Arguably the main reason many Christians reject evolution is that it precludes monogenism, the belief that the entire human race descended exclusively from two human beings. As the doctrine of Original Sin as formulated by the Catholic and Reformed faith traditions is contingent on monogenism, it is hardly surprising that there is opposition to evolution.
It should be pointed out that serious re-examination of the doctrine of Original Sin predated Darwin. As Tatha Wiley notes, post-Reformation Christians "[l]ike Pelagius, they felt the idea that human beings were born already guilty of sin was morally reprehensible..Moderns considered the doctrine of original sin unsound on ethical grounds, not only historical ones." [1] Needless to say, once the realisation that evolution ruled out monogenism, many Christians realised that Original Sin as classically formulated was untenable. If humans were not exclusively descended from two people, then it was impossible for the guilt and consequences of Adam's sin to be genetically passed down to the entire human race.
Given that traditional Christadelphian views on the atonement are not contingent on monogenism, the fact of evolution does not affect our theology, which is why it is frustrating to see such vehement opposition to evolution. With this in mind, it is fascinating to read the essay "Evolution and Original Sin" by the philosopher Robin Collins, Distinguished Professor of Philosophy at Messiah College which is currently being serialised at the BioLogos blog. The insights Collins offers are fascinating, but for us, the realisation that our theological position needs no essential modification in the light of the reality of common descent is even more reassuring.
The entire series to date can be found here.
References
1. Wiley, Tatha. Original Sin: Origins, Developments, and Contemporary Meanings. Paulist Press, New Jersey, 2002, p 111
