By "species essentialism" maybe you mean the 'typological species concept'. The typological spcies concept distinguish between species of organisms by means of their 'type', which is influenced by Platonic philosophy. The assumption here is that the diversity between life-forms can be reduced to a number of distinct and immutable 'types' or 'essences'. It is believed that there are an 'ideal type' for each species, while all variations are deviations from the ideal type.
The findings of natural history, and Darwinian variational evolution, put an end to species essentialism in biology generally. But some of its practices are still based on the essentialist assumption, for instance the designation of a holotype for new species description, or even 'The Human Genome Project'.
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