The following material is flagged Green Level. It is intended to reflect material that the author believes to be a matter of consensus among experts in the field. This belief may be incorrect, however; and as the author is not an expert and does not have an expert fact-checking the article, errors may creep in.As with the previous example, let us say that we are looking at a population of bears in a cold place.
Now, let us suppose that the bears somehow split into two populations. Maybe the ice floes they live on are broken and begin drifting apart farther than the bears can swim. After millions of millions of generations, the bears build up a large number of mutations. Now, suppose that one mutation in one population is incompatible with another mutation in the other population. If those two mutations occur in the same individual, that individual will die off.
Contrary to how it may appear, this is not a disadvantage. As was mentioned above, a trait does not have to be beneficial in all situations in order to be beneficial; it only must be beneficial in the current situation. Similarly, it does not have to avoid being harmful in all situations in order to be harmless; it only must be harmless in the current situation. And the inability to breed with an individual one could not otherwise breed with is not a harmful trait. If the trait is beneficial (or even just allowed to spread over a few thousand more generations or so), the bears on the two floes will be separate species.
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