Sex is always well worth its two-fold cost

Sex is considered as an evolutionary paradox, since its evolutionary advantage does not necessarily overcome the two fold cost of sharing half of one's offspring's genome with another member of the population. Here we demonstrate that sexual reproduction can be evolutionary stable even when its Darwinian fitness is twice as low when compared to the fitness of asexual mutants. We also show that more than two sexes are always evolutionary unstable. Our approach generalizes the evolutionary game theory to analyze species whose members are able to sense the sexual state of their conspecifics and to switch sexes consequently. The widespread emergence and maintenance of sex follows therefore from its co-evolution with even more widespread environmental sensing abilities.Comment: 8 pages, 3 figure

Evolution of sexual reproduction as development of sex switching and sensing abilities.

<p>Contour plot of <i>R</i> (probability to possess sex complementary to the environment) in populations with varying individual switch and sensor characteristics. The switch/sensor complex is described by and , representing conditional probabilities to possess sex <i>F</i> in <i>M</i> and <i>F</i> environments respectively. The single point × denotes a fully developed sexual population () while there exist multiple possibilities for asexual populations (). Evolution is equivalent to the motion of a point, denoting a population, from an asexual state to the sexual endpoint. Specific evolutionary mechanisms correspond to different evolutionary pathways.</p

Evolutionary stability of sexual reproduction.

<p>The equation for evolutionary stability (13) defines positive and negative semi-planes, corresponding to favorable and non-favorable mutations, dependent on payoffs <i>b</i> and <i>c</i>. A population with arbitrary and (I) will always dispose of a positive region, precluding so an evolutionary stable solution. A sexual population (II) is evolutionary stable since no positive direction is available. The population on the (III) and (IV) edges are also unstable. If we assume opposite signs for the semi-planes as consequence of different payoff values, the asexual populations (positioned on the edges or ) become stable, while the sexual population becomes unstable.</p