The biological species concept (that species are empirically defined as whether or not they interbreed and produce fertile offspring) is convenient but does not always reflect actual gene flow. First of all, it completely falls apart when applied to asexually reproducing organisms, such as bacteria, some fungi and even a few reptiles and insects. Then there's the difference between "can" and "do"; there are many organisms that are considered different species that can, under artificial conditions such as captivity or through artificial insemination, be made to produce fertile hybrids. This doesn't happen in nature, however, because of behavioral or temporal/spatial barriers to their breeding. Perhaps one species only breeds in late May, while another that otherwise would produce fertile hybrids only enters estrus in early March. The two will never (or extremely rarely) interbreed and the two populations will remain distinct. There are many different control methods to keep species distinct and avoid outbreeding depression (where the hybrid between two groups is less fit than either of its parents). Changes in chromosome number generally results in speciation, and is especially common in plants, where polyploidy is thought to have been a major factor in plant evolution.
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The biological species concept (that species are empirically defined as whether or not they interbreed and produce fertile offspring) is convenient but does not always reflect actual gene flow. First of all, it completely falls apart when applied to asexually reproducing organisms, such as bacteria, some fungi and even a few reptiles and insects. Then there's the difference between "can" and "do"; there are many organisms that are considered different species that can, under artificial conditions such as captivity or through artificial insemination, be made to produce fertile hybrids. This doesn't happen in nature, however, because of behavioral or temporal/spatial barriers to their breeding. Perhaps one species only breeds in late May, while another that otherwise would produce fertile hybrids only enters estrus in early March. The two will never (or extremely rarely) interbreed and the two populations will remain distinct. There are many different control methods to keep species distinct and avoid outbreeding depression (where the hybrid between two groups is less fit than either of its parents). Changes in chromosome number generally results in speciation, and is especially common in plants, where polyploidy is thought to have been a major factor in plant evolution.