Abstract
The number of times an organism reproduces (i.e., its mode of parity) is a fundamental life‐history character, and evolutionary and ecological models that compare the relative fitnesses of different modes of parity are common in life‐history theory and theoretical biology. Despite the success of mathematical models designed to compare intrinsic rates of increase (i.e., density‐independent growth rates) between annual‐semelparous and perennial‐iteroparous reproductive schedules, there is widespread evidence that variation in reproductive allocation among semelparous and iteroparous organisms alike is continuous. This study reviews the ecological and molecular evidence for the continuity and plasticity of modes of parity—that is, the idea that annual‐semelparous and perennial‐iteroparous life histories are better understood as endpoints along a continuum of possible strategies. I conclude that parity should be understood as a continuum of different modes of parity, which differ by the degree to which they disperse or concentrate reproductive effort in time. I further argue that there are three main implications of this conclusion: (1) that seasonality should not be conflated with parity; (2) that mathematical models purporting to explain the general evolution of semelparous life histories from iteroparous ones (or vice versa) should not assume that organisms can only display either an annual‐semelparous life history or a perennial‐iteroparous one; and (3) that evolutionary ecologists should base explanations of how different life‐history strategies evolve on the physiological or molecular basis of traits underlying different modes of parity.
Highlights
Semelparity describes the life history defined by a single, highly fecund bout of reproduction, and can be contrasted with iteroparity (“polycarpy”), the life history defined by repeated (i.e., “iterative”) bouts of reproduction throughout life
Populations of Lobelia inflata are capable of producing a range of different modes of parity, from a nearly instantaneous annual- semelparity, where plants produce many similar flowers quickly and simultaneously, to facultative biennial-iteroparity, where as much as half of all reproductive effort is invested in a second reproductive episode
Models rooted in the conception of parity as a binary trait do a good job of accounting for the fitness differences between discrete semelparous-annual and iteroparous perennial alternative strategies, and, even when they do not make accurate quantitative predictions, they have heuristic value
Summary
Semelparity (and the related botanical term “monocarpy”) describes the life history defined by a single, highly fecund bout of reproduction, and can be contrasted with iteroparity (“polycarpy”), the life history defined by repeated (i.e., “iterative”) bouts of reproduction throughout life. Two modes of parity are considered, classified by whether they express all reproductive effort in a single year (semelparity), or in more than one (iteroparity).
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