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When we think of evolution, we think of change. The species living at any particular time are different from their ancestors, having changed in form and function by the mechanisms reviewed up to now in the discussion of the genetics of the evolutionary process. But there is a second feature of the diversity of life, one that Darwin took as an important argument for the reality of evolution. Not only have present organisms descended from previous, different organisms; but, if we go back in time, organisms that are currently very different are descended from a single ancestral form. Indeed, if we go back far enough in time to the origin of life, all the organisms on earth are descended from a single common ancestor. 

Thus we expect to find that apparently different species have underlying similarities, attributes of their common ancestor that have been conserved through evolutionary time despite all the changes that have taken place. Before the tools of modern biochemistry and genetics were available, the chief evidence of the underlying similarity of apparently different structures in different species was taken from anatomical observations of adult and embryonic forms. So, the similar bone structures of the wings of bats and the forelimbs of running mammals make it evident that these structures were derived evolutionarily from a common mammalian ancestor. Moreover, the anatomy of the wings of birds points to the common ancestry of mammals and birds (Figure 21-15). 

It is even argued that the basic segmentation of the bodies of insects and of vertebrates are evolutionary variants on a common ancestral pattern derived from the common ancestor of invertebrates and vertebrates. Although this argument may seem to push the claim of evolution- ary conservation too far, it turns out, as we have seen in the discussion of the Hox and HOM-C genes in Chapter 18, that genetic analysis of patterns of development pro-vides a powerful demonstration of the common ancestry of animals as different as insects and mammals. We saw in Chapter 18 that such disparate organisms as the fly, the mouse, and human beings have similar sequences for the genes controlling the development of body form. (The same is true for the worm C. elegans.) The simplest explanation is that the Hox and HOM-C genes are the vertebrate and insect descendants of a homeobox gene cluster present in a common ancestor some 600 million years ago. 

The evolutionary conservation of the HOM-C and Hox genes is not a singular occurrence. Many examples have been uncovered of strongly conserved genes and even entire pathways that are similar in function. For example, the pathways for activating the Drosophila DL and mammalian NFB transcription factors are essentially completely conserved from a common ancestral pathway (Figure 21-16).The Drosophila protein at any step in the DL ac- activation pathway is similar in amino acid sequence to its counterpart in the mammalian NFB activation pathway. (Don’t worry about what the particular proteins do; just appreciate the incredible conservation of cellular and developmental pathways as demonstrated by the similarity between the corresponding components of the two pathways indicated by similarly shaped objects in the diagrams. 

We do indeed know that DL and NFB participate in some equivalent developmental decisions.) Indeed, as can be seen from a selection from the known examples, such evolutionary and functional conservation seems to be the norm rather than the exception. What has made developmental genetics into an extraordinarily exciting field of biological inquiry is the demonstration, by means of genetic analysis, that basic developmental pathways and their genetic basis have been conserved over hundreds of millions of years of evolution.

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