A tail with a larger span has more drag when it moves laterally than does a tail with a smaller span. When the small span is increased to the large, the only way to keep the amplitude of the caudal fin constant in the face of the increased drag is to generate more power internally, in the machinery that is driving the tail. Guess what? That internal machinery includes the vertebral column, which we know stores and releases elastic energy as it bends. So here’s the basis of our concerted-evolution prediction: a stiffer vertebral column could compensate for the increased drag that accompanies the increased span of the caudal fin.
We set out to test the hypothesis that selection for enhanced feeding performance and predator avoidance would increase the number of vertebrae. Next thing you know, we are yammering on about two other characters that we are going to evolve: the predator-detection threshold and the span of the caudal fin. We predicted that both of these other traits would evolve in concert with the number of vertebrae, with predation-detection evolving before vertebrae (the sequential-contingent pattern of concerted evolution) and the span of the caudal fin evolving at the same time (just plain old concerted evolution). The alternative hypotheses are that predation-detection may evolve at the same time as number of vertebrae (concerted evolution) or may not be correlated with the number of vertebrae (mosaic evolution). Span of the caudal fin, too, may not be correlated with the number of vertebrate (mosaic evolution).
The results of our evolutionary experiments in the predator-prey world of Tadro4 are fascinating (Figure 6.9). Our amazing Tadro4 teams met and exceeded all expectations. Led by Gianna in the summer of 2007, Hannah, Elise, Andres, and Hassan perfected the biomimetic-vertebral-column production line and conducted the first evolutionary run, which lasted for five generations before we had season-ending injuries to the robots.[132] Led by Hannah and Andres in the summer of 2008, Sonia Roberts and Jonathan Hirokawa conducted the second evolutionary run, which went for eleven generations. Because we started the two populations with the same average values for their evolving characters, the two runs are independent replications of the same experiment. It turns out that comparing the runs is critical for testing our hypotheses.
First and foremost, in both runs of the predator-prey world, the population of PreyRos quickly evolves more vertebrae, moving from the starting average of 4.5 to an average of 5.5 by the third generation. In the second run an equilibrium average of 5.7 vertebrae appears to have been reached. These early directional and positive increases, even though they are modest, provide tentative support for our big-picture hypothesis that the number of vertebrae increase when the population is under selection for enhanced performance in feeding and fleeing.
I can only say “tentatively support” because, as we talked about in Chapter 4, strictly speaking you can only falsify a hypothesis—you can’t prove it. So the phrase “tentatively support” is meant to recognize (1) that we have failed to falsify and (2) that over time repeated failures to falsify will eventually lead us to conclude that the hypothesis is probably true. Caution and consideration are required.
But it’s really, really hard not to get totally stoked when you see this pattern of evolutionary change. Emotionally, we—okay, I—want to yell, “Kick ass! We’ve proven that this specific type of selection on these fish-like autonomous agents works as we predicted!” But we mustn’t do that. And we mustn’t holler, “And we did it twice, you cynical bastards, and it worked both times! Evolvabots rock!” So we don’t. And we won’t. Ahem. What was I saying?
Right. “Dignity, always dignity.”[133] Please take the time to notice in Figure 6.9 that by reaching an apparent equilibrium, the population of PreyRos creates an evolutionary pattern with their average number of vertebrae that resembles the evolution pattern that we saw in the Digi-Tad3s and their average tail stiffness (see Figure 6.1). Is this coincidence? I think not. Once again we see evidence that is consistent with the hypothesis that the stiffness of the vertebrate axial skeleton has evolved to balance the mechanical demands of maneuverability, wherein a flexible axis works best, with those of speed, whereby a stiff axis outperforms.
Because we were evolving three characters—(1) number of vertebrae, (2) predator-detection threshold, and (3) span of the caudal fin—at the same time, the patterns of evolutionary change shared or not shared between them also inform. For example, in both runs of the predator-prey world, the PreyRo population’s changes in the predator-detection threshold are positively and strongly correlated with the changes in the number of vertebrae, at least over the first five generations (Figure 6.9, middle graph). This strong correlation tentatively supports the hypothesis of concerted evolution between these two characters. What we don’t see is a threshold-first-then-vertebrae-next pattern—with vertebrae lagging in time—that would support the sequential-contingent pattern.
This in-phase pattern of apparent concerted evolution is evidence that a functional codependence exists between sensing a predator and moving away from it.[134] Are we surprised? No. But that’s why you run the experiments. We aren’t surprised because we knew, from Chapter 5, that tightly linked perception-action feedback loops (PAFL) create behavioral modules. What’s new here is that we’ve shown that this particular PAFL—Escape!—is well characterized by the ability to detect predators and then flee from them. What’s also very interesting is that we’ve connected the Escape! PAFL to the evolution of vertebrae. This connection, measured here as a strong and positive correlation between predator-detection threshold and the number of vertebrae, allows us to understand how selection acting on behavior changes a feature of the skeleton.
FIGURE 6.9. (facing page) The evolution of vertebrae in PreyRo is directional, concerted, and mosaic. If you look at the evolution of the average number of vertebrae, N, in the population (top graph), you’ll see that in both evolutionary runs N increased over the first five generations. This is a pattern of directional selection. In the second run Nthen reached a plateau, just like we saw with the tail stiffness in the digi-Tad3s (see Figure 6.1). Points represent the populations’ average, and the error bars represent the standard error of the average. Only one side of the error bar is shown, by the way, so that they don’t overlap when the means are close.
The predator-detection threshold (middle graph) increased over five and then three generations in the first and second runs, respectively. In both runs predator-detection threshold is strongly and positively correlated with N over the first five generations (r values of 0.93 and 0.92, where r can vary from 1 to -1). This pattern of correlated evolution with N is consistent with a hypothesis of concerted evolution for these characters with respect to each other.
132
When they started the predator-prey trials for generation six, the students noticed a change in the behavior of the robots that turned out to be related to the servo motors breaking down. The wisdom of their observations had been informed, in part, by the fact that at the start of every generation they ran positive controls on PreyRo and Tadiator, using a fixed “control tail” to assess any degradation in the hardware. Although we normally keep spare and identical parts on hand for just such a breakdown, we were fresh out of servos and so was the supplier. This pause in evolutionary activities allowed us to analyze this first run, which we published: N. Doorly, K. Irving, G. McArthur, K. Combie, V. Engel, H. Sakhtah, E. Stickles, H. Rosenblum, A. Gutierrez, R. Root, C.-W. Liew, and J. H. Long Jr., “Biomimetic Evolutionary Analysis: Robotically-Simulated Vertebrates in a Predator-Prey Ecology,”
133
Spoken by Don Lockwood, in the movie
134
I’m playing a bit fast and loose with my use of “concerted” here, so please help me out by keeping in mind that a correlation is only the first piece of evidence for concerted evolution. Because we’ve defined concerted evolution to be causally based, we also want to show how, in this case, the number of vertebrae improves the acceleration ability of the PreyRo. I’ll show you evidence for this in some experiments that we run on the Tadro-derived MARMT system in the next chapter.