I cannot grasp the structure of this sentence.
Natural selection will tend to maximize r for the environment in which the species lives, for any mutation or gene combination which increases the chance of genotypes possessing them contributing more individuals to the next generation (that is, of increasing r) will be selected over genotypes contributing fewer of their kind to successive generations. (Birch 1960, p. 10).
(r is rate of increase.)
Is "for" after comma is in parallel with former "for" after "maximize r" or just a conjunction like "because" ?
What is the meaning of "possessing them" here?
Why could "possessing" and "contributing" be appeared in succession ?
What (is the subject of) "will be selected over"?