Evolutie. Deel XIV

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Op zondag 14 oktober 2007 14:44 schreef LB06 het volgende:
Wat zijn eigenlijk de verklaringen die biologen geven voor de macro-evolutie? D.w.z. van vis naar vogel? Bij biologie op het VWO heb ik min of meer geleerd dat dit een opstapeling van micro-mutaties onder druk van natuurlijke selectie is, maar ik heb laatst een boekje gelezen waarin werd beweerd dat dit niet mogelijk is en dat er dus een andere, al dan niet wetenschappelijke verklaring moet zijn hiervoor.

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Op zondag 14 oktober 2007 15:09 schreef Triggershot het volgende:
Ja, ja, ja ik weet het, strict wetenschappelijk, maar dees is leuk.
Ken Miller vs Harun Yahya

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Op zondag 14 oktober 2007 14:44 schreef LB06 het volgende:
Wat zijn eigenlijk de verklaringen die biologen geven voor de macro-evolutie? D.w.z. van vis dino naar vogel? Bij biologie op het VWO heb ik min of meer geleerd dat dit een opstapeling van micro-mutaties onder druk van natuurlijke selectie is, maar ik heb laatst een boekje gelezen waarin werd beweerd dat dit niet mogelijk is en dat er dus een andere, al dan niet wetenschappelijke verklaring moet zijn hiervoor.

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Writing Oct. 11, 2007 in the journal Nature, Howard Hughes Medical Institute investigator Sean B. Carroll and former UW-Madison graduate student Chris Todd Hittinger document how, over many generations, a single yeast gene divides in two and parses its responsibilities to be a more efficient denizen of its environment. The work illustrates, at the most basic level, the driving force of evolution.

"This is how new capabilities arise and new functions evolve," says Carroll, one of the world's leading evolutionary biologists. "This is what goes on in butterflies and elephants and humans. It is evolution in action."

The work is important because it provides the most fundamental view of how organisms change to better adapt to their environments. It documents the workings of natural selection, the critical idea first posited by Charles Darwin where organisms accumulate random variations, and changes that enhance survival are "selected" by being genetically transmitted to future generations.

The new study replayed a set of genetic changes that occurred in a yeast 100 million or so years ago when a critical gene was duplicated and then divided its nutrient processing responsibilities to better utilize the sugars it depends on for food.

"One source of newness is gene duplication," says Carroll.

"When you have two copies of a gene, useful mutations can arise that allow one or both genes to explore new functions while preserving the old function. This phenomenon is going on all the time in every living thing. Many of us are walking around with duplicate genes we're not aware of. They come and go."

In short, says Carroll, two genes can be better than one because redundancy promotes a division of labor. Genes may do more than one thing, and duplication adds a new genetic resource that can share the workload or add new functions. For example, in humans the ability to see color requires different molecular receptors to discriminate between red and green, but both arose from the same vision gene.

The difficulty, he says, in seeing the steps of evolution is that in nature genetic change typically occurs at a snail's pace, with very small increments of change among the chemical base pairs that make up genes accumulating over thousands to millions of years.

To measure such small change requires a model organism like simple brewer's yeast that produces a lot of offspring in a relatively short period of time. Yeast, Carroll argues, are perfect because their reproductive qualities enable study of genetic change at the deepest level and greatest resolution because researchers can produce and quickly count a large number of organisms. The same work in fruit flies, one of biology's most powerful models, would require "a football stadium full of flies" and years of additional work, Carroll explains.

"The process of becoming better occurs in very small steps. When compounded over time, these very small changes make one group of organisms successful and they out-compete others," according to Carroll.

The new study involved swapping out different regions of the yeast genome to assess their effects on the performance of the twin genes, as well as engineering in the gene from another species of yeast that had retained only a single copy.

"We retraced the steps of evolution," the Wisconsin biologist explains.

The work shows in great detail how the ancestral gene gained efficiency through duplication and division of labor.

"They became optimally connected in that job. They're working in cahoots, but together they are better at the job the ancestral gene held," Carroll says. "Natural selection has taken one gene with two functions and sculpted an assembly line with two specialized genes."

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Charlesworth, J. and Eyre-Walker, A. (2007) The other side of the nearly neutral theory, evidence of slightly advantageous back-mutations. PNAS, advanced online publication.

We argue that if there is a category of slightly deleterious mutations, then there should be a category of slightly advantageous back-mutations. We show that when there are both slightly deleterious and advantageous back-mutations, there is likely to be an increase in the rate of evolution after a population size expansion. This increase in the rate of evolution is short-lived. However, we show how its signature can be captured by comparing the rate of evolution in species that have undergone population size expansion versus contraction. We test our model by comparing the pattern of evolution in pairs of island and mainland species in which the colonization event was either island-to-mainland (population size expansion) or mainland-to-island (contraction). We show that the predicted pattern of evolution is observed.

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Bergthorsson, U. et al. (2007) Ohno's dilemma: Evolution of new genes under continuous selection. PNAS, advanced online publication.

New genes with novel functions arise by duplication and divergence, but the process poses a problem. After duplication, an extra gene copy must rise to sufficiently high frequency in the population and remain free of common inactivating lesions long enough to acquire the rare mutations that provide a new selectable function. Maintaining a duplicated gene by selection for the original function would restrict the freedom to diverge. (We refer to this problem as Ohno's dilemma). A model is described by which selection continuously favors both maintenance of the duplicate copy and divergence of that copy from the parent gene. Before duplication, the original gene has a trace side activity (the innovation) in addition to its original function. When an altered ecological niche makes the minor innovation valuable, selection favors increases in its level (the amplification), which is most frequently conferred by increased dosage of the parent gene. Selection for the amplified minor function maintains the extra copies and raises the frequency of the amplification in the population. The same selection favors mutational improvement of any of the extra copies, which are not constrained to maintain their original function (the divergence). The rate of mutations (per genome) that improve the new function is increased by the multiplicity of target copies within a genome. Improvement of some copies relaxes selection on others and allows their loss by mutation (becoming pseudogenes). Ultimately one of the extra copies is able to provide all of the new activity.

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NIJMEGEN - Nederlandse en IJslandse onderzoekers hebben min of meer bij toeval de genetische varianten gevonden, die bepalen welke oog-, haar- en huidskleur een Europees mens heeft. Tot nu toe kon op basis van DNA-materiaal alleen geslacht en ras van een persoon worden vastgesteld.

De onderzoeksgroep, die in Nederland is gevestigd in het Universitair Medisch Centrum St Radboud (UMC) in Nijmegen, is eigenlijk op zoek naar genetische determinanten van prostaat- en borstkanker. In dit onderzoek zijn toevallig de genvarianten aangetroffen, die uiterlijke kenmerken van Europees mensen bepalen.

De vondst van de genetische variant is volgens de onderzoekers onder meer belangrijk voor onderzoek naar misdrijven. Uitsluitend op basis van aangetroffen DNA-materiaal kan nu behalve het geslacht worden vastgesteld welke haarkleur de verdachte heeft, wat voor soort huid en welke kleur ogen.

Eén gevonden genetische variant bepaalt het in Europa veelvoorkomende blonde haar met blauwe ogen. Een tweede variant zorgt voor een lichte huid met sproeten en bruin haar. Er zijn tevens varianten gevonden die zorgen voor sproeten, een huid die gemakkelijk verbrandt en blauwe ogen. Eerder waren al genen gevonden die rood haar, een lichte huid en sproeten veroorzaken.

De onderzoekers stellen dat er veel genetisch materiaal is betrokken bij het bepalen van de kleuren die iemands uiterlijk heeft. Dat verklaart tevens waarom het Europese ras zoveel verschillende types telt.

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Pigliucci, M. (2007) Do we need an extended evolutionary synthesis. Evolution, advanced online publication

The Modern Synthesis (MS) is the current paradigm in evolutionary biology. It was actually built by expanding on the conceptual foundations laid out by its predecessors, Darwinism and neo-Darwinism. For sometime now there has been talk of a new Extended Evolutionary Synthesis (EES), and this article begins to outline why we may need such an extension, and how it may come about. As philosopher Karl Popper has noticed, the current evolutionary theory is a theory of genes, and we still lack a theory of forms. The field began, in fact, as a theory of forms in Darwin's days, and the major goal that an EES will aim for is a unification of our theories of genes and of forms. This may be achieved through an organic grafting of novel concepts onto the foundational structure of the MS, particularly evolvability, phenotypic plasticity, epigenetic inheritance, complexity theory, and the theory of evolution in highly dimensional adaptive landscapes.

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Let me again be clear on a fundamental point underlying this whole discussion: one can reasonably argue that none of this contradicts any tenet of the MS, although it seems to me at least reasonable to concede that the new concepts and empirical findings I have briefly outlined above may eventually force a shift of emphasis away from the population genetic-centered view of evolution that characterizes the MS. On the other hand, to attempt to go further and state that there is not much new here and that all of this is already part of the MS, implicitly or not, would be intellectually disingenuous and historically inaccurate.

What I think will happen over the next several years is a process analogous to the MS itself: not a paradigm shift a la Kuhn (1970), but another series of complex developments that build on its three major predecessors: Darwinism, neo-Darwinism, and the MS itself. This is not the place for a philosophical analysis of whether paradigm shifts ever happen, even in astronomy and physics—Kuhn's original sources for his examples. Nevertheless, I argue that biology has, in fact, never undergone a true paradigm shift, and very likely never will. If one really wishes to identify anything in biology equivalent to the move from the Ptolemaic to the Copernican system, then one has to go back in time before Darwin: we have to reach as far back as the early 19th century transformation from a natural theological to a natural philosophical understanding of the history and diversity of life, to which Darwin greatly contributed but that he did not initiate, and consider the earlier evolutionary works of Lamarck, Chambers, and Erasmus Darwin. Perhaps truly incommensurable paradigm shifts only happen when a proto-science turns into a mature science, as astronomy did during the 16th and early 17th centuries (Copernicus and Galileo), physics during the 17th and 18th centuries (Galileo and Newton), and both biology and geology during the 19th century (Darwin and Lyell). Once a science is established, conceptual frameworks tend to expand, more than being replaced, and even when replacement can be argued for (as in the transition from Newtonian to relativistic mechanics at the onset of the 20th century), the new model is not incommensurable with the older, as the latter can be interpreted as a limit case or approximation of the new and broader one.

Be that as it may, the proof for the EES is in the pudding: it is up to those who advocate a significant expansion of theoretical biology (in the broader sense of conceptual understanding of the discipline's foundations) to show tangible progress. It would be a shame, however, to curtail or dismiss the discussion in its infancy. Evolutionary biologists, whether they advocate the MS or the EES, have a lot of work to do to make sense of a rapidly growing amount of findings in molecular, developmental, and organismal biology. Instead of looking forward to retirement, we should appreciate what a truly exciting time is awaiting us all.

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Op woensdag 24 oktober 2007 14:49 schreef Monolith het volgende:
Tijd voor een hernieuwd raamwerk in de evolutionaire biologie?
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Conclusie van het artikel:
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Interessante ontwikkeling m.i.

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Op woensdag 24 oktober 2007 15:01 schreef Papierversnipperaar het volgende:

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Ik mis de dramatiek. We gaan verder waar we gebleven waren, zo te lezen. Mis ik iets?

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Op zondag 28 oktober 2007 22:00 schreef Triggershot het volgende:
'Menselijk ras evolueert in twee soorten'

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Op zondag 28 oktober 2007 22:00 schreef Triggershot het volgende:
'Menselijk ras evolueert in twee soorten'

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In the summer of 2006 I was commissioned by Bravo Television to write an essay on the future of human evolution. The essay was intended as a 'science fiction' way of illustrating some aspects of evolutionary theory.

Bravo then sent out a press release on the essay, but did not release the essay itself. As a result, a wildly distorted version of what I had written ended up being reported as ‘science fact’ in the media. I do not endorse the content of these media reports. Here's the actual essay.

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Divided by oceans of empty space, our distant descendants would find themselves in a similar situation. We should expect to see different species of humans adapting to their local planetary environments. Descendants who end up on big, slow, heavy, dark cold planets might come to resemble modern day Inuits: short, stocky, with slow metabolisms, longer circadian rhythms, and pale skin. Those who end up on small, fast, light, warm planets might come to resemble the fabled Nubians: tall, thin, with fast metabolisms, shorter circadian rhythms, and darker skin. In all cases, we should expect the different
populations of humans to adapt their metabolism to the peculiarities of the local foodstuffs, and the local atmosphere; and to adapt their immune systems to local diseases. Under these conditions we would see the evolution of new species of humans, perhaps named after their celestial location: perhaps Homo andromeda, Homo betelgeuse. Our own species, Homo sapiens, would be long gone.

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The future
So, these might be some of the highs and lows of human evolution over the next million years. But remember, that there is nothing inevitable about the evolutionary process. It is not aiming at anything, it has no goals, and no inertia. What evolves will be a product of the mutations that arise, and the circumstances in which they find themselves. To the extent that we will be able to invent new genes, and to create our own conditions, we have some control over our future. In this sense, the future is not a place that we will go to, it is a thing that we will build.

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Modern-day birds have a highly specialised respiratory system, made up of a small rigid lung and around nine air sacs.

The bellows-like movement of the sternum and ribs moves air through the system.

Bony projections on the ribcage known as uncinate processes play an important role in both respiration and locomotion.

The small bones act as levers to move the ribs and sternum during breathing. They have become adapted in different types of birds to deal with different ways of getting around.

The bones are shortest in runners like emus that don't need large breast muscles for flight, intermediate in flying birds and longest in divers such as the penguin.

The Manchester team studied a wealth of fossil remains of dinosaurs and extinct birds such as Archaeopteryx, and compared these with skeletons of living birds.

They found that uncinate processes are also found both in the extinct ancestors of birds, the theropod dinosaurs, and in modern species.

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Op zaterdag 10 november 2007 20:37 schreef Karmijne het volgende:
Wat verklaart het blonde haar van Europeanen en dan met name Scandinaviers?

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Op zaterdag 10 november 2007 20:47 schreef Iblardi het volgende:

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Dat was een tijdje terug in het nieuws: het zou om een vrij recente (c. 10.000 jaar geleden) aanpassing gaan die het gevolg was van strengere seksuele selectie van vrouwelijke partners. Zie http://www.timesonline.co.uk/tol/news/uk/article735078.ece.

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Op zaterdag 10 november 2007 20:47 schreef Iblardi het volgende:

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Dat was een tijdje terug in het nieuws: het zou om een vrij recente (c. 10.000 jaar geleden) aanpassing gaan die het gevolg was van strengere seksuele selectie van vrouwelijke partners. Zie http://www.timesonline.co.uk/tol/news/uk/article735078.ece.

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Op zaterdag 10 november 2007 20:55 schreef Karmijne het volgende:

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hmm, hij opent niet...

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Op zaterdag 10 november 2007 21:12 schreef Iblardi het volgende:

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Probeer het eens zo: http://www.timesonline.co.uk/tol/news/uk/article735078.ece
Macht der gewoonte, had afgesloten met een punt.