And this sort of technology could easily go so far as to toss around embryos for the sake of getting a baby with the "right" eye color..
That's kinda the point I've been trying to refute for the whole length of this thread. It isn't easy. As MP_Ryan said the technology to do this has existed for a long time. If it was easy why would it have been sitting on the shelf for so long?
are we saying that those who exhibit such traits (for a nice and prominent example, let's say Down's Syndrome) are somehow "unworthy" in some sense, belonging to a prior and inferior class of humanity?
If you can find me one person with Down's Syndrome who hopes their child has Down's Syndrome too I might believe that we live in a world where people with it don't already consider themselves in some sense "unworthy". Down's is actually a bad choice as if I understand it correctly anyway, as it's caused by a failure of the chromosome to replicate properly and not due to any actual problem with the gene itself.
But let's suppose we are dealing with a proper hereditary condition. How is not having children at all because you are worried about passing it on an improvement? If you really feel that this could make someone feel unworthy then the fact that people willingly give up their chance to be parents just to avoid having children with the same condition is surely going to make them feel unworthy too?
I don't see why there has to be a dichotomy between saying that people with crippling genetic conditions are fully human and in no sense "unworthy" and saying that I wish we'd never have another child born with Cystic fibrosis or Tay-Sachs.
This is a sticky issue and you've raised a lot of good points that the field of genetics itself has to contend with too.
On the one hand, we have conditions like Tay-Sachs, CF, SCID, MS, MD, etc which are crippling single-gene diseases that PGD can eliminate entirely (at enormous expense) provided the parents are screened for the disease well in advance. I would be right there in the crowd cheering if we could ensure that a child afflicted with one of these diseases is never born with a pre-imposed early death sentence again.
That said...
I have a degree in this stuff and have been wrestling with the ethics in the field since my first year, and even I am not sure if we are meddling in areas we shouldn't be. PGD opens an enormous can of worms. For example, the destroyed embryos aside, we have the enormous expense of IVF with its associated chances of success, while there are large numbers of unwanted kids the world over. Should we be genetically-screening potential births for parents who could adopt? The second question is do we have the right to tell people to adopt when we have the technology to allow them to have healthy children? That's just one of hundreds of ethical dilemmas regarding PGD.
Beyond that, we have the issue of regulation. While kara has mentioned we have plenty of time for regulation of this area, we don't. In fact, global regulations on this science aren't even in the works. It is up to individual countries. So, while developed nations will probably choose to heavily regulate what is and isn't allowed, there is always going to be someplace in this world where unethical bastards can get away with genetic modification of embryos for the people willing to pay for it. I should mention that we have not yet identified the genetic basis of things like eye colour, hair colour, skin tone, etc - so while cosmetic changes aren't yet possible, if someone wanted to and had the money they could choose, today, whether they wanted a boy or a girl and make that happen. That may seem like not such a big deal, but consider this: Males are born at a slightly higher frequency than girls already (51%ish to 49%ish, depending on your statistician) because of the naturally higher mortality rate in young men - this results in roughly 51-49 split of girls to boys when people reach sexual maturity. Many cultures prefer boys to girls. Even a slight imbalance in that gender ratio has enormous implications for a country's demographics. We can already see this effect in China. While their population is still growing and their economy is still expanding, China is going to see a major demographic shift in the next century... which is part of the reason they are already beginning to relax the one-child policy in certain areas of the country.
The problem with the attitude of "we still have time" is that we don't. People who have money can make this happen today, and the cost of these treatments decreases significantly every year. Indeed, as the frequency of use increases the overall cost decreases too. While IVF was an option for the rich or the heavily subsidized as little as 5 years ago, today any couple in the middle-class income bracket can afford it (or at least can pay for it without being forced to declare bankruptcy).
So, while I'm heavily in favour of advances in this field, this particular area requires a great deal of caution and heavy global regulation on its use. And that just isn't happening fast enough.
As for the rate of human evolution...
As a species, with a global society we have effectively ended divergent evolution - speciation from Homo sapiens into multiple new species will not occur. That said, with the rapid mixing of genes from different allelic populations, as someone else said we have actually sped up directional evolution - gene flow is occurring at a rate we've never seen before, and it has interesting implications for us as a species down the road. So human biological evolution has actually sped up as a result of globalization rather than slowed down or stopped. Human behavioural evolution, on the other hand, has slowed down dramatically and behaviour can have much larger impacts than biology in the short term.
