American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

Symposium: “When synchrony makes the best of both worlds even better: how well do we really understand facultative sex?”

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Hanna Kokko (Feb 2020)

Read the Article (Just Accepted)

One can often hear evolutionary biologists describe sexual reproduction as one of the major puzzles in evolutionary theory, sometimes with the addition that facultative sex is an even bigger problem. The problem is that creating offspring sexually is surprisingly inefficient (compared with asexual reproduction), but whenever we are tempted to claim we have understood why sex is on balance selected for, we should also deal with an extra complication: the genetic benefits and economic (demographic) costs scale nonlinearly and do not increase at the same rate when the frequency of sex increases. Very rare sex – for example, once every 100 or 1000 generations – appears, for many models, to be almost as good as obligate sex at producing the adaptive benefit, but it avoids paying the cost in most generations. This is why facultative sex is sometimes stated to offer the best of both worlds, and the consequent puzzle is why not all life is organized that way. This paper does not claim to have found the ultimate solution. Instead, it highlights that existing theories sometimes make simplifications that make the problem mathematically more tractable, but ignore real-life complications that may really matter. One is that facultative sex occurs in synchrony, which links theories of sex with those of bet-hedging. This paper shows why synchrony makes facultative sex even better: if sex is rare, synchrony alleviates mate-finding problems, and if sex has – due to some ecological change – become costlier than before, synchrony helps finding a better rate of sex much more quickly.


Biological diversity abounds in potential study topics. Studies of model systems have their advantages, but reliance on a few well understood cases may create false impressions of what biological phenomena are the ‘norm’. Here I focus on facultative sex, which is often hailed as offering the best of both worlds, in that rare sex offers benefits almost equal to obligate sex, and avoids paying most of the demographic costs. How well do we understand when and why this form of sexual reproduction is expected to prevail? I show several gaps in the theoretical literature, and by contrasting asynchronous with synchronous sex, I highlight the need to link sex theories to the theoretical underpinnings of bet-hedging on the one hand and to mate limitation considerations on the other. Condition-dependent sex, and links between sex with dispersal or dormancy, appear understudied. Simplifications on the one hand are justifiable as a simple assumption structure enhances analytical tractability, but on the other hand, much remains to be done to incorporate key features of real sex to the main theoretical edifice.