In this series, we feature excerpts from biologist Sean Carroll’s book, Endless Forms Most Beautiful: The New Science of Evo Devo, a well-wrought account of how the field of evolutionary development biology, or Evo Devo, has shaped our understanding of howanimals evolve, by studying their development. The Psalmist wrote of how God knits us together in our mother’s womb, and thanks to the methods and tools now available to biologists, these processes that were once hidden are being revealed through this exciting field. While we cannot attest to the author’s faith persuasion, Sean Carroll does an outstanding (and witty) job of making a complex set of ideas more manageable to understand, and we encourage you to read this book in its entirety.
In this excerpt, Carroll summarizes a few significant facets of evolution that Evo Devo has helped illuminate: the “master tool kit,” (an ancient set of genes and their protein products shared across the animal kingdom that controls development, which is powerful evidence for descent with modification), why understanding the relationship between animals based on appearances alone can be misleading, and how new animal forms do not require new genes. This excerpts comes from the final chapter of the book and refers to earlier chapters.
The editorial policy used in these excerpts can be found at the bottom of this post.
The Tools for Making the Kingdom are Ancient
The first and still perhaps the most stunning discovery of Evo Devo is the ancient origin of the genes for building all sorts of animals (chapters 3 and 6). The fact that such different forms of animals are shaped by very similar sets of tool kit proteins was entirely unanticipated. The ramifications of these revolutionary findings are powerful and manifold.
First of all, this is entirely new and profound evidence for one of Darwin’s most important ideas—the descent of all forms from one (or a few) common ancestor. The shared genetic tool kit for development reveals deep connections between animal groups that were not at all appreciated from their dramatically different morphologies.
Second, the discovery that organs and structures that were long viewed as independent analogous inventions of different animals, such as eyes, hearts, and limbs, have common genetic ingredients controlling their formation has forced a complete change in our picture of how complex structures arise. Rather than being invented repeatedly from scratch, each eye, limb, or heart has evolved by modification of some ancient regulatory networks under the command of the same master gene or genes (chapter 3). Parts of these networks trace back to the last common ancestor of bilaterians (Urbilateria), and earlier forms (chapter 6).
Third, the deep history of the tool kit reveals that the invention of these genes was not the trigger of evolution. The bilaterian tool kit predated the Cambrian (chapter 6), the mammalian tool kit predated the rapid diversification of mammals in the Tertiary period, and the human tool kit long predated apes and other primates (chapter 10). It is clear that genes per se were not “drivers” of evolution. The genetic tool kit represents possibility—realization of its potential is ecologically driven.
Existing Genes and Structures Provide the Means for Innovation
We have seen that insects, pterosaurs, birds, or bats did not invent “wing” genes (chapter 7), butterflies a “spot” gene (chapter 8), or humans a “bipedalism” or “speech” gene (chapter 10). Rather, innovation in all of these groups has been a matter of modifying existing structures and of teaching old genes new tricks.
The key to innovation at the genetic level is the multifunctionality of tool kit genes. The multifunctionality of tool kit genes stems from their deployment at different times and places through batteries of genetic switches. In this manner, a protein such as Distal-less can act at one time to promote limb formation, and at another to promote eyespot development. The protein made each time is identical, so the difference in function is due to its action on different switches in these different contexts.
At an anatomical level, multifunctionality and redundancy are keys to understanding the evolutionary transitions in structures. We saw this especially in arthropods, where the shifting of a function such as feeding to one of a battery of appendages freed other appendages to become specialized for walking, swimming, or other activities. In a similar fashion, the gill branches in aquatic arthropod ancestors became modified into book gills, book lungs, tubular tracheae, spinnerets, and wings.
Evo Devo has revealed the continuity among forms that was masked or about which there were uncertainties based on appearance alone. By revealing the developmental similarities among structures, Evo Devo presents a wholly new kind of evidence that is far more objective than morphology alone. These insights into the evolution of novelty strengthen aspects of Darwin’s original ideas that some have found most difficult to grasp.
The history of these structures also illustrates how “endless forms” evolve through cycles of invention and expansion. New structures open up new ways of living. The insect wing led to the evolution of dragonflies and mayflies, butterflies and beetles, fleas and flies, and more. The expansion of these groups was catalyzed in turn by a cycle of innovation and expansion by making modifications to the wings or body plan—scale coloration systems in moths and butterflies, a hard covering in beetles, a sophisticated balancing hindwing in flies.
Why are existing body parts and genes the more frequent pathway to innovation? This is a matter of probability. Variation in existing structures and genes is more likely to arise than are new structures or genes, and this variation is therefore more abundant for selection to act upon. As François Jacob explained so eloquently, Nature works as a tinkerer with available materials, not as an engineer does by design. The invention of wings never occurred from scratch, but by modifying a gill branch (insects) or forelimbs (three times). Trends in evolution reflect the paths that are most available and therefore those taken most frequently.
Evo Devo has revealed that evolution can and does repeat itself at the levels of structures and patterns, as well as of individual genes. If evolution takes the most probable path, via existing structures and genes, then when confronted with similar selection pressures, different species may follow the same path to adaptation. We saw this in the evolution of feeding appendages in crustacea (chapter 6), pelvic spine reduction in sticklebacks (chapter 7), and other cases of limb reduction in vertebrates. We also saw that melanic fur or plumage patterns can arise through mutations in the very same gene in different species, and even the very same position in this gene (chapter 9).
These instances of evolution repeating itself directly address difficulties some have had in grasping the role of random mutation in the evolutionary process. Some people have found it hard to imagine how novelty and complexity arise from “a random process.” The key distinction is that while the generation of genetic variation by mutation is a completely random process, the sorting of these variations as to which will persist and which will be discarded is determined by a powerful, selective nonrandom process. Of the hundreds of millions or billions of individual base pairs in an animal genome, all are equally susceptible to random copying errors or physical damage that cause mutations. But only a tiny fraction of all possible mutations can alter a mammal’s coat in a viable manner, or reduce a stickleback’s spines without causing catastrophic collateral damage. In large populations of animals, over eons of time, such mutations will arise simply as a matter of probability. When they do occur, positive selection upon the trait they affect will cause them to spread in populations over time.
Jacques Monod captured this interplay of randomness and selection in evolution most eloquently in the title of his landmark book, Chance and Necessity (a reference to the Greek philosopher Democritus who said, “Everything existing in the Universe is the fruit of chance and necessity”). Evolution is indeed a matter of chance, but in the random lottery of mutations, some numbers and combinations better meet the imperatives of ecological necessity, and they arise and are selected for repeatedly.
We also saw in rock pocket mice that the same species can use different paths to a similar solution. And, while pterosaurs, birds, and bats evolved wings out of their forelimbs, they did so in fundamentally different ways. Similar ecological demands and opportunities have selected for similar adaptations, but the developmental solutions will sometimes differ in detail.
By revealing the genetic and developmental mechanisms underlying change, Evo Devo allows us to compare and contrast the evolutionary paths of different groups. Long-standing mysteries such as Batesian mimicry in butterflies, melanism in moths, and even the evolution of finch beak size and shape now lie within our grasp. We shall soon have detailed pictures of many of the classic examples of natural selection and understand in depth how variation arises and is selected for.