Sentence:  Plant breeding is always a numbers game....The wild species we use are rich in genetic variation, [...]. In addition, we are looking for rare alleles, so the more plants we try, the better. These rarities may be new <WSD>mutations</WSD>, or they can be existing ones that are neutral—or are even selected against—in a wild population. A good example is <WSD>mutations</WSD> that disrupt seed dispersal, leaving the seeds on the heads long after they are ripe. 
Meaning of the word marked with <WSD> tags:  Any heritable change of the base-pair sequence of genetic material.