Now, let's go on over to the Republican side of the fence and do some more sacred cow tipping. I could pick on them for their mirror-image myopia on the same issues of environmental stewardship, but let's go for something more fun. Let's take the classic Republican relationship with tradition and history.
Republicans believe, with good justification, that freedom and prosperity grow from the same tree, and the roots of this tree are fundamental rights to life, liberty, and the pursuit of happiness. If you've never been a Republican or associated with them extensively, you're not likely to understand just how important history is to them. Right-wing Republicans have a profound respect for their received history and traditions. They learn from history that the kind of social order that allows freedom to flourish can be a fragile thing. Common criticisms to the contrary, they really do put an amazingly high premium on the value of human life - it's their respect for life and love of freedom that makes them ideologically amenable to militarism and capital punishment, and chilly towards abortion, stem cell research, and cloning. Republicans see clearly in history how human attempts to meddle in human biology have gone disastrously wrong, and assume a straight-line correlation between "eugenics was monstrous and resulted in untold suffering" and "therefore abortion, cloning, and embryonic stem-cell research must not be tolerated."
This recent opinion piece on abortion illustrates the point nicely, although the language is very religious and the whole essay is shot-through with magical thinking. Even removing those magical elements, the view articulated there holds true even for many Republicans whose worldview is primarily secular (yes, they really do exist).
Of course, this view of abortion doesn't just rest on religious authority, it claims to be rooted in a clear understanding of history and to take seriously the view that if we mess around with our biology we are playing God (a job we're not qualified for). A zygote is a living organism that, if left alone, will develop into a human, therefore abortion ends a human life, therefore it must be murder, and any ethical gerymandering to the contrary can't change that fundamental fact. Ditto for stem cell research, which destroys human embryos, or for hybrid experimental cloning, and for dozens of other biotech research techniques.
At first blush, that seems to be a pretty solid rooting in biology - but it's not. A true ethical grounding in biology has to contend with a few other facts that make the connections between point A and point B very tendentious.
The first problem is evolution: All life is made from the same stuff, and human life on a biological level is in no way distinctive. Human nature and human biology are subject to the same selective pressures as the rest of the biosphere, plus the internally imposed selective pressures of human culture. It's not impossible to make a case for human exceptionalism (I'm a human exceptionalist myself), but it's not axiomatic.
The second problem is embryology: only somewhere between 25% and 60% of all zygotes become viable pregnancies, and 8% of those that do fail to make it to term without any intervention. Not every conception results in a life - and most wouldn't even if medical abortion were never discovered. George Carlin had it right: If life begins at conception, then every sexually active woman who's had at least three periods is a serial killer.
The third problem is technology: Since the conception of a zygote creates a life, and if that life is seen to have value because it is a potential human being, then technology poses a new and frightening problem. A zygote has only a minority potential of surviving to birth - and so does a clone. Although cloning tech is still in its infancy, it is now possible to artificially split embryos in vitro, making every IVF procedure the potential ancestor of countless offspring in one generation. More importantly, it is now possible to take the genetic material from an adult skin cell and implant it in the nucleus of an ovum, throw a few hormonal switches, and have a viable zygote. With this the reality, every time I scratch my arm I've engaged in a holocaust of potential human beings.
The fourth problem is medical: We now know beyond any doubt that the seat of human consciousness is the central nervous system (i.e. the brain). You can argue about souls all you want - whether there is a ghost operating the machine or whether we are all machine - but the machine does not operate at all without a brain. Before the 22nd week of gestation, there isn't enough of a brain there to operate the machine. Any ghost that may exist can't have moved in yet. Citation.
These four problems are not the only problems with Republican attitudes towards biotech. There's also the question of those who die from potentially curable diseases if research is suppressed - are their lives worth less than, or more than, the lives of potentially viable zygotes and blastocysts?
Banning pre-viability abortions, banning biotech procedures, or banning government funding of either will neither reduce the number of murders in the world, nor will it reduce eugenics. It will not further respect for human life - in fact, as demonstrated in the book Freakanomics, an abundance of unwanted children leads directly to an increase in violent crime and a lessening of the social value of human life. Therefore here, as with the Democratic equation of "mitigate global warming by reducing energy consumption," the policy prescriptions will not - and can never - achieve the aims they are meant to achieve. And yet right-wing Republicans and abortion, just like left-wing Democrats and global warming, the prescriptions themselves are a matter of doctrine, not of reason, and it's a damn shame.