According to Surah 51:49 everything is created in pairs, but then why do there exist species that only consist of a single gender?
"In all things We have created pairs so that you may take admonition." - Qur'an 51:49
Everything is created in pairs does not mean necessarily in male and female because millions of things in the universes do not fall under two genders but still exist in pairs e.g. day and night.
Species with out male and female still have pairs within itself as the DNA two parts divides in to two parts in reproduction. With out that pairs of DNA, no reproduction can take place at all.
Wassalam.