Biology teachers and students are generally comfortable in understanding the small, gradual evolutionary changes that distinguish closely related species, but many of us feel rather like fish out of water when it comes to explaining the evolution of complexity, such as how some of our aquatic ancestors became land-dwellers. Is it simply that the smaller ““micro-evolutionary”” changes (e.g., alterations in an enzyme) are easier to understand and explain than the bigger ““mega-evolutionary”” changes (e.g., having limbs instead of fins)?
Arthur, a zoology professor at the National University of Ireland, emphasizes that some of the general resistance to teaching about the evolution of complexity arises from its threat to creationist or intelligent-design beliefs. He urges readers to leave behind their philosophical baggage about the supernatural in order to travel unhindered and unprejudiced along the pathway of life, as revealed by the facts, while recognizing the historically significant thinkers who have led the journey so far. His book is full of examples of scientists who challenged doctrine, including Johann Meckel, Karl von Baer, Charles Darwin, A. R. Wallace, Ernst Haeckel, Robert Hooke, Matthias Schleiden, Theodor Schwann, William Bateson, Stephen Jay Gould, and Richard Dawkins. Since teachers are usually asking students to see evolution in a new light, it can be helpful for students to review some of the breakthroughs that visionaries and paradigm-shifters went through as they upset comfortable dogma.
Arthur's book is an engaging 20-chapter adventure into animal evolution. Much as a teenager who has just learned about the birds and the bees comes to the shocking realization that he or she would not exist but for an almost unimaginable sequence of ancestral couplings, Arthur finds questions about the mega-evolution of animal complexity awe-inspiring and vast. He manages to bring the discussion back to earth, though, in the same way that our now-enlightened teenager might later look at a family pedigree.
Arthur begins the journey by noting that the complexity of life increases over the vast stretches of evolutionary time. Finding our own species to be a very complex life form, and being naturally interested in ourselves as if we were the center of the universe, we cannot help but accept his challenge. He appropriately emphasizes the central role of variation in differential reproductive success as the grist of evolutionary change.
Many of the milestones of the evolutionary journey to complexity are discussed: unicellular to multicellular, irregular body form to symmetrical form, radial development to bilateral development, headless to protohead, the onset of gas-exchange specializations, the development of the nervous system. All of these changes are based on the shifting dance of the genes: mutations, networks of developmental reprogramming, and diverging functions of redundant parts.
There are several emerging topics, such as the evolution of evolvability, facilitated variation, epigenetics, and multilevel inheritance, which, although not necessarily named as such in the book, will further expand knowledge about the evolution of complexity. As noted by Arthur, some species appear to change phenotype more rapidly than others. He also notes that there are certain critical sets of developmentally regulated genes, which, if any is mutated, can radically alter phenotype.
The book concludes with some observations on human evolution. Arthur writes persuasively, encouraging students to evaluate the evidence for evolution and to more clearly recognize their place in the world. Can your students do this? I urge you to give them a try, using this book. The writing style is engaging while still being straightforward. The tone is encouraging and optimistic, showing what we have learned and why it is interesting to pursue this field.