DIAGNOSING SENESCENCE - INFERRING EVOLUTIONARY CAUSES FROM PHENOTYPICPATTERNS CAN BE MISLEADING

Based on the predictions of two theories for the evolution of senescen ce, the 'antagonistic pleiotropy' and the 'mutation accumulation' theo ry, an age-specific increase in mortality and a decrease in fecundity are widely used criteria to diagnose senescence in natural and laborat ory populations. In this study we question the reliability of these cr iteria. Using a simple model we show that similar phenotypic patterns result from optimal life histories without senescence. With a tradeoff between reproduction and period survival, optimal life histories prod uce patterns of increasing mortality and decreasing fecundity as organ isms age, even if the tradeoff does not deteriorate with age, so that we are not forced to invoke genetic effects such as antagonistic pleio tropy or accumulation of deleterious mutations to explain such pattern s. Furthermore, if optimal life history theory is applied to senescent organisms, phenotypic patterns can result that are usually not associ ated with senescence. We conclude that the reliability of a diagnosis of senescence based on phenotypic patterns and the comprehension of th e phenomenon senescence depends critically on understanding to what ex tent tradeoffs are determined by the effects of segregating genes.