BY FAZALE RANA – SEPTEMBER 12, 2017
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, “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 appears to be violations of Dollo’s Law. These violations call into question the sufficiency of the evolutionary paradigm to fully account for life’s history. On the other hand, the return to ‘ancestral states’ finds 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, basing it on patterns he unearthed from the fossil record. Today, his idea finds undergirding in contemporary understanding of genetics and developmental biology.
Evolutionary biologist Richard Dawkins explains the modern-day conception 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 . . . 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, the corresponding genes can be “reactivated” with reasonable probability over time scales of five hundred thousand to six million years. But once a time span of ten million years has transpired, 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 (Duplicated genes serve as a proxy for dormant genes rendered useless because the trait they encode has been lost.) 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, sixteen to twenty-four million years after the duplication event, the duplicated gene will have completely lost its function. Presumably, this result applies to dormant, unexpressed genes rendered unnecessary because the trait they specify is lost.
Both scenarios assume neutral evolution and the accumulation of mutations in a clockwise 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 on the order of about one million years if natural selection influences gene loss.4 This research team studied the loss of eyes in the cave fish, the Mexican tetra. Because they live in a dark cave environment, eyes serve no benefit for these creatures. The team discovered that eye reduction offers an advantage for these fish, because of the high metabolic cost associated with maintaining eyes. The reduced metabolic cost associated with eye loss 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 reversals is on the order of about 20 million years.
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
Despite this expectation, over the course of the last several years, researchers have 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. When examined from an evolutionary framework, mandibular teeth were present in ancient frogs and then lost in the ancestor of all living frogs. It also looks as if teeth have been absent in frogs for 225 million years before they reappeared in Gastrotheca.5
The re-evolution of oviparity in sand boas. When viewed from an evolutionary perspective, it appears as if live-birth (viviparity) evolved from egg-laying (oviparity) behaviors in reptiles several times. For example, estimates indicate that this evolutionary transition has occurred in snakes at least thirty times. As a case in point, there are 41 species of boas in the Old and New Worlds that give live births. Yet, two recently described sand boas, the Arabian sand boas (Eryx jayakari) and the Saharan sand boa (Eryx muelleri) lay eggs. Phylogenetic analysis carried out by researchers from Yale University indicates that the egg-laying in these two species of sand boas re-evolved 60 million years after the 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. Compared to transverse sex combs, rotating sex combs result 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 the two exceptions. These fruit fly species possess rotating sex combs. Phylogenetic analysis conducted by investigators from the University of California, Davis indicates that the rotating sex combs in these two species re-evolved, twelve 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 conclude this group re-evolved the capacity for sexual reproduction.8
The re-evolution of shell coiling in limpets. From an evolutionary perspective, the coiled shell has been lost in gastropod lineages 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 represents 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 short list gives just a few recently discovered examples of Dollo’s Law violations. Surveying the scientific literature, evolutionary biologist J. J. Wiens identified an additional eight examples in which Dollo’s Law was violated and determined that in all cases the lost trait reappeared after at least 20 million years had passed and in some instances after 120 million years had transpired.10
Violation of Dollo’s Law and 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 twenty million years, the numerous discoveries of Dollo’s Law violations provide a basis for skepticism about the capacity of the evolutionary paradigm to fully account for life’s history. The problem is likely worse than it initially appears. J. J. Wiens points out that Dollo’s Law violations may be more widespread than imagined, but difficult to detect for methodological reasons.11
In response to this serious problem, evolutionary biologists have offered two ways to account for Dollo’s Law violations.12 The first is to question the validity of the evolutionary analysis that exposes the violations. To put it another way, these scientists claim that the recently identified Dollo’s Law violations are artifacts of the evolutionary analysis, and not real. However, this work-around is unconvincing. The evolutionary biologists who discovered the different examples of Dollo’s Law violations were aware of this complication and took painstaking efforts to ensure the validity of the evolutionary analysis they performed.
Other evolutionary biologists argue that some genes and developmental modules serve more than one function. So, even though the trait specified by a gene or a developmental module is lost, the gene or the module remains intact because they serve other roles. This retention makes it possible for traits to re-evolve, even after a hundred million years. Though reasonable, this explanation still must be viewed as speculative. Evolutionary biologists have yet to apply the same mathematical rigor to this explanation as they have when estimating the timescale for loss of function in dormant genes. These calculations are critical given the expansive timescales involved in some of the Dollo’s Law violations.
Considering the nature of evolutionary processes, this response neglects the fact that genes and developmental pathways will continue to evolve under the auspices of natural selection, once a trait is lost. Free from the constraints of the lost function, the genes and developmental modules experience new evolutionary possibilities, previously unavailable to them. The more functional roles a gene or developmental module assumes, the less likely it is that these systems can evolve. Shedding one of their roles increases the likelihood that these genes and developmental pathways will become modified as the evolutionary process explores new space now available to it. In this scenario, it is reasonable to think that natural selection could modify the genes and developmental modules to such an extent that the lost trait would be just as unlikely to re-evolve as it would if gene loss was a consequence of neutral evolution. In fact, the study of eye loss in the Mexican tetra suggests that the modification of these genes and developmental modules could occur at a faster rate if governed by natural selection rather than neutral evolution.
Violation of Dollo’s Law and the Case for Creation
While Dollo’s Law violations are problematic for the evolutionary paradigm, the re-evolution—or perhaps, more appropriately, the reappearance—of the same biological traits after their disappearance makes sense from a creation model/intelligent design perspective. The reappearance of biological systems could be understood as the work of the Creator. It is not unusual for engineers to reuse the same design or to revisit a previously used design feature in a new prototype. While there is an irreversibility to the evolutionary process, designers are not constrained in that way and can freely return to old designs.
Dollo’s Law violations are at home in a creation model, highlighting the value of this approach to understanding life’s history.
- Richard Dawkins, The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design (New York: W.W. Norton, 2015), 94.
- Charles R. Marshall, Elizabeth C. Raff, and Rudolf A. Raff, “Dollo’s Law and the Death and Resurrection of Genes,” Proceedings of the National Academy of Sciences USA 91 (December 6, 1994): 12283–87.
- Michael Lynch and John S. Conery, “The Evolutionary Fate and Consequences of Duplicate Genes,” Science 290 (November 10, 2000): 1151–54, doi:10.1126/science.290.5494.1151.
- Meredith Protas et al., “Regressive Evolution in the Mexican Cave Tetra, Astyanax mexicanus,” Current Biology 17 (March 6, 2007): 452–54, doi:10.1016/j.cub.2007.01.051.
- John J. Wiens, “Re-evolution of Lost Mandibular Teeth in Frogs after More than 200 Million Years, and Re-evaluating Dollo’s Law,” Evolution 65 (May 2011): 1283–96, doi:10.1111/j.1558-5646.2011.01221.x.
- Vincent J. Lynch and Günter P. Wagner, “Did Egg-Laying Boas Break Dollo’s Law? Phylogenetic Evidence for Reversal to Oviparity in Sand Boas (Eryx: Boidae),” Evolution 64 (January 2010): 207–16, doi:10.1111/j.1558-5646.2009.00790.x.
- Thaddeus D. Seher et al., “Genetic Basis of a Violation of Dollo’s Law: Re-Evolution of Rotating Sex Combs in Drosophila bipectinata,” Genetics 192 (December 1, 2012): 1465–75, doi:10.1534/genetics.112.145524.
- Katja Domes et al., “Reevolution of Sexuality Breaks Dollo’s Law,” Proceedings of the National Academy of Sciences USA 104 (April 24, 2007): 7139–44, doi:10.1073/pnas.0700034104.
- Rachel Collin and Roberto Cipriani, “Dollo’s Law and the Re-Evolution of Shell Coiling,” Proceedings of the Royal Society B 270 (December 22, 2003): 2551–55, doi:10.1098/rspb.2003.2517.
- Wiens, “Re-evolution of Lost Mandibular Teeth in Frogs.”
- Wiens, “Re-evolution of Lost Mandibular Teeth in Frogs.”
- Rachel Collin and Maria Pia Miglietta, “Reversing Opinions on Dollo’s Law,” Trends in Ecology and Evolution 23 (November 2008): 602–9, doi:10.1016/j.tree.2008.06.013.