Biologist: Some speculate that the unusually high frequency of small goats found in island populations is a response to evolutionary pressure to increase the number of goats so as to ensure a diverse gene pool. However, only the reproductive success of a trait influences its frequency in a population. So, the only kind of evolutionary pressure that can reduce the average size of the members of a goat population is that resulting from small goats achieving greater reproductive success than their larger cousins.

to increase the number of goats 修饰哪一个？

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

that是什么意思，替代pressure？

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

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？？

同意！！！