Senescence: an example

Most of us mere mortals realize that the older we get, the more likely it is that we will die. Here are some data in support of that claim...

Figure 1. Age- and sex-specific mortality rates in Canada from 1995–1997. Data from Statistics Canada.

Each dot in the above figure represents the percentage of Canadians that achieved a given age between the years 1995–1997, but died before reaching their next birthday. For example, 0.4% of Canadian males that celebrated their 50th birthday sometime between 1-Jan-1995 and 1-Jan-1997, died before celebrating their 51st birthday. For 95-year-old males, this mortality rate jumps to 28%. In general, these data demonstrate that among Canadians, mortality rates increase with chronological age, a phenomenon termed senescence.

Presumably, senescence occurs because our bodies deteriorate over time. But why has natural selection not eliminated this fitness-limiting phenomenon? All else being the same, shouldn't individuals that live longer have higher lifetime fitness and inject more of their genes into the population's gene pool than shorter-lived individuals??

So WTF is going on... why can't I haz immortality? Does this mean that I was intelligently designed by a supernatural father-figure whose first creations accidentally committed the original sin thus condemning all of humanity to a miserably mortal existence dominated by ignorance and concupiscence?

Well, uhh, I'll leave that for you to decide. I will simply note that there are good evolutionary reasons for the occurrence of senescence, which I will discuss in a future post.