Observations, Science.

Evo devo: where a little means a lot

-PD-US, G.J. Romanes

Redrawing of Haeckel. Attrib: G.J. Romanes, PD-US.

Ernst Haeckel’s old idea that “Ontogeny recapitulates phylogeny” is raising its head again; he suggested that the stages of a complex organism’s development correspond roughly to the stages of its evolutionary history, so that, for example, the presence of gill slits in the early development of the mammalian embryo was considered evidence of a distant fish ancestor for mammals.  Haeckel’s early explanation and justification for his idea has not survived into modern biology. (note 1) However, the burgeoning discipline of “evo devo”, or evolutionary developmental biology, provides new ways of testing this idea, and it is bearing serious fruit; it would appear that a small number of genetic changes can produce very large, and species specific, changes, particularly those mutations occurring during an organism’s development or to the gene regulatory mechanisms of that organism; this is strong new support for the theory of evolution.

Scientists for a long time have hypothesized that different animals, say apes or dragonflies or worms, had some common genes that would account for the universality of glycolysis, for example, but also very different genes to code for significantly different body plans and metabolisms; it was thought that the more complex the animal or plant, the larger and more complex the genome would need to be to describe the relative complexity of the organism.  Working against part of this idea is that there is new and strong evidence that a great many disparate species have many of the same genes, and roughly the same number of genes!!! How is it, then, that a human has perhaps 23,000 genes, and a common garden worm about the same number?  How is it that the chimpanzee has an almost identical set of genes that a human does (they are currently estimated to be about 1% different)?   If big differences in the genome don’t provide a strong basis for the bewildering diversity of life, or the basis for apparently large differences in complexity, say between a human and a worm, then what does?

The past fifteen years has seen the sequencing of many organism’s genomes, the collective DNA of a species, to include the human genome.  Subsequent comparative studies of these genomes, genomics, has raised the aforementioned and perplexing questions.  Combining these genomic studies with the techniques of molecular genetics and developmental biology, including embryology, has provided some early answers, which have had unexpected relevance in the explanation of the origin of species:  hence “evo devo.”

Nova’s recent film, What Darwin Never Knew, provided a highly recommended exposition of the emerging evo devo approach to these questions, and also provided a succinct summary, part of which is excerpted here:  “The new science of “evo devo” (evolutionary developmental biology) is applying lessons from the development of individual organisms to a broader understanding of changes in groups over the course of evolution. By studying developing organisms, scientists have learned about areas of DNA that “regulate” genes by controlling where, when, and to what degree they are turned on and off. Thus, the same gene can produce very different traits in two species, depending on how it is regulated during development.  Moreover, if species diversity comes in part from differences in how genes are regulated, then it follows that mutations in regulatory mechanisms—not just in genetic code—can explain how species develop new traits.”

Developmental biologists are now carefully studying specific genetic differences in the different stages of embryonic development in organisms to eke out more evidence supporting evo devo.  They have located and characterized the HOX genes, regulatory genes that control the large scale structural orientation of organisms during embryological development, such as the location of the head, appendages, gut, etc.; these genes have been found in all of the complex organisms studied so far.  Small mutations in the HOX genes, or in the switching regions that the HOX genes control, have already been found to produce large scale, species level changes!

-PD-US,

stickleback, Culaea inconstans. PD-US.

In one example, the stickleback fish, a fresh water and salt water fish, has sharp and stiff ventral and dorsal fins, making it difficult to eat.  There is one species of fresh water stickleback that is found only in one large lake, once an inland sea; there are presently no serious predators for this stickleback, and it no longer forms the stickle fins.  It still has the same genes for the stickle fins as its cousins, but researchers have isolated the switching region on the DNA that controls the expression of the stickle fin genes, and it has mutated and become inoperable for that one species. In another example, a small number of genes code for the expression of skin appearance in fish, reptiles and mammals (generating spots, stripes, coloration, etc.), and they are the same genes!  It is mutations in the regulatory and switching genes that appear to account for some of the wide variety of these phenotypic expressions.

In Ernst Haeckel’s and Darwin’s time, molecular genetic techniques, which underlie much of the new approach, were unavailable; by using modern molecular biological techniques to extend embryology , the “Ontogeny recapitulates phylogeny” concept has been given fresh life. The same genes for gill slits are present in fish, amphibians, reptiles and mammals, and the early presence of proto-gills in the embryos of all of these highly varied organisms happens because they each do develop, for a short period, the same early gill structures, coded for by highly similar genes, but, due to small but significant differences in their respective regulatory and switching genes that control the expression of these cross-species gill genes and/or the timing of when those genes are expressed, these proto-gills further develop into much different structures in different organisms; e.g. gills in fish, or the bones of the inner ear in mammals.  It would seem that, in evolutionary developmental biology, evo devo, a little means a lot.

 

Notes


1. The evolution of Haeckel’s ideas, prior to “evo devo,”, is described here.

Leave a Reply

Your email address will not be published. Required fields are marked *

Solve the puzzle to post a comment *