--- "Chris J. Whitcomb" <[EMAIL PROTECTED]> wrote: > David Scheidt wrote: > > On Jan 26, 2008 7:10 PM, Susan Koziel > > <[EMAIL PROTECTED]> wrote: > >> > >> --- > >>> Does that number even > >>> come close to modeling a similar circumstances > in > >>> humans? How quickly would I have almost all > 7-foot > >>> humans if we only allowed seven-footers to > reproduce > >>> with each other, and nobody else? Is it 1-2 > >>> generations, or 10, or more? I think probably > only > >>> 1-2, to be honest. > >> > >> Height is a multigene thing that is largely > influenced > >> by environment. Breeding only 7ft isn't going to > > > > Dutch men are the current favorite example of > this. They're about 20 > > cm taller (average height is a bit more than > 185cm) than they were > > about 100 years ago. Given that it is > > rather unlikely that they've been engaging in > selective breeding, > > that's down entirely to factors like diet. > > It all depends on how you define selectice breeding. > If the women of the > country on average prefer taller men, then for the > most part the taller men > will breed... > > I'm not saying it definitely is a factor, but that > it can't be completely > ruled out. > I'm not saying that height isn't heritable. It is. The issue is that selecting for it is not straight forward, and it does not necessarily hold true that if you take two tall people you will get taller offspring. You might. Or you might not.
If you take two tall individuals you are just as likely to get offspring that are the same height or shorter then their parents. If you happen to stumble on a set of parents that breed true and their offspring are taller you will only get so tall before the maximum effect of that combination of genes is reached. After that you have to bring in another set of genes - and that's a gamble... that set of genes commonly cancels the original mutation (making your offspring shorter) or in more rare cases (and this is what you want) they will make your offspring taller... but this taller may or may not be heritable. You need to check the next two generations to know for sure.... then you will eventually max out the height of that cross. Eventually you will actually max out height for the species and you'd have to turn to genes from other mammals. This is what the plant people have encountered. Plants have been breed and tinkered with over time more then any mammal breeds. For example: With Canola and Soy they have been trying for years to decrease the amount of bad oil in the seeds... they maxed it out by crosses fairly early on (because there isn't that much natural variation in the crops). Now the only way to change them is to either intentionally mutagenize populations or bring genes in from other sources. There are indications with animal breeds that the same effect occurs - that each species has a max level within it's genome for certain things like size, IQ, speed, etc. Much of these maximums have to do which the sheer physical nature of our parts. Too tall a human and you have both back and knee problems - to push past these limits you need to bring in a number of outside systems. I digress.... (getting back on topic) If you are talking natural selection and normal human evolution - yes sex selection is a very strong push - but this isn't something that happens in a small number of generations. What you are likely seeing in the Dutch community is the increased prevalence of a mutation rather then the effects of sex selection (sort of like Tay Sachs disease being prevalent in certain ethic communities)... it's not selection you are seeing - but rather an increase or decrease in frequency of certain genes within a semi-closed population. This can lead to a trait changed by sex selection; but since the Dutch can and do breed outside their community ;) suggests that the current effect isn't directly due to sex selection. If you wanted to breed tall humans then starting with a tall Dutch man from a family of tall Dutch men is an ideal choice. Then crossing him with an average human and comparing his gene profile to his wife's profile then to his children's profiles (iding the children being tall or short) is how you'd start to get a gene profile for tallness. Once you have that you can start looking at what happens when you combing different tallness profiles. Of course this sort of thing isn't the same as looking at what happens in normal society (nor would I consider it a proper use of genetics - since variability in a genome helps a species survive in the long term). The term Selective Breeding, as I am using it, is when someone forces a plant, or an animal to breed with chosen plant or animal. So that the experimenter can track the parentage. The term Natural selection, means what happens in nature, without the interference of an experimenter. Sex selection is a factor in natural selection... not in selective breeding. (As I am using the terms). I realize these terms seem to be splitting hairs - but the different terms in genetics are important because they tell you which factors you are looking at and which ones you can discount when you look at a system. -Sue _______________________________________________ GurpsNet-L mailing list <[email protected]> http://mail.sjgames.com/mailman/listinfo/gurpsnet-l
