Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Sentence1: Examples of gametes are ova and spermatozoa. 
Sentence2: fertilization of ova and spermatozoa produces a diploid zygote.
ova

Sentence1: high mountains with well-rounded peaks can be formed by collision of crustal plates. 
Sentence2: high mountains with well-rounded peaks can be formed by collision of plate tectonics.
collision

Sentence1: Behaviors that improve fitness increase through natural selection. 
Sentence2: Natural selection is the mechanism of evolution.

natural