Imagine you're a female cardinal and you're looking for a mate. Before you are three males. Which one should you choose?
If you think dating is tough, just be glad you aren’t a garden snail. When you live life at a snail’s pace, you’d better be able to mate with the next snail you meet without worrying if it’s male or female. That’s why garden snails are hermaphrodites and can take on both the male and female roles in reproduction.
After all, for the basic body plan to change, not only do you need a random genetic mutation that results in a sixth finger, but that mutation must also provide an evolutionary advantage when it comes to reproduction.