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March 1, 2013

Copying in birds, dolphins, and viruses; evolution & environmental change; evolution of mutation rate

Learning and signal copying facilitate communication among bird species "where only two species regularly interact, one species' [alarm] calls incorporate the call of the other."

Vocal copying of individually distinctive signature whistles in bottlenose dolphins "Copying occurred almost exclusively between close associates such as mother-calf pairs and male alliances during separation... copies were clearly recognizable as such because copiers consistently modified some acoustic parameters of a signal when copying it... no evidence for the use of copying in aggression or deception."

A bacteriophage encodes its own CRISPR/Cas adaptive response to evade host innate immunity "the only documented bacterial adaptive immune system is the CRISPR/Cas... a phage-encoded CRISPR/Cas system is used to counteract a phage inhibitory chromosomal island of the bacterial host. "

Evolutionary rescue from extinction is contingent on a lower rate of environmental change" "By assessing fitness of these engineered [E. coli] strains across a range of drug concentrations, we show that certain genotypes are evolutionarily inaccessible under rapid environmental change."

Fossil evidence for a hyperdiverse sclerophyll flora under a non-Mediterranean-type climate "sclerophyll hyperdiversity has developed in distinctly non-Mediterranean climates... likely a response to long-term climate stability."

A trade-off between oxidative stress resistance and DNA repair plays a role in the evolution of elevated mutation rates in bacteria "The dominant paradigm for the evolution of mutator alleles in bacterial populations is that they spread by indirect selection for linked beneficial mutations when bacteria are poorly adapted... [but] hydrogen peroxide, generates direct selection for an elevated mutation rate in the pathogenic bacterium Pseudomonas aeruginosa as a consequence of a trade-off between the fidelity of DNA repair and hydrogen peroxide resistance."


November 9, 2012

Cooking key to cognition; corals recruit bodyguards; etc.

Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution "This limitation was probably overcome in Homo erectus with the shift to a cooked diet."

Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds "Mutualistic gobies and corals appear to represent a marine parallel to terrestrial ant-plants, in that the host provides shelter and food in return for protection from natural enemies."

Bateman in Nature: Predation on Offspring Reduces the Potential for Sexual Selection "substantial yearly variation in the Bateman slope [number of offspring as a function of the number of mates] due to predation on fawns was evident."

Drift-barrier hypothesis and mutation-rate evolution "selection appears to reduce the mutation rate... to a level that scales negatively with both the effective population size (Ne)... and the genomic content "


For a list of my upcoming talks around the world, see my new Darwinian Agriculture Blog.

July 30, 2012

Correcting Science and PNAS, evolution of more-deadly suicide bombers, blind composers, transcending tradeoffs, light-responsive rhizobia

Absence of Detectable Arsenate in DNA from Arsenate-Grown GFAJ-1 Cells
GFAJ-1 Is an Arsenate-Resistant, Phosphate-Dependent Organism
Surprise! They don't use arsenic instead of phosphorus after all.

Unobserved time effects confound the identification of climate change impacts "PNAS reported statistical evidence of a weather-driven causal effect of crop yields on human migration from Mexico to the United States. We show that this conclusion is based on a different statistical model than the one stated in the paper."

Evolution of music by public choice ...as in natural selection, variation is random but selection isn't.

Explosive Backpacks in Old Termite Workers ...they weren't going to reproduce anyway, but I wonder how long these "suicide vests" took to evolve.

Compensatory mechanisms for ameliorating the fundamental trade-off between predator avoidance and foraging "enhanced nutritional physiology allows caterpillars to compensate when threatened. However, we report physiological costs of predation risk, including altered body composition (decreased glycogen) and reductions in assimilation efficiency later in development." Similarly, reducing investments in research may not affect economic development much between now and the next election.

Light regulates attachment, exopolysaccharide production, and nodulation in Rhizobium leguminosarum through a LOV-histidine
"illumination of bacterial cultures before inoculation of pea roots increases the number of nodules per plant and the number of intranodular bacteroids."

January 12, 2012

Flycatchers use early birds (great tits) to crowd-source clutch size

A bird that tries to raise too many chicks this year may not survive to reproduce again. So there's a tradeoff between current and future reproduction. Earlier, I discussed how stress hormones over-worked mothers pass to their chicks decrease survival of this year's chicks, whereas the reduced work-load can increase the mother's life-long reproduction. But how many chicks is too many? It depends on food supply and other conditions that vary from year to year.

Rather than researching conditions themselves, flycatchers could wait to see how many eggs other flycatchers lay. This is the approach many humans use to decide how much to invest in stocks, but it doesn't work any better for the flycatchers than it does for us. By the time they figure out what others are doing, it's too late to take advantage of the information.

An alternative crowd-sourcing approach is to see how many eggs were laid by another species of bird that finishes egg-laying a bit earlier. Indeed, Jukka Forsman and colleagues report, in a recent paper in Biology Letters, that "Observed heterospecific clutch size can affect offspring investment decisions."

Continue reading "Flycatchers use early birds (great tits) to crowd-source clutch size " »

July 8, 2011

Resistance is futile!

Pathogens and pests evolve resistance to our control measures, from antibiotics and pesticides to crop rotation and pest-resistant crop varieties. Slowing the evolution of resistance is an important practical application of evolutionary biology.

An iconic agricultural example, discussed in my forthcoming book, is the "high-dose/refuge strategy" to slow the resistance of crop-eating insects to the bacterial toxin, Bt, which has been genetically engineered into corn, cotton and other crops. The "high dose" refers to crop Bt levels high enough that only insects with two resistance genes (genotype rr) can survive. Bt-free refuges serve as a source of so many susceptible (ss) insects that any rs mutants that arise will mate with them (producing susceptible ss and rs progeny) rather than with each other (with 25% of their progeny resistant rr).

But rs mutants could arise in the Bt-free refuge, not just in the Bt crop. If, in the refuge, the fitness of rs mutants is as high as that of ss insects (i.e., if there is no cost to Bt resistance), then rs individuals could become common enough that two of them could mate, producing rr progeny that could then devastate the nearby crop. So it would be good if, in the refuge, rs insects had lower fitness than ss insects.

This week's paper shows one way that this goal might be achieved. "Fitness Cost of Resistance to Bt Cotton Linked with Increased Gossypol Content in Pink Bollworm Larvae" was published recently in PLoS One.

Continue reading "Resistance is futile!" »

April 8, 2011

This week's picks

Workers influence royal reproduction
"worker aggressive and non-aggressive behaviour towards queens predicted which queen monopolized reproduction. In contrast, among-queen interactions were rare and did not predict queen reproduction. Furthermore, parentage analysis showed workers favoured their mother when present"
[Maybe "inclusive fitness" is useful after all!]

Updated chronology for the Miocene hominoid radiation in Western Eurasia
"Eurasian pongines [orangutans and extinct relatives] and African hominines [humans, chimps, bonobos, and extinct relatives] might have independently evolved in their respective continents from similar kenyapithecin ancestors [apes living 14 million years ago], resulting from an early Middle Miocene [5-23 MYA] intercontinental range extension followed by vicariance [geographic separation, reducing or eliminating interbreeding so allowing evolutionary divergence]. "

Ribozyme-Catalyzed Transcription of an Active Ribozyme "we recombined traits evolved separately in different ribozyme [catalytic enzyme made of RNA rather than protein] lineages. This yielded a more general polymerase ribozyme that was able to synthesize a wider spectrum of RNA sequences, as we demonstrate by the accurate synthesis of an enzymatically active RNA, a hammerhead endonuclease ribozyme. "

An evolutionary process that assembles phenotypes through space rather than through time "assortative mating between fast-dispersing individuals at the invasion front results in an evolutionary increase in dispersal rates in successive generations"

Fork-tailed drongos use deceptive mimicked alarm calls to steal food
"false alarm calls when watching target species handling food, in response to which targets flee to cover abandoning their food"

Moving calls: a vocal mechanism underlying quorum decisions in cohesive groups
"a sharp increase in the probability of changing foraging patch when the number of group members joining the chorus increased from two up to three"

Differences in the temporal dynamics of phenotypic selection among fitness components in the wild "The consistency in direction and stronger long-term average strength of selection through mating success and fecundity suggests that selection through these fitness components should cause more persistent directional evolution relative to selection through survival."

Rapid Spread of a Bacterial Symbiont in an Invasive Whitefly Is Driven by Fitness Benefits and Female Bias "Rickettsia sp. nr. bellii swept into a population of an invasive agricultural pest, the sweet potato whitefly, Bemisia tabaci, in just 6 years. Compared with uninfected whiteflies, Rickettsia-infected whiteflies produced more offspring, had higher survival to adulthood, developed faster, and produced a higher proportion of daughters. The symbiont thus functions as both mutualist and reproductive manipulator. "

The evolutionary biology of child health "cancer, the primary cause of non-infectious childhood mortality, mirrors child growth rates from birth to adolescence, with paediatric cancer development impacted by imprinted genes"

Tradeoffs associated with constitutive and induced plant resistance against herbivory "Across all 58 plant species, we demonstrate a tradeoff between constitutive and induced resistance, which was robust to accounting for phylogenetic history of the species. Moreover, the tradeoff was driven by wild species and was not evident for cultivated species."

Towards a quantitative understanding of the late Neoproterozoic carbon cycle
"all of the main features of the carbonate and organic carbon isotope record can be explained by the release of methane hydrates from an anoxic dissolved organic carbon-rich ocean into an atmosphere containing oxygen levels considerably less than today"

March 31, 2011

This week's picks

Chimpanzees help conspecifics obtain food and non-food items "...given that the donor cannot get the food herself.... the key factor... is the recipients' attempts to either get the food or get the attention of the potential donor."

On the earliest evidence for habitual use of fire in Europe
"...spectacular cases of Neandertal pyrotechnological knowledge..."

Sizing up your enemy: individual predation vulnerability predicts migratory probability
"trade-off between seasonal fluctuations in predation risk and growth potential... Smaller, high-risk individuals migrate with a higher probability"

Plant-ants feed their host plant, but above all a fungal symbiont to recycle nitrogen
"In many ant-plant symbioses, a fungal patch grows within each domatium."

More closely related species are more ecologically similar in an experimental test
"Species also competed more with close relatives than with distant relatives in field soils; however, in potting soil this pattern reversed..."

Assassin bug uses aggressive mimicry to lure spider prey "vibrations from bugs had a temporal structure and amplitude... similar to vibrations generated by leg and body movements of prey and distinctly different... from courting males or leaves..."

Social and Ecological Synergy: Local Rulemaking, Forest Livelihoods, and Biodiversity Conservation "participation in forest governance institutions by local forest users is strongly associated with jointly positive outcomes"

Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates "cold local climatic conditions linked to the paleolatitudinal position of northeastern China and global icehouse climates..."

Adaptation to local ultraviolet radiation conditions among neighbouring Daphnia
"we separated the effects of shared population ancestry and environmental variables in predicting phenotypic divergence among populations."

June 25, 2010

Are scientists smarter than squirrels?

"Monkey-watchers often use the word "aunt" for an adopting female." -- Richard Dawkins, The Selfish Gene
The willingness of animals to adopt and care for orphans has been shaped by past natural selection. Often, Dawkins suggested, adoption represents "misfiring of a built-in rule... a mistake that happens too seldom for natural selection to have 'bothered' to change the rule by making the maternal instinct more selective." This seems a reasonable explanation for the failure of bird parents to kick "brood parasites" out of their nests, a situation I discussed recently.

But this week's paper, by Jamieson Gorrell and colleagues, seems to show that squirrels have a more-sophisticated understanding of selfish-gene theory than I would have expected. "Adopting kin enhances inclusive fitness in asocial red squirrels" was recently published in the new online journal Nature Communications. The authors analyzed five cases of orphaned squirrels being adopted, all by close relatives, and two cases where they were left to die, even though a relative had a territory nearby. In each case, they asked whether adopting would likely increase or decrease the frequency of the adopter's genes in future generations.

Closely related individuals tend to share gene variants (alleles) even if those alleles are rare in the overall population, so adopting a younger sister or a nephew who would otherwise die could increase one's genetic representation in future generations. On the other hand, adding an orphan to one's litter puts one's own offspring at somewhat greater risk. The authors were able to estimate this risk and compare it to the increased survival chances of the adoptee, weighted by its relatedness to the foster mother. If this benefit exceeds the risk, then Hamilton's rule (the fundamental equation of social evolution) predicts adoption. All of the adoptions that did occur met this criterion -- two cases were right on the line -- whereas the two potential adoptions that didn't occur failed the Hamilton's-rule test. Yet another example of squirrels solving challenging problems.

At least, that's what the data seemed to show. But the "relatedness" term in Hamilton's rule isn't necessarily equal to the relatedness we could calculate from a family tree or from genetic similarity. It would be, if helping an orphan had no negative effect on anyone outside one's current litter. But if there are more red squirrels than red-squirrel territories, then a surviving orphan may end up displacing another squirrel. So the question is, how closely related is that displaced squirrel likely to be to the adoptive mother? In the cases studied, 1/4 to 1/2 of the lactating females nearby were kin to the adopting mother. If that's a representative sample, then a surviving orphan might often end up displacing another squirrel that was as closely related to the mother as the orphan was. In such cases, the mother would have exposed her own litter to increased risk, without doing much to increase her genetic representation in future generations. Even so, the adoptive mothers aren't acting as maladaptively as Dawkins suggested (as if they adopted orphans at random), but their behavior wouldn't be optimal (by Hamilton's rule) unless there were unoccupied territories available nearby. Thanks to Dr. Carin Bondar, whose blog alerted me to this interesting paper.

Meanwhile, over at Science, Jeff Smith and colleagues propose "A generalization of Hamilton's rule for the evolution of microbial cooperation." When one cooperative act (releasing an expensive enzyme, say) benefits all microbes nearby, it's common to assume we can add up all the costs and benefits over a population. But what if twice the enzyme gives three times the benefit? The authors developed some high-powered math to deal with such problems and concluded that certain kinds of cheaters would have a harder time getting established than we would have expected from the simpler version of Hamilton's rule. Scientists are definitely smarter than squirrels, but they can't jump as well.

March 5, 2010

Evolution via less-fit intermediates

A central hypothesis in my forthcoming book, "Darwinian agriculture: where does Nature's wisdom lie?" is that past natural selection is unlikely to have missed simple, tradeoff-free improvements. This implies (as discussed in a recent post on drought-tolerant wheat) that tradeoff-blind biotechnology is less likely to succeed, relative to crop-improvement methods that consider tradeoffs, as long as biotechnology is limited to simple changes, like increasing the expression of an existing gene.

More complex improvements (those whose evolution would require a series of steps) are another story, however. Just because some hypothetical horse would kick ass, if it did evolve, doesn't guarantee that it will evolve. The problem is that you can't get from genotype A to some very different genotype Z, except through one or more generations of individuals with intermediate genotypes.

It's fairly easy to get from A to Z, provided that B is at least as fit as A, while C is at least as fit as B, and so on. This can be the case, as shown by experiments on the five-step evolution of antibiotic resistance, discussed in a previous post. But is this the only way a population can evolve a superior genotype? Or does evolution sometimes reach new heights (faster-flying birds, scummier pond scum, etc.) through intermediates that are significantly less fit?

Evolution via less-fit intermediates would expand evolution's options, making it even harder for biotechnology folks to come up with something missed by evolution. And that's what this week's paper seems to show.

"Compensatory evolution in mitochondrial tRNAs navigates valleys of low fitness" was recently published in Nature by Margarita Meer and colleagues.

Continue reading "Evolution via less-fit intermediates" »

November 11, 2009

Experimental evolution of bet hedging

Will headshot.jpg
Guest blogger: Will Ratcliff

This week's paper, "Experimental evolution of bet hedging" by Hubertus Beaumont, Jenna Gallie, Christian Kost, Gayle Ferguson and Paul Rainey, published in Nature, shows that a trait that initially evolves for non bet hedging purposes can be maintained in the population through bet hedging.

The theory of bet hedging was first mathematically developed by Daniel Bernoulli (yes, the Bernoulli we all learned about in high school physics) in 1738. Because the basic idea is so simple - uncertain future conditions make conservative strategies beneficial - it is likely that folk wisdom advising bet hedging long predates Bernoulli's maths. The phrase "Don't put all your eggs in one basket" is one example of a widespread but anachronistic reminder to spread risk. Before we dive into this week's paper, I want to briefly cover the theory of bet hedging.

Like investing in the stock market, evolution is a multiplicative process, not an additive one. Steve Stearns (2000) illustrates this well....

Continue reading "Experimental evolution of bet hedging" »

July 9, 2009

Has natural selection been asleep at the switch?

"This new forage has great insect resistance", effused a former colleague, "we just need to eliminate the toxins that keep sheep from eating it."

Genetically engineered drought-tolerant crops are introduced with great fanfare, only to disappear when they turn out to have low yield under nondrought conditions.

When natural selection falls short of perfection, it may be because "you can't get there (some desirable adaptation) from here (current genotypes)" without passing through a series of intermediate generations that would have lower fitness. Natural selection favors genotypes best-adapted to current conditions, which are not necessarily steps towards any long-term improvement.

But natural selection often seems to miss even "simple" improvements, that might be achieved by changing as little as one DNA base. Such small changes are often enough to increase or decrease expression of key genes, for example. This sort of evolutionary progress may be blocked by tradeoffs, e.g., between seed production under different conditions (e.g., wet vs. dry), or between the competitiveness of individual plants and their collective seed production.

So what are we to make of two recent papers (in Science and Nature, respectively, discussed in Science News) on extending lifespan, one using calorie restriction and the other using the antibiotic, rapamycin?

Calorie restriction has been shown to increase longevity in model species like nematode worms and mice, but this latest study shows clear benefits in monkeys. The obvious question -- at least, it was obvious to me -- is why has past natural selection given monkeys (and fruitflies, and nematodes, and mice...) appetites that make them eat more than is good for them?

At least, that seemed to be the question, until it was shown that food odors can reverse the beneficial effects of calorie restriction, at least in fruitflies and nematodes. In humans, soft drinks with artificial sweeteners turn out to be just as likely to cause "metabolic syndrome" (related to diabetes) as those with sugar. So apparently our lives can be shortened by a perception of abundance, not just by actually eating too much. What is going on here?

In this case, the evolutionary tradeoff seems to be between current and future reproduction. As discussed in last week's post, delaying reproduction usually decreases fitness (representation in the next generation, relative to others) when population is increasing, but delaying reproduction can increase fitness when population is decreasing. Calorie restriction predicts population decline, triggering physiological responses that delay reproduction and thereby increase longevity. So do bitter-tasting foods, traditionally eaten only during famines. Food odors or sweet tastes have the opposite effect, because they predict population increase.

But what about life extension by rapamycin? One known tradeoff is suppression of the immune system, so we might get longer lives only in a hypothetical germ-free environment. But could the protein target of rapamycin (TOR) also be important to reproduction? Is this yet another example of a longevity-vs.-reproduction tradeoff?

July 6, 2009

Throwing the longevity switch

If you could choose a longer, healthier life, but only by having fewer kids, would you? What if you could eventually have the same number of kids, but only by having sex more often, and with no possibility of becoming a parent as a teen-ager?

Is this really possible? Based on the paper we published last week, we are pretty sure it is, although we don't yet know how much of an increase in lifespan is achievable, nor how much it will "cost" in reduced fertility.

A key assumption is that there are tradeoffs between longevity and reproduction, especially early reproduction. There is plenty of evidence for this antagonistic pleiotropy hypothesis: some gene variants that increase longevity nonetheless stay rare, because individuals with those variants have fewer kids. There are many possible reasons for this tradeoff. Calories used for reproduction aren't available for maintaining our bodies. Blood pressure and insulin levels optimal for reproduction are unlikely to be exactly optimal for longevity. Other risks associated with reproduction include sexually transmitted diseases and direct risks of childbirth. When there is a conflict between reproduction and longevity, natural selection will often favor reproduction.

There are, however, two ways we may be able to choose differently, increasing longevity at the expense of (potential, but maybe not actual) reproduction. First, once germ-line gene therapy is perfected and available (initially, perhaps, only in one or two "outlaw states"), maybe we could reverse some of the effects of past natural selection. We might be able to produce genetically engineered kids who would reach puberty later and with low enough intrinsic fertility that occasional unprotected sex would rarely lead to pregnancy, but who would still be healthy at age 100.

Second, what about people already born? Is there some biological "switch" we can throw, that tilts the longevity-vs.-reproduction tradeoff more towards longevity? Or has past natural selection welded the switch in the "reproduce now" position?

We think the switch is free to move, depending on environmental cues that affected our ancestors' survival and reproduction. Our paper shows that the switch position that maximizes Darwinian fitness depends on whether the overall population is increasing or decreasing. If population is decreasing, then individuals that live longer and reproduce later can contribute a larger fraction to their species' (shrunken) gene pool than those that reproduce earlier, on average, even if a few of them die before they get a chance to reproduce, and even if their lifetime reproduction is less than they might have achieved earlier.

Therefore, even though gene variants that always sacrifice early reproduction to increase longevity may not have persisted in the gene pool, variants that delay reproduction (thereby increasing longevity) only when populations were decreasing are likely to be with us, in each of our DNA molecules, today.

If this is true, all we need to do to increase our longevity is to give our bodies (false) cues that, over our evolutionary history, usually predicted population declines. To the extent that population declines were caused by food shortage, eating less may work, as it does in most species tested. Eating "famine foods" (leaves rather than meat, maybe) may also trigger physiological responses that reduce fertility but extend lifespan. On the other hand, if population declines were usually caused by cold winters, is there some reasonably comfortable way to trigger similar responses?

Delaying reproduction can only increase fitness if it increases the chances of surviving the famine or cold winter and reproducing later. So stresses that often predicted the death of the stressed individual (those associated with violent conflict, perhaps) won't necessarily delay reproduction or increase longevity. But there are lots of examples of mild stress increasing longevity. These stresses presumably trigger health-and-longevity-promoting mechanisms, but we may be the first to explain why such beneficial mechanisms aren't turned on all the time: they tend to reduce fertility.

Now, here's a question for you: would increasing human longevity be a good thing? I've seen this issue discussed in various places, but rather superficially. Assume that this option was made available to everyone, given that the cost could be quite low: inexpensive drugs or lifestyle changes that might even save money. Death rates would go down, in the short run, but so would birth rates, especially in countries where birth control is now rare. Death from old age is a fairly small component of overall population trends in these countries (relative to birth rate and infant mortality), so their rate of population increase might actually slow. But, if people expected to live longer, would they have more children (despite lower intrinsic fertility) or fewer, and at what age? Assuming some increase in population, we might need to grow more food -- a significant challenge -- but how would the overall impact of two healthy 90-year-olds who are still working (perhaps as doctors or nurses) and driving compare to that of one 90-year old who doesn't drive but needs expensive medical care? If professors keep working into their 90's, will that slow the spread of good new ideas, or only of stupid ideas that younger faculty may not know were debunked long ago? Would a longer-lived population produce too many bloggers?

September 20, 2008

Who suffers from stress?

Recently, I wrote about how grooming each other can reduce levels of stress hormones, for example, in baboons and birds. But I asked, “why should natural selection allow excessive levels of this stress hormone??
This week’s paper shows one way that natural selection can lead to harmful levels of stress hormones. The question, of course, is “harmful to whom??

Writing in American Naturalist, Oliver Love and Tony Williams report that stressed mother birds pass stress hormones to their offspring. (Passing your stress on to others seems to be popular in humans also.) These hormones increase the risk of chicks dying, especially male chicks. But they may also increase the mother’s lifetime reproductive success.

Continue reading "Who suffers from stress?" »

June 29, 2008

Evolution 2008: sexy plants, battling bacteria, durable cooperation

About 1500 scientists attended Evolution 2008 here last week. The four-day meeting was filled with 15-minute talks (usually ten at once, in different rooms), plus two evening poster sessions (like a science fair, for grownups, with discussions rather than judging), scenically located on a pedestrian bridge over the Mississippi. Reports that “scientists are abandoning evolution�? appear to be exaggerated.

Here are summaries of some of the talks I enjoyed.

Continue reading "Evolution 2008: sexy plants, battling bacteria, durable cooperation" »

February 18, 2008

Natural enemies complicate reproductive tradeoffs

Semelparous plants and animals are those that reproduce only once, whether after a few months of growth (annual plants, like wheat) or after years (“century plant? or most salmon). Iteroparous species iterate. That is, they reproduce repeatedly. For example, perennial grasses may produce seeds every year for a decade or more.

One reason this difference matters is that perennial crops may have some environmental benefits, relative to annual crops. Plowing, traditionally more common with annual than perennial crops, can greatly increase soil erosion, especially on steep slopes. So there is increasing interest in developing perennial grain crops as an alternative to wheat.

However, perennial plants have lower seed yield than their annual relatives, so we would need to devote more land to agriculture to get the same amount of grain. One reason for the yield difference is that an annual plant can transfer most of the carbon (energy) and nitrogen (needed for protein) from its leaves, stem, and roots into its seeds. It’s going to die anyway, so the next generation gets its accumulated wealth. A perennial plant needs to hold back some carbon and nitrogen for winter survival and spring regrowth. The more resources it puts into this year’s seed production, the less it can carry forward to support reproduction next year.

This week’s paper shows that iteroparous plants face additional costs when they reproduce, namely, ecological costs. “Herbivore-mediated ecological costs of reproduction shape the life history of an iteroparous plant? was written by Tom Miller and colleagues at the University of Nebraska (where I’ll be speaking on Darwinian Agriculture in April) and published in American Naturalist.

Continue reading "Natural enemies complicate reproductive tradeoffs" »

September 27, 2007

Cooperation and cheating in microbes: quorum sensing and persisters

Two papers on cooperation this week. If you were trying to help someone, but end up causing problems for them, were you being cooperative? I have no idea, so I like to study cooperation in microbes. Microbes don't have brains, so "intent" isn't a factor. And the only definition of "benefit" that makes sense is an increase in Darwinian fitness or reproductive success, which is often easy to measure in microbes; just count them.
I like these definitions:

Cooperation: a behaviour which provides a benefit to another individual (recipient), and which is selected for because of its beneficial effect on the recipient. [Exhaling CO2 isn't cooperation; it evolved as a side-effect of breathing oxygen, not to benefit plants.]
Cheaters: individuals who do not cooperate (or cooperate less than their fair share), but are potentially able to gain the benefit of others cooperating. ["Equal share" might be less ambiguous.]

Continue reading "Cooperation and cheating in microbes: quorum sensing and persisters" »

September 20, 2007

Menopause trade-offs

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" »

August 17, 2007

Almost a no-brainer

How sophisticated behavior would you expect from an animal with a brain as small as a wasp's? Few, if any, female wasps have read David Lack's classic paper on the optimum number of eggs to lay, or even John Dennehy's clear summary of it. This week's paper asks whether they, nonetheless, adjust egg numbers optimally in response to competition from other wasps and resource availability.

"Encountering competitors reduces clutch size and increases offspring size in a parasitoid with female–female fighting" was written by Marlene Goubault, Alexandra Mack, and Ian Hardy, of the University of Nottingham, and published in Proceedings of the Royal Society.

Continue reading "Almost a no-brainer" »

June 24, 2007

Trade-offs in defense against retroviruses

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" »

April 6, 2007

Darwinian agriculture I

Next week, I'm speaking at a one-day symposium on "Darwinian Agriculture: the evolutionary ecology of agricultural symbiosis", in Wageningen, The Netherlands. So, instead of reviewing a recent paper, this week I'm going to discuss some of the not-quite-so-recent papers on which my talk will be based. The following week, I plan to summarize some of the talks I hear at the meeting.

I may do the same thing in August, when my grad students and I speak at the much larger Ecological Society of America meetings in San Jose, California. Feel free to comment if you feel cheated of your weekly paper review, and I might reconsider. The Evolution meetings are in Christchurch, New Zealand, this year, but my grant won't stretch that far.

"Darwinian Agriculture: when can humans find solutions beyond the reach of natural selection?" was the title of a paper that Toby Kiers, Stuart West, and I published in 2003. Our answers to the title question suggested how increased understanding of past and ongoing evolution could improve: 1) breeding of crops and livestock, and 2) design of agricultural ecosystems.

Continue reading "Darwinian agriculture I" »

February 20, 2007

Evolutionary trade-offs: how are soybeans like salmon?

Answer: they're both semelparous (reproduce once, then die), so evolutionary trade-offs between number and size of offspring are expected to be similar.

This week's paper is "Evolutionary aspects of the trade-off between seed size and number in crops" (Field Crops Research 100:125-138) by Victor Sadras. You can read the abstract on the web for free. For the full version, you can pay $30 to download, visit your nearest agricultural research library (in the U.S., often at a state university), or email the author at: sadras.victor@saugov.sa.gov.au. My discussion is mostly based on a shorter version presented at the Australian Agronomy Conference.

Demand for grain is increasing, to feed growing human and livestock populations and more recently for ethanol production. Unless those trends are reversed, we will either need to expand the land area used for agriculture or increase grain yields per unit area. Grain yield is the product of plants per area, seeds per plant, and weight per seed. Unfortunately, increasing any one of these (by increasing seeding rate, or through plant breeding) tends to decrease the others.

This paper looks at how natural selection (in the wild ancestors of crop plants and in fish) and plant breeding (especially in maize and sunflower) shape trade-offs between seed number per plant and seed size. The similar patterns in plants and fish show that, as predicted by the relevant aspects of evolutionary theory, we are dealing with fundamental constraints that we are unlikely to change.

Continue reading "Evolutionary trade-offs: how are soybeans like salmon?" »