Quite varied life trajectories can emerge from evolutionary processes, not just differences in longevity between species. The older, simpler evolutionary theories of aging that only account for some of these outcomes have to be extended and refined to explain new data, and so it goes - this is science at work:
The classic evolutionary theories of aging provide the theoretical framework that has guided aging research for 60 years. Are the theories consistent with recent evidence?
At the heart of the theories lies the observation that the old count less than the young: Unfavorable traits are weeded out by evolution more slowly at higher ages; traits that are beneficial early in life are selected for despite late life costs; and resources are used to enhance reproduction at younger ages instead of maintaining the body at ages that do not matter much for evolution. The decline in the force of selection with age is viewed as the fundamental cause of aging. It is why, starting at reproductive maturity, senescence - increases in susceptibility to death and decreases in fertility - should be inevitable in all multicellular species capable of repeated breeding.
Yet, this is not the case. Increasing, constant, and decreasing mortality (and fertility) patterns are three generic variants that compose the rich diversity of life trajectories observed in nature.