Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.

Fact 1: Behaviors that improve fitness increase through natural selection. 
Fact 2: Darwinism is strictly, selection theory.
behaviors that improve fitness increase through Darwinism.

Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers cause erosion by transporting weathered rock and soil downstream.
Valleys are formed by erosion.

Fact 1: Thyroid hormones increase the rate of metabolism in cells throughout the body. 
Fact 2: Thyroid hormones are produced by the thyroid gland.

a gland produces hormones in cells.