there the similarity would end.

Induction, deduction, and the subtle implications of the notion of 'intermediate' forms
Stott's book,  Darwin and the Barnacle, discusses Darwin's work in terms of the presumed intermediate barnacle stages he found.  But the very use of such terms carries subtle implications. It conflates induction with deduction, it assumes what is past will be repeated.  It makes of evolution what Darwin also made of it: a deterministic, force-like phenomenon.  Indeed, it's not so different from a form of creationism.

This has deeper implications.  Among them are repeatability of environments and genomes, at least to the extent that their combination in local areas--life, after all, operates strictly on local areas--will be repeated elsewhere and else-times.  Only by assuming not only the repeatability of environments but also of genomic variation, can one see in current states of barnacle species today stages in a predictable evolutionary parade.  The inductive argument is the observation of what happened in the past, and the deductive argument is that what we see is intermediate, on its way to becoming what some present-day more 'advanced' stage is like.

This kind of view, which is implicitly and (as with Darwin) sometimes explicitly invoked, is that we can use the past to predict the future.  And yet we routinely teach that evolution is by its essential nature locally ad hoc and contingent, based on random mutations and genetic drift--and not driven by any outside God or other built-in specific creative force.

And 'force' seems to be an apt word here.

The idea that a trait found in fossils, that was intermediate between some more primitive state and something seen today, implies that a similar trait today could be an 'intermediate stage' today for a knowable tomorrow, conflates inductive observation with deductive prediction.  It may indeed do so, but we have no way to prove it and usually scant reason to believe it.  Instead, equating induction with deduction tacitly assumes, usually without any rigorous justification, that life is a deductive phenomenon like gravity or chemical reactions.

The problem is serious: the routine equating of induction with deduction gives a false idea about how life works, even in the short-term.  Does a given genotype, say, predict a particular disease in someone who carries it, because we find that genotype associated with affected patients today?  This may indeed be so, especially if a true causal reason is known; but it cannot be assumed to be.  We know this from well-observed recent history: Secular trends in environmental factors with disease consequences have indeed been documented, meaning that the same genotype is not always associated with the same risk.  There is no guarantee of a future repetition, not even in principle.

Darwin's worldview
Darwin was, in my view, a Newtonian in his view.  That was the prevailing science ethos in his time.  He accepted 'laws' of Nature and their infinitesimally precise action.  That Nature was law-like was a prevailing, and one may say fashionable view at the time. It was also applied to social evolution, for example, as in Marx's and Engels' view of the political inevitability of socialism.  That barnacles can evolve various kinds of sexual identities and arrangements doesn't mean any of what Darwin observed in them was on the way to full hermaphrodism or even later to fully distinct sexes...or, indeed, to any particular state of sexuality.  But if you have a view like his, seeing the intermediate stages even contemporaneously, would reinforce the inevitabilistic aspect of a Newtonian perspective, and seemingly justify using induction to make deductions.

Even giants like Darwin are products of their times, as all we peons are.  We gain comfort from equating deduction with induction, that the past we can observe allows us to predict the future.  That makes it comfortingly safe to make assertions, the feeling that we understand the complex environment in which we must wend our way through life.  But in science, at least, we should know the emptiness of the equation of the past with the future.  Too bad we can't seem to see further.