Adaptive Radiation and Evolution of New Organisms

        When a species moves into new habitats, natural selection (i.e., the mechanism of evolution) is bound to take place.  Natural Selection acts on a proportion of the original population.  It acts in such a way as to cause a change in the frequency of phenotypes represented in a population due to differential reproductive success.
        Suppose for example, there is a population of squirrels.  In this population, the majority has bushy tails, due to these being able to hide more from predators.  However, keep in mind that in this population, there will be a range of phenotypes. Thus there will be ones with bushy and not-so-bushy tailed ones in the population, yet the squirrels expressing this phenotype will be the most numerous (i.e., have a higher frequency).
         Now, suppose this population moves into new territory.  Furthermore, the ones that have the not-so-bushy tails are able to evade predators more easily than the bushy tailed ones.  This now means that the not-so-bushy tailed ones will increase in numbers.  Conversely, the ones with bushy tails phenotype will now decrease in numbers.  In natural selection terms – the bushy tailed ones are being selected against; the not-so-bushy ones are being selected for.
        So here’s the hard part.  Those being selected for will produce more offspring than those being selected against – remember differential reproductive success.  The not-so-bushy ones live longer and have more offspring.  Given enough generations in this new territory, we would then expect the not-so-bushy tailed ones be the more numerous in population (i.e., have a higher frequency).  So now, the frequency of the not-so-bushy phenotypes has increased and most of the represented organisms caught will express this.