asasasas1

to increase the number of goats 修饰哪一个？

Modifies "evolutionary pressure"

so as to ensure a diverse gene pool是不是to increase的目的状语。

Correct!

that是什么意思，替代pressure？

"evolutionary pressure"

achieving是做small goats的定语，还是result from的动名词宾语？？

做small goats的定语