Breaking Dollo's Law Brings Home the Case for Creation
Published posthumously, Thomas Wolfe's 1940 novel, You Can't Go Home Again—considered by many to be his most significant work—explores how brutally unfair the passage of time can be. In the finale, George Webber, the story's protagonist, concedes that "You can't go back home" to family, childhood, familiar places, dreams, and old ways of life.
In other words, there's an irreversible quality to life. Call it the arrow of time.
Like Wolfe, most evolutionary biologists believe there is an irreversibility to life's history and the evolutionary process. In fact, this idea is codified in Dollo's Law, which states that an organism cannot return, even partially, to a previous evolutionary stage occupied by one of its ancestors. Yet several recent studies have uncovered what appear to be violations of Dollo's Law. These violations call into question the sufficiency of the evolutionary paradigm to fully account for the history of life. On the other hand, the return to "ancestral states" does find an explanation in an intelligent design/creation model approach to life's history.
French paleontologist Louis Dollo formulated the law that bears his name in 1893, before the advent of modern-day genetics. He based it on patterns he unearthed from the fossil record. Today, his idea finds undergirding in the contemporary understanding of genetics and developmental biology.
Evolutionary biologist Richard Dawkins explains the modern-day concept of Dollo's Law this way:
Dollo's Law is really just a statement about the statistical improbability of following exactly the same evolutionary trajectory twice . . . in either direction. A single mutational step can easily be reversed. But for larger numbers of mutational steps . . . [the] mathematical space of all possible trajectories is so vast that the chance of two trajectories ever arriving at the same point becomes vanishingly small.1
If a biological trait is lost during the evolutionary process, then the genes and developmental pathways responsible for that feature will eventually degrade, because they are no longer under selective pressure. In 1994, using mathematical modeling, researchers from Indiana University determined that once a biological trait is lost, there is a reasonable possibility that the corresponding genes could be "reactivated" within a timescale of five hundred thousand to six million years. But once a time span of ten million years has elapsed, unexpressed genes and dormant developmental pathways become permanently lost.2
In 2000, a scientific team from the University of Oregon offered a complementary perspective on the timescale for evolutionary reversals when they calculated how long it takes for a duplicated gene to lose function.3 According to the evolutionary paradigm, once a gene becomes duplicated, it is no longer under the influence of natural selection. That is, it undergoes "neutral evolution," and eventually becomes silenced as mutations accrue. As it turns out, the half-life for this process is approximately four million years. To put it another way, 16 to 24 million years after the duplication event, the duplicated gene will have completely lost its function. Presumably, this same result applies to dormant, unexpressed genes that have been rendered unnecessary because the trait they specify has been lost.
Both scenarios assume neutral evolution and the accumulation of mutations in a linearly temporal manner. But what if the loss of gene function is advantageous? Collaborative work by researchers from Harvard University and NYU in 2007 demonstrated that loss of gene function can take place over a time span of about one million years if natural selection influences gene loss.4 This research team studied the loss of eyes in the Mexican tetra, a cave fish. Because they live in a dark, subterranean environment, having eyes serves no benefit to these creatures. In fact, the research team discovered, eye reduction actually offers an advantage to these fish, because there is a high metabolic cost associated with maintaining eyes. For the Mexican tetra, eye loss reduces this metabolic cost, and that reduction, in turn, accelerates the loss of gene function through the operation of natural selection.
Based on these three studies, it is reasonable to conclude that once a trait has been lost, the time limit for evolutionary reversal is on the order of about 20 million years. In any case, the very nature of evolutionary mechanisms and the constraints of genetic mutations make it extremely improbable that evolutionary processes would allow an organism to revert to an ancestral state or to recover a lost biological trait. You can't go home again.
Violations of Dollo's Law
Yet despite this expectation, researchers have, over the course of the last several years, uncovered several instances in which Dollo's Law has been violated. A brief description of a handful of these occurrences follows.
• The re-evolution of mandibular teeth in the frog genus Gastrotheca
This group is the only one that includes living frogs with true teeth on the lower jaw. According to the evolutionary framework, mandibular teeth were present in ancient frogs and then lost in the ancestor of all living frogs. It also appears that, before they reappeared in Gastrotheca, such teeth had been absent in frogs for 225 million years, far longer than "permissible" under Dollo's Law.5
• The re-evolution of oviparity in sand boas
From an evolutionary perspective, it appears that, among reptiles, live-birth (viviparity) reproductive behaviors evolved from original egg-laying (oviparity) behaviors. Among snakes, for instance, it is estimated that this evolutionary transition has occurred in at least thirty cases. There are 41 species of boa alone, in both the Old and New Worlds, that once laid eggs but now give live birth to their offspring. Yet two recently described sand boas, the Arabian sand boa (Eryx jayakari) and the Saharan sand boa (Eryx muelleri) have reverted back to laying eggs. Phylogenetic analysis of these boas carried out by researchers from Yale University indicates that the egg-laying in these two species re-evolved 60 million years after the initial transition to viviparity took place.6
• The re-evolution of rotating sex combs in Drosophila
Sex combs are modified bristles unique to male fruit flies, used for courtship and mating. There are two kinds: transverse sex combs and rotating sex combs, the latter resulting when several rows of bristles undergo a rotation of ninety degrees. In the ananassae fruit fly group, most of the twenty or so species have simple transverse sex combs, with Drosophila bipectinata and Drosophila parabipectinata being the two exceptions. These fruit fly species possess rotating sex combs. But what is most interesting about them is that phylogenetic analysis, conducted by investigators from the University of California, Davis, indicates that the rotating sex combs in these two species re-evolved, 12 million years after being lost.7
• The re-evolution of sexuality in mites belonging to the taxa Crotoniidae
Mites exhibit a wide range of reproductive modes, including parthenogenesis. In fact, this means of reproduction is prominent in the group Oribatida, clustering into two subgroups that display parthenogenesis almost exclusively. However, residing within one of these clusters is the taxa Crotoniidae, which displays sexual reproduction. Based on an evolutionary analysis, a team of German researchers concluded that this group re-evolved the capacity for sexual reproduction.8
• The re-evolution of shell coiling in limpets
According to the evolutionary perspective, the coiled shell in gastropod lineages has been lost numerous times, producing a limpet shape consisting of a cap-shaped shell and a large foot. Evolutionary biologists have long thought that the loss of the coiled shell represented an evolutionary dead end. However, researchers from Venezuela have shown that coiled shell morphology re-evolved at least one time, in calyptraeids, 20 to 100 million years after its loss.9
This list gives just a few recently discovered examples of Dollo's Law violations. Surveying the scientific literature, evolutionary biologist John J. Wiens identified an additional eight violations, and, in each of these cases, he determined that the lost trait reappeared after at least 20 million years had passed, and in some instances after 120 million years had elapsed.10
Violations & the Theory of Evolution
Given that the evolutionary paradigm predicts that re-evolution of traits should not occur after the trait has been lost for 20 million years, the numerous discoveries of Dollo's Law violations present a problem. At least they provide a basis for skepticism about the capacity of the evolutionary paradigm to fully account for life's history. And the problem is likely worse than it appears: Wiens points out that these violations may be more widespread than imagined, but difficult to detect for methodological reasons.11
Evolutionary biologists have responded to this challenge by offering two ways of accounting for Dollo's Law violations.12
(1) The first is to question the validity of the evolutionary analysis that exposes them. That is, some scientists claim that the recently identified Dollo's Law violations are artifacts of the evolutionary analysis, and not real.
This work-around, however, is unconvincing. The evolutionary biologists who discovered the various violations were aware of this complication and took painstaking efforts to ensure the validity of the evolutionary analysis they performed.
(2) The second response is to argue that some genes and developmental modules serve more than one function. According to this scenario, even though the trait specified by a gene or a developmental module may be lost, the gene or module itself could remain intact because it serves other roles. This retention makes it possible for traits to re-evolve even after a hundred million years.
Although this proposal is reasonable, it still must be viewed as speculative. Evolutionary biologists have yet to apply the same mathematical rigor to this explanation as they have to estimating the timescale for loss of function in dormant genes. These calculations are critical, given the extensive timescales involved in some of the Dollo's Law violations.
Also, considering the nature of evolutionary processes, this response neglects the fact that, after a trait is lost, genes and developmental pathways will continue to evolve under the auspices of natural selection. Free from the constraints of the lost function, the genes and developmental modules will experience new evolutionary possibilities that were previously unavailable to them. The more functional roles a gene or module assumes, the less will be its capacity to evolve. Hence, shedding one of its roles increases the likelihood that the gene or module will become modified, as the evolutionary process will now have new space available to explore.
It is reasonable to think that natural selection could modify the gene or developmental module to such an extent that the lost trait would be just as unlikely to re-evolve under this scenario as it would be if gene loss had been a consequence of neutral evolution. In fact, the study of eye loss in the Mexican tetra suggests that the modification of these genes and modules could occur at a faster rate under natural selection than under neutral evolution.
Violations & the Case for Creation
While Dollo's Law violations are problematic for the evolutionary paradigm, they fit right in with a creation model/intelligent design perspective. The re-evolution—or perhaps, more appropriately, the reappearance—of a biological trait long after its initial disappearance makes sense from the latter standpoint, because such a reappearance can be understood as the work of a Creator. It is not unusual for engineers to reuse or revisit a previously used design feature in a new prototype. Irreversibility may be inherent to the evolutionary process, but designers are not constrained in that way and can freely return to old designs whenever they find them useful.
Dollo's Law violations are at home in a creation model, highlighting the value of this model in understanding life's history.Fazale Rana
is a biochemist and Vice President of Research and Apologetics at Reasons to Believe (www.reasons.org). His books include The Cell's Design (2008) and Creating Life in the Lab (2011).This article originally appeared in Salvo, Issue #43, Winter 2018 Copyright © 2019 Salvo | www.salvomag.com https://salvomag.com/article/salvo43/no-going-back