These next, fairly short articles are about evidence from the FOSSIL RECORD, plus a few other bits about new studies going on in evolution today. These are very interesting to remember when anyone asks you the questionâ€¦â€¦ Where is your evidence?
"Fossils offer crucial clues for evolution, because they reveal the often remarkable forms of creatures long vanished from Earth. Some of them even document evolution in action, recording creatures moving from one environment to another.
Whales, for example, are beautifully adapted to life in water, and have been for millions of years. But, like us, they are mammals. They breathe air, and give birth to and suckle live young. Yet there is good evidence that mammals originally evolved on land. If that is so, then the ancestors of whales must have taken to the water at some point.
As it happens, we have numerous fossils from the first ten million years or so of whale evolution. These include several fossils of aquatic creatures such as Ambulocetus and Pakicetus, which have characteristics now seen only in whales - especially in their ear anatomy - but also have limbs like those of the land-living mammals from which they are clearly derived. Technically, these hybrid creatures were already whales. What was missing was the start of the story: the land-living creatures from which whales eventually evolved.
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Work published in 2007 might have pinpointed that group. Called raoellids, these now-extinct creatures would have looked like very small dogs, but were more closely related to even-toed ungulates - the group that includes modern-day cows, sheep, deer, pigs and hippos. Molecular evidence had also suggested that whales and even-toed ungulates share a deep evolutionary connection.
The detailed study, by Hans Thewissen at Northeastern Ohio Universities Colleges of Medicine and Pharmacy in Rootstown and his colleagues, shows that one raoellid, Indohyus, is similar to whales, but unlike other even- toed ungulates in the structure of its ears and teeth, the thickness of its bones and the chemical composition of its teeth. These indicators suggest that this raccoon-sized creature spent much of its time in water. Typical raoellids, however, had a diet nothing like those of whales, suggesting that the spur to take to the water may have been dietary change."
This study demonstrates the existence of potential transition forms in the fossil record. Many other examples could have been highlighted, and there is every reason to think that many others await discovery, especially in groups that are well represented in the fossil record." Gets you thinking, doesn't it?
From water to land
"The animals we are most familiar with are tetrapods - they are vertebrates (they have backbones) and they live on land. That includes humans, almost all domestic animals and most of the wild ones that any child would recognize: mammals, birds, amphibians and reptiles. The vast majority of vertebrates, however, are not tetrapods, but fish. There are more kinds of fish, in fact, than all the species of tetrapods combined. Indeed, through the lens of evolution, tetra pods are just one branch of the fish family tree, the members of which just happen to be adapted for life out of water.
The first transition from water to land took place more than 360 million years ago. It was one of the most demanding such moves ever made in the history of life. How did fins become legs? And how did the transitional creatures cope with the formidable demands of land life, from a desiccating environment to the crushing burden of gravity?
It used to be thought that the first land lubbers were stranded fish that evolved to spend more and more time ashore, returning to water to reproduce. Over the past 20 years, palaeontologists have uncovered fossils that have turned this idea upside down. The earliest tetrapods, such as Acanthostega from eastern Greenland around 365 million years ago, had fully formed legs, with toes, but retained internal gills that would soon have dried out in any long stint in air. Fish evolved legs long before they came on land. The earliest tetrapods did most of their evolving in the more forgiving aquatic environment. Coming ashore seems to have been the very last stage.
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Researchers suspect that the ancestors of tetrapods were creatures called elpistostegids. These very large, carnivorous, shallow-water fish would have looked and behaved much like alligators, or giant salamanders. They looked like tetrapods in many respects, except that they still had fins. Until recently, elpistostegids were known only from small fragments of fossils that were poorly preserved, so it has been hard to get a rounded picture of what they were like.
"In the past couple of years, several discoveries from Ellesmere Island in the Nunavut region of northern Canada have changed all that. In 2006, Edward Daeschler and his colleagues described spectacularly well- preserved fossils of an elpistostegid known as Tiktaalik that allow us to build up a good picture of an aquatic predator with distinct similarities to tetrapods - from its flexible neck, to its very limb-like fin structure. The discovery and painstaking analysis of Tiktaalik illuminates the stage before tetrapods evolved, and shows how the fossil record throws up surprises, albeit ones that are entirely compatible with evolutionary thinking." These cold, quasi-arctic environments helped greatly in discovering the more preserved fossils found in science today.
In fact, there are amazing finds that contribute to mans knowledge of evolution through finding remains in ice. The best-known fossil is that of an 'ice-man" who was found in an Alpine glacier and is 5,000 years old. Mammoths and 60,000 year-old bison bones have given researchers DNA fragments and bone proteins that can be used for comparing them to the modern species. "Are we having fun yet?"
The origin of feathers
"One of the objections to Charles Darwin's theory of evolution was the lack of 'transitional forms' in the fossil record - forms that illustrated evolution in action, from one major group of animals to another. However, hardly a year after the publication of On the Origin of Species, an isolated feather was discovered in Late Jurassic (about 150 million years old) lithographic limestones of Solnhofen in Bavaria, followed in 1861 by the first fossil of Archaeopteryx, a creature with many primitive, reptilian features such as teeth and a long, bony tail - but with wings and flight feathers, just like a bird.
Although Archaeopteryx is commonly seen as the earliest known bird, many suspected that it was better seen as a dinosaur, albeit one with feathers. Thomas Henry Huxley, Darwin's colleague and friend, discussed the possible evolutionary link between dinosaurs and birds, and palaeontologists speculated, if wildly, that dinosaurs with feathers might one day be found.
In the 1980s, deposits from the early Cretaceous period (about 125 million years ago) in the Liaoning Province in northern China vindicated these speculations in the most dramatic fashion, with discoveries of primitive birds in abundance - alongside dinosaurs with feathers, and feather-like plumage. Starting with the discovery of the small theropod Sinosauropteryx by Pei-ji Chen from China's Nanjing Institute of Geology and Paleontology and his colleagues, a variety of feather-clad forms have been found. Many of these feathered dinosaurs could not possibly have flown, showing that feathers first evolved for reasons other than flight, possibly for sexual display or thermal insulation, for instance. In 2008, Fucheng Zhang and his colleagues from the Chinese Academy of Sciences in Beijing announced the bizarre creature Epidexipteryx, a small dinosaur clad in downy plumage, and sporting four long plumes from its tail. Palaeontologists are now beginning to think that their speculations weren't nearly wild enough, and that feathers were indeed quite common in dinosaurs.
The discovery of feathered dinosaurs not only vindicated the idea of transitional forms, but also showed that evolution has a way of coming up with a dazzling variety of solutions when we had no idea that there were even problems. Flight could have been no more than an additional opportunity that presented itself to creatures already clothed in feathers."
The evolutionary theory of teeth
"One motivation in the study of development is the discovery of mechanisms that guide evolutionary change. Kathryn Kavanagh at the University of Helsinki and her colleagues investigated just this by looking at the mechanisms behind the relative size and number of molar teeth in mice. The research, published in 2007, uncovered the pattern of gene expression that governs the development of teeth - molars emerge from the front backwards, with each tooth smaller than the last.
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The beauty of the study lies in its application. Their model predicts the dentition patterns found in mouse- like rodent species with various diets, providing an example of ecologically driven evolution along a developmentally favoured trajectory. In general, the work shows how the pattern of gene expression can be modified during evolution to produce adaptive changes in natural systems.
We owe much of what makes us human to remarkable tissue, found only in embryos, called the neural crest. Neural-crest cells emerge in the developing spinal cord and migrate all over the body, effecting a remarkable series of transformations. Without the neural crest, we would not have most of the bones in our face and neck, or many of the features of our skin and sensory organs. The neural crest seems to be unique to vertebrates, and helps to explain why vertebrates have distinctive 'heads' and 'faces'.
Untangling the evolutionary history of the neural crest is especially hard in fossil forms, as embryonic data are obviously absent. One key mystery, for example, is how much of the vertebrate skull is contributed by neural- crest cells and how much comes from deeper layers of tissue.
New techniques have allowed researchers to label and follow individual cells as embryos develop. They have revealed the boundaries of the bone derived from the neural crest, down to the single-cell level, in the neck and shoulders. Tissue derived from the neural crest anchors the head onto the front lining of the shoulder girdle, whereas the skeleton forming the back of the neck and shoulder grows from a deeper layer of tissue called the mesoderm.
Such detailed mapping, in living animals, casts light on the evolution of structures in the heads and necks of animals long extinct, even without fossilized soft tissue such as skin and muscle. Skeletal similarities that result from a shared evolutionary history can be identified from muscle attachments. This allows the tracing of, for example, the location of the major shoulder bone of extinct land vertebrate ancestors, the cleithrum. This bone seems to survive as part of the shoulder blade (scapula) in living mammals.
This kind of evolutionary scan may have immediate clinical relevance. The parts of the skeleton identified by Toshiyuki Matsuoka from the Wolfson Institute for Biomedical Research in London and his colleagues as being derived from the neural crest are specifically affected in several developmental disorders in humans, providing insights into their origins.
Matsuoka's study shows how a detailed analysis of the morphology of living animals, informed by evolutionary thinking, helps researchers to interpret fossilized and now-extinct forms."
Natural selection in SPECIATION
"Evolutionary theory predicts that divergent natural selection will often have a key role in speciation. Working with sticklebacks (Gasterosteus aculeatus), Jeffrey McKinnon at the University of Wisconsin in Whitewater and his colleagues reported in 2004 that reproductive isolation can evolve as a by-product of selection on body size. This work provides a link between the build-up of reproductive isolation and the divergence of an ecologically important trait.
The study was done on an extraordinary geographical scale, involving mating trials between fish taken in Alaska, British Columbia, Iceland, the United Kingdom, Norway and Japan. It was underpinned by molecular genetic analyses that provided firm evidence that fish that have adapted to living in streams had evolved repeatedly from marine ancestors, or from fish that live in the ocean but return to fresh water to spawn. Such migratory populations in the study had larger bodies on average than did those living in streams. Individuals tended to mate with fish of a similar size, which accounts well for the reproductive isolation between different stream ecotypes and their close, seafaring neighbours.
Taking into account the evolutionary relationships, a comparison of the various types of stickleback, whether stream or marine, strongly supports the view that adaptation to different environments brings about reproductive isolation. The researchers' experiments also confirmed the connection between size divergence and the build-up of reproductive isolation - although traits other than size also contribute to reproductive isolation to some extent."
A case of co-evolution
"Species evolve together, and in competition. Predators evolve ever deadlier weapons and skills to catch prey, which, as a result of Darwin's canonical 'struggle for existence', become better at escaping them, and so the arms race continues. In 1973, evolutionary biologist Leigh Van Valen likened this to the Red Queen's comment to Alice in Lewis Carroll's Through the Looking Glass, "it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!" The 'Red Queen' hypothesis of co-evolution was born.
A problem with studying Red-Queen dynamics is that they can be seen only in the eternal present. Discovering their history is problematic, because evolution has generally obliterated all earlier stages.
Happily, Ellen Decaestecker from the Catholic University of Leuven in Belgium and her colleagues discovered a remarkable exception, in the co-evolutionary arms race between water fleas (Daphnia) and the microscopic parasites that infest them; their research was published in 2007. As the water fleas become better at evading parasitism, the parasites become better at infecting them. Both prey and predator in this system can persist in dormant stages for many years in the mud at the bottom of the lake they share. The sediments of the lake can be dated to the year they were formed, and the buried predators and prey can be revived. Thus, their interactions can be tested, against one another, and against predators or prey from their relative pasts and futures.
Confirming theoretical expectations, the parasite adapted to its host over a period of only a few years. Its infectivity at any given time changed little, but its virulence and fitness rose steadily - matched at each stage by the ability of the water fleas to resist them.
This study provides an elegant example in which a high-resolution historical record of the co-evolutionary process has provided an affirmation of evolutionary theory, showing that the interaction of parasites and their hosts is not set in time but is instead the result of a dynamic arms race of adaptation and counter-adaptation, driven by natural selection, from generation to generation."
Differential dispersion in wild birds
Gene flow caused, for instance, by migration, can disrupt adaptation to local conditions and oppose evolutionary differentiation within and between populations. Indeed, classical population genetics theory suggests that the more that local populations migrate and interbreed, the more genetically similar they will be. This concept seems to accord with common sense, and it assumes that gene flow is a random process, like diffusion. But non-random dispersal can actually favour local adaptation and evolutionary differentiation, as Ben Sheldon of the Edward Grey Institute of Field Ornithology in Oxford, UK, and his colleagues reported in 2005.
Their work was part of a multi-decade study of the great tits (Parus major) that inhabit a wood in Oxfordshire, UK. The researchers found that the amount and type of genetic variation in nestling weight in this songbird differs from one part of the wood to another. This pattern of variation leads to varying responses to selection in different parts of the wood, leading to local adaptation. The effect is reinforced by non-random dispersal; individual birds select and breed in different habitats in a way that increases their fitness. The authors conclude that "when gene flow is not homogeneous, evolutionary differentiation can be rapid and can occur over surprisingly small spatial scales".
In another study of great tits on the island of Vlieland in the Netherlands, published in the same issue of Nature, Erik Postma and Arie van Noordwijk from the Netherlands Institute of Ecology in Heteren found that gene flow, mediated by non-random dispersal, maintains a large genetic difference in clutch size at a small spatial scale, again illustrating, as these scientists put it, "the large effect of immigration on the evolution of local adaptations and on genetic population structure".
Selective survival in wild guppies
"Natural selection favours traits that increase fitness. Over time, such selection might be expected to exhaust genetic variation by driving advantageous genetic variants to fixation at the expense of less advantageous or deleterious variants. In fact, natural populations often show large amounts of genetic variation. So how is it maintained?
An example is the genetic polymorphism seen in the colour patterns of male guppies (Poecilia reticulata). As reported in 2006, Kimberly Hughes from the University of Illinois at Urbana-Champaign and her colleagues manipulated the frequencies of males with different colour patterns in three wild guppy populations in Trinidad. They showed that rare variants have much higher survival rates than more common ones. In essence, variants are favoured when rare, and selected against when common.
Such 'frequency-dependent' survival, in which selection favours rare types, has been implicated in the maintenance of molecular, morphological and health-related polymorphisms in humans and other mammals."
Out of all the articles in the ones I was researching here, this one proved, to me, the most interesting. I mentioned the controversy over micro and macro-evolution in a previous chapter. Micro-evolution has been readily documented, as smaller, incremental changes are relatively simple to research. "There is a mountain of evidence for such changes". Here is the article for you:
Darwin conceived of evolutionary change as happening in infinitesimally small steps. He called these 'insensible gradations', which, if extrapolated over long periods of time, would result in wholesale changes of form and function. There is a mountain of evidence for such small changes, called microevolution - the evolution of drug resistance, for instance, is just one of many documented examples.
We can infer from the fossil record that larger species-to-species changes, or macroevolution, also occur, but they are naturally harder to observe in action. That said, the mechanisms of macroevolution can be seen in the here-and-now, in the architecture of genes. Sometimes genes involved in the day-to-day lives of organisms are connected to, or are even the same as, those that govern major features of animal shape and development. So everyday evolution can have large effects.
Sean Carroll from the Howard Hughes Medical Institute in Chevy Chase, Maryland, and his colleagues looked at a molecular mechanism that contributes to the gain of a single spot on the wings of male flies of the species Drosophila biarmipes; they reported their findings in 2005. The researchers showed that the evolution of this spot is connected with modifications of an ancestral regulatory element of a gene involved in pigmentation. This regulatory element has, over time, acquired binding sites for transcription factors that are ancient components of wing development. One of the transcription factors that binds specifically to the regulatory element of the yellow gene is encoded by engrailed, a gene fundamental to development as a whole. This shows that a gene involved in one process can be co-opted for another, in principle driving macro-evolutionary change." Pretty cool, huh?
Here is another one of those 'theories-within-a-theory' I talked of before:
"Species can remain mostly unchanged for millions of years, long enough for us to pick up their traces in the fossil record. But they change, too, and often very suddenly. This has led some to wonder whether species - usually those developing along specific tracks - store the potential for sudden change 'under the hood', unleashing a flood of otherwise hidden variation at times of environmental stress - variation on which selection can act.
This idea of such 'evolutionary capacitance' was first mooted by Suzanne Rutherford and Susan Lindquist in startling experiments on fruitflies. Their idea was that key proteins involved in the regulation of developmental processes are 'chaperoned' by a protein called Hsp90 that is produced more at times of stress. On occasion, Hsp-90 is overwhelmed by other processes and the proteins it normally regulates are left to run free, producing a welter of otherwise hidden variation.
"Aviv Bergman from the Albert Einstein College of Medicine in New York and Mark Siegal at New York University explored whether evolutionary capacitance is particular to Hsp90 or found more generally; their study was published in 2003. They used numerical simulations of complex gene networks and genome- wide expression data from yeast strains in which single genes had been deleted. They showed that most, and perhaps all, genes hold variation in reserve that is released only when they are functionally compromised. In other words, it looks as if evolutionary capacitance might go wider and deeper than Hsp-90".
If you do a little research you will find that there is a MOUNTAIN of evidence out there to support the idea of macro and micro-evolution. In reality it would take a lifetime to study all that is involved in our complex 'family tree'. I, for one, am glad that we have the best and brightest minds delving into this, and more. One day we will have so much obvious evidence that theists will unanimously come together to support the idea of evolution.
Reason, and evidence, might win out after allâ€¦â€¦â€¦..