“Evolutionary hysteresis and ratchets in the evolution of periodical cicadas”

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Jaakko Toivonen and Lutz Fromhage (July 2019)

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Evolutionary hysteresis and ratchets promote the evolution of long synchronous life cycles in periodical cicadas

Interesting evolutionary mechanisms underlying the evolution of periodical cicadas

The authors show how two interesting evolutionary mechanisms, hysteresis and ratchets, promote the evolution of long and synchronous life-cycles in the periodical cicadas. Evolutionary hysteresis is a phenomenon wherein evolution not only depends on the current environmental conditions but also on the history of environmental change. Evolutionary ratchets are mechanisms that cause evolution to become “locked” such that it can only proceed in one direction.

The authors show that the evolution of perfectly synchronous life-cycles in the periodical cicadas is unlikely in contemporary climates. However, the harsh environmental conditions encountered during past ice ages may have been instrumental in the evolution of synchronicity. Further, once a synchronous life-cycle is attained it cannot be lost even when the environmental conditions become favorable again (evolutionary hysteresis). The authors show that once periodical cicadas evolved synchronous life cycles they would only have been able to develop ever longer life cycles and it became impossible for them to return to non-synchronous or shorter life cycles (evolutionary ratchet).

While this paper concentrates on a specific case study, the authors hope to inspire others to look for evolutionary ratchets and hysteresis in other contexts as well.


Abstract

It has been previously hypothesized that the perfectly synchronized mass emergence of periodical cicadas (Magicicada spp.) evolved as a result of a switch from size-based to age-based emergence. In the former case cicada nymphs emerge immediately (at the first opportunity) upon reaching maturity whereas in the latter case nymphs wait in order to emerge at a specific age. Here we use an individual-based model to simulate the cicada life cycle and to study the evolution of periodicity. We find that if age-based emergence evolves in a constant abiotic environment, it typically results in a population that is proto-periodic and synchronous emergence of the whole population is not achieved. However, perfect periodicity and synchronous emergence can be attained, if the abiotic environment changes back-and-forth between favorable and unfavorable conditions (hysteresis). Furthermore, once age-based emergence evolves, generally it can only be invaded by other age-based emergence strategies with longer cycle lengths (evolutionary ratchet). Together, these mechanisms promote the evolution of long periodic life cycles and synchronous emergence in the \textit{Magicicada}. We discuss how our results connect to previous theories and recent phylogenetic studies on Magicicada evolution.