Barriers

... a definition The processes leading to the origin of new species have long been of interest and often a source of debate, e.g., whether ecological barriers to gene flow are essential for populations to diverge into distinct species http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=22088 ...

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... keratinised barrier that protects it from patogens. Natural openings are protected by mucus secretions and ciliated epithelia. If the above barriers are penetrated the body first responds with innate (non-specific) defence mechanisms mentioned above. If the non-specific mechanisms prove ...

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http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Energy,Cells,Catalysis&rid=mboc4.section.229 see the "Enzymes Lower the Barriers That Block Chemical Reactions" section, perhaps it can explain to you in a more easily understood way?.

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... become more benign and more equal, genetic sources of variation will become larger. For example, over the last 50 years, as environmental barriers to health and educational attainment have fallen, the variance in health and educational attainment accounted for by genetic factors has increased ...

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... But in general, although very closely related but very different looking species may be able to have offspring they do not because of ecological barriers(say modern man and something like a Homo erectus : i don't know if they would be able to reproduce, but even if so, I doubt they would.)

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