This is a post originally made on the old Pharyngula website; I’ll be reposting some of these now and then to bring them aboard the new site.
There are a number of reasons why the current theory of evolution should be regarded as incomplete. The central one is that while “nothing in biology makes sense except in the light of evolution”, some important disciplines within biology, development and physiology, have only been weakly integrated into the theory.
Raff (1996) in his book The Shape of Life(amzn/b&n/abe/pwll) gives some of the historical reasons for the divorce of evolution and development. One is that embryologists were badly burned in the late 19th century by Haeckel, who led them along the long and unproductive detour of the false biogenetic “law”. That was a negative reaction; there was also a positive stimulus, the incentive of Roux’s new Entwicklungsmechanik, a model of experimental study of development that focused exclusively on immediate and proximate causes and effects. Embryology had a Golden Age of experimentation that discouraged any speculation about ultimate causes that just happened to coincide with the time that the Neo-Darwinian Synthesis was being formulated. Another unfortunate instance of focusing at cross-purposes was that the Neo-Darwinian Synthesis was fueled by the incorporation of genetics into evolutionary biology…and at the time, developmental biologists had only the vaguest ideas about how the phenomena they were studying were connected to the genome. It was going to be another 50 years before developmental biology fully embraced genetics.
The evolutionary biologists dismissed the embryologists as irrelevant to the field; furthermore, one of the rare embryologists who tried to address evolutionary concerns, Richard Goldschmidt, was derided as little more than a crackpot. It’s an attitude that persists today. Of course, the problem is mutual: Raff mentions how often he sees talks in developmental genetics that end with a single slide to discuss evolutionary implications, which usually consist of nothing but a sequence comparison between a couple of species. Evolution is richer than that, just as development is much more complex and sophisticated than the irrelevant pigeonhole into which it is squeezed.
In her book, Developmental Plasticity and Evolution(amzn/b&n/abe/pwll), West-Eberhard (2003) titles her first chapter “Gaps and Inconsistencies in Modern Evolutionary Thought”. It summarizes the case she makes in the rest of her 700 page book in 20 pages; I’ll summarize her summary here, and do it even less justice.
She lists 6 general problems in evolutionary biology that could be corrected with a better assimilation of modern developmental biology.
- The problem of unimodal adaptation. You can see this in any textbook of population genetics: the effect of selection is to impose a gradual shift in the mode of a pattern of continuous variation. Stabilizing selection chops off both tails of the distribution, directional selection works against one or the other tail, and disruptive selection favors the extremes. This is a useful, productive simplification, but it ignores too much. Organisms are capable of changing their specializations either physiologically, in the form of different behaviors, morphologies, or functional states, developmentally in the form of changing life histories, or behaviorally. Evolution is too often a “theory of adults”, and rather unrealistically inflexible adults at that.
- Cohesiveness. There is a long-standing bias in the evolutionary view of development, that of development imposing constraints on the organism. We can see that in the early favor of ideas like canalization, and the later vision of the gene pool as being cohesive and coadapted, which limits the magnitude of change that can be permitted. It’s a view of development as an exclusively conservative force. This is not how modern developmental biologists view the process. The emerging picture is one of flexibility, plasticity, and modularity, where development is an innovative force. Change in developmental genes isn’t destructive—one of the properties of developmental systems is that they readily accommodate novelty.
- Proximate and ultimate causation. One of the radical secrets of Darwin’s success was the abstraction of the process away from a remote and unaccessible ultimate teleological cause and to a more approachable set of proximate causes. This has been a good strategy for science in general, and has long been one of the mantras of evolutionary biology. Organisms are selected in the here and now, and not for some advantage many generations down the line. Evolutionary biology seems to have a blind spot, though. The most proximate features that affect the fitness of an organism are a) behavioral, b) physiological, and c) developmental. These are the causes upon which selection can act, yet the focus in evolutionary biology has been on an abstraction at least once removed, genetics.
- Continuous vs. discrete change. This is an old problem, and one that had to be resolved in a somewhat unsatisfactory manner in order for Mendelian genetics (an inherently discrete process) to be incorporated into neo-Darwinian thought (where gradualism was all). Many traits can be dealt with effectively by quantitative genetics, and are expressed in a graded form within populations, and these are typically the subject of study by population geneticists. We often see studies of graded phenotypes where we blithely accept that these are driven by underlying sets of graded distributions of genes, such as the studies of Darwin’s finches by the Grants, yet those genes are unidentified. Conversely, the characters that taxonomists use to distinguish species are usually qualitatively distinct are at least abruptly discontinuous. There is a gap in our thinking about these things, a gap that really requires developmental biology. Wouldn’t it be useful to know the molecular mechanisms that regulate beak size in Darwin’s finches?
- Problematic metaphors. One painful thing for developmental biologists reading the literature of evolutionary biology is the way development is often reduced to a metaphor, and usually it is a metaphor that minimizes the role of development. West-Eberhard discusses the familiar model of the “epigenetic landscape” by Waddington, which portrays development as a set of grooves worn in a flat table, with the organism as a billiard ball rolling down the deepest series. This is a model that completely obliterates the dynamism inherent in development. Even worse, because it is the current metaphor that seems to have utterly conquered the imaginations of most molecular biologists, is the notion of the “genetic program”. Again, this cripples our view of development by removing the dynamic and replacing it with instructions that are fixed in the genome. This is bad developmental biology, and as we get a clearer picture of the actual contents of the genome, it is becoming obviously bad molecular biology as well.
- The genotype-phenotype problem. Listen to how evolutionary explanations are given: they are almost always expressed in the language of genes. Genes, however, are a distant secondary or tertiary cause of evolutionary solutions. Fitness is a collective product of success at different stages of development, of different physiological adaptations, and of extremely labile interactions with the environment. Additionally, a gene alone is rarely traceable as the source of an adaptive state; epigenetic interactions are paramount. We are rarely going to be able to find that a specific allele has a discrete adaptive value. It’s always going to be an allele in a particular genetic background in a particular environment with a particular pattern of expression at particular stages of the life history.
One unfortunate problem with discussing these issues in venues frequented by lay people is, you guessed it, creationism. Any criticism of a theory is seized upon as evidence that the theory is wrong, rather than as a sign of a healthy, growing theory. The Neo-Darwinian Synthesis is not wrong, but neither is it dogma. It was set up roughly 70 years ago with the knowledge that was available at the time, and it is not at all surprising that the explosion of new knowledge, especially in molecular biology, genetics, and developmental biology, means that there are radically different new ideas clamoring to be accommodated in the old framework. The theory is going to change. This isn’t cause for creationists to rejoice, though, because the way it is changing is to become stronger.