Evolution 101

Monday, March 13, 2006

What is Evo-Devo?

Evo-Devo is a combination of two disciplines within the field of biology: evolutionary biology and developmental biology. The realm of evolutionary concepts should be fairly familiar to you by now, but what is developmental biology? Developmental biology is the study of how organisms develop from a single cell through all the intermediate embryological stages, all the way to birth. Evolutionary developmental biology, then, or Evo-Devo for short, is a way to look at the way that the mechanisms of development have been influenced by evolutionary forces.

And this, of course, is a logical collaboration between two different biological camps. And it’s a fairly typical overlap, as well- remember that in biology, you almost never find things that are black or white, instead you find things that are various shades of grey. Likewise, different biological disciplines find themselves overlapping with others all the time. My field is molecular biology, which is pretty general really, because it focuses on the molecular pathways, the individual genes and gene products that contribute to physiological function and pathological condition. But our entire bodies are made up of molecules, so depending on the project of interest, any given molecular biologist could be overlapping in cardiology, in neurology, in gastroenterology, in immunology, or any number of other “ologies.” The same could be said about a cell biologist, or a biological chemist. Or even an ecologist, I suppose, although my own personal paradigm in biomedical research is preventing me from thinking of a good example for some kind of collaboration between a molecular biologist and an ecologist, I’m sure there are plenty out there. My point is that a collaboration between evolutionary biology and developmental biology is not odd or unique, and the only reason it’s become recently popular is because of some pretty powerful discoveries, which I’ll get to a little bit later.

First, I’d like to defuse or debunk one of the criticisms of Evo-Devo from the creationist camp. It seems like there’s really no aspect of evolutionary biology that creationism hasn’t taken pot shots at over the years, and most of them are more pervasive in the popular consciousness than the actual science, for a number of reasons. This particular criticism, like most of the others put forth by creationism, has sort of been left behind scientific progress, but since so many are unaware of current scientific thought, it can be somewhat successful. You may remember that I mentioned the Jack Chick tract “Big Daddy,” when I was talking about “What is NOT Evolution,” several weeks ago. Well, you can find this creationist criticism here also (by the way, not to get off on a tangent, but it does seem to me that it’s very rare to find an actual Argument put forth by creationism- much more often, it’s a criticism of one aspect of evolutionary theory). Midway through the comic, the Evil Evolutionary Professor tells his class that, “Here is proof of evolution. Human embryos have gill slits proving man evolved through the fish stage millions of years ago.” All this while thinking, “I hate him,” about the Saintly Creationist Student who is challenging him. And the Student replies, “Sir, Ernst Haeckel made up those drawings in 1869 and they were proven to be wrong in 1874. Those folds of skin are not gills. They grow into bones of the ear and glands in the throat.” And another student comments, “Wow, 125 years wrong and still in our book!”

So what is the truth behind this criticism? Well, Ernst Haeckel was a German scientist who accepted evolutionary theory fairly strongly, although he was somewhat torn between Darwin’s theory of natural selection and Lamarck’s theory of evolutionary ontology. He won some popularity with his analysis of the embryological stages of different animals, and in fact published a theory which is now currently referred to as “recapitulation theory.” According to Haeckel, if you line up embryos from different vertebrates at similar stages of development, there are obvious anatomical similarities between all organisms. He published drawings that he had made of these embryos in 1874, to back up his claim. Now, the “Big Daddy” comic says that Haeckel “made up” these drawings. This is not true. At worst, Haeckel deliberately overemphasized the similarities between the different organisms in the way he drew them, and at best, he didn’t realize that he had drawn his conclusions into the figure subconsciously. They’re clearly not made up out of whole cloth, as the comic implies. It would be more accurate to think of Haeckel treating his drawings the way magazines airbrush pictures of models to remove blemishes and overemphasize certain characteristics. A magazine photographer might airbush a model to have larger breasts and thinner thighs, and Haeckel airbrushed a drawing of a human embryo to have larger gill slits and a longer tail. That doesn’t excuse the inaccuracy, of course. A magazine might be only be selling you perfume or lingerie, but a scientific paper is supposed to be a pretty clear representation of the truth.

So, the guy fudged his drawings to support his theory. But what is recapitulation theory anyway? Haeckel thought that the evolutionary development of an organism was carried out again, or recapitulated, during its embryological development. A short, catchy way to say this is, “Ontogeny recapitulates phylogeny.” Ontogeny means the development of an organism from embryo to adult, and phylogeny means the evolutionary development of an organism from ancestral to modern species. Essentially, this means that as an organism develops from embryo to adult, it passes through a series of intermediate forms which approximate ancestral species. For example, a human embryo would pass through a fish stage, then an amphibian stage, then a reptile stage, then a bird stage, then a general mammal stage, and finally the human stage. It’s pretty clear that this theory is bunk- evolutionary science has rejected this theory almost completely… almost. While it’s pretty clear that human embryos don’t actually become fish, they do share a number of characteristics with fish embryos. For example, the notorious gill slits. In the “Big Daddy” comic the Student criticizes the Professor for mentioning gill slits, because they’re not gills. Excuse my glibness, but DUH! It’s well known in developmental biology that gill slits aren’t gills. I’m not sure why this is supposed to be such a shocking revelation by the Student, other than the fact that the comic was likely written by someone who has no knowledge of developmental biology or willingness to look it up. Gill slits are not gills- they’re often called “pharyngeal pouches” because they occur in the throat, which is technically known as the pharynx. They look somewhat like gills, hence the name. The Student is correct in saying that they develop into ear bones and throat glands, but he leaves out a little bit. Ear bones develop in mammals only- in reptiles, these bones are part of the jaw, and this intermediate stages of this is an excellent example of evolution in the fossil record. Otherwise, the first two slits become the jawbone, and the other slits become different anatomical structures in different organisms. But importantly, in fish, surprise, surprise, the gill slits become… GILLS!

And this really gets to the essence of what we can retrieve from Haeckel’s theory. Clearly, Ontogeny does NOT recapitulate phylogeny. However, ontogeny does organize according to phylogeny. By which I mean, we can look at the developmental forms of different organisms and infer evolutionary relationships between them. So, to compare a fish embryo, a chimpanzee embryo, and a human embryo, is to show pretty clearly that there are more similarities in the way a chimpanzee and a human develop compared to either one and a fish. All three start off with the same number of pharyngeal slits, but only chimpanzees and humans form ear bones, and only the fish forms functional gills. And we don’t have to rely on Haeckel’s drawings, either- most biology textbooks use photographs of embryos, which may be a bit harder to interpret, but which are at least more accurate than Haeckel’s drawings.

But evo-devo isn’t limited to examining embryos. Nowadays the most exciting research in this field is directly or indirectly related to the domain of molecular biology and genetics. That’s right- just like virtually everything else in biology, it comes down to genes. Hox genes, specifically. Hox is short for homeobox, and refers to a region of DNA within a particular gene that allows that gene to turn on or off other genes once it’s been translated into a protein. Hox genes function to promote embryonic development and to structure the developing body plan. Different Hox genes are expressed at different locations along the body, from head to tail, and these signals allow for the expression of other genes which activate anatomical characteristics that are specific to one region of the body. The early work in understanding Hox genes was done (as was most genetics research) in fruit flies. By changing the order in which Hox genes were turned on or off, researchers could cause legs to grow where antennae should be, or to cause the generation of a second pair of wings. Not surprisingly, Hox genes are remarkably highly conserved among vertebrates, and even to a lesser extent among invertebrates. Even more interestingly, the Hox genes are first activated at the stage in embryologic development just prior to observable differences between different organisms. These show that the regulation of gene transcription is a remarkably potent force in evolutionary development, and may have a higher impact than direct mutations on specific genes.

So, let’s review. Evolutionary biology and developmental biology join forces to study how evolution affects embryologic development. The creationist criticism of Ernst Haeckel’s embryo analysis is over a century too late, since evolutionary theory rejects Haeckel’s theory that ontogeny recapitulates phylogeny. However, work on evo-devo shows that not only are evolutionary relationships evident when comparing the development of different organisms, but there exists a genetic mechanism for these relationships in the modulation of the Hox genes.


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