Wednesday, March 09, 2011

What Darwin Got Wrong

Life evolves seemingly endless varieties. To account for the varieties, Charles Darwin invented natural selection. He gave his invention the task of bestowing upon organisms whatever traits they have or had or will have.

What Darwin Got Wrong argues that the mechanism of natural selection is inadequate to this task. The book’s authors, Jerry Fodor and Massimo Piattelli-Palmarini (avowed atheists we learn), argue that Darwin overstated the power of natural selection, that it cannot account for how organisms got to be how they got to be. The authors don’t cite missing fossils of transitional forms or appeal to irreducible complexity, a la intelligent design argument. They just pick away at the putative logic of natural selection until nothing remains but grandma’s common-sense intuitions. They conclude that Darwin granted a truism wings to which it was not entitled.

The book attacks selectionism on various fronts, from its inability to field counterfactuals (if the arctic environment had been green, would polar bears have green fur?) to limitations placed on creaturely form by physical mechanics. But the star larvae hypothesis is interested primarily in the accounts of internal, or endogenous, constraints on the variability of phenotypes, the observable forms of organisms. The internal constraints leave environmental, or exogenous, influences with little from which to select. As the authors put it, natural selection at most can tune the piano; it cannot compose the melody.

The book, in short, is about the conceptual rigor, or lack of, of the NeoDarwinian theory. The Neo- part is important, because the authors support their case with findings from genetic sequencing and analysis. In particular, they lean on a new discipline called evolutionary developmental biology, or evo-devo, which has evolved from the discovery that DNA is conserved during evolution. This means that the genetic makeup of organisms, their genotypes, varies little across species, relative to the great diversity of phenotypes across species. How does a relatively limited genetic toolkit translate into so many forms of creatures? That is the question.


The answer apparently lies in the action of “master” genes and their protein-based “switches.” These systems control whole suites of genes, turning them on and off during development. The authors cite, for example, a master gene designated Otxi, which influences the development of several seemingly unrelated organs. They point out, ". . . in particular, since the Otxi ‘master’ gene controls the development of the larynx, inner ear, kidneys, and external genitalia and the thickness of the cerebral cortex, selective pressures sensitive to changes in the functions of the kidneys (due to bipedal station, or different liquid intake and excretion resulting from floods or droughts), or the fixation of different sexual patterns, may have had in turn secondary effects on the expansion of the cerebral cortex and the structure and function of the larynx.”

They use this example to show that the key theoretical construct of “selected for,” such as selection for long necks among giraffes or for a complex cerebral cortex among humans, cannot deliver what it is supposed to deliver. It can’t tell an adaptive trait from a trait that coincidentally rides along with an adaptive one. Too few genes, it turns out, are available singly for selection. Genes tend to come hierarchically bundled.

The authors argue, “[E]volutionary theory purports to account for the distribution of phenotypic traits in populations of organisms; and the explanation is supposed to depend on the connection between phenotypic traits and the fitness of the creatures whose phenotypes they belong to. But, as it turns out, when phenotypic traits are  (locally or otherwise) coextensive, selection theory cannot distinguish the trait upon which fitness is contingent from the trait that has no effect on fitness (and is merely a free rider). Advertising to the contrary notwithstanding, natural selection can’t be a general mechanism that connects phenotypic variation with variation in fitness. So natural selection can’t be the mechanism of evolution.”

Nature cannot “select for” particular adaptive traits because in any given generation, only whole phenotypes are available for selection. All of the genes underlying a selected phenotype are selected. And evo-devo further complicates this already complicated picture. It asserts that only whole ontogenies, entire sequences of phenotypes expressed during life cycles, are available for selection. The sequential expression of sometimes wildly divergent phenotypes (think caterpillar to butterfly) during a life cycle is tightly constrained by developmental regulation, further restricting the potential formative power of natural selection.

The authors don’t mince words: “Contrary to traditional opinion, it needs to be emphasized that natural selection among traits generated at random cannot by itself be the basic principle of evolution. Rather there must be strong, often decisive, endogenous constraints and hosts of regulations on the phenotypic options that exogenous selection operates on.”

And

“[N]atural selection badly underestimates the significance of endogenous factors in the determination of phenotypes: we think that the thesis that organisms are random generators of phenotypes can’t be sustained even as a first approximation to an explanation of why there are the phenotypes there are.”

If natural selection cannot explain how creatures got to be how they got to be, what can explain it?

The authors don’t propose an alternative to selectionism and look to research to throw out more clues.  But an alternative might be right under their, and our, noses. Evolution might be just what the new science of evo-devo indicates that it is: the developmental unfolding of a life cycle, the ontogeny of an organism.

Darwin’s phrase, “descent with modification,” describes evolution and development equally well. Both processes involve descent with modification from a common ancestor. In the case of evolution the descent is of varieties of species, and in the case of development the descent is of varieties of cell/tissue types.

When regulatory proteins serially activate and inhibit various genetic “switches” in an organism the result is the ontogenetic development of the organism. The sequence of genes being turned on and off and their locations in the body steer development in a predictable direction, generating forms characteristic of the species. All of the descendant cells, no matter how modified from their common ancestor, the zygote, inherit the ancestor’s entire genetic allotment. That is, during this process of descent with modification, DNA is conserved.

Now evo-devo comes along and paints a similar picture with regards to evolution. The new science points to “toolbox” genes discovered to be common across diverse species (and families, orders, classes, and even phyla) and the regulatory systems that turn these foundational genes on and off.  DNA is conserved not only in ontogeny, but also in phylogeny, it turns out. Evolution generates new species in much the same way that an embryo generates new structures and tissue types, by flipping genetic switches. In this way, evo-devo weakens the selectionist model and suggests a developmental model instead. No doubt few evo-devo advocates would endorse such a radical interpretation.

Nonetheless, when the authors of What Darwin Got Wrong point to master genes, toolbox genes, genetic switches, protein-based regulatory systems, and other endogenous factors as the primary directors of phenotypic form, they are telling us that in the new, evo-devo, model of evolution the origin of species is an ontogenetic, developmental process. If it walks like a duck and it quacks like a duck . . . .

Classifying evolution as an ontogeny relieves the environment from having to account for phenotypes, something the authors insist that it cannot do. They assert, “. . . multiple levels of internal constraints on possible phenotypes make the notion of evolution as the product of external selection operating on phenotypic variations generated at random radically untenable.” In a developmental model of evolution, however, the environment doesn’t bestow medals of fitness on adaptive phenotypes, but functions as it does in ontogeny. A developmental model of evolution demotes the environment, subordinating it to the needs of ontogenetic programs. In this supportive role, it can function well or poorly, and in so doing facilitate or retard phenotypic expression. Nature in this model cannot select, as in the Darwinian model; it can only nurture or neglect. The environment does not pick any particular path, but it will feed or starve whoever ventures.

The old, and since discarded, formula was “ontogeny recapitulates phylogeny.” The arrival of evo-devo and the arguments in What Darwin Got Wrong stand the formula on its head. Embryos don’t replay the record of their evolutionary history. Rather, evolution unfolds as a process of development. Evo-devo, it seems, is a misnomer. The new understanding should be called devo-evo, or developmental evolutionary biology, with the emphasis on developmental. When we observe modification with descent managed endogenously, we are observing development and so should feel justified in adopting a developmental model of evolution and in retiring the theory of natural selection.

The arguments in What Darwin Got Wrong support the general argument of the star larvae hypothesis. The hypothesis argues explicitly that evolution is an ontogenetic process—a stage in the life cycle of an organism. Terrestrial evolution is the larval stage of the stellar life cycle.