too many question !!!!
migration makes a groups within a population reproductively isolated and so gene flow between them stops since migration brings a group of them or both in another region environmental conditions change and each group adapt to new condition changing allelic frequency and the best fitted individuals pass on alleles to offspring and so two sub populations are formed and when they meet again they won't breed together but maybe Competition occur or one group is best fitted and the other whole group dies.
isolation bring about the same results as about if any reproductive isolation occur, gene flow stops and gene pool is not maintained
if no selection occur all individuals survive and since individuals reproduce sexually therefore variation occur due to random assortment of chromosomes in metaphase 1 and their consequent random distribution if them in the gamete cells. allelic frequency will then increase a lot since all individuals survive and are capable to reproduce and bring about new variation of offspring
no random mating is like inbreeding in artificial selection where only closely relatives are allowed to breed in order to maintain the gene pool. Allelic frequency are then too minor only considering a double recessive character the express itself.
very small population is taken as above where allelic frequency is too minor
mutations are really different may happen is a sequence on the DNA or in whole set of chromosome or brings about polyploidy organisms. allelic frequency will absolutely vary if the new mutant organism id capable to survive but if not allelic frequency won't change
Hope I helped
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