Evolution: convergence or parallelism (Introduction)

by David Turell @, Wednesday, August 09, 2017, 23:39 (223 days ago) @ David Turell

A book review looks at parallel developments rather than convergence for comparative results:


" Improbable Destinies is deep, broad, brilliant and thought-provoking. Losos explores the meaning of terms such as fate, chance, convergence and contingency in evolution. Why do similar solutions — morphological, genetic and molecular — crop up again and again? He became intrigued by these questions when, as a student, he began to study the Caribbean Anolis lizards, following groundbreaking work by ecologist Thomas Schoener. These lizards inhabit a great range of island sizes and habitats, and tend to evolve similar adaptations and roles in similar circumstances. However, species on different islands that resemble each other aren't each other's closest relatives. Why not?

"The answer, we think, is that closely related lineages have similar genetic components, so under comparable ecological conditions they are likely to produce similar mutations that are then selected for. Many call this convergence; I prefer the term parallelism for closely related lineages. 'Convergence' is appropriate for reinvention in very different groups — the superficially similar wings of birds and pterosaurs, or the elongated grub-seeking fingers of the aye-aye (Daubentonia madagascariensis) and striped possum (Dactylopsila trivirgata). We can catalogue examples all day, but is there any real theory of convergence? We cannot assert that some lineages are 'fated' to converge on these features. Biological ideas of determinism went out with Jean-Baptiste Lamarck in the late eighteenth century.

"Evidence against determinism is the prevalence of creatures whose adaptations have never been duplicated: the kangaroo, the platypus (Ornithorhynchus anatinus), the century plant (Agave americana, which blooms only once in its multidecade life) — and humans. Primates have the equipment apparently needed to evolve flight: arboreal habits, large brains, good coordination and active metabolisms. Yet no primate seems to have evolved gliding, let alone powered flight. No evolutionary duplication is inevitable.

"And how are we to predict these convergences? Losos resurrects palaeontologist Dale Russell's fantastic “dinosauroid” of the 1980s, a conception of what would have happened if bipedal, carnivorous dinosaurs with large brains and grasping hands had been selected for, had the asteroid not struck (Russell accepted the asteroid hypothesis). This three-fingered creature with a huge brain, a beak and chicken feet seems to have inspired the aliens in the 1980s science-fiction television series V, but scientists were uninspired.

"Evolutionary biologist Stephen Jay Gould wondered about the unique, fragile fauna of the Cambrian period (541 million to 485 million years ago) in his 1989 book Wonderful Life (W. W. Norton). Had the period's chordates — which included the ancestors of vertebrates — become extinct, we wouldn't exist. But as Losos points out, it's not a fair comparison: you're not replaying the tape, but running a different one.

"The idea of contingency is perhaps best based on palaeontologist Dolf Seilacher's theory of constructional morphology. In this, features such as the elephant's trunk or the osprey's habit of catching fish with claws rather than beak result from three factors: adaptation (the selective component), evolutionary history (organisms must work with what they've inherited) and construction (how the material properties of living structures empower and constrain their form). From there, history takes over. Evolution is not a preordained, inevitable narrative. Neither is it a crapshoot, with random particulates disporting themselves until something works. Rather, it is like the game Monopoly. Where you go next is in part determined by where you are now; who you are is where you've been (your acquisitions); where you can go is determined by the throw of the dice, with limited possibilities and probabilities."

Comment: This is in direct contrast to Simon Conway Morris who firmly believes convergence is built into evolution. However this book (despite its atheist approach) fits my thought that there are built in patterns which would produce parallel devedlopments.

Complete thread:

 RSS Feed of thread

powered by my little forum