Evolution and Ethics

First I just want to state my opinion on Darwin's theories (again). I don't know who I thinks is more absurd Nietzsche's or Darwin's. I mean this is just my opinion; I know there are people out there who agree with Darwin, but I fail to see how I come from a baboon or monkey or whatever he says we evolved from.
Now that got that out of my system, I wanted to take a closer look at the at pages 217-219 which talks about ethics. I do not understand what Pojman is trying to get across. He tells us about three types of birds, who usually attract parasites and the only way to free themselves of the parasites is to clean themselves. Technically they cannot literally clean themselves, but they rely on other birds to clean them. I see a connection to humans we don't rely on others to clean us, however, we do rely on other people to survive. Now these "birds" have three different personality traits; the sucker (who cleans everyone whether they reciprocate or not), the cheater (who never cleans any other bird but expects other birds to clean them) and the grudgers (who as their name implies only cleans those who cleans them and holds grudges against the cheaters). Since I believe the birds are suppose to represent a community of people which the book says that the suckers are the Christians, being totally altruistic, what is Dawkins implying? Is humanity suppose to be grudgers? Why are the Christians labeled as the suckers? Is it because we believe in love for all no matter what they have done to us? Why is that so wrong? Should we only do good deeds as long as we get something in return? Only do things for gain and not for the morality we hold? This is what I do not understand. I believe that good deeds should be rewarded on occasion, but don't let that be the only reason you do something. Do things because you know its morally right, not for fame or fortune. For the bad you do, yea you should be punished. I don't know, all I do know is that I found this interesting and it sparked many questions for me.