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?