Theological essentialism is a special case of the wider phenomenon of philosophical essentialism.
Theological essentialism is Bible-based. It holds that every species of plant and animal, and race of human being, has an essential nature that makes it what it is. These essential natures were defined during the period covered by the Book of Genesis and are unchangeable, save by God's grace or curse. Theological essentialism ultimately depends on the truth of creationism.
Philosophical essentialism is not in conflict with theological essentialism, but covers a wider range of phenomena. Philosophical essentialism derives from the theories of the Greek philosopher Plato, who taught that every functioning thing - whether plant, animal, human or inanimate - derives its existence by reference to some defining essence.
The main opposing view to theological essentialism is Darwinism, which shows that species are changeable, and the concept of different human races has no objective reality (see the discussion of skin pigmentation later in this article).
The main opposing views to philosophical essentialism are the process philosophies, most notably Buddhism, which state that no functioning thing has any defining essence. All phenomena are impermanent and empty of inherent existence. They derive their existence purely in relation to their history, structure and the way they are viewed.
In this article I shall start by reviewing theological essentialist teachings, as these deluded views have been, and continue to be the source of much divisiveness and suffering .
The second part of the article will review
philosophical essentialism, with subject matter ranging from Darwinism through genetics,
chemistry, classical and quantum physics and the nature of objectivity.
Theological essentialism comes in two major aspects - inherited sin and sexism.
The curse of Ham.
One son, Ham decided to leave him like that to teach him a lesson, but the other two sons, Shem and Japheth, were worried what the neighbours would think and covered him over. In the morning Noah woke up with the mother of all hangovers. When he found out what Ham had done he cursed him and all his descendents to be dark skinned slaves and menials, who would serve the descendents of the other brothers.
Well that's the Biblical justification for racism, and no, I don't believe it either. But apparently this story was at one time a great favourite with the South African Reformed Church 
Race and Rationaity
Populations living in inland forests in northern latitudes were selected towards light skin. So according to Darwinism, pigmentation has nothing to do with any other supposedly inherited qualities such as divine curses. The degree of pigmentation in a population is purely a reflection of the evolutionary tradeoff between rickets and melanoma  . Being black, white, brown, red, yellow, green or purple has no relevance to any other aspect of your personality, apart from your essentialist stereotype .
The inherited guilt of the Jews
Does this sound crazy? Well it was official infallible Vatican doctrine until 1965 (No, I haven't got the 9 upside down. I did mean 1965 not 1665. The Middle Ages ended later for some people than for others )
Not surprisingly, in view of the geographical location of the Vatican, the present day Romans have somehow escaped inheriting this sin.
Sexism and machismo
1 Corinthians 11:7-9 "For a man
indeed ought not to cover his head, forasmuch as he is the image and glory of God: but the
woman is the glory of the man. For the man is not of the woman, but the woman of the man.
Neither was the man created for the woman, but the woman for the man."
Another inherited curse! This time the transgression gene is passed down from mother to daughter (X chromosome recessive?).
Theological essentialism - conclusion
For the best part of two thousand years essentialism held sway over the Western mind, firstly in the form of Platonic essences, then as the unchanging species and races of the Bible, and finally as nineteenth century atomic substantialism.
Before Newton, the heavenly bodies wandered around the firmament according to their essential natures as decreed by the 'Unmoved Mover'.
After Newton, the stars, planets, moons, comets and asteroids moved according to the same mathematical relationships.
Before Newton, stars, planets, moons, comets and asteroids were separate entities. After Newton there was a continuity in size and composition from the tiniest 'grain of sand shooting star' through meteorites, asteroids, comets, moons, miniplanets, small planets, gas giants, brown dwarfs and all the different sizes of stars.
Before Newton there was the concept of the 'Unmoved Mover'. After Newton every action had an equal and opposite reaction. As a consequence anything that produced a change was itself changed. Therefore ALL functioning things must be impermanent. These observations were never taken to their logical conclusion by European philosophers in Newton's day, possibly because heresy still attracted severe punishment in most European countries.
The first hint of atomic substructures came from the work of Mendeleev, who published his periodic table in 1869. He left gaps in his table for as yet undiscovered elements and was able to predict their properties.
Work on radioactivity in the early 20th century demonstrated that atoms were not fundamental but were composed of elementary particles - electrons, protons and neutrons. But these elementary particles did not act like classical 'things'. They were only knowable by interactions with other particles, and the mere act of observation changed their properties in an indereminate way.
Even worse, their 'essential nature' seemed to change radically according to how they were observed. If you set up your experiment to observe them as particles, then they behaved as particles. If you set it up to observe them as waves, then they behaved as waves.
Evolution and Genesis
And if Genesis was untrue, what about the rest of the Bible? Was queer-bashing still mandatory? Were the Jews really damned for all time? Although the seed was planted in 1859, the tree took a century to come to fruition with the liberation movements of the 1960's
'The Origin of the Species' was the first major blow against essentialism in the West. In 'Darwin's Dangerous Idea'  Daniel Dennett says ' Even today Darwin's overthrow of essentialism has not been completely assimilated .... the Darwinian mutation, which at first seemed to be just a new way of thinking about kinds in biology, can spread to other phenomena and other disciplines, as we shall see. There are persistent problems both inside and outside biology that readily dissolve once we adopt the Darwinian perspective on what makes a thing the sort of thing it is, but the tradition-bound resistance to this idea persists.'
As mentioned previously, the traditional Copenhagen interpretation of Quantum mechanics regards subatomic entities as having a dual nature - sometimes they behave as particles ('things') and sometimes as waves ('processes' ). However recent work by Shahriar S. Afshar  suggests that there is 'no such thing as a photon' , and the fundamental nature of all 'particles' is actually wavelike processes.
So, when you get down to the actual nitty-griitty of what all 'things' are made of, you find there's 'no-thing' there. It's processes and impermanence all the way up, and processes and impermanence all the way down.
Buddhism and science
Mid-nineteenth century Europeans regarded Buddhism as a nihilistic, heathen superstition devised by some brown-skinned son of Ham, and of interest only to anthropologists and missionaries who were sent out to refute it. These attitudes began to change following the publication of 'The Light of Asia' in 1879, though among some Christians old habits die hard.
Today, Buddhism is the only major religion which is capable of holding a rational discourse with science without the need for 'separate magisteria'.
Notes and references
 Essentialist stereotyping
 The inheritance of sin is a rather odd concept in terms of the modern understanding of genetics, see
 The Curse of Ham
The 'Curse of Ham' has not been accepted by all Christians. The Friends were strong campaigners for the abolition of slavery, see:
 Rickets, melanoma and skin pigmentation
 The inherited guilt of the Jews for the Crucifixion of Jesus
Nor is Anti-semitism a purely Christian phenomenon, see
 Sexism and homophobia
Again, not all Christians are physical or verbal queer-bashers, see:
(In fairness it should be noted that although the Judeo-Christian treatment of women in the past may not always have been sweetness and light, it doesn't begin to compare with another religion (no prizes for guessing which) in the depths of depravity, institutionalised pedophilia and sexual exploitation of girls and women that are still committed in the name of the Almighty. Visit http://www.cathud.com/LINKS/pages_GL/Islam.htm and learn how 'God' condemns child rape victims to execution by flogging to death, followed by eternal torment in hell. )
 Darwin's dangerous idea
Dennett, Daniel C in 'Darwin's Dangerous Idea ' p 39; publ Penguin, London 1996, ISBN 0-14-016734-X
 Quantum processes
If we regard Buddhism as a combination of a philosophy, psychology and religion, then how much mileage can we get from the first two aspects before we have to start invoking religious faith?