<P> If sexual creatures avoid mates with strange or unusual characteristics, in the process called koinophilia, then mutations that affect the external appearance of their carriers will seldom be passed on to the next and subsequent generations . They will therefore seldom be tested by natural selection . Evolution is, therefore, effectively halted or slowed down considerably . The only mutations that can accumulate in a population are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "neutral mutations", which can be, and are, used to trace the relatedness and age of populations and species .) </P> <P> This implies that evolution can only occur when mutant mates cannot be avoided, as a result of a severe scarcity of potential mates . This is most likely to occur in small, isolated communities . These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves, or during the aftermath of a mass extinction . Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition . Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes . If an isolated population such as this survives its genetic upheavals, and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come in being . In geological terms this will be an abrupt event . A resumption of avoiding mutant mates will, thereafter, result, once again, in evolutionary stagnation . </P> <P> Thus the fossil record of an evolutionary progression typically consists of punctuated equilibrium, with species that suddenly appear, as if by macromutation, and ultimately disappear, in many cases close to a million years later, without any change in external appearance . However, this is compatible with phyletic gradualism, because periods of a few tens of thousands of years can barely be distinguished in the fossil record: relatively rapid evolution will always appear as a sudden change in a sequence of fossils . Charles Darwin indeed noted in On the Origin of Species that periods of change would be short compared to the overall existence of a species . During each species' existence new species appear at random intervals, each lasting many hundreds of thousands of years before disappearing without a change in appearance . The exact relatedness of these concurrent species is generally impossible to determine, as seen in the evolution of modern humans . </P>

In which of the following circumstances will evolution always take place