Revisiting convergence: a new view (Introduction)

by David Turell , Friday, September 15, 2017, 01:21 (2626 days ago) @ David Turell

Real convergence does occur but why are there 'one-ofs'?

http://nautil.us/issue/52/the-hive/why-hasnt-evolution-made-another-platypus?utm_source...

"For many decades, the accepted narrative in evolutionary biology, as formulated by Stephen Jay Gould, was one of such contingency: Change any one event in the history of life, and all life might have looked entirely different. Life as it exists today was not inevitable or even likely, but a roll of the evolutionary dice. 

"In recent years, however, a cadre of scientists, led by Simon Conway Morris, has taken the opposite view, arguing that certain evolutionary solutions are indeed quite likely—as evidenced when entirely different species repeatedly evolve the same adaptive solutions to problems posed by the environment—such as the very similar eye structure of humans and octopuses. These repeated solutions are called evolutionary convergence.

"We can easily understand convergence, species adapting in the same way to similar circumstances. But what about the evolutionary one-offs? Why haven’t other species convergently evolved similar adaptations to them?

"One explanation for evolutionary one-offs is that these species occur in unique environments. Perhaps they have no analogs because no other species has experienced a similar environment. This, possibly, explains the koala. Its entire lifestyle revolves around living in eucalyptus trees and eating their leaves, which are loaded with poisonous compounds. The koala’s digestive system is extremely long, providing ample time to slowly detoxify the leaves and extract the nutrients. ...Eucalyptus trees naturally occur only in Australia, so maybe the singularity of the koala reflects the uniqueness of its environment.

"But I suspect this is not the explanation in most cases. Platypuses occur in the streams, ponds, lakes, and rivers of eastern Australia, where they eat crayfish and other aquatic invertebrates. When they’re not out paddling around, they retire to their rest chambersdug into the stream bank. The platypus lifestyle would seem to be possible in many places beside Australia. The streams they occupy are much like the creek that ran behind my friend’s house when I was growing up in Saint Louis.

***

"The other explanation for evolutionary one-offs is that natural selection is either not as predictable or as powerful as some make it out to be. That is, even when species experience identical environments, they might not evolve in the same way.

"A key reason for lack of convergence is that there may be more than one way to adapt to a problem posed by the environment. Think about the way vertebrate animals swim. Many use their tail for thrust, but not all tails are the same. Fish tails are vertically flattened and are moved back and forth. Crocodiles swim in the same way. But whale tails are horizontally flattened and are moved up and down.

***

" To be considered convergent, how similar do the traits of two species have to be? Squid and dolphins use very different anatomical structures to move rapidly through the water—there’s no question that they are not convergent. The foot-propelled locomotion of some aquatic birds is yet another non-convergent means of rapid underwater propulsion.

***

"So, are the forearm-modified wings of birds, bats, and pterosaurs convergent adaptations for powered flight that are built in different ways? Or do they represent alternative, non-convergent means of evolving powered flight?

***

"Where one draws the line between convergence and non-convergence among structures that are grossly similar and produce the same functional advantage is arbitrary. My inclination is to consider the wings of birds, bats, and pterosaurs to be convergent. Similarly, I view baleen whales and whale sharks convergent overall because both are large-mouthed, filter-feeding planktivores; however, I consider their filter-feeding structures to be non-convergent, alternative adaptations for filter feeding.

***

"Non-convergence can result for another reason. Often there are different functional ways to adapt to an environmental condition. As an example, consider how potential prey species may adapt to the presence of a predator such as lions. One option is to evolve great sprinting ability to outrun them, but there are other options, too, like camouflage,
***

" We all knew about convergence as a neat trick of natural history, a striking example of the power of natural selection. But Conway Morris and company have made clear that evolutionary duplication is much, much more common than we realized. We now recognize that it’s a frequent occurrence in the natural world, with examples all around us. Still, it’s far from ubiquitous.

"Seemingly just as often, maybe more often, species living in similar environments don’t adapt convergently. We need to go beyond documenting the historical pattern, chronicling yet more examples pro and con. Rather, we need to ask whether we can understand why convergence occurs in some cases and not others—what explains the extent to which convergence does or doesn’t occur."

Comment: The author makes an interesting point. Convergence exists, but it is not just a response to environmental challenges, and many look-alikes are not convergent. The process of evolution is very inventive as this illustrates.