This week’s BEACON Researchers at Work blog post is by University of Washington graduate students Katie Dickinson and Sarah Hammarlund and postdoc Brian Connelly.
Brian Connelly, Katie Dickinson, and Sarah Hammarlund are all thumbs!
Hold your hand out in front of you and examine it closely. Five digits, four fingers and a thumb… What a useful appendage, the thumb! You can give a friend an encouraging thumbs-up or maybe a death sentence in a gladiator coliseum with a thumbs-down. You can twiddle your thumbs during a boring seminar, thumb wrestle with your brother, tend your garden with your green thumb, or stick out like a sore thumb. We use our thumbs to text and type and write, to hold a spoon, to touch. But arguably the most important use for a thumb is to hitchhike.
Humans can hitchhike to get to Burning Man, robots can hitchhike around the world (http://m.hitchbot.me/), and even genes can hitchhike during evolution. In the case of genes, a trait can become more common if it is genetically linked with another trait that is favored by natural selection. Otto and Hartfield[i] have shown that even costly, maladaptive traits can survive by “genetic hitchhiking.” One such costly trait is cooperation.
Evolutionary biologists including Darwin have been puzzled by the evolution of cooperation. We would expect natural selection to favor selfish individuals that maximize their own fitness. In contrast, cooperative behaviors benefit others at a cost to the cooperator. Furthermore, cooperators continually face the risk of being exploited by “defectors,” individuals that don’t cooperate but reap the benefits of cooperation. So how did cooperation come to be so prevalent in our world?
One way for cooperation to succeed is through genetic hitchhiking. If cooperation, a costly trait, becomes linked to an adaptive trait (one that increases survival in a harsh environment, for example), cooperation can hitchhike with that trait. For this to work, the selective advantage of the adaptive trait must outweigh the cost of cooperation. This process has been demonstrated to support both yeast[ii] and bacterial cooperators[iii],[iv].
The challenge with genetic hitchhiking is that cooperators and defectors are equally likely to gain these adaptations through mutation. Once one type gets lucky and catches a mutational ride, it leaves the other in the dust. So how can cooperators consistently get a thumbs-up? We have recently been exploring ways in which cooperators can actively increase their chances of gaining adaptations.
One way cooperators can more reliably catch a ride is by sticking together. When cooperators preferentially interact with other cooperators, their growth is boosted by the benefits of cooperation. With more growth comes more mutations, and each mutation offers an opportunity to gain an adaptation. By working together, cooperators increase their chances of hitchhiking—they are now much more likely catch a ride than defectors. So in a sense, cooperators have larger thumbs—they are more visible to passing cars and therefore have a higher likelihood of catching a ride with an adaptive trait.
We call this phenomenon the “Hankshaw effect” after the fictional character Sissy Hankshaw from Tom Robbins’ novel Even Cowgirls Get the Blues. Hankshaw was born with extremely oversized thumbs. She’s teased for her thumbs as a child and has trouble even just buttoning up her sweaters, but she eventually discovers that her thumbs make her an excellent hitchhiker. For Hankshaw, a trait that is an impairment becomes her salvation on the open road. We constructed a model to see how the Hankshaw effect might allow cooperators to hitchhike their way to success[v].
Cooperation and the Hankshaw effect. Although equally present in the beginning, cooperators are quickly driven to extinction when they are not more likely to gain adaptations than defectors (above). However, when cooperators improve their chances for gaining adaptations, they can hitchhike along with these non-social traits to dominance by the Hankshaw effect (below). The ride abruptly ends once cooperators become fully adapted, and adapted defectors eventually take over.
We found that the Hankshaw effect can allow cooperators to consistently hitchhike and escape the threat of defectors, but only as long as there are beneficial traits to be gained. Once cooperators become fully adapted to their environment and the ride ends, mutations create equally-adapted defectors that take over. This makes the Hankshaw effect only temporary.
However, if the environment changes and new opportunities for adaptation are created, the ride may not be over for cooperators. And if environmental change continually occurs, then cooperators can be maintained by the Hankshaw effect as long as there are opportunities for adaptation.
We have taken this idea one step further and allowed organisms themselves to change the environment instead of passively waiting for change to occur. We found that this “niche construction”[vi],[vii] can allow cooperators to create opportunities for adaptation that keep the ride going indefinitely[viii].
When the environment changes (at each vertical line), creating a continual supply of potential adaptations, cooperators can continue to hitchhike and are maintained indefinitely.
Although at first, from an evolutionary perspective, cooperation seems like a losing strategy, there are ways that cooperators can succeed. Maybe Sesame Street had it right: “Cooperation—Makes it Happen.”
[i] Hartfield, M. and Otto, S. P. 2011. Recomination and hitchhiking of deleterious alleles. Evolution, 65: 2421–2434. doi: 10.1111/j.1558-5646.2011.01311.x
[ii] Waite, A. J. and W. Shou. 2012. Adaptati
on to a new environment allows cooperators to purge cheaters stochastically. Proc. Natl. Acad. Sci. USA 109:19079-19086.
[iii] Morgan A.D., B. J. Z. Quigley, S. P. Brown, and A. Buckling. 2012. Selection on non-social traits limits the invasion of social cheats. Ecol. Lett. 15:841-846
[iv] Asfahl, K. L., J. Walsh, K. Gilbert, and M. Schuster. 2015. Non-social adaptation defers a tragedy of the commons in Pseudomonas aeruginosa quorum sensing. ISME J. doi:10.1038/ismej.2014.259.
[v] Hammarlund SP, Connelly BD, Dickinson KJ, Kerr B. 2015. The evolution of cooperation by the Hankshaw effect. bioRxiv. doi:10.1101/016667.
[vi] Odling-Smee, F. J., K. N. Laland, and M. W. Feldman. 2003. Niche construction: the neglected process in evolution (No. 37). Princeton University Press.
[vii] Laland, K. N., F. J. Odling-Smee, and M. W. Feldman. 1999. Evolutionary consequences of niche construction and their implications for ecology. Proc. Natl. Acad. Sci. USA 96:10242- 10247.
[viii] Connelly, B. D., Dickinson, K. J., Hammarlund, S. P., & Kerr, B. 2015. Negative Niche Construction Favors the Evolution of Cooperation. Evol. Ecol. doi:10.1007/s10682-015-9803-6.
