Sentence:  Plant breeding is always a numbers game....In addition, we are looking for rare alleles, so the more plants we try, the better. These <WSD>rarities</WSD> may be new mutations, or they can be existing ones that are neutral—or are even selected against—in a wild population. A good example is mutations that disrupt seed dispersal, leaving the seeds on the heads long after they are ripe. 
Meaning of the word marked with <WSD> tags:  (rare object)A rare object.