Figure 1. Basic life history configuration of animals, for example, the tropical butterfly Bicyclus anynana. The figure represents the relative importance of genes affecting aging and longevity through time in a single cohort. At old age, the relative importance of longevity assurance genes is much increased relative to the contribution of deleterious mutations.

Figure 2. (a) Survival (l_x) and reproductive rate (m_x), and (b) the probability of reproduction and proportion of reproduction remaining for an individual aged x, for a hypothetical nonaging population. The probability of reproduction at age x is taken as the product of l_x and m_x. The proportion of reproduction remaining can be taken as a measure for the strength of natural selection at age x. It appears that the strength of natural selection declines with age even in the absence of aging. Genes can escape the scrutiny of natural selection if their effects occur in the selection shadow.

Figure 3. Change in the survival curve of human populations from a curve where few aging deaths occurred to a typical rectangular aging curve (redrawn with permission from Kandel et al., 2000).

Figure 4. The enzymes superoxide dismutase and catalase are involved in a pathway that neutralizes reactive oxygen species (ROS). The figure is not exhaustive; other enzymes exist that have the same function, such as glutathione peroxidase.

Figure 5. The insulin/IGF-1 signaling pathway. In bold are the identified C. elegans genes that are homologous to the vertebrate (and D. melanogaster) genes. Some of the names of the components of the pathway are short-hand notations.

Figure 6. The role of the gonadal cells in the insulin/IGF-1 signaling pathway and how it influences the life history of C. elegans. Active signals, genes, proteins and traits are shown in italics, and those that are inactive in bold (redrawn with permission from Leroi, 2001).