This Week in Evolution

"A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase... It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms" -- Charles Darwin, Chapter 3, The Origin of Species

Although this paper focuses on Easter Island, it also discusses many of the same societies in Jared Diamond's 2005 book "Collapse." The book includes much that is not in the paper, but the paper has the advantage of being shorter and of supporting specific points with specific citations, in contrast to the diffuse "Further Reading" approach used in Collapse.

Continue reading "Oestrus Island" »

Posted by R. Ford Denison at 10:34 PM | Permalink | Comments (0)

Punishing cheaters selects against cheating, but what selects for punishing? Are the answers different, depending on whether the species involved have brains? A recent internet experiment suggests that altruistic punishment, perhaps unique to humans, doesn't promote cooperation as effectively as previously thought.

My own research focuses on cooperation in species without brains. We showed that “sanctions” imposed by legume plants limit the evolution of “cheating” rhizobium bacteria (those that divert more plant resources to their own reproduction, relative to other rhizobia, by investing less in fixing the nitrogen needed by the plant). We think individual plants help themselves by imposing sanctions that limit wasteful resource use by less-beneficial rhizobia – they don’t do it for the benefit other legumes.

In theory “altruistic punishment” (paying some cost or taking some risk to punish noncooperators) could help explain why there is more cooperation among unrelated humans than might otherwise be expected. (Cooperation among relatives is explained by kin selection.) But how much are individuals willing to pay to punish noncooperators?
The latest experiments attempting to answer this question were just published on-line in Proceedings of the Royal Society, by Martijn Egas and Arno Riedl: The economics of altruistic punishment and the maintenance of cooperation.

Continue reading "Altruistic punishment? Maybe not." »

Posted by R. Ford Denison at 11:46 PM | Permalink | Comments (0)

There are enough examples of ‘‘cheating’’ in bacteria ... that mindless obedience to such [quorum-sensing] chemical signals cannot be assumed. Mindlessness can be assumed, but not obedience. -- Denison et al. (2003) Ecology 84:838-845

Quorum sensing, an exchange of chemical signals among bacteria, can solve the coordination problem. But this week's paper Cooperation and conflict in quorum-sensing bacterial populations shows that quorum sensing doesn't automatically solve the problem of cheaters. The paper is by Stephen Diggle, Ashleigh Griffin, Genevieve Campbell, and Stuart West and published in Nature.

Continue reading "Communication doesn't automatically prevent cheating" »

Posted by R. Ford Denison at 11:08 PM | Permalink | Comments (0)

Ed Yong beat me again, this time discussing an interesting paper in Nature on the evolution of language, but I'm going to comment anyway. Actually, there are two papers in the same issue, both showing that frequently used words change more slowly. For example, irregular past tenses of rarely-used verbs (bide, delve, etc.) have tended to disappear, but we still say "came and saw" not "goed and seed." Lieberman et al. (Nature 449:713) note that:

It is much rarer for regular verbs to become irregular: for every ‘sneak’ that ‘snuck’ in there are many more ‘flews’ that ‘flied’ out.

Languages evolve, but analogies with evolution of genes may be misleading. Genes pass only from parent(s) to offspring and their frequency changes over generations. Words and ideas can spread much more rapidly, including from child to parent. Cultural evolution seems more analogous to the spread of viruses, only some of which come from our parents.

Some ideas (e.g., religions) come packaged with explicit instructions to proselytize, like the rabies virus making its victims bite others. But the desire to spread our tastes in music, for example, seems intrinsic to us, not to the music. So cultural evolution seems similar enough to epidemiology that analogies will sometimes be useful, but only sometimes.

Posted by R. Ford Denison at 06:11 PM | Permalink | Comments (0)

Why do women, in contrast to our closest relatives, stop giving birth while they are still relatively young and healthy? This week's paper. "Testing Evolutionary Theories of Menopause", by Daryl Shanley and coauthors, published in Proceedings of the Royal Society, uses data from people living in The Gambia to test two different hypotheses.

Continue reading "Menopause trade-offs" »

Posted by R. Ford Denison at 07:12 PM | Permalink | Comments (2)

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Posted by R. Ford Denison at 04:14 PM | Permalink | Comments (3)

Usually, those alleles (versions of a gene) that become more common over generations are those that are most beneficial to the organisms in whose cells they live. But not always.

The latest issue of PLoS Biology has an open-access article on a particularly selfish gene responsible for Apert syndrome in humans.

Continue reading "Selfish sperm cells" »

Posted by R. Ford Denison at 05:13 PM | Permalink | Comments (0)

This week's paper, published in Science (317:679) is "Immune-like phagocyte activity in the social amoeba" by Guokai Chen, Olga Zhuchenko, and Adam Kuspa of the Baylor College of Medicine.

Cells of the social amoeba, Dictyostyleium discoideum forage individually, but eventually group together into a "slug", which crawls through the soil for days before eventually forming a spore-tipped stalk. Previous work with this species has looked at conflicts of interest over which cells have to sacrifice future reproduction (as spores) and become part of the stalk. This week's paper uncovers another example of apparent altruism in Dictyostelium, which may shed light on the evolution of a key part of our immune system.

As a Dictyostelium slug crawls through the soil, some cells are left behind. Are these just random sluggards? Or do they function like human phagocytes, the immune system cells that gobble up bacteria?

Continue reading "Left behind: social amoebae" »

Posted by R. Ford Denison at 06:25 PM | Permalink | Comments (5)

This week's paper is "Palaeogenetic evidence supports a dual model of Neolithic spreading into Europe" by M.L. Sampietro and others, published online in Proceedings of the Royal Society. The paper is interesting both for its findings and for its methods.

We know that agriculture spread from the Near East -- do people in Asia call this the Near West? -- to western Europe, starting around 10,000 years ago. But did this mostly involve farmers moving, or the spread of agriculture without major movement of people?

People have tried to figure out past population movements using genetic differences among modern populations, but it would help to have genetic information from people who lived thousands of years ago, as well. This is technically challenging, however...

Continue reading "Tracing the spread of agriculture with stone-age human DNA" »

Posted by R. Ford Denison at 04:51 PM | Permalink | Comments (0)

I have written about evolutionary trade-offs before, starting with early posts about trade-offs between seed size and seed number in plants, and trade-offs between the ability of insects to escape predators by flying away, versus the ability to hide from them by playing dead. I have also given some examples of the increasing use of sophisticated experimental (often molecular) methods in evolutionary biology. This week's paper combines both themes.

The paper is "Restriction of an extinct retrovirus by the human TRIM5-alpha antiviral protein" by Shari Kaiser, Harmit Malik, and Michael Emerman, published in Science (vol.316 p.1756).

Retroviruses are made of RNA, but make DNA copies of themselves that can insert into the DNA of host cells they infect. HIV, the cause of AIDS, is a well-known example, but there are many others. If DNA copies of the retrovirus are inserted into cells giving rise to sperm or eggs, they can be passed to the next generation, as endogenous retroviruses. If the DNA inserts somewhere where it turns an important gene on or off, it may kill the host. Or, once in a while, this change may turn out to be beneficial. The few beneficial changes are the ones that survive and spread, just as the few mutations that are beneficial are the ones that persist.

VWXYNot has an interesting discussion of how a creationist web site misused one of her papers as evidence of "intelligent design." She shows how shared endoviruses can be used to infer shared ancestry, providing yet more evidence that we share a recent ancestor with apes, less-recent ancestors with monkeys, etc. But that's not what this week's paper is about....

Continue reading "Trade-offs in defense against retroviruses" »

Posted by R. Ford Denison at 08:28 PM | Permalink | Comments (4)

What did our early ancestors and related species eat? Different data seemed to give different answers. This week’s paper may have helped to solve this mystery.

Isotope data suggest that tropical grasses were a big part of the diet of the hominins Australopithecus africanus and Paranthropus robustus. These grasses have CO2-concentrating C4 photosynthesis. As a result, they have a little more of the rare carbon-13 isotope, and a little less C12, relative to most other plants. So do the fossil teeth of these early human relatives, as if they ate these grasses. But the shape of their teeth, and wear patterns, are wrong if they mostly ate grass leaves or animals that ate grass. What about roots, or underground storage organs? These are an important food for some human foragers today, especially in dry climates. If our early relatives mostly ate these “USOs”, then the isotope ratios in their teeth should be like those of other species with a similar diet. Mole rats, for example.

Continue reading "Roots" »

Posted by R. Ford Denison at 05:48 PM | Permalink | Comments (0)

Last week, I was at a meeting in the Netherlands on “Darwinian agriculture: the evolutionary ecology of agricultural symbiosis.” Topics included: the effects of cows on human evolution, the independent invention of “agriculture” by ants and termites, and some disadvantages of diversity. As promised, here are a few highlights.

Continue reading "Darwinian agriculture II" »

Posted by R. Ford Denison at 06:15 PM | Permalink | Comments (5)

This week’s paper, "Emergence of novel color vision in mice engineered to express human cone pigment", by Gerald Jacobs and colleagues at UC Santa Barbara and Johns Hopkins Medical School (Science 315:1723), is yet another experimental study that increases our understanding of how repeated cycles of natural selection, each producing a fairly small change, can lead to adaptations that may seem irreducibly complex.

Most humans have three different photopigment color sensors, as do our closest relatives. Many other mammals, including mice, have only two. Three-color vision is useful for many purposes, from identifying higher-protein leaves to eat (Nature 410:363) to telling which wire to cut to disarm the nuclear bomb buried under the stadium. But eventual usefulness isn’t enough for a trait to evolve. If a series of steps is required, each step must be beneficial, or at least not lethal. Such a series of steps has been worked out for the evolution of optically sophisticated eyes from light-sensitive spots (Proc. Roy. Soc. B 256:53), but what about color vision?

Continue reading "Evolution of color vision: transgenic mice see red" »

Posted by R. Ford Denison at 04:15 AM | Permalink | Comments (7)