Simplify the following text for a Grade 8 native English-speaking person.
Original: Fisher 's book The Genetical Theory of Natural Selection also had a major influence on the evolutionary biologist W. D. Hamilton and the development of his later theories on the genetic basis for the existence of kin selection .
Simplified: Fisher 's book also had a major influence on W.D. Hamilton and the development of his theories on the genetic basis for kin selection .