Pin eye |
I remember as a child being fascinated by the two different forms of primroses, which, unless you're aware of it, you probably wouldn't notice unless you looked especially closely. What I didn't realise until much later was that this had evolved to overcome a problem potentially faced by all hermaphrodite species and especially plants - the problem of losing the benefits of sexual reproduction by self-fertilisation.
Such is the advantage of mixing genes with those of another individual and so being able to 'try out' new combinations, or to acquire more advantageous alleles and pass these on to your offspring, that there is evolutionary pressure to ensure it happens frequently.