system
Given the following post, respond as if you're commenting on Reddit:
human
Why haven't we selected for Avocados with smaller stones?

For many other fruits and vegetables, farmers have selectively bred varieties with increasingly smaller seeds. But commercially available avocados still have huge stones that take up a large proportion of the mass of the fruit. Why?
gpt
Avocado's are not well suited for selective breeding. This is for a number of reasons.

1. Avocados can take a very long to produce fruit and reproduce. Growing from seed, it can take 5 to 10 years for it to create fruit. That means you have to wait a long time to try to breed better avocado

2. It's phenomenally difficult to control pollination. They create thousands and thousands of blooms, only a few which actually become fruit, meaning hand pollination is almost impossible. So you're at the mercy of the winds and of the other trees nearby - it's very tough to control the gene lines

3. Avocados are incredibly heterozygous, meaning even selecting for two parents of a certain type (say, two small trees) can produce wildly different offspring. So even if you can control the pollination, you have to grow hundreds of seedlings to see if one or two are worth keeping.

4. Finally, the avocado craze is fairly new. Humans haven't domesticated that many plants, and probably haven't put much effort into avocado until the last few decades. 

That's why when you get a good avocado like Hass (which was a total accident) you propagate it by grafting, not by seed.