Many Pokemon fans know that in the video game series it is more likely to find a male Eevee than a female. A quick check to Bulbapedia confirms that the gender ratio for Eevees is 87.5% male, 12.5% female[1]. In real-world biology, the gender of a baby is determined mostly from the genes of the male producing the offspring. This is the reason why Henry VIII, no matter how many wives he had, only had healthy daughters. If this fact is also present in the Pokemon world, the gender of a baby Eevee would be determined by the male. Now, let's say that hundreds of years ago, the gender ratio for Eevees was 50/50, there were around the same number of male and female Eevees. Let's say that, at this time, the reproductive genes of a male Eevee mutated, which is quite likely, seeing that it happens all the time, to make this certain Eevee more likely to produce male offspring. This gene would be passed down to his offspring, say 2 males and 1 female. The 3 children would posses this gene, but it would only affect the males, as it is their genes that determine their offspring's gender. Now, let's say that these three Eevees grow up and reproduce. The 2 males with the genetic mutation will birth 2 sons and 1 daughter each, and the female will birth, say a son and a daughter. These numbers are highly exaggerated, as the mutated genes would be passed down much more gradually, but this current generation contains 5 males and 3 females, where the past generation had 2 males and 1 female and the first had 1 male and 1 female. If this trend continues, eventually the birth of a female Eevee would become so unlikely that the species would be in danger of extinction, as you cannot produce offspring with no females (or Dittos :P). Only Eevees who do not have this mutation could continue to thrive without human interaction. For example, if you were a Pokemon breeder in charge of breeding Eevees, you would want to catch purebred Eevees, ones without the mutation, so you could keep the Eevee population alive and save them from extinction. So if you see a female Eevee in the tall grass, don't catch it, wait for a male, protect the Eevee population, they're too cute to go extinct.

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