Features in individuals causing high population variation
I believe there are evidences in both directions in terms of mechanisms to favor variation in a population: outbreeding and crossing within individuals that look alike. There is a fine line between the two. Outbreeding helps in keeping variation and mixing variants that appeared in different parts of the population, but too much outbreeding can generate inviable offspring. Too much inbreeding also reduces viability. Certain plants have features that are selected to favor outbreeding and make inbreeding incompatible by rejecting incompatible ("self") pollen (self-incompatibility mechanisms).