September 9, 2018 at 10:46 am

Morris, Co-Authors Examine Reproductive Tactics of Swordfish at Evolutionarily Stable State

Oscar Rios Cardenas and Molly Morris, portrait of the two of them

Drs. Oscar Rios-Cardenas and Molly Morris

Dr. Molly Morris, Professor of Biological Sciences, co-authored an article in Animal Behaviour that examined reproductive tactics of swordfish at an evolutionarily stable state.

Oscar Rios-Cardenas from Mexico’s Instituto de Ecología and Lisa Bono from the Department of Ecology and Evolutionary Biology at Yale University were the other authors of the article, titled Frequency-dependent selection and fluctuations around an equilibrium for alternative reproductive tactics in a swordtail.

“The evolution and maintenance of alternative phenotypes have been of interest since bimodal distribution of traits within a sex was first recorded. Gadgil (1972) described how the escalation of investment in weaponry and combat by some males could come to an end when these individuals are no longer more successful than the individuals that opt out of such investments,” they write in their introduction.

“One mechanism that could precisely equalize the selective advantage of the two alternatives was negative frequency-dependent selection (NFDS), which relies on advantages of the rare genotype (e.g. less competition within alternative phenotypes). Extensive theory and empirical work have expanded on these ideas to include a better understanding of the different trade-offs involved in the evolution of intrasexual phenotypic and genetic variation; predation; female preference, the inheritance and evolution of tactics, and the dynamics involved when there are more than two alternatives,” they write.

“And yet, after almost half a century of extensive research, the circumstances under which competitively mediated NFDS is the main mechanism maintaining equal fitness of alternative morphs as well as the degree to which NFDS is responsible for maintaining reproductive polymorphisms at an evolutionarily stable state are still prominent questions.”

Abstract: Negative frequency-dependent selection (NFDS) should have an important role at maintaining equal fitness among different alternative reproductive tactics (ARTs) at an evolutionarily stable state (i.e. equilibrium). Empirical evidence to support both NFDS and equal fitness in the same system is limited. Using the swordtail fish Xiphophorus multilineatus we examined the hypotheses that NFDS may play an important role in maintaining the genetically influenced ARTs in this species at equal fitness. With a mesocosm study we found that sneaker males had a significantly lower reproductive success when they were more common, as expected. Field estimates of relative reproductive success also supported NFDS: the reproductive success of each tactic decreased when they were more common. We then used the field estimates of relative reproductive success and age to sexual maturity in a modified version of the Euler equation to determine whether the relative fitness of the two ARTs was equal in two different periods. Our analysis incorporated both the zero class of males and the invisible fraction, which are often missing from studies of the maintenance of polymorphisms. We found that the population was not at equilibrium during one sample (sneaker frequency 86%) but was at equilibrium during another (sneaker frequency 38%). We discuss factors that may be driving the fluctuations around the equilibrium, including variation in female preferences for courter males, while NFDS may be producing a stable limit cycle around it.

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