Reproduction involving the union or fusion of a male and a female gamete. The hypothesis that organisms are constantly struggling to keep up with one another in an evolutionary race between predator and prey species. The process by which the genomes of an asexual population accumulate deleterious mutations in an irreversible manner. The genetic or environmental process by which the sex of an individual is established. States that for any species the sex ratio should relate to the ratio of parental investment in each of the two sexes. Anything and everything an individual does that functions to allow it to reproduce. This includes its production of gametes, how and when and with whom it mates, parental care and so on. Behavior of a parent that increases the probability of offspring survival at the cost of the parent"s ability to produce more offspring.