A couple of weeks ago on BioLogos, biologists Darrel Falk and David Kerk posted a very nice essay on the evolution of the heart. The main point of their essay is found in this paragraph:
So in the fruit fly, that which controls the “make-a-heart” switch is TINMAN. What controls the same switch in other organisms? The zebra fish is a little aquarium fish which produces transparent embryos. This transparency trait makes it the embryologist’s dream organism and thereby it has been widely studied. Fish, like fruit flies, have a heart. We talked about the structure of the fish heart in our last post. What controls the “make a heart” switch in zebra fish? You guessed it. Out of the hundreds of proteins that operate various switches in fish embryos, the one which controls the “make-a-heart” switch is almost the same as the fruit fly, TINMAN. In fact, when one puts one of the several zebra fish versions of TINMAN into mutant fruitfly embryos which lack their own TINMAN, one of the zebra fish versions will throw the switch, and cause the mutant embryo to produce a heart.2 Zebra fish and fruit flies have been on separate lineages for over 500 million years, however each, despite hundreds of possible switches, still retain a switch that can only be activated by a variety of the TINMAN protein. So do other organisms like frogs and mammals have this same switch operated in the same way? Yes, all tested vertebrates, including humans make their heart in response to the TINMAN signal. It has changed a little more in mammals so the mammalian varieties are not interchangeable with the fruit fly’s TINMAN, but it is still the same gene which makes almost the same protein.
Once again, the concept of deep homology emerges to play a key role in evolution. Yet Falk and Kerk did make one claim that I found to be misleading:
Just as the structure of the heart of mammalian embryos goes through stages where the developing heart almost certainly resembles the old heart of ancient organisms (see our previous post), so we would expect to see certain “old” genes still in use as the cells in the developing embryo “read” the heart chapter of the instruction book and proceed to assemble into a heart.
I noted that yes, we would expect to see “old” genes at play given the hypothesis that evolution was front-loaded. After all, such “old” genes would constitute key elements that would help frame the context of subsequent evolution and thus help to guide it. But mainstream evolutionary theory did not have this expectation.
As Dr. Sean Carroll, an expert of evo-devo, explained:
So what this means is in some ways, some sense, evolution is a simpler process than we first thought. When you think about all of the diversity of forms out there, we first believed this would involve all sorts of novel creations, starting from scratch, again and again and again. We now understand that, no, that evolution works with packets of information and uses them in a new and different ways, and new and different combinations, without necessarily having to invent anything fundamentally new, but new combinations.
I then expanded on this point by noting that Kerk and Falk were correct in pointing to TINMAN as evidence of evolution. Where they erred is in framing TINMAN as some type of prediction of evolution. For example, evolution could have occurred such that the last common ancestor of flies and fish did not have a heart-like structure and thus, the heart evolved independently after the two lineages split. After all, this is what mainstream evolutionary theory used to teach. The last common ancestor of flies and mice was some type of flatworm that did not have a heart. So the heart evolved independently and there was thus no reason to think the genetic circuit used to make the analogous hearts would be the same. There are many roads to Rome.
I’ll let Mike Levine explain the shift in thinking:
Nobody thought that this bag of genes would be conserved in radically different animals like us, like vertebrates.
It led to a change in all of our thinking. When I was a student, there was a sense that all animals did things differently. If a gene was conserved between different animals, or between plants and animals, it was considered to be boring. How could it be interesting, because all animals and plants are different from each other?
So, certainly, creating distinct forms of life couldn’t depend on a common set of molecules. And so to see that genes that are doing such profound things in the fruit fly — making head from tail, stomach from back, thorax from abdomen — are conserved, related in other animals … this was just not predicted by anybody. At least nobody that I ever read.
In the case of the discovery of common homeotic genes among all animals, there was a strong sense in the ’70s and the ’80s that embryonic development among different animals involved completely different molecules, completely unrelated. This was such a strongly held view. And so, yes, it came as a huge surprise not only to people like my mother who says, “My God, an earthworm and a mouse? An earthworm and me, sharing things in common?” But it came as a surprise to other scientists that there was this profound conservation of mechanism of building embryos among all these different kinds of animals.
After pointing this out, Dr. Falk replied to me as follows:
Mike Gene is correct that 25 years ago we had no idea that master genes would be conserved. However, we had no idea what master genes were or how they worked. Now that we know how they work, it fits together beautifully with what we would expect “if evolution were true.”
So don’t be misled by the fact that in our ignorance we were shocked. Now that we know how embryonic development works nothing could more beautifully point to the reality of evolution than the way the switches work and the manner in which they are conserved. This finding is one of the greatest unifying themes in biology.
None of this detracts from my point that while TINMAN is evidence of evolution, evolutionary theory did not predict this. On the contrary, it predicted something like TINMAN did not and would not exist. Evolutionary biologist Arlin Stoltzfus explains what other evolutionary biologists immersed in the Modern Synthesis expected:
Given the abundance of variation in the “gene pool”, and the ability of selection to shape this gene pool to fit circumstances, it was not safe to assume that shared characters had a shared genetic basis, as Mayr (1963) argued in one of his more famous erroneous claims:
“In the early days of Mendelism there was much search for homologous genes that would account for such similarities. Much that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives (Dobzhansky, 1955). If there is only one efficient solution for a certain functional demand, very different gene complexes will come up with the same solution, no matter how different the pathway by which it is achieved.” (p. 609)
So when Dr. Falk wrote, “However, we had no idea what master genes were or how they worked,” I replied that this was the “perfect place to flex the muscle of theory.” This was the very time for mainstream evolutionary theory, as the prime expression of the non-teleological perspective, to guide science and predict “old genes.” But it did not. It taught us otherwise – “Given the abundance of variation in the “gene pool”, and the ability of selection to shape this gene pool to fit circumstances, it was not safe to assume that shared characters had a shared genetic basis.”
As I noted to Dr. Falk, there is a deeper point at work here – evolution is slowly taking on a form. These new discoveries have enhanced the plausibility of evolution being front-loaded and guided. Evolution is looking more and more predictable. Almost like it was……just maybe………designed.