A fundamental scientific fallacy of eugenics is the assumption that it would be a good thing to specifically choose certain genetic characteristics in human beings and selectively breed for them. This would be accomplished either by encouraging or enforcing “desirable unions” (positive eugenics) or by forbidding reproduction by – or sterilizing – the carriers of “undesirable traits” (negative eugenics). The analogy of selective breeding of plants for agriculture or of farm animals was often invoked: both Darwin and Galton employed this metaphor; and supporters of eugenics frequently issued dire warnings that humans with “undesirable traits” were reproducing in greater numbers than “desirables”, thus supposedly thwarting human evolution and causing deterioration (devolution) of the human species.
NOTE: It has not escaped the attention of the opponents of eugenics that “desirable traits” almost invariably tended to be those possessed by the advocates of eugenics; who, strangely-enough, never seemed to be among those condemned to restricted breeding or sterilization programs.
As the sciences of genetics and ecology developed in the twentieth century, it became increasingly clear that the evolution of species depends not only on the natural selection described by Darwin, but but even more dynamically on processes that are almost the exact opposite of the programs favored by the promoters of eugenics. The survival and health of a species is dependent upon genetic diversity, the subject we will consider here. The absolutely-essential diversity of a species results from the constant reshuffling and flinging far and wide throughout the species of inherited characteristics and new mutations, rather than the deliberate concentration of a few allegedly-desirable characteristics in a selected few. Indeed, the actual results of long-standing human experiments in "selective breeding" are both sobering and tragic.
Ten thousand years of human tinkering with the genes of dogs have resulted in numerous breeds, all of which are in most ways inferior to the original wolves from which they descend. Most modern breeds of dogs are by and large unfit for survival in the wild. The characteristics favored and bred by their human masters have not resulted in a better wolf, but rather a more docile creature, friendlier and more attractive to humans, but not in any biological sense "better". Deafness and digestive-tract defects requiring special and expensive diets are common among modern breeds of dogs, all of which represent the human effort to emphasize "desirable characteristics" - according to the aesthetic standards of the canine eugenicists.
The same can be said for most other human efforts to improve animals and plants through selective breeding. We create species to serve our needs, but not necessarily species that are superior to those that nature is constantly reshaping and improving without our meddling.
We will focus here on two cellular mechanisms that promote genetic diversity: the phenomena of  crossing over; and  independent assortment. For some this will be a review. Those who have not previously studied the biology of reproduction should not worry not worry about technical terminology. The principal point is that during the meiosis, the specialized cell divisions that produce germ cells (spermatazoa in the male and oocytes/eggs in the female), there are two processes that serve to recombine the genetic information inherited by the parent into germ cells that will transmit unique combinations of that information to the children who will be conceived by those cells.
Please note: the video-animations accompanying this lecture demonstrate these processes dynamically. Please compare the videos of mitosis (ordinary cell division) and meiosis (reduction-division that produces germ cells), together with the video entitled Genetic Diversity that highlights both crossing over and independent assortment. These can be replayed and stopped at any point, as you wish.
1. CROSSING OVER
During the process of crossing over the chromosomes originally inherited from the individual's two parents actually intertwine and exchange portions with each other, forming unique chromosomes, each different from that inherited from either parent:
2. INDEPENDENT ASSORTMENT
During Independent Assortment the number of chromosomes reduces to half the normal number (haploid). During the two cell divisions that accomplish this the chromosomes can line up in a variety of different combinations (independent assortment), this increasing the diversity of the genetic material already recombined during crossing over:
This Webpage was created for a workshop held at Saint Andrew's Abbey, Valyermo, California in 2003