Evolution of Evolution

Atavism: The reappearance of an ancestral trait after many generations of absence. Dollo’s Law claims that “reverse evolution” never occurs for complex traits, but in fact there are exceptions. Atavisms can arise suddenly by mutation, but only rarely do they spread to fixation in a species. Sporadic throwbacks include hen’s teeth, horse’s toes, dew claws (extra digits) in dogs, premolars in mice, hindlimb flippers in dolphins and whales [148], and tails or extra nipples in humans. Possible reversions at the species level include frog teeth, lizard digits, reptile scales (= revived fish scales?), and dorsal fins in ichthyosaurs (= revamped fish fins?). One confirmed species-level atavism is sex-comb rotation in flies. One putative species-level atavism that was disproven involves wings in stick insects [2231].” (Held 2017, 264, How the Snake Lost its Legs: Curious Tales from the Frontier of Evo-Devo)

Held, Lewis I. Jr. (2017) How the Snake Lost its Legs: Curious Tales from the Frontier of Evo-Devo

How the vampire bat reinvented running

Bats are adept fliers, but most are clumsy on the ground. One exception is the vampire bat, Desmodus rotundus. When tested on a treadmill these bats run amazingly well. An analysis of their gait, published in 2005, showed that they “bound” into midair like mice, with their forefeet and hindfeet touching the ground at different times [1862]. The study’s authors argue that (1) the founders of the bat order had no need to run after they specialized as aerial insectivores, so (2) the neural circuits for running atrophied over millions of years (use it or lose it), but (3) vampire bats adapted to a new (blood-drinking) niche by reacquiring a running capability [1863]. Whether this talent is an atavism (old circuits revived) or a novelty (new circuits created) is unclear.

— Held, Lewis I. Jr. (2017) How the Snake Lost its Legs: Curious Tales from the Frontier of Evo-Devo

As it turns out, the miracle of complex life is more amazing, yet ironically simpler, than anyone ever expected. Researchers now know that life’s building materials are few, and they were “invented” near the dawn of animals. More specifically, a surprisingly small number of genes—”tool kit genes”—are the primary components for building all animals, and these genes emerged at a time before the Cambrian Explosion, some 600 million years ago. Thus the amazing diversity of the animal kingdom is the result of the flexibility of a small number of building blocks that have existed for eons.

This means, for example, that the gene that controls the formation of an arm on a human is the same gene that controls the formation of a wing on a bird, a fin on a fish, and a leg on a centipede, and that this gene has been around since the first animals grew the first appendage of any kind. Some prominent scientists have argued that if we could rewind the tape of life and start over again, the result would be a totally different world from that which exists today. They are wrong. Tool kit genes conserve the essence of animals, and they react to ecological cues in very consistent ways.

Carroll, Sean B. (2005) Endless Forms Most Beautiful: The New Science of Evo Devo

Introduction: Birth of a Scientific Field

Every scientific field, no matter how broad its reach, has one or more basic goals that define its shape and direction. The central aim of evolutionary developmental biology is to delineate the precise mechanisms, processes, and events that have been responsible for generating the astonishing diversity of animal and plant forms that characterize our planet. If this exploration is productive, we should eventually come to comprehend the evolutionary routes by which, for example, mice came to differ from men, spiders from butterflies, and dandelions from sequoias. Despite all that has been learned about the general nature of evolution since the publication of Charles Darwin’s The Origin of Species in 1859, we are only just beginning to fathom such divergent evolutionary trajectories at the level of the actual genetic, developmental, and historical details. (Wilkins 2002, 3)

Evolutionary Developmental Biology

By 1959, the year of the centenary of the publication of The Origin, the atmosphere within evolutionary biology circles was quietly but profoundly celebratory.18 The puzzle of evolution was considered essentially solved; what remained was the good, steady work of filling in the details. Indeed, one would have had to search diligently to find suggestions that there were any major gaps or omissions in evolutionary theory. With Schmalhausen silent and in retirement, and Goldschmidt dead, Waddington (1959) remained a rather isolated dissenting voice, calling out to the field that something important, the evolution of developmental processes, had been badly neglected. (Wilkins 2002: 31)

And then, starting in the early 1970s, the consensus about the sufficiency of the evolutionary synthesis began to melt and break up like an iceberg that has drifted into warmer waters. Disputes began to arise about the best ways to reconstruct evolutionary history (a detailed and provocative history of this debate is given in Hull, 1988) and about the significant modes of genetic mechanisms of evolutionary change (Eldredge and Gould, 1972; Stanley, 1979; Dover, 1982). One contributory factor, certainly, was the advent of new methodologies. If, to change the metaphor, the evolutionary pot was beginning to simmer again, one important source of heat was the new science of molecular biology. (Wilkins 2002: 31)

(….) Many years ago, from a detailed phylogenetic survey of the distribution of eye structures within the Metazoa, Salvini-Plawen and Mayr (1977) argued that eyes had evolved independently, perhaps as often as 20 times, in different metazoan phyla. Thus, in their reconstruction, there was no single ancestral eye form in the Metazoa. Furthermore, in their scheme, continuity of appearance of structures was a cardinal test of homology. In cephalopods, for instance, it seems certain that ancestral molluscan forms did not possess eyes. Therefore, the eyes of cephalopods must have originated in lineages derived from forms lacking eye development (Salvini-Plawen and Mayr, 1977). In addition, there might not even have been a common ancestral metazoan photoreceptive field. The phylogenetic analysis of Salvini-Plawen and Mayr indicates that photoreceptor cells themselves may have evolved independently between 40 and 65 times during metazoan evolution. (Wilkins 2002: 166)

The only, or at least the simplest, way to reconcile this pattern with the equally unambiguous general role of Pax6 and its associated genes in eye development throughout the Bilateria is to posit shared genetic potential for development of eyes, even when the potential is not expressed. From this perspective, both the multiple independent occurrence of certain traits in different related lineages (cases of “parallelism”) and the reacquisition of a particular trait in a lineage that had seemingly lost it (a “reversal”) can, in principle, be accommodated. The key is the retention of genes and genetic architectures within those lineages and their suppression or revocation through certain genetic changes (Simpson, 1983; Wake, 1991; Butler and Saidel, 2000). Loss-of-function mutations that abrogate the operation of networks are easy to imagine. Similarly, suppression of the effects of such mutations, leading to a restoration of the network’ functions, can also be readily envisaged. In this conceptual framework, the widespread usage of Pax6 and its associated network of genes is a significant fact, while the polyphyletic pattern of eye development drawn by Salvini-Plawen and Mayr is not dismissed, but can be interpreted as reflecting losses and gains of the use of this network. (Wilkins 2002: 167)

Altogether, these considerations require some changes in the ways in which we view, and use, the term “homology.” The basic concept of shared possession of a trait through common descent remains intact, but the idea that the “same” trait in two different organisms may actually exhibit more points of visible difference than of discernible identity seems counterintuitive, to put it mildly. Nevertheless, the idea that homologous morphological traits and genes need not share tight, invariant relationships had been anticipated long ago by de Beer (1938, 1958, 1971) …. In effect, a set of decoupled relationships would involve the sharing of part of the same regulatory circuitry, but without visible (morphological) homology being involved. As Fernald (2000) has expressed it:

Recently, the discovery of conservation of many of the genes used during ontogeny of the eye, particularly Pax6, has led to the proposal that all eyes are monophyletic that is, they arose from an “Ur” eye. However, our current level of understanding of the genetic control of eye development does not support this conclusion. Instead, there appears to be a continuity of genetic information that regulates the development of similar but nonhomologous eyes.

Perhaps, however, it is the traditional framework that needs reevaluation. In cladistic terms, a homologous trait is a synapomorphy. If morphological similarity is not a reliable guide to homologous relationships, what precisely would such a snyapomorphy consist of? In light of the material presented here, one can suggest that it would be a combination of shared key genes (one or more) plus a shared biological or developmental function for which those genes are crucial. This is a rather radical revision of the concept of homology, which for 150 years has been tied to visible similarity, and which has specifically disavowed shared function as a criterion of homology. The formulation put forward here, however, allows one to incorporate the observations on conserved key regulatory genes without invoking convergent evolution, and it preserves the basic idea of homology as “continuity of [genetic] information.” Davidson (2001, p. 201) has reached a similar position. In his words, when it comes to assessing homologous relationships, “seeing is not [necessarily] believing.” (Wilkins 2002: 167)

From this perspective, the eyes of insects and vertebrates are homologous, even though they look different from each other, develop differently, and may have arisen independently in separate lineages from ancestors lacking eyes. What they share is the inherited regulatory machinery and the ancestral use (function) of that machinery dating to early in metazoan evolutionary history for light sensing or some rudimentary form of vision. (Wilkins 2002: 16)

~ ~ ~

18 Writing a few years afterward, Ernst Mayr captured the mood on the century of The Origin: “Symposia and conferences were held all over the world in 1959 in honor of the Darwin centennial, and were attended by all the leading students of evolution. If we read the volumes resulting from these meetings at Cold Spring Harbor, Chicago, Philadelphia, London, Gottingen, Singapore, and Melbourne, we are almost startled at the complete unanimity in the interpretation of evolution presented by the participants. Nothing could show more clearly how internally consistent and firmly established the synthetic theory is.” (Mayr, 1963, p.8)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s