Senescence, the decline in fitness components of an organism with age [1], is a nearly universal characteristic of living beings [2-6]. This ubiquity is challenging because natural selection does not favor the evolution of traits decreasing fitness [1, 7, 8]. Senescence may result from two nonexclusive mechanisms: the accumulation of deleterious mutations acting late in life, when the strength of natural selection against them declines [9-11] (mutation accumulation or MA hypothesis [12]) and the delayed cost of genes having beneficial effects early in life (antagonistic pleiotropy or AP hypothesis [13]). Few empirical studies have evaluated their contribution to the standing genetic variation in senescence. These studies focused on Drosophila and may be compromised by recent laboratory adaptation [14]. We here study genetic variation in aging patterns in snails (Physa acuta) freshly sampled in natural populations. Our results strongly support the MA theory by validating all its classical predictions, confirming previous results in Drosophila. We also report a striking, novel finding: interbreeding between natural populations alleviates the decline in survival with age. We provide new theoretical models showing this to be another consequence of MA. Our results offer interesting perspectives on how different populations may follow different genetic pathways to evolve senescence.