Tag: evolution

Scientists again see natural selection in real time

June 8, 2009 • 3:32 pm

In WEIT, I describe several studies showing that natural selection can change species over periods shorter than a human lifetime.  These studies have been important in convincing skeptics (although not creationists, who will never be convinced) that natural selection is more than just a speculation, but a force that can really mold animal behaviors, appearances, and reproductive traits in real time.  The most famous of these, of course, is the work of Peter and Rosemary Grant on a species of Darwin’s finch.  Their work showed that individuals’ beak and body characteristics could change over a single generation when an El Niño event changed the finch’s food.  (See The Beak of the Finch, by Jon Weiner, for a popular account of the Grants’ work.)

Now a group of researchers from Canada and the US have reported natural selection acting in guppies, molding their reproductive behavior over a period of only 8 years: roughly 13-26 guppy generations.  This work is based on the observation that in Trinidad, the common guppy (Poecilia reticulata) lives in streams both above and below waterfalls.  The guppy’s downstream predators cannot migrate past the waterfall barriers, so upstream guppies experience different environments from downstream guppies. In other words, downstream populations have to deal with much stronger predation.

This leads to some evolutionary predictions.  The body of evolutionary theory called “life history theory” predicts that when an animal species experiences higher predation, it should evolve a different reproductive strategy.  In short, guppies harassed by predators should reproduce earlier than un-predated guppies, for the greater your chance of being chomped, the less likely you are to leave offspring if you delay reproduction. Second, fish that experience less predation should produce larger embryos, since they have the luxury of delaying reproduction and because larger embryos make the newly hatched fish more competitive.  Finally, low-predation fish should have fewer offspring in each bout of reproduction, for it is to their advantage to spread their given lifetime allotment of reproductive effort over a longer time.

In 1996, some of these authors introduced guppies from downstream, high-predation populations into uninhabited upstream, above-waterfall areas. Eight years later, in 2004, they took samples from both ancestral and derived populations to see if any differences in life history had evolved.  And they did — in the predicted direction.  Upstream guppies had fewer but larger embryos than downstream guppies, as well as a small reproductive allotment (egg mass as a proportion of body mass).  Mark-recapture experiments on adults also showed that, when tested in the upstream environment, upstream-evolved guppies had higher survival than their downstream ancestors.

The analysis is arcane, but the results are clear:  guppies have changed their life histories in an adaptive way in only eight years.  This certainly reflects the action of natural selection, since previous studies have found similar results for life history, and also for color. (Guppies introduced to a low-predation regime evolve brighter colors in males; brightly colored males are favored everywhere by sexual selection but become disadvantageous in high-predator environments since they are more likely to be spotted and eaten.)

In toto, the guppy work is as powerful a body of evidence for selection, if not more so, than the work on finches.  This is not to denigrate the finch study, which is brilliant.  That work, however, was an uncontrolled “natural experiment” that affected two characters (bill and body size), while the guppy work has involved many groups of investigators doing controlled introductions — and all finding the predicted evolutionary changes in many characters.  Birds, of course, are more charismatic, but the humble guppy has a lot to show us about evolution.

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S. P. Gordon et al. 2009.  Adaptive changes in life history and survival following a new guppy introduction.  The American Naturalist, Volume 174, pp. 34-45.

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Deep time

March 6, 2009 • 2:19 pm

by Greg Mayer

Seed Magazine has a nice video showing The Evolution of Life in 60 Seconds.  It gives you a good feel for “deep time“: the immense span of ages that is the history of the Earth.  It is especially notable how brief the entire Phanerozoic (from the Greek for “visible life”), the last 550 million years or so of largish animals with hard parts, is.

Skell pwned again

March 5, 2009 • 12:13 pm

by Greg Mayer

I think I’ve been able to figure out why chemist Philip Skell’s attack on Jerry in Forbes was so unresponsive to what Jerry actually wrote: he probably wrote most of it before seeing Jerry’s article!  P.Z. Myers noted a piece in the Eugene, Oregon, Register-Guard by teacher Stuart Faulk rebutting Skell’s arguments.  Addressing Skell’s claim that evolution is irrelevant to medicine, Faulk (who is Skell’s son-in-law!) does some research:

The contention that evolutionary science is not useful is easily shown false by counter-example. The necessary research is accomplished by walking the five feet to my coffee table and picking up the March edition of Scientific American magazine, in which the article “New Tactics Against Tuberculosis” describes progress against the spread of drug-resistant TB….As the authors state of one promising approach, “It allows us to harness the power of natural selection in our quest to thwart (drug-resistant TB).”

He also notes that Skell’s claim that for evolution to be relevant  to medicine, then paleontology must drive its research agenda, is an “absurd idea”,  “introduced by Skell, not evolutionary scientists.” Faulk goes on to note that Skell’s real concerns are religious, not scientific, as “any Web search will show”.

That Skell’s arguments are easily rebutted is not surprising; what is surprising is that Faulk was responding to something Skell wrote in the Register-Guard that appeared February 12, the same day as Jerry’s piece in Forbes, and 11 days before Skell’s piece in Forbes.  The Register-Guard piece is only available on the web as an excerpt, but it seems to be much the same as what appeared in Forbes. You compare:

The Register-Guard, Feb. 12: “Darwin was great, but too often he’s oversold”

In 1942 Nobel Laureate Ernst Chain wrote explicitly that his discovery (with Florey and Fleming) of penicillin, and the development of bacterial resistance to that antibiotic, owed nothing to Darwin’s and Alfred Wallace’s evolutionary theories. The same can be said about a variety of other 20th century discoveries: that of the structure of the double helix; the characterization of the ribosome; the mapping of genomes; research on medications and drug reactions; improvements in food production and sanitation; and various new surgeries.

Forbes, Feb. 23, “The dangers of overselling evolution”

In 1942, Nobel Laureate Ernst Chain wrote that his discovery of penicillin (with Howard Florey and Alexander Fleming) and the development of bacterial resistance to that antibiotic owed nothing to Darwin’s and Alfred Russel Wallace’s evolutionary theories.

The same can be said about a variety of other 20th-century findings: the discovery of the structure of the double helix; the characterization of the ribosome; the mapping of genomes; research on medications and drug reactions; improvements in food production and sanitation; new surgeries; and other developments.

I can understand why Jerry found Skell’s Forbes piece off-point:

The curious thing is that Skell’s piece is not, as it pretends to be, a critique of what I said in Forbes, but merely a repetition of the argument, which he has been making for years, that evolution is of no practical use for humanity and of no use to experimental biology

The one thing I would add to these critiques of Skell is to point out his curious use of the phrase “experimental biology”, and his disdain for what he seems to consider unobservable or uncertain knowledge.  He seems to imply that chemistry and “experimental” biology, are good science, because they are observable; other sorts of biology (i.e. evolution), and (if the apparent criterion is to be applied uniformly), geology and astronomy are not, because we have not seen a live trilobite, or Gondwanaland, or a star moving along the main sequence.  Thus creationists seek not just to eliminate biology, but much of the rest of science as well. All knowledge in empirical science, including chemistry, is tentative; and the changes in the kinds of plants and animals you see as you travel up slope on a fossiliferous exposure are much more observable than any chemical bond.

A brand-new whale fossil

February 5, 2009 • 2:22 pm

Phil Gingerich and his colleagues at the University of Michigan, the Geological Survey of Pakistan, and the University of Bonn have just described a brand new and intriguing pair of whale fossils in the journal PLoS ONE, which you can find here. Tis whale Maiacetus inuus, lived about 47.5 million years ago, which puts it roughly at the time of the fossil whale ancestor Rodhocetus (see pp. 50-51 of WEIT). Rodhocetus was probably an amphibious creature, living on both sea and land, and this is almost certainly true of the new find as well.
The really intriguing thing about Maiacetus is that the female fossil contained an embryonic individual, apparently near term. This is the first known example of a fossil embryonic whale. What’s more, the embryo was positioned with its head facing the rear, so that it would be born head first. That birth position is characteristic of land-dwelling animals (probably an evolutionary feature to allow the embryo to start breathing as soon as possible), but is not found in marine mammals. In the latter group, babies are born tail first, probably so that a.) they won’t drown during birth and b.) so that they are born in the right position to immediately bond with and start following their mother.

There are other interesting features of this whale, too, like the permanent first molar teeth in the fetus, indicating precocial development
From the structure of its limbs and body, Maiacetus obviously lived on both sea and land, since the hind limbs are still substantial but reduced. The position of the fetal whale shows that it did, however, give birth on the land. Just another link in the ever-growing chain of fossils documenting the evolution of whales.

Genes for surviving after reproduction

February 3, 2009 • 1:03 pm

An alert reader has written me about a statement I made in WEIT:

You write that “a gene that knocks you off after reproductive age incurs no evolutionary disadvantage.” And you go on to say that selection would not favour genes that helped survival after reproduction has finished. “One example would be a gene that helps human females survive after the menopause.” I understand and accept this point. But could there be an exception (in theory) as follows? If women were giving birth right up to menopause, they would need to survive after menopause to bring up their last children. Also, if a woman who survived post-menopause helped her daughter to bring up her grandchildren and had the effect of improving the survival rate of those grandchildren, wouldn’t her “live longer genes” then get passed on?

The reader is absolutely right–I glossed over the nuances of this idea in the interest of space, and probably shouldn’t have.  Indeed, a woman will be selected to live until all the benefits she can confer upon her children (the most important being maternal care) have already been bestowed, and that means staying alive until she has brought up all her offspring.  To the extent that children are grown and independent before menopause, selection will be very small against a gene that bumps mom off.  That is, of course, unless she can help her grandchildren grow up, for in that case she is still contributing to the survival of her own genes in her offspring’s offspring.  So there are exceptions to what I said.  Let me then rephrase my statement to say that “a gene that knocks you off after you’ve made all possible contributions to rearing related individuals incurs no evolutionary disadvantage.”

Is “The Hobbit” a fraud?

February 1, 2009 • 10:50 am

As recounted in WEIT, one of the most remarkable hominin fossils is that of Homo floresiensis, discovered on the island of Flores in Indonesia in 2003. This creature was remarkable in that although it lived only 18,000 years ago, when modern H. sapiens had already evolved, it was only a meter tall, weighed 50 pounds, and had a brain of less than 500 cc.–similar in size to of our distant cousin Australopithecus afarensis (“Lucy”). It seemed that some relict populations of Homo had survived on this Indonesian island, bypassed by modern humans.

Ever since H. floresiensis (dubbed “The Hobbit”) was found, it has been the center of heated controversy. Some have said that rather than being a long-surviving ancient hominin, for example, the one good specimen found is simply that of a modern human afflicted with a growth disease (such as goiterious cretinism) that produced a small skull. Others counter-claim that the wrist bones of the hobbit are clearly not that of a modern human, but of an earlier relative.

Now another criticism has surfaced–the claim that the hobbit’s teeth show dental work! In particular, an anthropologist at the University of Adelaide in Australia, Maciej Henneberg, claims that a lower molar of H. floresiensis shows a filling (and possibly a root canal) of the type performed in Indonesia in the 1930s. (See the articles about this claim here and here.) Could the hobbit be another Piltdown Man, a fraud foisted on a credulous scientific community?

Well, probably not. In a careful analysis of the dentition of H. floresiensis and a comparison with other ancient skulls, Peter Brown, one of the hobbit’s discoverers, debunks Henneberg’s claims. X rays and careful analysis (see the pictures on Brown’s page) show absolutely no evidence of dental work. Thus this claim, at least, has been debunked.

It is starting to look as if H. floresiensis really was a genuine species, but an anomalous one: a small population of tiny humans who hunted dwarf elephants with miniature spears. There will undoubtedly be more argument before this is settled.

flores_sapiens

H. floresiensis (l.), H. sapiens (r.). Photograph courtesy of National Geographic news.

More on the evolution of flight

January 30, 2009 • 4:51 pm

As I discuss in WEIT, the evidence shows that birds evolved from theropod dinosaurs–gracile, carnivorous beasts that walked on two legs. Some of the important evidence comes from Chinese fossils showing theropods with various types of feathers. The incipient stages of feather evolution appears to be filamentous feathers (T. rex might well have been covered with fluff!), implying that flight feathers originated as devices to help insulate the theropods. In a recent paper in Nature, however, F. Zhang et al. found a theropod fossil (Epidexipteryx hui), about 160 million years old, which had not only the downy feathers (feathers completely unsuitable for flight) but also four very long tail feathers that could not have been used for either flight or insulation. E. hui also showed a number of morphological features that seem to make it closely related to modern birds. The most likely explanation for these tail feathers is that they were ornaments–ornaments that evolved for either species recognition or via sexual selection. The evolution of flight, then, may have begun with feathers that were used for display. A somewhat fanciful reconstruction of the beast is shown below.

Epidexipteryx hui (reconstruction)
Epidexipteryx hui (reconstruction)

Review of WEIT in Wall Street Journal

January 29, 2009 • 9:38 am

A very nice review of WEIT appeared today in the Wall Street Journal.  It was written by the distinguished philosopher Philip Kitcher, who has written extensively on evolution, creationism, and evolutionary psychology.  I highly recommend his recent book, Living with Darwin: Evolution, Design, and the Future of Faith, which not only dismantles intelligent design, but deals with the thorny problem of how nonreligious people can find the same kind of solace and social networking that is provided by religion. Philip also wrote what I consider the definitive critique of evolutionary psychology (then called “sociobiology”): Vaulting Ambition: Sociobiology and the Quest for Human Nature.  Finally, his avocation is James Joyce, and he’s just penned an introduction for the general reader to Finnegans Wake: Joyce’s Kaleidoscope: An Invitation to Finnegans Wake, an accomplishment which I can regard only with astonishment.