Tag Archives: Evolution

Biology of Color Preference

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Color is funny. Anthropologists have long known that different cultures have different relationships, linguistically and in day to day practice, to the color spectrum. For example, the Efe Pygmy Hunter-Gatherers of the Ituri Forest describe things as white, black, or red, and that’s it. They live in a world of green, so it does not get a mention. Going with the model for “Eskimos” having a hundred words for snow because snow is so important in their environment, one would expect that the Efe would have a hundred words for green. On the other hand, the Efe Hunter-Gatherers must have a fairly primitive culture, compared, say, to those of us living in Coon Rapids Minnesota, that of course they have fewer words for different colors.

Of course, this is all a bunch of hooey. First, we don’t call them “Eskimos.” We call them “Inuit.” “Eskimo” is a bad word. It would be like calling the Irish “Drunken Leprechauns.” Second, the “fact” that the Inuit have a hundred words for snow is simply not true. It is an Urban Legend. Third, the great variation in something — any thing — does not necessarily demand a rich lexicon to describe it. You — yes, you — rely heavily on computers, right? Many computer users presumably need to have a concept of “memory” and the memory their computers use — related to the choices you make when buying or using a computer, where and how you store your documents, etc. But this concept is often appallingly simplified. I know many people who can’t distinguish between storage of data on a hard drive from storage of data in RAM. And there are many kinds of hard drive and many kinds of RAM. And then there is processor cache, video memory, and so on. There are probably over two dozen kinds of memory, it matters to any computer user, and most computer users have either one word for memory or two. (“Memory” or “Hard drive” and “Memory”)

Finally, if the Efe are Primitive, then I’m a monkey’s uncle. Indeed, both are untrue. The Efe are far from primitive and I’m a monkey’s great great great …. great great nephew, not uncle. The Efe have one of the largest brain to body ratios of any people. I’ve never met an Efe man who knew fewer than four different languages. I’ve never met an Efe who was not very smart. I can’t say any of these things for the population living in Coon Rapids, or even Edina, Minnesota.

So why do the Efe not seem to even have a word for the color green?

I can think of two answers to that question. One is that they do but have not bothered to teach this to us. I spent years living with them, and there were basic, day to day things that I learned right up to the last day I was with them. Sure, linguists presumably asked them about this, but that means little considering that only a handful of linguists have actually worked with them. The other explanation is that this is a stupid question. We only think that one needs a large number of words for the color green (if you are an Efe) because we mistakenly think the Inuit have a hundred words for snow.

Then there is the issue of gender and color. I am not color blind, but I am a man. Therefore I have only a few words for color. Let’s see. There’s black and white, green red and yellow, and pretty much that’s it. OK, maybe purple as well. Brown is a form of lightish black. I am not color blind. I’m simply not that interested.

Boys = blue, girls = pink. We know this because these are the colors of clothing, decorations and wall paper or paint in nurseries, etc. Anthropologists will tell you that this blue/pink gender thing is cultural, and that you can find exceptions to it, even reversals, if you look around the world and across history. For instance, the color association with the emperor of Rome was some girley color like purple.

A current study in the journal Current Biology claims to have found a non-culturally generated (but nonetheless culturally modified) gender difference in color preference.

The long history of color preference studies has been described as “bewildering, confused and contradictory”. Although recent studies … tend to agree on a universal preference for ‘blue’, the variety and lack of control in measurement methods have made it difficult to extract a systematic, quantitative description of preference. Furthermore, despite abundant evidence for sex differences in other visual domains, and specifically in other tasks of color perception … there is no conclusive evidence for the existence of sex differences in color preference. This fact is perhaps surprising, given the prevalence and longevity of the notion that little girls differ from boys in preferring ‘pink’. Here we report a robust, cross-cultural sex difference in color preference, revealed by a rapid paired-comparison task. Individual color preference patterns are summarized by weights on the two fundamental neural dimensions that underlie color coding in the human visual system. We find a consistent sex difference in these weights, which, we suggest, may be linked to the evolution of sex-specific behavioral uses of trichromacy.Anya C. Hurlberta and Yazhu Linga. “Biological components of sex differences in color preference.” Current Biology. Volume 17, Issue 16, 21 August 2007, Pages R623-R625.

I think the study design is good and the results convincing, that there is a sex difference in color preference along one aspect of the way color is perceived. Here is what the difference looks like in the figure the researchers provide:Color Preference

As implied in the summary, this difference corresponds to male-female differences in visual processing. This is believable.

The ultimate (evolutionary) explanation that the researchers give is weaker. It is a fairly typical post-hoc Environment of Evolutionary Adaptedness argument. Females prefer or are more perceptive of red because they are gathering berries found in a background of green leafy stuff. Or, alternatively, females evolved to be sensitive to social signals (blushing).

We speculate that this sex difference arose from sex-specific functional specializations in the evolutionary division of labour. The hunter-gatherer theory proposes that female brains should be specialized for gathering-related tasks and is supported by studies of visual spatial abilities. Trichromacy and the L-M opponent channel are ‘modern’ adaptations in primate evolution thought to have evolved to facilitate the identification of ripe, yellow fruit or edible red leaves embedded in green foliage. It is therefore plausible that, in specializing for gathering, the female brain honed the trichromatic adaptations, and these underpin the female preference for objects ‘redder’ than the background. As a gatherer, the female would also need to be more aware of color information than the hunter. This requirement would emerge as greater certainty and more stability in female color preference, which we find. An alternative explanation for the evolution of trichromacy is the need to discriminate subtle changes in skin color due to emotional states and social-sexual signals; again, females may have honed these adaptations for their roles as care-givers and ’empathizers’ .

Why do I say this is a weak post-hoc argument? For the simple reason that a reversal of their findings could be equally well explained. The game sought by hunter-gatherers is distinguished from a green foliage-rich background by its reddish-brown hue. Most meat actually collected by male hunter-gatherers is not from shooting an animal dead with an arrow, but by wounding it and following an often very subtle blood (red) trail. Also, among the Efe, men gather wild fruit about as often as do women, and the food women gather from the wild tends to be either green or underground. If the Efe were the forager culture used to represent the human past by modern evolutionary psychologists, many of the ultimate, evolutionary, explanations attached to various findings would be very different.

Regarding the blushing: Since white skin against which blushing is most obvious is a recent mutation (and a rather harmful one at that), I think this argument can be rejected out of hand. Nonetheless, this is a good piece of research, well done, and of great interest.

HURLBERT, A., LING, Y. (2007). Biological components of sex differences in color preference. Current Biology, 17(16), R623-R625. DOI: 10.1016/j.cub.2007.06.022

Hybrids of Blind Fish Can See

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The loss of sight in cave dwelling species is widely known. We presume that since sight in utter darkness has no fitness value, the mutation of a gene critical to the development of the sense of sight is not selected against. Over time, any population living in darkness will eventually experience such mutations, and these mutations can reach fixation.

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Astyanax mexicanus: Top is the surface, sighted form, bottom is the cave-dwelling, blind form. From the Jeffery Lab.

Beyond this, we may hypothesize that a mutation “turning off” sight could be beneficial. By definition, an adaptation (such as sight) has a cost. When a trait that is adaptive is no longer adaptive, individuals with that trait “turned off” should experience an increase in fitness. It may also be the case, however, that such an increase in fitness is so small that it may be irrelevant. This line of thinking needs further investigation and what one finds in such an investigation may vary a lot from system to system. For example, a mutation that simply causes a particular protein to no longer be produced in what would have been a small quantity would save the individual with that mutation the use of a few tens of thousands of amino acids over some fixed period of time. This would have very little fitness value. But if a system is exploitable by a pathogen — such as a receptor site on a cell used by a common virus — turning that gene off may have enormous benefits. But this is a bit of a digression from the research at hand.

Borowsky, in his paper “Restoring sight in blind cavefish,” provides a test case for how we think evolution works. In Mexico, the species Astyanax mexicanus, is known to exist in 29 distinct populations. Genetic studies indicate that the turning off of the sense of sight in these fish has involved a deleterious (as in loss of function) of genes in at least three different lineages, or to put it a different way, sightlessness has evolved three or more separate times in these Mexican blind cavefish.When Borowsky cross breeds some of these cavefish, crossing them between these populations, he gets a certain percentage of fish that have functional, if not fully developed, eyes.This should not be at all surprising. Several different genes are involved in the development of sight, so by cross breeding strains that have experienced mutations in different genes, one would expect a certain number of offspring to have a set of functioning genes sufficient to make the sense of sight develop at least to some extent. When Borowsky breeds the blind cavefish with the non-blind version of this fish (“surface fish”) he gets restoration of the sense of sight in all of the offspring.

F1 hybrids between surface fish and cave fish have smaller eyes than surface fish, but are fully visual, even into adulthood … Thus, one surface allele at each of the population-specific eye loci is sufficient for restoring vision.

This is also expected, although not necessarily inevitable (This depends on the dosage required for each genetically coded step in the development and function of sight).

It seems to me that one could test the hypothesis mentioned above that turning off any fitness-free gene is adaptive. If simple production of unused proteins is costly, the rate at which particular genes are found to be turned off should be correlated with that cost. Perhaps the genes coding for longer proteins, or proteins that are produced more often in a particular system, should be more likely turned off. Or, some measure of the total mass of amino acids turned into proteins when a gene functions, should be correlated to the likelihood of having a gene turned off. At a most basic level, one would need to show that the mutant genes are in fact turned off and are not simply producing a non-functional protein.In short, this study (and others by this and other research teams) demonstrates in empirical reality what is expected from commonly held evolutionary theory. Creationists often cite blind cave dwelling organisms as evidence against evolution, because, they say, it is “devolution.” This point of view is absurd, and relies on a teleological view of, in this case, teleost (bony fish) evolution.

Darwin wrote about cave blindness and disuse, and through various observations notes the potential complexity of the problem:

It is well known that several animals, belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse. In one of the blind animals, namely, the cave-rat, the eyes are of immense size; and Professor Silliman thought that it regained, after living some days in the light, some slight power of vision. In the same manner as in Madeira the wings of some of the insects have been enlarged, and the wings of others have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work.[On the Origin of Species…, 1859, pp 137-138]

You might be wondering how these fish got into these caves to begin with. I can’t describe the exact process for the fish studied in this paper, but there is a general way in which this can happen. Underground lakes or streams in caves may be connected to each other during less arid periods, in some cases running from the deeps of large lakes that later try up almost entirely. In this way, a continuous population in a river or lake is broken into relict populations that are separate from each other and perhaps living in habitats that are different from the original, continuous habitat, and possibly different from each other as well. Under these conditions evolution’s just gotta happen.

BOROWSKY, R. (2008). Restoring sight in blind cavefish. Current Biology, 18(1), R23-R24. DOI: 10.1016/j.cub.2007.11.023

Hopeful Monsters and Hopeful Models

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A hopeful monster is a mutant born with a genetically determined and large novel trait (compared to its parents) which confers enhanced fitness on that individual. This enhanced fitness increases the likelihood that the new mutant gene that determines this trait will be passed on and spread throughout the evolving population, so in a single generation a rapid process of speciation is initiated. For example, a fish with a mutation that causes both its eyes to grow on one side of its head could become the flounder of a new generation of flatfish. Well, just for the halibut, it might be fun to further examine this notion.

The hopeful monster idea is attractive for three reasons. First, we already know that some of the most profound differences between larger scale taxonomic groups (like the phyla) are about aspects of body plan that are controlled by early stages in embryonic development. So, if a simple mutation in one of those stages can cause a taxon that has a certain number of limbs to give rise to a new species with a totally different number of limbs, then that would be cool.

The second reason is that the fossil record seems to have the property whereby many species stay roughly similar for long periods of time, then suddenly, there is lots of evolutionary change. You’ve heard of this, it’s called “punctuated equilibrium.” If hopeful monsters — also called saltational (dancing, leaping) evolution — occurred generally, we might postulate that these moments of dramatic change, these punctuations, are moments in time where for some reason a lot of hopeful-monstering was going on all at once. That would be cool.

The third reason that this is an attractive idea is just that it would be cool, especially if you are the guy who discovers the next hopeful monster.

Olivia Judson recently wrote a column talking about the Hopeful Monster idea. She proposes that the idea, which has been mostly discredited and put aside as not possible, or at least, so unlikely as to not be a factor in explaining overall patterns in evolution, is coming back. Her column, “The Monster is back, and its Hopeful” … is here.The article is interesting, but she does not really provide any new information that would lead one to suspect that we should be rethinking the unlikelihood of hopeful monstering. She gives a few examples of single, possibly small genetic changes that have huge (perhaps) phenotypic effects, within populations, that she suggests could be analogs for hopeful monster events.

One of these I want to discuss in more detail in a moment is the loss of feathers on a chicken over the neck and head. This, she suggests, is analogous to vultures having naked heads and necks (a presumed adaptation). So, a population of carrion eating eagles could give rise to a hopeful monster gene that produces a carrion eating vulture.O

livia Judson’s article has been read and taken to task by Jerry Coyne, a blogless curmudgeon who happens to be an expert on this topic, and who is friends of Carl Zimmer. Zimmer has given Coyne a spot on The Loom (Zimmer’s Blog) to respond to Judson. This post is here.

Coyne explains how Judson is totally wrong. The hopeful monster idea has been thrown out, and in fact its debunking happened some time ago. Nobody believes this crazy idea now, and what the heck is she doing bringing this up again.

I want to say a couple of things about both bits of writing, but especially Coyne’s.

First, I think Judson’s column is unconvincing and very fluffy, and I agree with Coyne’s critique that this kind of looks like a journalist getting readers more than the heralding of a new way of looking at evolution. However, it is a column in a newspaper and Judson is acting as a journalist, so maybe this approach can be forgiven. Regarding Judson’s examples, I think they are all reasonable examples of a genetic mutation that could indeed arise and spread in a population, but they are not species- or (and this is very important) higher taxa-determining differences. A vulture is different from an eagle for a very large number of reasons, and the naked head is only one of them, perhaps not even the most important. If a population of naked-headed eagles arose, then we’d have a subspecies of “truly bald” instead of “looks bald” eagles. They would not really be hopeful monsters.

On the other hand, I think that Coyne’s treatment of Judsons’s paper is a bit heavy handed and rather sanctimonious. And given this holier than thou attitude (which may well be justified, but I’m just sayin…) it is appropriate to hold him to a very high standard of perfection.

Coyne mentions that the naked-necked chickens are an example of a mutation in a domestic animal (true) and that mutations occur in domestic populations with great frequency compared to wild populations (maybe) and that these mutations only persist because the animals are coddled in their domestic setting.

This last part may often be true, and I think I agree with that. But not for the naked necked chickens. Naked necked chickens are common in the African tropical rain forest. They are not coddled. They are not kept in coops, they are not provided with water, they are not fed. Maybe they are kept from predators a little, but in fact, that is hard to argue since they are often eaten by eagles, hawks, snakes, civets, wild cats, and so on. It is even possible that in that setting, where they do eat some carrion, and where there is a high parasite load, that the naked neck and head are adaptive (though I’m not advocating for that idea, it is merely a tenable hypothesis).

So, I agree with Judson that the naked necked chicken, a simple genetic mutation, may be a good example of a one-shot trait that need not have arisen feather by feather over generations of time. But I disagree that a naked necked chicken is a hopeful monster. As hopeful as such a trait may be, it just isn’t that monstrous.

Now, if you try eating one of these chickens, that that is a different story. They are as tough as buffalo hide. Truly monstrous, as cuisine.

Culture influences brain function

Race and Racism,

People from different cultures use their brains differently to solve the same visual perceptual tasks, MIT researchers and colleagues report in the first brain imaging study of its kind.

This is not that surprising, but it is very interesting research. We already knew, for instance, that people who read and write different “kinds” of languages … pictographic vs. non-pictographic … use different regions of their brain for this function, and thus are differentially affected by strokes or other damage. Continue reading Culture influences brain function →

The Origin of Syphilis

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Blogging on Peer-Reviewed ResearchSyphilis is first clearly seen in Europe in 1495, when it appeared as a plague (though it was not “the plague” … Yersinia pestis) among Charles VIII’s troops. When these troops went home shortly after the fall of Naples, they brought this disease with them, staring an epidemic. The level of mortality in Europe was truly devastating. Is it the case that syphilis was brought to Europe by Columbus and his men just prior to the plague-like outbreak of 1495?

The origin of syphilis has been debated for years, really since the actual 1495 event itself. Some researchers have asserted that syphilis is present in the writings of Hippocrates, placing it squarely in the old world thousands of years prior to Columbus. Others, as suggested above, have argued that Columbus brought syphilis over to the Old World . A third (Crosby’s “combination theory”) asserts, essentially, that syphilis is both an Old World and New World disease, and that the history of the disease is complicated by the innately complex relationship between any pathogen and human populations with variable immunities, both of which tend to evolve.

A new paper is being published as we speak in PLoS Neglected Tropical Diseases, by Harper et al , called “On the Origin of the Treponematoses: A Phylogenetic Approach.” Studiously ignoring Crosby’s discussion (and I’m sure there is some unseemly story of academic infighting to explain that), the paper examines the Old World vs. New World origins hypotheses.Here is the author’s summary from the paper:

 

For 500 years, controversy has raged around the origin of T. pallidum subsp. pallidum, the bacterium responsible for syphilis. Did Christopher Columbus and his men introduce this pathogen into Renaissance Europe, after contracting it during their voyage to the New World? Or does syphilis have a much older history in the Old World? This paper represents the first attempt to use a phylogenetic approach to solve this question. In addition, it clarifies the evolutionary relationships between the pathogen that causes syphilis and the other T. pallidum subspecies, which cause the neglected tropical diseases yaws and endemic syphilis. Using a collection of pathogenic Treponema strains that is unprecedented in size, we show that yaws appears to be an ancient infection in humans while venereal syphilis arose relatively recently in human history. In addition, the closest relatives of syphilis-causing strains identified in this study were found in South America, providing support for the Columbian theory of syphilis’s origin.

 

The authors looked at 21 strains of the bacterium Treponematoses pallidum and conducted a detailed genetic (phylogenetic) study of these genomes to come to the conclusion that syphilis originates in the new world. However, a commentary on the paper, published along side it in PLoS, brings the conclusion into question. The commentary by Lukehart and Norris notes that the genetic data from the New World (which the main paper’s authors assert points to a New World origin) is weak. In addition, there are problems, partly outlined in the commentary and partly fairly obvious to anyone who reads the paper, that the issue of evolutionary change in both the pathogens and the humans who harbor them has not been sufficiently taken into account.In my view, a detailed phylogenetic study such as the one presented here is fundamentally important, but is very unlikely on its own to definitively answer the question of origin and evolution of syphilis.Need more research…

Updated: See The Science Behind Pre-Columbian Evidence of Syphilis in Europe: Research by Documentary, by Armelagos, Zuckerman, and Harper, on “Science by documentary” and “Science by press release”

This article discusses the presentation of scientific findings by documentary, without the process of peer review. We use, as an example, PBS’s “The Syphilis Enigma,” in which researchers presented novel evidence concerning the origin of syphilis that had never been reviewed by other scientists. These “findings” then entered the world of peer-reviewed literature through citations of the documentary itself or material associated with it. Here, we demonstrate that the case for pre-Columbian syphilis in Europe that was made in the documentary does not withstand scientific scrutiny. We also situate this example from paleopathology within a larger trend of “science by documentary” or “science by press conference,” in which researchers seek to bypass the peer review process by presenting unvetted findings directly to the public. © 2012 Wiley Periodicals, Inc.

See: Syphilis May Have Spread Through Europe Before Columbus.

Summary: A recent study conducted at the University of Zurich now indicates that Europeans could already have been infected with this sexually transmitted disease before the 15th century. In addition, researchers have discovered a hitherto unknown pathogen causing a related disease. The predecessor of syphilis and its related diseases could be over 2,500 years old.

Lukehart S, Mulligan C, Norris S (2008) Molecular Studies in Treponema pallidum Evolution: Toward Clarity? PLoS Negl Trop Dis 2(1).Harper KN, Ocampo PS, Steiner BM, George RW, Silverman MS, et al. (2008) On the Origin of the Treponematoses: A Phylogenetic Approach. PLoS Negl Trop Dis 2(1): e148. doi:10.1371/journal.pntd.0000148

PKU: An exploration of a metabolic disease

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Phenylketonuria (fee-null-keet-o-noo-ria), mercifully also known as “PKU” (pee – kay – you) is a disorder in which phenylalanine, an essential amino acid, is not broken down as it normally would be by an enzyme (phenylalanine hydroxylase) and thus accumulates (in the form of phenylpyruvic acid) in the body. Normally, Phenylalanine hydroxylase coverts phenylalanine into tyrosine, another amino acid, which has a number of different functions.

This is bad because buildup of phenylpyruvic acid has several negative effects, the most important being to interfere with normal development of neural tissues. Continue reading PKU: An exploration of a metabolic disease →

Charles Darwin Bicentennial – Iguanas, a “most disgusting, clumsy lizard…

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…They are as black as the porous rocks over which they crawl & seek their prey as from the Sea. — Somebody calls them “imps of darkness”. — They assuredly well become the land they inhabit. — When on shore I proceeded to botanize & obtained 10 different flowers; but such insignificant, ugly little flowers, as would better become an Arctic, than a Tropical country. — The birds are Strangers to Man & think us him as innocent as their countrymen the huge Tortoises. Little birds within 3 & four feet, quietly hopped about the Bushes & were not frightened by stones being thrown at them.” [Darwin’s Beagle Diary (1831-1836)].

And thus we get a hint of Darwin’s impressions of the Galapagos, and in particular, that Island’s marine iguanas.

The Iguana family is Iguanidae, but most Iguana’s you’ve cuddled in the pet store are members of the genus Iguana (and most likely species Iguana iguana.) The Galapagos Islands have two or three species of iguana: The Land Iguana is Conolophus subcristatus and Conolophus pallidus, or perhaps is actually the subspecies Conolophus subcristatus pallidus. The marine iguana is Amblyrhynchus cristatus.

The two genera of iguana on the Galapagos seem able to interbreed, though they otherwise also seem to make good, distinctive species. (No, it is not really true that inability to inbreed is “THE biological definition of species….” it is more complex than that. A topic for another time, perhaps.) The phylogenetic relationship among the Galapagos iguanas and continental iguanas is similar to that among the finches and other Galapagos animals… complex and more complex because of the apparent fact that while the oldest of the Galapagos islands is about four million years old, earlier islands, perhaps going back twice that age, formerly existed but are now eroded down below sea level. One wonders what will happen next ice age (or what happened last ice age) when a 120 -150 meter drop in sea level exposes some of these islands! The point is that these volcanic islands have a complex history, and it is likely that the islands themselves have a complex relationship to the distant continent. Again, the topic of another post perhaps.

The following passages from Darwin (1839) Continue reading Charles Darwin Bicentennial – Iguanas, a “most disgusting, clumsy lizard… →

Charles Darwin Bicentennial – A Tangled Bank

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Last Darwin Post I gave you the famous “Tangled Bank” quote, in which Darwin links the concept of selection to the concept of ecology and thus derives “grandeur in this view of life.”

This is a theme of much of Darwin’s writing in The Origin, and in fact, the Phrase “Tangled Bank” shows up much earlier in the volume.

In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all concur in determining the average number or even the existence of the species. In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this! Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been observed that ancient Indian ruins in the Southern United States, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds as in the surrounding virgin forest. What a struggle must have gone on during long centuries between the several kinds of trees, each annually scattering its seeds by the thousand; what war between insect and insect—between insects, snails, and other animals with birds and beasts of prey—all striving to increase, all feeding on each other, or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is the problem where each shall fall compared to that of the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!

(Darwin, C. R. 1869. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. 5th edition. Pages 86-87)

This is a fantastic example of Darwin’s breadth of interest and integrated mind. He makes explicit reference to the fact that selection is context dependant (“widely-different checks act on the same species in different districts”). He is explicit about the fact that chance is NOT the operative force in organizing nature (a fact that creationists seem to ignore when they speak of the unlikelihood of a tornado passing through a junkyard creating a Boeing 747 and http://gregladen.com/wordpress/?p=264such hogwash). Continue reading Charles Darwin Bicentennial – A Tangled Bank →

Parasitic Birds and The Red Queen Effect

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The Avian Brood Parasites

The Avian Brood Parasites

Brood parasitic birds lay their eggs in the nests of other birds (the “hosts”) who then raise them as their own. Examples of parasitic birds includes the cuckoo, cow birds, widow (“whyda”) birds, honeyguides, and even the South American Black-headed Ducks. Brood parasitism is virtually a world wide phenomenon.

Many interspecific brood parasites are obligate for this strategy … this is the only way they raise their own young. There are many variants (beyond the scope of this post). Intraspecific parasitism is known in many colonially nesting birds.

The Red Queen effect is a concept now widely known by aficionados of biology. The phrase is from Alice Through the Looking Glass, but the biological concept was first developed by Leigh Van Valen, a biologist at the University of Chicago.

While the Red queen and Alice are discussing chess, the following dialog and events ensue: Continue reading Parasitic Birds and The Red Queen Effect →

Models of Sexual Selection

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Darwin was puzzled by exaggerated traits. (Aren’t we all, really?) For example, why would a widow bird male have a tail so long that he could scarcely fly away from predators? Indeed, speaking of birds:

What a contrast is presented between the sexes by the polygamous peacock or pheasant, and the monogamous guinea-fowl or partridge! Many similar cases could be given, as in the grouse tribe, in which the males of the polygamous capercailzie and black-cock differ greatly from the females; whilst the sexes of the monogamous red grouse and ptarmigan differ very little. Amongst the Cursores, no great number of species offer strongly – marked sexual differences, except the bustards, and the great bustard (Otis tarda), is said to be polygamous. With the Grallatores, extremely few species differ sexually, but the ruff (Machetes pugnax) affords a strong exception, and this species is believed by Montagu to be a polygamist. Hence it appears that with birds there often exists a close relation between polygamy and the development of strongly-marked sexual differences. On asking Mr. Bartlett, at the Zoological Gardens, who has had such large experience with birds, whether the male tragopan (one of the Gallinaceæ) was polygamous, I was struck by his answering, “I do not know, but should think so from his splendid colours.”

Darwin, C. R. 1871. The descent of man, and selection in relation to sex. London: John Murray. Volume. 1. 1st edition. Pages 269-270

I don’t want to give a comprehensive (or bullet proof) “definition” of sexual selection. Instead, I want to lay out a few key ideas and suggest a way to think of models of sexual selection.

Darwinian Sexual Selection.
Females possess a built in aesthetic Continue reading Models of Sexual Selection →

The Evolution of Human Diet

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Chimp, Australopith and
Human Teeth Compared.

The evolution of human diet followed a major zig (as in zig-zag) in a wholly unexpected direction, followed by the most significant biological innovation to ever occur among multi celled animals: The invention of cooking. I’m actually going to point you to two papers on this topic, and provide a brief summary of the ideas here.

Let’s start with the bold assumption that humans evolved from a chimpanzee-like animal. This is tantamount to saying that the last common ancestor of chimpanzees and humans was, essentially, pretty much like a chimpanzee. At another time, I’ll write a post on why this is a good assumption, but for now lets just go with it. Some large percentage of human evolution experts like this assumption, a bunch of others hate it (which is the usual pattern for most ideas in human evolution).

A mammal’s diet is reflected in physiological attributes that can be discerned from the fossil record. Body size, the nature of the teeth and associated muscles, possibly the shape of the mouth’s cavity, and even the overall size and shape of the gut may be closely connected with diet.

If we draw a direct line from a presumed chimpanzee-like ancestor to modern humans, Continue reading The Evolution of Human Diet →

The Modes of Natural Selection

Uncategorized,

There many ways of dividing up and categorizing Natural Selection. For example, there are the Natural Selection, Sexual Selection and Artificial Selection, and then there is the Modes of Selection (Stabilizing, Directional, and Disruptive) trichotomy.

We sense that these are good because they are “threes” and “three” is a magic number. Here, I’m focusing on the Mode Trichotomy, and asking that we consider that there are not three, but four modes of Natural Selection. This will cause tremors throughout the Evolutionary Theory community because Four is not a magic number, but so be it.

In Stabilizing Selection the extremes of a trait are selected against and the mean value of the trait remains the same. Mutations constantly introduced into the population tht produce traits out at the extremes are selected against. In Directional Selection the values of a trait at one end of the distribution are selected against and/or values at the other end are selected for, so that the distribution of values, and it’s mean, move in one direction. In Disruptive Selection the average values are selected against so that the distribution of the trait becomes bimodal.

That was pretty simple, but Continue reading The Modes of Natural Selection →

The Three Necessary and Sufficient Conditions of Natural Selection

Uncategorized,

Natural Selection is the key creative force in evolution. Natural selection, together with specific histories of populations (species) and adaptations, is responsible for the design of organisms. Most people have some idea of what Natural Selection is. However, it is easy to make conceptual errors when thinking about this important force of nature. One way to improve how we think about a concept like this is to carefully exam its formal definition.

In this post, we will do the following:

  • Discuss historical and contextual aspects of the term “Natural Selection” in order to make clear exactly what it might mean (and not mean).
  • Provide what I feel is the best exact set of terms to use for these “three conditions,” because the words one uses are very important (there are probably some wrong ways to do it one would like to avoid).
  • Discuss why the terms should be put in a certain order (for pedagogical reasons, mainly) and how they relate and don’t related to each other.

When you are done reading this post you should be able to:

  • Make erudite and opaque comments to creationists that will get you points with your web friends.
  • Write really tricky Multiple Choice Exam Questions if you are a teacher.
  • Evolve more efficiently towards your ultimate goal because you will be more in control of the Random Evolutionary Process (only kidding on this third one…)

Continue reading The Three Necessary and Sufficient Conditions of Natural Selection →

Of skinks and monkeys

Animals, Evolutionary Biology

I was recently looking at a practice AP biology test question on evolution, and sparing you the details, I found it interesting that two of the four parts dealt with genetic variation and speciation in such a way that it was difficult to tell them apart. As expected, students who answered these questions got confused as well, and tended to give perfectly good answers to Part B, but unfortunately, this was their answer to Part A. By the time they got to Part B they seemed a little confused, perhaps realizing that there was some overlap and conflation of concepts.

Inter and intra-specific variation is probably patterned such that the sum of variation among several species is greater than the partitioned variation within a given species. That’s pretty obvious.

(Just in case it is not: Imagine measuring the mass of several elephants. The variation can be represented by the standard deviation, range, or whatever you like, among your measurements. It is such and such. Now do the same thing with a bunch of mice. Again, you have some measure of variation. Now do it for the mice and elephants combined. Here, the variation will be larger than for either. This is not the same as if you want to compare variation or patterns of variation between mice and elephants. Do do that, you need to scale the variation, say by using the coefficient of variation. In this case, combining the coefficients of variation might show less variation when combined than for either group simply because of sample size effects. But what I’m talking about here is total variation. Mice are tiny, elephants are huge, so their total size variation runs Continue reading Of skinks and monkeys →