An analysis of convergent evolution -- why animals that are not closely related may have very similar morphology.
Below article from The New York Times reprinted for fair use purposes only -- copyright acknowledged.
This Megalania page has been visited times since April 3, 1999.
Copyright 1998 The New York Times
The New York Times
December 16, 1998
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 along, sticky, worm-like tongue, no teeth to speak of and scimitarclaws.
Each has bulging salivary glands, a stomach as rugged as a cementmixer 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 apowerful 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,
As a result, the predecessors of today's various anthunters gradually, and quite independently, converged on thebody 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 withfascination 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 flamboyantcases 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 onopposite 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
"Convergences keep happening because organisms keepwanting to do
similar things, and there are only so many ways ofdoing 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 specieswe 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, asort of kangaroo, that looked like a saber-toothed cat.
"The world is a rich and wonderful place, but it is not one of
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 manifoldanteaters.
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 tolap 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 muskox 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 tearsago, 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 objectlike a virus
and begin the process of destroying the enemy. But in camels and nurse
sharks, a subset of antibodies has lost its lightchains: 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 beenlengthened in both the
camel and the nurse shark antibodies.
"It's a case of structural convergence," Dr. Rouxsaid.
"If this wasn't the only solution to the problem, it was certainly the
The unorthodox antibodies of the sharks and camels may
look and actalike, but the genetic subunits that encode the proteins are decidedly dissimilar from one another -- that is, they have different amino acidsequences.
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 ofthe 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 ofantifreeze
proteins, and the sequences of these proteins are, as a rule, all over
the map. But in the case of the cod and thenotothenioids, the
antifreeze molecules retain their resemblances down totheir 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, whilethe 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
And the cod can be thankful that nature, at least,does not believe in
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