⇒Progeny continuosly gets weaker after every generation
⇒Less chances of the production of new species & varieties.
⇒Continued self pollination leads to INBREESING DEPRESSION. 
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The primary disadvantage of self-pollination is that it results in genetic uniformity of the population, which makes the population vulnerable to extinction by, for example, a single devastating disease to which all the genetically identical plants are equally susceptible. Another disadvantage is that beneficial genes do not spread as rapidly as in cross-pollination, because one plant with a beneficial gene can transmit it only to its own offspring and not to other plants. Self-pollination evolved later than cross-pollination, and may have developed as a survival mechanism in harsh environments where pollinators were scarce.