The yeast generation number defined by brewers is not necessarily related to actual reproductive generations. You need to realise a couple of things to make sense of this.
First, yeast reproduce by self-replication (mitosis, i.e., essentially just splitting to make two identical copies) every 1-2 hours under good conditions. So after a vigorous 24-hr starter, you're really on literal "generation" 12 or more. Second, yeast will begin a cycle of sexual reproduction under nutrient limited conditions, by undergoing meiosis (which makes four new genetically distinct cells, analogous to sperm and eggs, enclosed in a protective 'spore' wall). When conditions become favourable again, the spores will become active and eventually begin fusing together (to form zygotes, analogous to the first cell of an embryo). Spores, and consequently zygotes, will be vastly more genetically diverse than the cells that originally underwent meiosis. Third, yeast can evolve pretty quickly. This just means that the distribution of traits will change because conditions favour the survival and reproduction of individuals with certain traits. Fourth, new genetic diversity is always being spontaneously generated from scratch in yeast (and in all other organisms) by mutation, i.e., random changes in DNA sequences. Nothing can stop this. Fifth, bacteria will eventually get a foothold. You can't sterilise a yeast culture except by extracting a single cell and propagating it alone.
So suppose you start with a monoculture of yeast (truly genetically uniform, propagated very recently from a single cell). What happens over time as you make starters and brews? (1) sex happens. Every time you complete a fermentation and the yeast are temporarily knocked down by lack of nutrients and energy supply, a larger fraction of them than normal will undergo sexual reproduction, which creates new genetic diversity -- more 'meat' for evolution. (2) mutation happens. The simple passage of time, combined with yeast numbers. ensures that mutation will also generate 'meat' for evolution. (3) evolution happens. Because your wort is not identical to the conditions that Chris White used to create your monoculture, and because sex and mutation are creating new trait combinations anyway, your culture will evolve away from the original strain's characteristics. (4) contamination accumulates. You'll unavoidably accumulate a larger population of bacteria over time.
All four of these things will tend to turn a sterile monoculture (ideal for brewing) into a contaminated and genetically diverse culture (useless for brewing, unless you're a monk) over time.
RE your original question: The point is that the concept of a 'generation' as used by homebrewers probably doesn't have anything to do with biological generations, but is instead a proxy for the total amount of evolution and contamination that tend to occur over a single brewing session.
