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The trait is the same for all organisms.
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It's the natural process of species separating from other species. Two different species cannot interbreed.
No, populations of organisms evolve, individuals are naturally selected.
Because natural disasters eliminate most of organisms which results in reduced population and bottle neck effect .
Basically, natural selection, genetic drift and gene flow into and out of population of organisms.
What would there be to select from if all the organisms in a population were not different in morphology and behavior. Some of these differences are reproductively successful against the immediate environment, and that environment is the natural selector.
Biological isolation occurs when an individual or in most cases a population of individuals do not have contact with other species that they are capable of breeding with or infecting. This process can be man made or natural. In man made we isolate organisms in a sterile environment such as a level 4 biology lab, special greenhouses, and on special farms. Natural isolation can occur when a population of organisms travel to islands from the mainland and no longer breed with organisms from the mainland. Occasionally natural disasters such as earthquakes, mud slides, etc can force a population into a small portion of its previous environment separating the population into different areas. These populations can then evolve separately.
Evolution is the change in allele frequency over time in a population of organisms. Change over time and a fact. Natural selection is the nonrandom survival and reproductive success of randomly varying organisms and explains much about the fact of evolution.
Adaptive change is the province of natural selection and natural selection is one of the main drivers of evolution. Natural selection selects from the individuals variations in a population of organisms on, basically, reproductive success and this adaption is passed on to progeny which changes the allele frequency in the population which is evolution.
Without the subtle differences on organisms phenotypes what would natural selection select from? All organisms in a population are variants and some survive and reproduce better than other against the background of the immediate environment and these are selected by that environmental pressure.
Isolation prevents mating between the populations and in time different mutations will accumulate in each population so much that they will no longer be able to mate. The inability to interbreed is one of the definitions of different species.