Nature: "Convergent Evolution Seen in Hundreds of Genes" and "Surprisingly, Extensive"
Bats and cetaceans (whales and dolphins) are very different. One is the only mammal capable of sustained flight. The other is one of only two types of mammals that are fully aquatic (the other being sirenians, or sea cows). Despite their differences, bats and cetaceans share something unique: both use echolocation.
Typical evolutionary thinking holds that when two species share a complex trait, that's because they share a common ancestor that had the trait. In these cases, shared complex biological features are said to be homologous, or derived from a common ancestor.
But sometimes, complex traits exist in widely disparate species where it's unlikely that their supposed common ancestor had that trait. In those cases, shared complex biological features are said to be homoplastic, or derived independently through convergent evolution.
Initially, evolutionary biologists generally assume that shared biological similarities result from inheritance from a common ancestor. But there are so many instances where this rule fails, that evolutionary thinking amounts to shared complex similarities between two living organisms results from inheritance from a common ancestor...except for when they don't. The existence of echolocation in bats and whales is another exception to the rule.
In the past we've discussed the phenomenon of convergent genetic evolution, where (under a Darwinian evolutionary view), not only physical traits, but also genes seem to converge independently upon the same amino-acid sequences. A new article in Nature News, "Convergent evolution seen in hundreds of genes," reports on a paper that affirms prior claims that "many genes evolved in parallel in bats and dolphins as each developed the remarkable ability to echolocate." The article explains the results of a new study, also in Nature:
"These results imply that convergent molecular evolution is much more widespread than previously recognized," says molecular phylogeneticist Frédéric Delsuc at the The National Center for Scientific Research (CNRS) at the University of Montpellier in France, who was not involved in the study. What is more, he adds, the genes involved are not just the few, obvious ones known to be directly involved in a trait but a broader array of genes that are involved in the same regulatory networks.The Nature study itself states:
Here we analyse genomic sequence data in mammals that have independently evolved echolocation and show that convergence is not a rare process restricted to several loci but is instead widespread, continuously distributed and commonly driven by natural selection acting on a small number of sites per locus. Systematic analyses of convergent sequence evolution in 805,053 amino acids within 2,326 orthologous coding gene sequences compared across 22 mammals (including four newly sequenced bat genomes) revealed signatures consistent with convergence in nearly 200 loci. Strong and significant support for convergence among bats and the bottlenose dolphin was seen in numerous genes linked to hearing or deafness, consistent with an involvement in echolocation. Unexpectedly, we also found convergence in many genes linked to vision: the convergent signal of many sensory genes was robustly correlated with the strength of natural selection. This first attempt to detect genome-wide convergent sequence evolution across divergent taxa reveals the phenomenon to be much more pervasive than previously recognized.The Nature article says that its results show "surprisingly, extensive convergent changes." Why so surprising? It's difficult enough to evolve a complex structure once. But the odds of evolving a similar feature multiple times, independently, in different lineages, would seem very low. Darwinian evolution is supposed to have no goal, yet convergent evolution implies that species are evolving the same complex traits -- over and over again -- even at the sequence level. If the data are "surprising" even given the power of selection, how can we make sense of this?
I would suggest that the data fit much better under an intelligent design paradigm. Designers regularly re-use functional parts or components in modular fashion. The observed patterns of biological similarity we see across whales and bats -- patterns that bear no correspondence to common ancestry -- are best explained by common modular design. They're analogous to an Intel chip being plugged into different computer platforms. Hypotheses like common design can help us make sense of this sort of "unexpected" data.