system
Given the following post, respond as if you're commenting on Reddit:
human
What is the purpose of humans having different blood types?

To unpack that question - I understand the basics about blood types as far as antigen and antibodies, and why one blood type rejects another. But from an evolutionary perspective, why would we develop different blood types? Or is it possible they are just mutations?
gpt
Having different blood types, and having a significant amount of genetic variation at the molecular level in general, prevents parasites and infectious diseases from being able to spread as easily from person to person.  That's not to say that it's impossible for a parasite to adapt to multiple "environments", but it does make it significantly more difficult to spread.  In plants and animals where there is little to no genetic variation, diseases can spread much more quickly.

An example of this is the fungal diseases that can quickly wipe out genetically-identical cultivars of crops like bananas and oranges.  Since every plant is genetically identical, diseases can quickly evolve to be very efficient at infecting that particular cultivar.  (This doesn't happen as often in the animal kingdom because it's quite a bit harder to clone animals than it is plants)