Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
A new poll, cited in an online editorial from a Louisiana website, shows this:
. . . . .40 percent of the respondents believe evolution is not well-supported by evidence or generally accepted within the scientific community. Only 39 percent of the respondents said they believed evolution is well-supported by evidence. Twenty-one percent said they did not know.
No real surprise here — these statistics are in line with national polls in America.
P. Z. Myers, the beloved (and also despiséd) author of the popular science blog Pharyngula, has started producing a column on the Guardian website. His first column is on asymmetry in animals — in particular the gene nodal, which sets up directional (left-right) asymmetries in animals. P. Z. points out recent research (reference below) showing that snails, who have directionally coiled shells, lose the directionality when nodal is inactivated. The asymmetry of the human body is also generated in ways similar to that of snails, and again nodal plays a key role. The gene is somehow involved in determining the directionality of the way cilia (small hairs) beat in the early embryo, which sets up a concentration gradient that can make an embryo left- or right-handed.
I’ve always been fascinated by directional asymmetries — those traits that occur on a consistent side (right or left) in a species. These include the side of the body that harbors the narwhal’s tusk. and our own internal organs. Other such traits include the direction in which the ears of an owl are turned, and what side of its body a flounder comes to rest on when it changes into a bottom-dweller. Directional asymmetries are not rare in animals. But how are they formed? How does a gene “know” it’s on the right or the left? The finding that the direction of cilia movement can tell a gene which side it’s on goes a long way to solving this question, but still leaves open the final question: why do cilia spin in a given direction? How do they know whether to go clockwise or counterclockwise? This may, ultimately, reside in the asymmetry of molecules that make up cilia.
At any rate, P. Z.’s column is good, but his explanation of nodal on Pharyngula is even better. P. Z. has a real talent for explaining science clearly and engagingly, and too often this is overlooked by the hordes of people who visit his blog for the controversy, atheism, and trenchant attacks on religion. (One thing I’ve found from writing this blog is that visits are much more numerous when I’m attacking something than when I’m talking about science, a fact that’s a little bit sad.)
But the point I wanted to make relates to an earlier post I made about what counts as evidence for evolution. P. Z.’s elegant description of how nodal works was hijacked by the Guardian editors by putting it under the title:
Lopsided gene that proves
humans are distant cousins
of the humble snail
A gene shared by birds, fish, reptiles, people – and snails – reveals the fundamental relatedness of all living creatures
Well, we already knew, of course, that we were distant cousins of the humble snail. We don’t need nodal to tell us that. And the observation that the gene has similar functions in humans and snails is not, to me, dispositive evidence that humans and snails are related. After all, creationists could always say, “Well of course the gene does the same thing in humans and snails! That’s just the way the Creator decided to make asymmetries! It doesn’t say anything about common ancestry.” As I’ve mentioned before, the fact that related creatures use similar genes to do similar things does not count as strong evidence for evolution as opposed to a creationist/intelligent-design alternative. We might as well say that snails have a gene producing cytochrome c as part of their metabolic pathway, and proclaim that this “proves that humans are distant cousins of the humble snail.” We share hundreds of genes with the humble snail.
The choice of what to emphasize in a headline is the editors’, not P. Z.’s. And I suppose anything touting evolution is a good thing for readers. Still, the Guardian editors should realize that hundreds and hundreds of genes already testify to common ancestry — if you choose to use genic similarity as evidence. I prefer to look at dead genes that are active in relatives as far stronger evidence for evolution against the creationist alternative.
Anyway, congrats to P. Z. for his new gig and a good inaugural column.
Reference: Grande, C., and N. H. Patel. 2009. Nodal signalling is involved in left–right asymmetry in snails. Nature 457:1008-1011.
I’m back, with lots to say, but lots of catching up to do on the day job. Let me first thank Matthew Cobb for a terrific job of filling in. His students get the benefit of his omnivorous readings in the form of a Z (zoology)-letter he sends out weekly, detailing all sorts of interesting animal stuff.
For today, until I shovel myself out from under, I post something for a belated Happy Easter. In WEIT I describe the convergences between marsupial and placental mammals, resemblances that imply that some niches antedate the animals who have evolved to fill them. Although the Australian bilby looks like a rabbit, it isn’t really herbivorous but omnivorous, although it does burrow. There used to be two species, the greater and the lesser bilby (the word “bilby” is aboriginal), but the lesser appears to be extinct. The greater bilby, Macrotis lagotis, is highly endangered due to habitat loss and predation by, among other species, feral cats; you can read about its precarious status here. Only a few hundred remain in the wild. To save the animal, extensive efforts are underway; these include widespread annual sale of chocolate Easter bilbies, which provide revenues for conservation. (In WEIT I mistakenly say “Each spring, chocolate bilbies fill the shelves of Australian supermakets. . .”, and was roundly taken to task by Aussies who pointed out, rightly, that the Australian Easter occurs in the fall.)
So, belatedly, here are some baby bilbies from down under, and the chocolate replicas that are helping save them:
Chocolate bilbies (buy them here):
NB: Goofed again. I am informed that in Australia the penultimate season is called “autumn,” not fall.
by Matthew Cobb
Sex is an odd business. In some animals, like us, sex is determined by which combination of a pair of chromosomes the individual carries. Males are XY, females are XX. In birds (and butterflies, for some reason) things are the other way round – males are ZZ, females are ZW.
Sex determination by specific chromosomes is not the rule, not was it the ancestral state – in both plants and mammals it appears to be a relatively recent invention. Zsex can also be determined by overall chromosome number (eg ants and bees), and in some reptiles, like crocodiles, sex is determined by the temperature at which eggs are incubated. Some species are hermaphrodite, while others can change their sex in response to the social or environmental changes or the action of a parasite.
In those species that do have chromosomally-based sex determination (like us), there’s nothing particularly special about the sex chromosomes – they were originally just like the other chromsomes (“autosomes”). But as time goes on, the chromosome that cannot exchange genetic material (the Y chromosome in humans, or the W chromosome in birds) gradually loses its genes. In humans the Y chromosome used to have over 1,000 genes. Now it just has a few dozen. How does the corresponding X chromosome cope with the declining number of genes in its opposite number?
In the fruitfly species Drosophila miranda, a new X chromosome has recently been formed (the “neo-X”). A recent study by Doris Bachtrog and colleagues from Berkely, published in PLoS Biology, has looked at the genes on the neo-X and compared them with those on the ancestral X chromosome. They found clear signs that the genes on the neo-X had recently been the subject of intense selection, as they adapted to their interaction with the Y chromosome. The authors conclude:
“Thus, newly formed X chromosomes are not passive players in the evolutionary process of sex chromosome differentiation, but respond adaptively to both their sex-biased transmission and to Y chromosome degeneration, possibly through demasculinization of their gene content and the evolution of dosage compensation.”
As well as providing a fascinating example of how genes and chromosomes interact to form individuals, this kind of genetic study poses a massive problem for all those who refuse to accept the facts of evolution. What other explanation is there, but that genes, and populations, evolve over time? The only other interpretation is that these signatures of selection were put in the fly’s DNA by the Creator as a whim, a joke, or a way of testing our faith…
This is my last post as the vacation blogger – Jerry is back on dry land tomorrow. Thanks for the comments, and thanks for reading!
Citation: Bachtrog D, Jensen JD, Zhang Z (2009) Accelerated Adaptive Evolution on a Newly Formed X Chromosome. PLoS Biol 7(4): e1000082. Open Access here.
by Matthew Cobb
It’s often said that we share 99% of our DNA with our closest relatives, the chimpanzees. The evolutionary biologist and science writer, Jared Diamond, memorably called us ‘the third chimpanzee’ (the other two kinds are the chimp you probably think of – Pan troglodytes – and the delightfully sensual Bonobo chimpanzee, Pan paniscus).
There are lots of obvious differences which are related directly or indirectly to that 1% difference – physical, behavioural and cognitive. Now Jeremy DeSilva of Worcester State College, writing in the Proceedings of the National Academy of Sciences, has looked at an apparently minor difference that has a major significance: the ankle.
Using video analysis, DeSilva studied the “dorsiflexion” of chimp ankles as the apes climbed – this is the extent to which the ankle rotates when the toes are pointing upwards. If you try it now, you’ll find your foot moves at most 15-20º. See? Not much movement there. But if you were a chimp (and if you are, try it now), you’d find it moved around 45º. One of the reasons the chimps can do this is that their tibia (the larger of the lower leg bones) has a recess that enables the ankle to flex more.
This isn’t just a tedious piece of comparative anatomy – it has a major effect on the chimp’s behavior. They can climb using their feet in a way that is just impossible for us – and not just because they also have an opposable big toe.
Having established that the shape of the end of the tibia was a diagnostic difference between humans and chimps for their climbing ability, DeSilva turned his attention to our ancestors. He studied 29 tibia and ankle-bones from hominin skeletons of 4.12 to 1.53 million years old. None of them had the chimp-like shape, suggesting that their climbing behavior would have been more like our own than that of chimps.
This is important because there have been arguments about whether our ancestors had at least a partly arboreal life-style, like modern chimps. This study suggests that this was not the case. Or rather, as DeSilva puts it in the careful conclusion to the Abstract of his paper: “This study concludes that if hominins included tree climbing as part of their locomotor repertoire, then they were performing this activity in a manner decidedly unlike modern chimpanzees.”
Citation:
J. M. DeSilva (2009) Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences (USA). Published online before print April 13, 2009.
Abstract available here – you or your institution will need a subscription to read the original article.
by Matthew Cobb
Western Europe is being affected by a substantial decline in many species of songbirds. Two of the most common birds of my childhood – sparrows and starlings – have seen massive population crashes. Both species are closely linked with humans. Sparrows used to live in the eaves of houses, and were the symbol of working-class London (people would be described as “a chirpy Cockney sparrer”). Starlings used to blacken the skies of many cities with dusk displays of swirling hallucinogenic patterns. They are now largely limited to a few rural outposts (see below):
Why are they declining? The Royal Society for the Protection of Birds has understandably been preoccupied by this. Together with the RSPB, a group based at Leicester Univeristy has been trying to find out why the sparrows are disappering. Cats do not seem to be responsible. The data suggest a number of factors are involved – low ambient temperatures, extremes of rainfall, low aphid densities leading to high levels of vegetable matter in the birds’ diet, and high concentrations of air pollution from traffic. All these factors suggest that what’s actually causing the population to decline is the lack of insects. The British have bombarded their gardens and their fields with insecticides; the birds are suffering now, but decline in insect numbers could have a series of disastrous effects on plants and other wildlife.
The history of the USA shows the dangers. In the 19th century the Passenger Pigeon (Ectopistes migratorius) was present in such numbers that flocks numbered billions of birds and would take hours to pass overhead. For complex reasons that are still debated (habitat loss, disease, predation and hunting are some of the factors involved) the population size plummeted and Martha, the last known Passenger Pigeon, died a lonely death in Cincinatti Zoo.
British songbirds could face a similar fate. And, as the saying goes, extinction is forever.
To find out more, read this article from Animal Conservation about the decline of British sparrows (you or your institution will need a subscription to get past the abstract).
by Matthew Cobb
Unless you are a keen student of geology, you’ll probably have never heard of the Great Oxidation Event. And yet it was arguably the most important single thing that happened to that planet after the first appearance of life, 4 billion years ago. In Earth’s early history, the dominant gas was methane, produced by bacteria. Free oxygen was in short supply – it reacts so easily with other elements that it does not last long in a free state (think about how easily rust forms – this is a consequence of oxygen reacting with iron).
And yet for the world to look the way it does, oxygen had to be present in astounding quantities – about 21% of our current atmosphere is made up of oxygen. Without oxygen there would be no multicellular organisms, and even the continents would look different – oxidative weathering is an important process that has shaped the planet. The decisive change began around 2.4 billion years ago, in the Great Oxidation Event.
So where did the oxygen come from? Life. Or more precisely, blue-green algae (“cyanobacteria”). These organisms could photosynthesize, producing oxygen. They first appeared around 2.7 billion years ago, but the oxygen they produced was quickly consumed by the more numerous methane-producing bacteria.
We know that after the Great Oxidation Event (GOE), the cyanobacteria became dominant, paving the way for the development of life as we know it and, eventually, to the high levels of atmospheric oxygen found today, which are the result of plant and microbial respiration. But was the GOE a consequence of a slow growth of the cyanobacteria and weakening resistance from the methane-producing bacteria, or was there a more rapid appearance of the cyanobacteria?
A study in Nature this week, led by Kurt Konhauser from the University of Alberta, suggests that neither of these scenarios is right. Looking at the ratio of nickel and iron in sedimentary rocks found in what are called the “banded iron formations” (see picture below). They conclude that there was a massive decrease in oceanic levels of nickel around 2.7 billion years ago, because of changes in the Earth’s geological activity.
This had a catastrophic effect on the methane-producing bacteria, which require nickel to metabolise. This in set the stage for the rapid expansion of cyanobacteria and the accumulation of oxygen in the atmosphere and in the oceans. The GOE was on its way.
As well as offering an insight into the early evolution of life, this study also shows how geological cycles have played a decisive role in the evolution of the planet as we know it.
Citations (You or your institution will need a subscription to Nature to read more than the abstracts):
Original article: Konhauser, K. O. et al. (2009) Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature 458:750-753.
A Nature “News & Views” summary can be found here.
by Matthew Cobb
Every Saturday, Jerry posts something about cats. Some readers find this irritating. I have been instructed to keep up the tradition, so here’s a hypothesis for how they purr. The next job is to test it!
This drawing is by the late great B. Kilban, from his 1976 book of cartoons “Cat”. The official website for Kliban products can be found here. His Wikipedia biography can be found here, and much of it may actually be true. If Kliban has been dead for the last 19 years (he has), how come they can still churn out lucrative cat drawings? The answer probably lies in this cartoon. Anyone can draw a Kliban cat! Try it at home!
