Eli5, why do some fruit trees not self pollinate?
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Many plants can and do pollinate themselves, but this has the effect of leading to inbreeding. Inbreeding can be dangerous for a species, as it has a tendency to cause bad mutations to pile up over time, as the same bad mutations keep getting doubled up and passed down from inbred parent to inbred offspring.
As a result, a lot of plants have evolved various different methods to avoid inbreeding from self-pollination.
For example, some species of plants produce either only male flowers or only female flowers on a one individual but not both. Cannabis/Hemp is like this. Male plants can only fertilize female plants, and so no Cannabis plant can fertilize itself.
Another example is avocados. Avocados produce both male flowers and females flowers, but on some individual trees the male flowers open in the morning and the female flowers open in the afternoon, whereas on other individual trees, the female flowers open in the morning and the male flowers open in the afternoon. This prevents each tree from fertilizing itself.
Some plants prevent self-pollination through physiological mechanism called “self-incompatibility”. For example, in apples and pears, the ovaries in the flower have receptor proteins that can identify surface proteins on the pollen that mark the pollen as either genetically similar or genetically dissimilar to the ovum, and then they kill off similar pollen before it can fertilize the ovum. This is how apples and pears prevent self-pollination.
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