Natural selection cannot discriminate between coextensive properties. To see how the process that Darwinian selection-against works in a real case, consider an example: two distinct gene products, one of which is neutral or even harmful to an organism and the other of which is beneficial, which are coded for by genes right next to each other on the chromosomes. This is the phenomenon of genetic linkage. The traits that the genes coded for will be coextensive in a population because the gene-types are coextensive in that population. Mendelian assortment and segregation don't break up these packages of genes with any efficiency. Only crossover, the breaking up and faulty re-annealing of chromosomal strings or similar processes can do this. As Darwin realized, no process producing variants in nature picks up on future usefulness, convenience, need, or adaptational value of anything at all. The only thing evolution (natural selection-against) can do about the free-riding maladaptive or neutral trait, whose genes are riding along close to the genes for an adaptive trait, is wait around for the genetic material to be broken at just the right place between their respective genes. Once this happens, then Darwinian processes can begin to tell the difference between them. But only when environmental vicissitudes break up the DNA on which the two adjacent genes sit, can selection-against get started—if one of the two proteins is harmful.