Skip to content

Cairns’ Deciding Experiment

19 May 2012, 3:28 pm

Cairns Experiments Results

Cairns added lactose either immediately after spreading the bacteria on agar plates, 24 hours later or 72 hours later. The graph shows the number of colonies that appeared, on the left in real time and on the right relative to the time at which the lactose was added. Lactose causes the late mutants to appear Delaying the lactose delays their appearance, but does not affect the number of mutants.

The final experiment that Cairns and his colleagues report comes even closer to the sort of selection one might expect in the real world. It exploits a technique developed early this century for classfying bacteria according to their ability to use certain sugars. Wild E coli, for example, can ferment lactose whereas shigella and salmonella cannot. Some species, however, are so-called “late” fermenters of certain sugars. That is, it may take a week or more before bacteria start using an unusual food source. Shigella sonnei, for example, is a late fermenter of lactose.

In fact, bacteria possess several such cryptic genes, which are brought into play only when needed. The mechanism of activation varies. Sometimes, another piece of DNA is inserted upstream of the desired gene and switches that gene on.

In other cases, The DNA sequence needs several specific changes before the cryptic gene will function properly. Cairns and his group studied one such cryptic gene which allows E coli to ferment lactose even when its beta-galactosidase gene is not working.

The gene is called ebg, and it needs at least two mutations to turn it on. The first is a change in the repressor, a DNA sequence which codes for a protein that normally keeps ebg inactive. The second is a change to ebg itself. The enzyme produced by the usual version of the gene cannot, in fact, break down lactose. It needs a mutation to make it effective. Under normal circumstances, each of these two mutations happens roughly once in every 100 million generations. Both mutations are needed, which would happen by chance roughly once every 10 million billion generations. Cairns says:

That such events ever occur seems almost unbelievable.

Yet colonies do appear after about two weeks. That they do, without at the same time gathering a lot of neutral and outright harmful mutations, suggests to Cairns that bacteria must have access to some reversible process of trial and error.

A Mechanism Needed

Cairns stresses that the main purpose of his paper is “to show how insecure is our belief in the spontaneity of most mutations”. But he realizes that the experiments he reports are not going to settle the issue. What seems to be missing, at the moment, is a mechanism that would use what we already know about the workings of the cell to achieve the sorts of directed mutations that the group at Harvard has demonstrated.

Cairns suggests that the cell might make a set of variable RNA messages—which carry the genetic instructions from the DNA to the machinery that makes proteins according to those instructions—and reverse transcribe the most effective of these back into DNA.

It would need some way of monitoring “effective” RNA, but if it reverse transcribed only those messages present when it started to grow again, it would most likely capture the message that had indeed enabled growth to resume.

Critics of Darwinism have already leapt on Cairns’s work to support their belief that there is something rotten in the state of evolutionary biology. Biologists, however, are being more circumspect. They would like to see further demonstrations of the phenomenon—and preferably a mechanism too—in bacteria before they embrace it wholeheartedly. They also doubt that it could apply when the biochemical link between adaptation and gene is longer and more complex than that between an enzyme and its substrate.

Report by Jeremy Cherfas, New Scientist, 22 September 1988.

Originally posted 2009-06-17 01:14:34. Republished by Blog Post Promoter

Filed under Evolution. Tagged , , , ,
Comment

Brain to Body Ratio

12 May 2012, 2:46 pm

A better criterion of intelligence might be the ratio of the brain’s weight to that of the whole body. This allows for the tendency of larger creatures to have larger brains simply to regulate their larger bodies. Sagan gives the ratio of body to brain weights for the hominids as:

  • 90 for A robustus
  • 50 for A africanus
  • 60 for H habilis
  • 65 for H erectus
  • 45 for H sapiens

There seems to have been no significant decrease in this ratio since A africanus and indeed the early toolmaking hominids seem to have had smaller brains for their body size than the non-toolmaking A africanus. A robustus, if descended from A africanus, regressed considerably—though there is some margin of error in these ratios. More importantly, the European pygmy shrew with a body weight of 4.7 grams and a brain weight of 0.1 grams has a body to brain weight ratio of 47, very similar to our own. On this criterion pygmy shrews should be as intelligent as us! Evidently it is no criterion.

Sagan’s two remaining criteria, the quantitative change in the number of neural connections leading on to the qualitative change needed for the development of different specialisms by the brain are probably more relevant. Regrettably, these are not easy to measure, though some clues can be had from casts of the interior of skulls. Bulges in certain parts of the brain can be related to the development of certain functions—like speech.

Originally posted 2009-06-04 23:35:26. Republished by Blog Post Promoter

Filed under Anthroposaurus sapiens, Dinosaurs, Hominids, Homo. Tagged , , ,
Comment

Could Sauropods have “Farted Themselves to Extinction” (Fox News)?

12 May 2012, 1:49 am

“Dinosaurs may have farted themselves to extinction”, was the headline of Fox News, that media guardian of truth and enlightenment, to be echoed by other similar media like the Daily Mail and others keen to jump on the bandwagon. Since emissions from cows and other livestock contribute greenhouse gases to our warming atmosphere, British scientists, David Wilkinson, Euan Nisbet and Graeme Ruxton, wondered if sauropods might have had a similar effect on the Mesozoic world.

Sauropods

Brian Switek at the Dinosaur Tracking blog explained that Wilkinson et al did not say anything about dinosaur extinction in their paper. Carrying out some rough calculations based on various assumptions about sauropod habits and populations, and the release of methane by small herbivorous mammals like rabbits and guinea pigs, the authors decided that these giant, long necked dinosaurs would have produced 520 million metric tons of methane each year, comparable, they guessed, to the amount of methane being pumped into the atmosphere each year today. So much dinosaur flatulence must have added to greenhouse gases from other sources to sustain the warm world the dinosaurs inhabited.

Besides that, the extinction headline idea does not make any sense. Titanic sauropods were around for about 130 million years. If their cloacal eruptions were so deadly, why did it take so long for them to be overwhelmed? The Fox News headline and lead are outright lies. Moreover, Switek notes that this research:

…relies on a stack of assumptions and is, at best, a rough model. We don’t know what the gut flora of sauropods was like; therefore, we don’t know whether they farted at all.

The small, mammalian herbivores are unlikely to be the best models for sauropod emissions. Perhaps hippos and elephants would have been better. Birds (modern dinosaurs), in general, do not fart much, and nor do crocodiles, the closest living cousins of dinosaur predecessors. The whole work is an exercise in speculation, Switek concludes.

Filed under Dinosaurs, Environment, Extinction, Science. Tagged , , , ,
Comment

Display, Fighting and Flying

5 May 2012, 2:19 pm

Archaeopteryx

Archaeopteryx

Although Ostrom suggested archaeopterygid flight feathers developed as insect traps in fast running and leaping therapods, few people now take the idea seriously. The alternative, variants on some dinosaurs finding feathers helpful in parachuting from branch to branch in trees, is still held by many. Whether flight feathers developed in a tree dwelling creature or a running creature, the idea would work better if a proto bird already had long, strong feathers in the right places and already had powerful arm movements. Both could have evolved out of aggressive territorial display as suggested by Cowan and Lipps and described in Cowan’s book History of Life published by Blackwell Science in 1994.

A strong wing flap, directed forward and downward, is the power stroke that gives lift to a bird in takeoff. According to Lipps and Cowan strong wing flapping is a simple extension of aggressive display flapping. The proto bird will have held out its feathered arms and flapped them in territorial or otherwise aggressive display. But a threatening display could not have been an empty bluff. A rival might have called the bluff and fighting is the ultimate deterrent. The effectiveness of such displays in driving off the rival must therefore have linked to genuine fighting power and so would have selected stronger forelimbs and faster movement of them. Longer arms and the active waving or flapping of them would have evolved, and such flapping would have encouraged the evolution of powerful pectoral muscles.

Powerful flapping at the rival would have incidentally lifted the proto bird off the ground, allowing it to rake its opponent from above with its hind claws or thrust at it with its pointed snout. They might have looked rather like fighting cockerels. Hacking with clawed feet or pecking with the evolving hard beak would have precluded the need for clawed wings so the claws on the wings of primitive birds would have atrophied. The more rapidly the wings could be lifted for another blow, the more effective the fighting, encouraging a rapid wing-lifting motion that minimized air resistance, so the wing action would then be almost identical to a takeoff stroke.

A few modern birds use their wings as weapons. The steamer ducks of the South Atlantic live in shoreline habitats where food is plentiful all year round. They are large, powerful birds with heavy, bright orange, horny knobs, on the wings of both sexes, which they use in display and fighting.

Display and fighting in birds take a lot of energy, but only for short seasons, and yet provide an enormous payoff in survival and selection. The features selected for effective fighting and threatening would also have become sexually selected and been flaunted in courtship dancing, such as many birds still do, accelerating their rate of evolution. New behaviors through sexual selection are quick to evolve, and they are evolutionarily cheap because they usually do not require any important morphological changes in their early stages. Bowerbirds, for example, show distinct behavioral differences in display between closely related and very similar species. Plumage has become essential to the success of some bird species for purely sexual reasons, as in peacocks and birds of paradise.

So threatening display and fighting were selective agents that encouraged the evolutionary transition from small dinosaurs to birds. But most evolving birds came to a stage when actual fighting became counter productive. Display and sometimes fighting are important to bird species, but because the penalty for wing injury is high, many birds are intimidated by display into yielding rather than fighting. Once the evolution had got to the stage where flapping could lift the contenders out of harm’s way as well as offering them an elevated position for striking with claw or beak, some of the pugilists would have opted for the more secure but less aggressive option. The aggressive individuals would more often have damaged their newly evolving features and failed to reproduce as often as their more circumspect rivals. Flight then might have literally evolved as a flight mechanism for the poltroons of the proto bird world.

Archaeopteryx

In territorial encounters, living birds fight on the ground not on the wing, even those that fly well, suggesting that the practice evolved before the birds took to flight. The display hypothesis suggests that a proto bird gained flight behavior, anatomy, and experience at low ground speed and low height, ideal preflight training. The selective payoff for successful mastery of the flight motions gave significant advantages, even before flight itself was possible. From that point, the many advantages of flight were added to those of social or sexual competition.

Archaeopteryx fits this hypothesis well. It was well adapted both for display and, like any small theropod, for fighting having sharp teeth and claws on hands and feet. Archaeopteryx did not have long primary feathers on its fingers, probably because they would have hidden the claws in display and would most likely have broken in a fight.

But did Archaeopteryx Fly? No. Lipps and Cowan envisage Archaeopteryx as a small, fierce predator, capable of liftoff but not true flight. It was a fierce little fast-running, displaying beast, which probably spent its life scurrying around the Solnhofen shore, hunting for small prey such as crustaceans, reptiles, and mammals. Cowan thinks Archaeopteryx was, in hunting style, like the roadrunner of the dry country of the American Southwest , but its ecological setting was that of a steamer duck—a shoreline wader with year-round food supply. Archaeopteryx did not compete in the air with the pterosaurs that are also found in the Solnhofen Limestone.Once liftoff was achieved, flapping flight quickly followed. There is no need to suggest any difficult evolutionary sequence to complete the final transition to full powered flight.

In more advanced birds than Archaeopteryx, the pulley system of the shoulder evolved for quick wing upstrokes and the wishbone became a spring. The breastbone strengthened as the anchor for the flight muscles. Forearms became longer, lighter, and more fragile in bone structure, becoming specialized as wings, and losing the finger claws. Feathers became more aerodynamically suited to powerful swishes through air. Meanwhile, the feet and beak became the dominant fighting weapons, as in most living birds today.

Thus Cowan and Lipps threat display hypothesis for the origin of flight is is fully compatible with the morphology of Archaeopteryx and the biology of living birds.

Originally posted 2009-05-27 22:27:36. Republished by Blog Post Promoter

Filed under Birds, Dinosaurs. Tagged ,
2 Comments

Late Cretaceous Loss of Diversity in Dinosaurs

2 May 2012, 11:38 pm

Palaeontologist, Paul Barrett, a dinosaur expert at the Natural History Museum, found that most species, such as those in the theropod group, like Baryonyx and T rex, had already declined millions of years before the asteroid impact at the end of the Cretaceous period. He realized this because his team were studying how species diversity appeared in the fossil record, but the judging of species variation is complicated by the ease of fossilization, the chances of their survival, and the population of species at the time. In fact, at that time in Earth”s history plenty of rock and fossils were preserved, so the diversity decline was far from obvious, the large amount of preserved rock tending to suggest a high diversity of dinosaurs. Nevertheless, such diversity did not show up.

A clearer view is possible by looking at the change in diversity within a given dinosaur group over time. variability indicates the health of a stock, and loss of it, the opposite, health here meaning fitness, for more variation gives species better chances of being able to survive environmental changes. Such species are also therefore able to diversify into more environmental niches, thereby evolving into new species. On the other hand, decreasing variability points towards a greater chance of extinction as the species are less equipped to adapt into new circumstances.

The researchers calculated morphological “disparity”—a measure of variability of body parts—for seven major dinosaur groups using databases that include wide ranging characteristics about the intricate skeletal structure of nearly 150 different species. Among them, the hadrosaurs showed different levels of disparity in different locations. During the late Cretaceous, while declining in North America, the disparity of this dinosaur group seems to have been increasing in Asia. In North America, extreme fluctuations of the inland Western Interior Sea and mountain building might have affected the evolution of dinosaurs in distinct ways from species on other continents. Therefore, the authors say, the North American record might not be representative of a global pattern, if one exists.

Changes in Morphological Diversity in Late Cretaceous Dinosaurs

Hadrosaurs and ceratopsids, two groups of large bodied, bulk feeding herbivores—animals, like modern cows, that did not feed selectively—may have experienced a decline in biodiversity in the twelve million years before the dinosaurs ultimately went extinct. In contrast, small herbivores (ankylosaurs and pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and coelurosaurs), and enormous herbivores without advanced chewing abilities (sauropods) remained relatively stable or even slightly increased in biodiversity. Steve Brusatte of Columbia university said:

Contrary to how things are often perceived, the Late Cretaceous wasn”t a static “lost world” that was violently interrupted by an asteroid impact. Some dinosaurs were undergoing dramatic changes during this time, and the large herbivores seem to have been mired in a long-term decline, at least in North America.

But he noted there is no way to tell whether a declining dinosaur group would have survived if the asteroid had not struck Earth. Mark Norell, chair of the American Natural History Museum”s Division of Paleontology, added:

Even if the disparity of some dinosaur clades or regional faunas were in decline, this does not automatically mean that dinosaurs were doomed to extinction. Dinosaur diversity fluctuated throughout the Mesozoic, and small increases or decreases between two or three time intervals may not be noteworthy within the context of the entire 150 million year long history of the group.

Filed under Dinosaurs, Evolution, Extinction. Tagged , , , , , ,
Comment
« Older Entries