Organisms gradually undergo morphological change. How does an organism's brain adapt to be attracted to, and apply aspects of a fitness model to, new forms? It can be strictly what a person is exposed to, or the mother, and so on, as stuff llike desiring a waist:hip of 0.6 - 0.7 are fairly universal. That means either:
-The brain references its own "body map" to decide what to be attracted to. This would mean it's nopt ancestry or the mother and early childhood experiences, immuine system, etc that decides what we ought to be attracted to, but literally referencing one's own body. When you see someone, their body geometry is processed and weighed against your own body map.
-The fitness model naturally changes because the only changes spreading throughout a population are the ones people selected for already, and odds are, the majority of the population would also accept. However in the case of major outliers, this would bias against rapid or major changes. Yet it also appears humans don't really care that much, eg hentai and fetish porn.

But does any of this really fully explain why animals can change greatly, and yet the ability to identify animals of the same species, which of them are the opposite sex, and know what is and ought to be attractive? Why are we not turned on by an australopithecus or bonobo? (inb4 "but I am")