The standard view of evolutionary biology is that life’s aim is to maximise the number of genes (in reality, not genes but “alleles” is the correct term) that are passed on from one generation to the next. In reality, this disregards the imporance of time. In this model, popularized by R.Dawkins, life is a photocopier which focuses on making as many copies of an allele as possible.
In fact, a more accurate formulation of fitness would be this: the total amount of time that an allele is present in the DNA of a living organism. Thus, number of alleles x time or f= N x t.
If an organism generates 2 long- lived offspring that die when they’re 100 years old, its fitness would be: 200, the same level of fitness of an organism that has 200 offspring each dying after 1 year or 2400 offspring with a lifespan of 1 month.
This accounts for organisms’ tendency to extend their life well past their reproductive age – a phenomenon that classical evolutionary biology cannot explain without recourse to just-so stories (e.g. the fitness benefits of the elderly to subsequent generations) – or for the tendency of evolution to produce long-lived species which generate few offspring (K-selection).