Re: Why are certain diseases inherited, hence genetic ?

Natural selection is the driving force of evolution. Without selection, evolution cannot happen. The rate of evolution depends on many factors in addition to the stringency of the selection. A few of the factors are population size, reproductive rate, mutation rate, and mode of reproduction (sexual vs clonal for example).

There is no such thing as "perfection". Would the "perfect" human be able to run as fast as a Cheetah, or fly like an eagle? Would we be "better" if we could live on grass like cattle, or on wood chips like termites? There is always room for improvement in a nearly infinite number of directions. In general, successful species tend to diversify over time as population sizes increase due to that success. Once upon a time there was just one lineage of mammal, that was successful and has now evolved into thousands of mammaliam species from whales to rodents to primates. Likewise for ants and beetles and fish, etc.

Speciation events can happen in hundreds of different ways. Geographical isolation is one way. Sexual selection preference divergence is another way. A change in chromosome number leading to reproductive isolation is another.

Geneticists call "gene pool ratios" allele frequencies. And they measure changes in allele frequencies over time, or across a geographic range etc. For example, humans in northern latitudes have a lower frequency of alleles for excess melatonin skin pigment production, in order to allow more sunlight in for vitamin D metabolism. Humans in areas with more malaria parasite have an increased allele frequency for sickle cell anemia.

Highly lethal disease trait genes are quickly eliminated if they are dominant. If they are recessive, they can increase in frequency at least until many homozygous individuals are produced. All organisms have more than one gene per chromosome, so a detrimental allele for one gene can be on the same chromosome and therefor linked to a beneficial allele for another gene. Over time, recombination will occur to separate the beneficial allele from the detrimental allele, but this will on average take longer for genes that are closer together.

With your example of type A hot-adapted people mixing with a much larger population of type B cold-adapted people, if we assume that there is enough selection that the hot-adapted people bear more children who survive to reproductive age than the cold-adapted people, then the hot-adapted genes will increase in the new mixed population over time. The hot-adapted genes will only be wiped out if there is no interbreeding and the cold adapted people kill off the hot-adapted people or outcompete them for resources.

The rate of change of allele frequencies over time is a measurement of the forces of natural selection that are taking place on a population. Assuming that migration is not a major cause of the allele frequency change, i.e. assuming we are studying a population and not a fragment of a population.