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Posts Tagged ‘convergent evolution’

Now for one that just blew me away!

From Science Daily:

A study published online July 23 in the Proceedings of the National Academy of Sciences finds that in the case of insects that developed resistance to a powerful plant toxin, the same adaptations have occurred independently, in separate species in different places and times.

The paper examines 18 insect species across four orders — beetles, butterflies and moths, flies, and true bugs — that all feed on plants containing powerful toxins called cardenolides.

Common to milkweeds and foxglove, cardenolides are lethal to nearly all insects and function effectively as a defense against pests. Cardenolides work by binding to a cell’s sodium pump, one of the most fundamental systems found in all animal cells. The sodium pump works when an essential enzyme (Na,K-ATPase) carries important elements, sodium and potassium, across the cell membrane. Cardenolides bind to the enzyme and disable it, thereby shutting down cells, which results in severe damage.

Among the 18 insects surveyed, the researchers found a few methods that the insects use to resist cardenolides. In monarch butterflies and a species of leaf beetle, for example, resistance is due to a specific mutation — called N122H — of the Na,K-ATPase gene. The mutation reduces cardenolide binding to the sodium pump enzyme.

“Already knowing how monarchs deal with the toxin, we wanted to see if it was the same molecular solution used by beetles, flies and true bugs that are also resistant to cardenolides,” said Anurag Agrawal, a Cornell professor of ecology and evolutionary biology and a co-author on the paper. Susanne Dobler, a professor of molecular evolution at Hamburg University, is the paper’s lead author.

By examining molecular changes in the sodium pump gene, the researchers found the mutation N122H in all four orders of insects studied. Furthermore, they discovered a second mutation in the same gene that also conferred resistance in 11 of the 18 species.

“This is truly a remarkable level of evolutionary repeatability and suggests that evolving resistance to the plant toxin had very few effective options,” said Agrawal.

The researchers tested the effectiveness of these gene changes by inserting the single Na,K-ATPase mutations into cell cultures and then dosing those cultures with cardenolides. They found the mutations gave the cells resistance, and when cells were given the two mutations that repeatedly evolved together, they had twice the resistance as cells with a single mutation, implying a synergistic effect.

The standard gene for the sodium pump is essentially the same in all insects, and even mammals carry the gene in a relatively unmodified form. The sodium pump thus originated from a common ancestor hundreds of millions of years ago and is central to the functioning of most animals. Out of that background, insects from different orders over the last 300 million years specialized on plants with cardenolides and evolved resistance independently, and in numerous cases, through exactly the same gene change.

Convergent evolution isn’t anything new to science.

However, convergent evolution using the same mutation is!

This is pretty damn amazing. Here, you have animals that are very different in terms of their phylogeny and taxonomy, but they have exactly the same mutation that allows them to deal with these particular toxins.

I don’t know if such a thing has been discovered before, but it is pretty astounding.

Mutations are random. All of us have some mutations. Most are neutral. Some are advantageous. Some are deleterious.

The chances of organisms evolving a similar adaptation using the same mutation would appear to be quite low.

But here we have an example of that very unlikely phenomenon happening.

Now, I’m sure that the creationists will have a lot of fun with this one, but the truth is I don’t know of a single other case in which very organisms from very divergent ancestries evolve convergent adaptations through the same genetic mechanism.

It’s a weird case.

The authors think that the reason why this mutation has been implicated in all these different species is that evolving this particular adaptation just has such limited options.

It’s not likely applicable to other situations.

For example, the chances that all the hairless dog breeds in the world that have a dominant hairless allele that stems from the same mutation all developed that mutation independently are unbelievably low. Unlike these insects, they all derive from a common ancestor that lived in Mexico 4,000 years ago.

It’s a giant leap to use this particularly unusual discovery in insects and apply it to other organisms.

And I hope people can refrain from doing so.

But they won’t.

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From Live Science:

The extinct thylacine, more commonly known as the “Tasmanian tiger” or “marsupial wolf,” hunted more like a cat than a dog, meaning the tiger moniker may be the more appropriate nickname.

The thylacine had the striped coat of a tiger, the body of a dog and like other marsupials (including kangaroos and opossums) carried its young in a pouch. These carnivores were last seen in Australia 3,000 years ago, having died out after the introduction of dingoes by humans. The last remaining populations were sheltered by their isolation on the island of Tasmania, surviving until the 1900s, when a concentrated eradication effort wiped the thylacine out.

Researchers hypothesized that the dingoes were a main cause of the thylacine decline in Australia, because the two species were in direct competition — using the same hunting strategies to hunt the same prey.

Dingoes are a species [sic] of wolves, they are runners,” study researcher Borja Figuerido of Brown University said. “If the thylacines are ambushers, the hypothesis of the extinction of the thylacine outcompeted by dingoes is less probable.”

The elbow joint of the thylacine and the modern tiger, top, is wider and more rectangular than the dog-like wolf and fox, bottom, which are more toward the square. This suggests different styles of catching and subduing prey, cat-like or dog-like.

By looking at the elbow joint bones of the thylacine and 31 other mammals, the researchers noticed they resembled those of cats, which can rotate their paws upward to pounce and attack prey. Dogs and wolves don’t have this rotation capability.

“These anatomical characters reveal something about the hunting strategies of the thylacine. They are more ambushers than previously suspected,” Figueirido said. “Ambush predators usually manipulate the prey with the forearms, they have very good mobility. Running predators lack this ability, because the elbow is locked.”

The limited rotation of their arm bones makes dogs and wolves (including dingoes) faster runners, which changed their hunting behaviors. Dogs and wolves hunt in packs, following their prey over longer distances. The researchers determined that the thylacine was more of a solitary, ambush-style predator, similar to cats.

Marsupial mammals, found mainly in Australia and other areas of the Southern Hemisphere, are similar to placental mammals (such as humans, dogs and cats), but their evolution diverged from ours during the Cretaceous Period, the earliest example of a marsupial appearing about 125 million years ago.

The evolution of these two groups of mammals is an example of convergent evolution, where two separate groups in different locations evolve similar morphologies to deal with similar habitats. The thylacine was thought to be the marsupial equivalent, or ecomorph, of the wolf, with similar body size and eating habits.

Now, Figueirido said, “this designation will need to be revised.”

The study was published today (May 3) in the journal Proceedings of the Royal Society B: Biology Letters.

All of this information has been discussed before, so I don’t know why it’s making news now.  The convergence is in phenotype, not behavior or hunting style. I don’t think anyone ever made that claim.

The dingo outcompeted the thylacine because it was a pack-hunter. It was smarter, and it could attack both large and small prey.

It’s very similar to what happened to Hyaenadon when bear-dogs arrived in North America.

The notion of convergent evolution isn’t that animals from different ancestries evolve to be exactly the same. That’s a bizarre standard– and the one that seems to be put forth here.

But it is remarkable that the bulk of this animal’s adaptations– even if it were an ambush predator– closely resemble that of a dog.

It’s still convergent evolution. You’re never going to find convergent evolution that is 1:1 anywhere. Even with golden and marsupial moles, which are probably the closest anyone will ever get to that 1:1 convergence.

Convergent evolution can mean that only one trait is similar, such as the retractable claws in both modern cats and the thylacoleo, the extinct “marsupial lion,”  or the fact that koalas have the fingerprints, which are essentially a primate trait.

By this metric, the thylacine and the dingo are pretty strong examples of convergent evolution, for they share many traits, including the head shape, and the fact that both were digitigrade. Both use strong jaws that have evolved into the shape of a muzzle.

And a dingo isn’t a “species of wolf.”  Is is a subspecies that is sometimes called Canis lupus dingo or Canis lupus familiaris, which is probably more accurate, seeing as dingoes are more closely related to East Asian and Indonesian dog breeds than they are to wild wolves.

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(source for image)

Ah. I finally got you.

I said it was not a domestic dog. It is not.

It isn’t even canine.

It’s not even from a member of the order Carnivora.

It is a thylacine skull.

For those of you who don’t know this is what a thylacine looked like, here’s footage of the last one, Benjamin, who died in 1936:

Source.

Thlyacines were a wonderful example of convergent evolution.

It’s amazing how much their skulls resemble that of a placental wolf.

When Europeans colonized Australia, the only large predator on the mainland was the dingo, an animal derived from East Asian domestic dogs, which were derived from the placental wolf. Dingoes very strongly resemble certain subspecies of wolf, and they are currently recognized as a subspecies of wolf, Canis lupus dingo.

On Tasmania, there were no dingoes at all. Instead, there was this wolfish creature with stripes.

One would have thought that the two were related.

After all, they looked so much alike and lived in relative proximity to each other.

But evolution often produces similar animals from very different lineages. The closest relatives to the dingo are dogs and wolves. The closest relatives to the thylacine are the carnivorous marsupials. The two animals evolved to hunt relatively similar prey.

However, a study from 2007 found that although physically similar, the dingo is structurally much better equipped to attack large prey. It may have had more powerful bite, but the thylacine was forced to hunt mostly smaller prey.

When the dingo arrived in Australia around 4,000 years ago, the dingo could prey on all sorts of different animals, including the biggest kangaroos.

The dingoes were also pack hunters, which meant that they could more easily pursue larger prey.

It has been suggested that the natives of Australia were also regularly managing the land through burnings mean that there was less cover for the thylacine. The thylacine preferred to hunt in the dense forest, while the dingo was more at home hunting out in the open. This has been disputed, simply because aboriginal burning increases the number of species that inhabit the desert environment.  The thylacine would have benefited from the burnings and would not have become so stressed that it would become extinct.

However, the dingo also had a complex relationship with the people– in the form of a kind of semi-domestication. That meant that the dingoes would benefit from the successes of humans.

The thylacine had none of these advantages. Humans would have been a major competitor for the thylacine for prey, and high densities of people would mean that the thylacine would have faced really intense competition from both people and dingoes. Such stresses could cause the thylacine to become extinct on the mainland.

It was forced to hunt smaller prey. It was a solitary hunter, and it had no beneficial relationship with people. Its hunting habitat was fragmented, and it simply could not compete with the dingo.

Because dingoes never made it to Tasmania, the thylacine was able to hold on for much longer.

I have never understood why people called this animal a Tasmanian tiger. It may be that it looked like a giant quoll, which are also called “native cats.” A big native cat might as well be a tiger, right?

However, it was soon the target of hunters in Tasmania, for it was widely believed to a great menace to sheep. Most the killings likely happened as the result of free roaming domestic dogs, but it was easier to blame the wild tiger than the tame dog. After all, if that 2007 study is correct, it is very unlikely that a thylacine would have risked so much to tangle with a sheep.

It is really a shame that the thylacine went extinct, because it was such a wonderful example of convergent evolution. Through the history of the earth, the doggish form has appeared several times.  The earliest large hyenas actually looked more like dogs than modern hyenas do, and the Mesonychids looked a lot like wolves (in the case of Andrewsarchus mongoliensis, a very, very big wolf!).

It should not be a surprise that an animal similar to this form would pop up in the marsupial lineage.  Mesonychids were actually related the even-toed ungulates and whales and were of no relationship to modern wolves or dogs at all.

Evolution does amazing things. Convergence is one of its most interesting aspects.

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To read more about the thylacine, check out the wonderful online Thylacine Museum. It has lots of historical photos, including the oldest extant photograph of a thylacine.

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