Really I think it comes down to the way of presenting information. Zoos aren't well-positioned to have evolution-themed exhibits, or exhibits on Darwinian Origin of Species. It'd be cool for a zoo to exhibit Darwin finches to talk about evolution, but unfortunately there aren't any Darwin finches in US Zoos. Furthermore, looking at the origin of life doesn't make sense for zoos as they only focus on a rather small percentage of living things: the animals. Teaching about origin of life requires exhibits on bacteria life,, plants, protists, etc. For most zoos, the majority of collection is even more specific than animals. However, one thing I'd like to see more of is a focus on "relatedness". Exhibit rock hyraxes inside an elephant barn and have signs talking about how hyraxes are some of the closest living relatives of the elephants. Have exhibits comparing and contrasting giraffes and okapis. Both of these ideas would be exhibits on evolution, even if it's not the way people immediately think of evolution.