Carroll: Endless Forms Most Beautiful
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Sean B. Carroll, Endless Forms Most Beautiful : The New Science of Evo Devo. New York : W.W.Norton & Company, 2005. 350 pages with "Sources and Further Reading" and index; illustrated with drawings and color plates.
I suspect that the general understanding of how genes lead to variations in living beings is wholly naïve – mine certainly was. Carroll's book changed that significantly for the better, and clarified a number of other developmental issues for me. Understanding how genes affect development means seeing how evolution affects genetic material, and the shift in perspective approaches breathtaking.
When I was a young scientist in the early 80s, gene expression and particularly cell differentiation were big questions. Understanding how a blob of cells in a newly fertilized egg could become a well-organized system of brain cells, heart cells, lung cells, blood cells, muscle cells, etc., was mystifying and impenetrable. Now, with Evo Devo (i.e., "Evolutionary Developmental Biology), it seems that the question has been redefined and subsequently met with a powerful answer.
The key to answering such questions [about why different animals look different] is to realize that every animal form is the product of two processes—development from an egg and evolution from its ancestors. To understand the origins of the multitude of animal forms, we must understand these two processes and their intimate relationship to each other. Simply put, development is the process that transforms an egg into a growing embryo and eventually an adult form. The evolution of form occurs through changes in development. [p. 4]
A short summary of cell expression would introduce the ideas that a developing fetus has a well-defined geography (up, down, front, rear, left side, right side), and that there are switches in the DNA that tell genes when and where to turn on and off in doing their business creating proteins. Protein messages to the cells abound. It's an amazing picture. But this is only the summary; for a clear and commanding exposition of all the ideas, read the book.
Almost immediately after the first sets of fruit fly genes were characterized came a bombshell that triggered a new revolution in evolutionary biology. For more than a century, biologists had assumed that different types of animals were genetically constructed in completely different ways. The greater the disparity in animal form, the less (if anything) the development of the two animals would have in common at the level of their genes. One of the architects of the Modern synthesis, Ernst Mayr, had written that "the search for homologous genes is quite futile except in very close relatives." But contrary to the expectations of any biologist, most of the genes first identified as governing major aspects of fruit fly body organization were found to have exact counterparts that did the same thing in most animals, including ourselves. This discovery was followed by the revelation that the development of various body parts such as eyes, limbs, and hearts, vastly different in structure among animals and long thought to have evolved in entirely different ways, was also governed by the same genes in different animals. The comparison of developmental genes between species became a new discipline at the interface of embryology and evolutionary biology—evolutionary developmental biology, or "Evo Devo" for short.
The first shots in the Evo Devo revolution revealed that despite their great differences in appearance and physiology, all complex animals—flies and flycatchers, dinosaurs and trilobites, butterflies and zebras and humans—share a common "tool kit" of "master" genes that govern the formation and patterning of their bodies and body parts. [...] We now know from sequencing the entire DNA of species (their genomes) that not only do flies and humans share a large cohort of developmental genes, but that mice and humans have virtually identical sets of about 29,000 genes, and that chimps and humans are nearly 99 percent identical at the DNA level. [pp. 9—10]
Armed with this picture, how different animal forms can evolve is radically clarified. All those silly objections to natural selection along the lines of "half an eye is no good for anything" really become meaningless, because that idea presupposes a "gene for an eye", and it doesn't happen that way. Genes don't control form that way. Instead, genes control development, influencing form in an indirect way, once removed from the process. Evolution becomes at once a more subtle, more clearly delineated concept.
The simplest and, for a long time, the most commonly held idea relating genes to the evolution of complex form is that new genes must evolve in order for new kinds of body designs and structures to arise. The intuitive appeal of such a notion is understandable. Since the form of a given species is due to its unique genetic information, then new forms require new information—ergo new genes. but as we will soon see, despite its appeal, the invention of "new genes" is not the explanation for the origin or diversity of most animal groups. [p. 150]
Along the way we get a lot of insight about what, until recently, was thought of as "junk DNA", great swaths of the molecule that didn't seem to do anything. Well, it turns out that quite a bit of it does do something during development: those are the switches that turn the genes on and off in different places and at different times during development, and these switches are themselves subject to mutation, multiplication, and evolution.
I can praise this book for its clarity, its enthusiasm, its skill at exposition—those are all true. It's also fun and exciting to read, and a real page turner if you've wondered at all about how genes make animals and how evolution makes different genes.
Carroll's book is itself an example of the application of careful analytical thinking and empirical tests to some tough problems, so we can read it as a brilliant case-study of science in action. Plus, he shares some thoughts on the educational process:
Finally, at the university level, the evolutionary view of life should be as fundamental to a college degree as Psychology 101 or Western Civilization. But rather than asking students to memorize and regurgitate mountains of testable facts, we should emphasize study of the history of the discovery of evolution, its major characters and ideas, and the basic lines of evidence. This would do far more to inform citizens and prepare teachers than forcing students to remember the Latin names of taxa. We are stoning our children to utter boredom with little pebbles and missing the big picture. The drama of the story of evolution will recapture student interest. [p. 296]
Finally, my highest praise: I am still thinking about and applying the ideas I've learned from reading Carroll's book. I will be for some time to come, too. It has changed my understanding and my thinking about all things to do with evolutionary biology, distinctly for the better.
-- Notes by JNS