Simplify the following text for a Grade 12 native English-speaking person.
Original: Therefore the first four chapters lay out his case that selection in nature , caused by the struggle for existence , is analogous to the selection of variations under domestication , and that the accumulation of adaptive variations provides a scientifically testable mechanism for evolutionary speciation .
Simplified: The first four chapters lay out his case that selection in nature , caused by the struggle for existence , is analogous to the selection of variations under domestication .