Embryology and Evolution

From SkepticWiki

Contents 1 Introduction 2 Examples 3 Misconceptions 4 Related Articles

[edit] Introduction

As natural selection affects the development of organisms as well as their finished form, it is possible to use the Theory of Evolution to make predictions about the course of development. One of the most clear-cut predictions may be stated as follows:

If a feature, useless to the embryo, is acquired and then lost in the course of embryological development, then it must be the relic of an ancestral form for which the structure had a function, and this must agree with the lines of ancestry revealed by other methods (the fossil record, molecular phylogeny, et cetera.)

The reasoning behind this is that natural selection cannot produce a structure which is useless at the time of its production, but it can eliminate one when it becomes useless. The selective pressure to remove the unnecessary feature is much stronger on the mature form than on the embryonic form, since nothing much can inconvenience an embryo; all it has to do is lie there developing.

We should note that the theory of evolution does not predict that an organism must produce every discarded ancestral feature during embryonic development. Rather, it allows us to distinguish between things that can happen, and things that can't. For example, it would be sensible enough, according to the reasoning that we have given, if mammals, during embryonic development, were to grow and then reabsorb a dorsal fin. But in fact they don't.

The way that the prediction allows us to test evolution, then, is that according to the evolutionary prediction, all those useless features produced and then abolished during development should fall into the class of things that, according to the theory of evolution, can happen, and not one of them should be something that the theory of evolution predicts cannot happen.

[edit] Examples

This leads to some specific predictions. Humans can (and do) develop and then lose a caudal appendage, which is in line with our ancestry as reconstructed from the fossil record, morphology, molecular phylogeny, and so forth. We could not (and do not) grow and then lose feathers, because that is not in our ancestry. Whales can produce and reabsorb hind limb buds, because they are descended from four-limbed animals; they cannot develop and then reabsorb a pair of wings, because that is not in their ancestry. Embryonic birds can produce and then lose claws on their forelimbs; they cannot produce and then lose hooves. Vertebrates can develop and then lose a notochord, in line with their ancestry; invertebrates do not, in line with theirs. The embryos of baleen whales have teeth, though the adults do not: obviously teeth are of no use in the womb; similarly, embryonic cattle have upper front teeth, though cows do not. In both cases this reflects the evolutionary history of the species as revealed by the fossil record, whereas lineages which never had teeth, such as crustaceans, do not have this quirk of embryology.

[edit] Misconceptions

The claims of evolutionary theory concerning development should not be confused with Haeckel's so-called "law of biogenesis", that "ontogeny recapitulates phylogeny" (see our article on Haeckel's Embryos for more details). There is no reason in the theory of evolution why every ancestral feature should be recapitulated in the development of the embryo, still less why this should happen in a chronological order that mimics the course of evolutionary development. The hoatzin chick, for example, only ever develops two claws on the forelimb, while dromaeosaur dinosaurs and Archaeopteryx have three.

As Haeckel's "law" is known to be wrong, and the evidence he produced for it was fudged to the point of fraud, creationists like to conflate it with the predictions made by the theory of evolution, as a substitute for finding anything wrong with the actual theory.

[edit] Related Articles

• Theory of Evolution
• Haeckel's Embryos
• Biology