I will now train ViaVoice to understand my speech and know some of the technical words I am likely to use in my writing.
Senescence. I'm going to talk about senescence. Senescence has been defined many ways. In the biological literature definitions of senescence fall into 4 main categories. Physiological senescence, cellular and tissue senescence, actuarial senescence and reproductive senescence. These last 2 can be combined into the measure I will refer to as Hamiltonian senescence.
When most people think about senescence they think of physiological senescence. Hardening of the arteries graying of the hair gradually increasing dementia and a wide range of other traits are easily observable signs of physiological senescence. Physiological senescence can be defined as age related deleterious changes in the anatomy, biochemistry or physical functions an organism.
This is distinct from tissue senescence leaf senescence cellular senescence and other adaptive degradations of disposable subsets of an organism. In this meaning of senescence, the senescence of the whole organism is not being discussed, the breakdown, recycling or discarding of a particular piece of the organism. In a fitness context, senescence refers to the decrease in mean remaining individual fitness with age. This can be broken down into reproductive senescence, the decrease in mean reproductive output by age and actuarial senescence the decrease in mean survivorship by age. The definition of reproductive senescence can be further refined to include both direct reproduction (i.e. fertility) and indirect reduction ( i.e. aid and transfers of resources to related individuals).
as Hamilton argues, these means should be calculated across all individuals born into a particular population or cohort, rather than simply those individuals surviving to the age in question.
Much of the biological literature suffers from the conflation of these various definitions both with each other and with the observed variables used as proxies for senescence. Different authors studying the same organisms or even the same datasets have come to very different conclusion regarding the senescence those organisms because of confusion regarding the definition and measure being used.