In honor of Charles Darwin’s birthday today, and VD (Valentine’s Day) looming, I’m thinking about how we choose partners. A mobile dating app called Tinder, it turns out, seems to echo sexual selection, Darwin’s idea expounded in The Descent of Man.
This week’s timely post arose when my synapses connected an article just published in Nature’s Scientific Reports Journal, “Risk sensitivity as an evolutionary adaptation,” Darwin’s birthday, and the “Gray Matter” column in last Sunday’s New York Times by Eli J. Finkel, on Tinder.
The app is simple. Images of zillions of potential dates/mates appear on your phone screen. Swipe to the left to reject, swipe to the right to consider.
After observing a Tinder demo, the experience reminded me very much of scanning fruit flies for the phenotypes I was looking for. Or judging pigs at a state fair. Each of us likes certain physical traits, and avoids others. Why? What is the basis of attraction?
SETTLE OR WAIT? A DIGITAL EVOLUTIONARY SCENARIO
DNA Science last ventured into the dating arena with “DNA and Dating: Buyer Beware” a few months ago. So when a news release last week pitched “Evolutionary researchers have determined that settling for ‘Mr. Okay’ is a better evolutionary strategy than waiting for ‘Mr. Perfect,’ I was intrigued, with VD coming up. I requested the paper. (Alas I can’t reprint my classic “If 50 Shades of Grey Had Been Written By A Biology Textbook Author” in honor of tomorrow’s big film debut here because it is far too titillating for the sophisticated readers of PLOS Blogs. But the link still works.☺)
“Risk sensitivity as an evolutionary adaptation” sets up a simulation in a population of digital organisms. The researchers are Arend Hintze, Randy Olson, and leader Chris Adami from Michigan State University and Ralph Hertwig, a psychologist at the Max Planck Institute for Human Development in Berlin. Hintze got the idea to place his imaginary creatures in an evolutionary setting from Hertwig, on visits home to Germany.
As usual the math in the paper scared me, so I zeroed in on the genetics, and found the study’s premise a little weak: “strong evidence that risk-taking behavior has significant genetic components.” Suspicious of genetic determinism, I checked out the two citations.
One study refers to the genetic contribution to risk-taking behavior in the three-spined stickleback, a model organism. That paper, actually a book chapter, “describes some of the challenges in studying the genetic basis of individual differences in risk-taking behavior, arguing new insights will emerge from studies which take a whole-genome approach and which simultaneously consider both genetic and environmental influences on the behavior.” So the genomic link to risk-taking behavior, in fish or us, hasn’t yet been established.
The second paper is in the Quarterly Journal of Economics, but is indeed a genetic investigation. It’s a classic twin study (of humans, not fish) that found a heritability of 20% for “experimentally elicited preferences for risk and giving.” Heritability is often misinterpreted to mean the genetic contribution to a trait. Instead, it refers to the genetic contribution to the variability of a trait in a group. So 20% isn’t much.
But the hypothesis of a relationship between risk-taking and evolution is worth exploring, because we likely give a little more thought to mate choice, Tinder notwithstanding, than do fish.
To explore under what conditions mate choice is random or favors the evolutionary currency of ability to have fertile offspring, the researchers followed the choices of digital organisms put computationally through thousands of generations. (Here’s how you can adopt your own digital organism.)
They measured various things, such as conditions and group size. And they found that situations that prompt risk-taking are rare, perhaps once-in-a-lifetime, with a possible high payoff.
Each digital organism made one lifetime decision: 1 = “the safest gamble” or “live” and 0 = the opposite, death or failure to leave fertile offspring. If it lived, the digital organism passed its status (survivability) to offspring.
“Such a life or death decision is akin to a rare lifetime event that has a large
impact on an individual’s fitness, such as mating and mate competition,” the researchers write. The simulation included a low mutation rate, like in life, so that genomes could change and offer new fodder for selection.
The startling results were that the digital beings were less likely to take risks in collections of fewer than 150 or so – which gibes pretty well with the size of fledgling pre-human groups a million or so years ago. Experiments also simulated small groups within larger ones. “We found that it is really the group size, not the total population size, which matters in the evolution of risk aversion,” Hintze said. Dating in the neighborhood.
Adami translates the results into modern human behavior. “An individual might hold out to find the perfect mate, but run the risk of coming up empty and leaving no progeny. Settling early for the sure bet gives you an evolutionary advantage, if living in a small group. Primitive humans were likely forced to bet on whether or not they could find a better mate. They could either choose to mate with the first, potentially inferior, companion and risk inferior offspring, or they could wait for Mr. or Ms. Perfect to come around. If they chose to wait, they risk never mating.”
In contrast, with Tinder, one might hold out for perfection because the canvas of choices is huge and ever-expanding.
Talk of evolution is often obfuscated by teleology, “the explanation of phenomena by the purpose they serve rather than by postulated causes,” says a standard definition. The phrase “evolves to,” for example, is a 4-letter word for us biology textbook authors, for it implies intent. Natural selection just happens, although it can be directional.
The digital organism paper, I think, has an underlying hint of teleology. The title of the first, unpublished version I read last week said “risk aversion,” which seems to have been changed to “risk sensitivity,” so perhaps the authors were aware of the issue of teleology.
Doubts aside, the findings got me thinking. I imagine myself, an australopithecine perhaps, living in a group of 150 or so, and dating. I’d be, say, 14 or 15 years old, approaching my peak fertility. Would I have had the mental capacity to evaluate who among my limited choices would most likely give me healthy, fertile kids? Do we have that capacity today?
That’s where sexual selection comes in, courtesy of Charles Darwin – the traits that attract us tend to be in fertile individuals. (See this article this article for relevant parts of the tome).
Sexually-selected traits are surrogates for fertility. It’s not as if a woman can peer through a man’s jeans into the labyrinth of his seminal vesicles to see whether his spermatids are maturing into healthy swimmers or not.
A female moose might seek a mate with big antlers.
A female 3-spined stickleback favors a male with a red throat.
Ancient hippos living in what is now the UK grew to enormous lengths (64 feet!) because size seemingly mattered to the ladies.
So while the new study is intriguing in that it matches up the group size of 150 in affecting risk-taking choice with what’s known about early humans, I think the conclusion leads towards teleology – that we did what we did, and do what we do, for a purpose, to intentionally influence evolution.
Happy Valentine’s Day!