archive: SETI Fw: convergent

SETI Fw: convergent

RonBlue ( (no email) )
Sat, 19 Dec 1998 12:46:52 -0500

>Reprinted for fair use purposes only. Copyright 1998 The New York
>Times; acknowledged.
>
> * * *
>When Evolution Creates the Same Design Again and Again
>
>By NATALIE ANGIER
>
>Nature is like Henny Youngman: She writes great jokes, and then flogs
>them again and again.
>
>Take the spiny anteater of Australia, the pangolin of Africa, and the
>giant anteater of Latin America (please!). Each of these mammals has a
>long, sticky, worm-like tongue, no teeth to speak of and scimitar
>claws.
>
>Each has bulging salivary glands, a stomach as rugged as a cement
>mixer and an absurd, extenuated, hairless snout that looks like a
>cross between a hot dog and a swizzle stick.
>
>Despite their many resemblances, the three creatures are unrelated to
>one another; the spiny anteater, in fact, lays eggs and is a close
>cousin of the duck-billed platypus.
>
>What has yoked them into morphological similitude is a powerful and
>boundlessly enticing process called evolutionary convergence. By the
>tenet of convergence, there really is a best approach and an ideal set
>of tools for grappling with life's most demanding jobs.
>
>The spiny anteater, pangolin and giant anteater all subsist on a diet
>of ants and termites, and myrmecophagy, it turns out, is a taxing,
>specialized trade.
>
>As a result, the predecessors of today's various ant hunters
>gradually, and quite independently, converged on the body plan most
>suited to exploit a food resource that violently resists exploitation.
>
>Scientists from Charles Darwin onward have been aware of convergent
>evolution and described examples of it with fascination and joy: the
>architectural parallelism of the wings of the bat, the bird, and the
>extinct pterodactyl, all having arisen independently but all having
>resulted from a similar modification of the vertebrate forelimb; and
>the concordantly streamlined profile of the shark -- a fish -- and the
>whale -- a descendant of a ratty, wolflike land mammal.
>
>Lately, the study of evolutionary convergence has taken on a new
>twist, as researchers look beyond such flamboyant cases of anatomical
>homology to detect subtle instances of convergence among molecules.
>They have found striking analogies between the antifreeze proteins
>that allow two unrelated groups of fish swimming on opposite ends of
>the globe to endure in icy waters. They have detected a bizarre form
>of antibody protein in species as different as camels and sharks --
>antibodies that look eerily like each other, and unlike the antibodies
>of most vertebrate creatures, yet that evolved their unorthodox
>proteinous conformations along entirely autonomous pathways.
>
>
>"Convergence is a really interesting part of the machinery of
>evolution," said Dr. Rudolf A. Raff, an evolutionary developmental
>biologist at the Molecular Biology Institute of Indiana University in
>Bloomington.
>
>"Convergences keep happening because organisms keep wanting to do
>similar things, and there are only so many ways of doing them, as
>dictated by physical laws."
>
>The issue of convergence also plays into a recent philosophical debate
>between two prominent evolutionary biologists, Dr. Stephen Jay Gould
>of Harvard University and Dr. Simon Conway Morris of Cambridge
>University.
>
>In his best-selling book, "Wonderful Life" (W. W. Norton, 1989), about
>the discovery of the Burgess Shale, a trove of 70,000 fossils half a
>billion years old, Dr. Gould emphasizes the importance of what he
>calls contingency, the idea that many of the species we see today are
>here by dint of a series of accidents -- an asteroid that struck the
>earth, for example, thereby eliminating the dinosaurs and making way
>for the rise of mammals.
>
>If you could rewind the tape of life and run the whole program over
>again, Dr. Gould said, you would end up with a radically different set
>of organisms, one almost certainly devoid of anything as cortically
>overendowed as we Homo sapiens are. He has criticized many of his
>colleagues for engaging in what he considers to be excessive
>adaptationist thinking, a "Panglossian" faith that the fittest
>survive, that evolution invariably progresses from simple to complex
>and from stupid to clever, and that what is, is for the best.
>
>Earlier this year, however, Dr. Conway Morris, one of the discoverers
>of the Burgess Shale, took issue with many of Dr. Gould's ideas in a
>new book, "The Crucible of Creation" (Oxford University Press).
>
>Rewinding the tape of life may not result in such a drastic change,
>Dr. Conway Morris insisted, one reason being the principle of
>convergence.
>
>
>"I would certainly not contest the reality of contingency and luck,"
>Dr. Conway Morris said recently in a telephone interview.
>
>"We're all the product of one very, very lucky sperm.
>
>On the other hand, when you look at the broad structure of the history
>of life, you can't help but be impressed by the number of organisms
>that began at different starting points and have come together -- the
>whale that looks like a fish, an extinct marsupial, a sort of
>kangaroo, that looked like a saber-toothed cat.
>
>The world is a rich and wonderful place, but it is not one of
>untrammeled possibilities."
>
>The relative degree to which the world's fauna and flora have been
>shaped either by contingency or by the slow hand of natural selection,
>as expressed most starkly in cases of convergent evolution, remains
>unclear.
>
>What is clear is that the more scientists look, the more examples of
>convergence they find. Sometimes the reasons for a particular
>convergence are easy to parse.
>
>Consider the shared traits of the world's manifold anteaters.
>
>Ants are tiny and must be consumed en masse, said Kent Redford of the
>Wildlife Conservation Society in the Bronx, who has studied anteating
>mammals -- hence the need for a long sticky tongue to lap up hundreds
>at a pop, and for enlarged salivary glands to help keep the tongue
>gummy and to wash the ants down.
>
>For moving that long tongue in and out rapidly, a muzzle improves the
>aim. And it is best for the snout to be hairless, to make sure that
>the pincered ants and termites have nothing to grab onto.
>
>Ants live in soil and sand, which requires powerful claws for digging.
>
>There is need of a digestive system that can readily pass the sand and
>dirt that will be lapped up with each tongueful of food, and that can
>metabolize the blistering chemical defenses with which ants and
>termites are loaded.
>
>Finally, sand grinds down enamel, so teeth can be dispensed with
>altogether.
>
>"It's a pretty weird bioplan," Dr. Redford said, "but it works." And
>the ultimate proof is sitting on his desk, in the form of a newly
>issued Beanie Baby toy with a telltale tubular schnoz.
>
>"Even the Beanie Baby phylogeny now has an anteater in it."
>
>Other cases of convergence are not so readily explained.
>
>Pamela Groves, a research associate at the University of Alaska's
>Institute of Arctic Biology in Fairbanks, has compared the musk ox of
>northern North America with the takin of China.
>
>Both are members of a large ungulate subfamily that includes sheep and
>goats, and biologists had long assumed that the two species were
>closely related, for they have several peculiar features in common.
>
>They are both the biggest and most barrel-chested members of the
>subfamily, and have unusual horns that grow out of the center of the
>forehead and hook off to the side. They also display an exceptional
>form of group defense behavior when confronted by a predator. Rather
>than bounding off in the manner of goats or gazelles, they instead
>fall with military precision into a circle formation, the adults
>facing outward, their sharp-tipped horns at a ready, and the young
>safely sequestered within the center.
>
>
>That the takin practice group defense is particularly surprising; the
>animals live in the dense vegetation of remote mountain regions, and
>as a rule herbivores in such environments tend to be solitary and rely
>on forest cover rather than herd life for protection.
>
>Thus, biologists had proposed that the takin and musk ox were
>descendants of a common ancestor, which arose in Asia under different
>habitat conditions than exist today and then radiated into North
>America across the Bering strait about 20,000 tears ago, during the
>Pleistocene.
>
>But in a recent DNA analysis of the two species, published in the
>journal of Molecular and Phylogenetic Evolution, Dr. Groves found that
>the animals are not close kin after all, but in fact diverged from one
>another nearly 10 million years ago, long, long before the
>Pleistocene. "No matter how I analyzed the data, the results always
>showed the musk ox and takin had other species that were genetically
>more similar to them than they were to each other," she said. "So why
>all the resemblances? My theory, after pondering it for a while, is
>that this is another example of convergent evolution."
>
>How the convergence occurred, though, and what the selective pressures
>were that resulted in each species having big bodies, the same sort of
>horn structure and the same circling-the-wagons approach to defense,
>she cannot say.
>
>Equally piquant are some of the recent discoveries of molecular
>convergence. Dr. Kenneth H. Roux, a structural biologist at Florida
>State University in Tallahassee, and his colleagues recently described
>in the Proceedings of the National Academy of Sciences a baffling
>similarity between certain antibody proteins in camelids -- the group
>that includes camels and llamas -- and nurse sharks.
>
>Throughout most of the animal kingdom, the antibodies of the immune
>system are built of two types of chains, called heavy and light, and
>each chain has three loops. Together the triple-looped heavy and light
>chains allow an antibody to attach to a foreign object like a virus
>and begin the process of destroying the enemy. But in camels and nurse
>sharks, a subset of antibodies has lost its light chains: All three
>loops are missing, and only the three loops of the heavy chains
>remain. The scientists cannot say why the loss occurred in the first
>place, whether by accident or by unfathomable selective design. In any
>event, the antibodies of the camels and the nurse sharks responded to
>the change in cognate ways.
>
>To compensate for their absence of light chains, both animals expanded
>the size of one of the loops in their heavy chains.
>
>Remarkably, it is the same loop that has been lengthened in both the
>camel and the nurse shark antibodies.
>
>"It's a case of structural convergence," Dr. Roux said.
>
>"If this wasn't the only solution to the problem, it was certainly the
>most efficient."
>
>The unorthodox antibodies of the sharks and camels may look and act
>alike, but the genetic subunits that encode the proteins are decidely
>dissimilar from one another -- that is, they have different amino acid
>sequences.
>
>Many combinations of amino acids can be strung together to construct
>proteins that behave in nearly identical ways.
>
>For statistical reasons, though, said Dr. Russell F. Doolittle, a
>molecular evolutionist at the University of California at San Diego,
>true sequence convergence -- where two independently evolved proteins
>not only perform the same task but have the same underlying building
>blocks -- is likely to be extremely rare.
>
>But odds, like hearts and eggs, are made to be broken, and so
>scientists recently announced what they think is the first
>illustration of bona fide sequence convergence. Dr. Chi-Hing C. Cheng
>of the University of Illinois at Urbana-Champaign and her co-workers
>reported in the Proceedings of the National Academy of Sciences on
>their analysis of antifreeze proteins found in two groups of fish: the
>notothenioids of the Antarctic and the Northern cod of the Arctic.
>
>The proteins help keep a fish's blood from freezing while it swims
>through frigid waters by binding onto a bit of ingested icicle and
>preventing the ice crystal from growing larger.
>
>A number of polar-dwelling creatures have versions of antifreeze
>proteins, and the sequences of these proteins are, as a rule, all over
>the map. But in the case of the cod and the notothenioids, the
>antifreeze molecules retain their resemblances down to their cores.
>They consist of the same three amino acids -- threonine, alanine and
>proline -- repeated over and over.
>
>In a painstaking series of experiments, Dr. Cheng and her colleagues
>demonstrated that the proteins arrived at their analogous sequences
>during entirely independent episodes of genetic shuffling. The
>notothenioid protein arose about 7 million to 15 million years ago,
>when Antarctic oceans were chilling to freezing, while the cod version
>probably evolved about 3 million years ago, during the glaciation of
>the Arctic seas.
>
>The simplicity of the protein sequence, Dr. Cheng said, explains how
>it was possible for it to have arisen on two separate occasions.
>
>And the cod can be thankful that nature, at least, does not believe in
>copyrights.
>
>