I'm also interested in this, because it certainly seems to me that a lot of modern day racism is deeply rooted in pseudo-science( thinking specifically of the Nazi "scientific" ideology). How did the development of Darwin's theory and other models of genetics affect racism?

It is important to stress that the Enlightenment was not a uniform school of thought. The various thinkers which today are labelled Enlightenment thinkers often have very dissimilar views on the most fundamental philosophical issues. Historian Jonathan I. Israel is probably the most prominent historian to have studied and highlighted this difference, and he has proposed two major schools of thought during the period, namlely Radical Enlightenment, which originated from the works of Spinoza and the various thinkers directly or indirectly influenced by him, and a Moderate Enlightenment, which includes thinkers who applies scientific and/or secular thought to some areas but adhere to conservative thoughts in others. Examples of thinkers from the moderate strain according to Israel would be John Locke, Montesquieu and Voltaire, the last one not least because of such thoughts as the one quoted by OP, but also because of his belief in enlightened absolutism, deism and so on.

It is legitimate to criticise Israels theory in that his classifications can be too simplistic, and at times seems forced to fit his model, not least in his attempts at linking Spinoza to thinkers by proxy or middlemen. However there is no getting around the fact that he has a valid point in that the Enlightenment was an extremely hetereogenous movement, which included all sorts of ideas, and he has amply documented it in his currently 5 volume work on the Radical Enlightenment. Basically the Enlightenment is a very difficult concept to define with any precision, and despite the overwhelming amount of studies on the subject produced in the last century or so, scholars today are still conflicted on the issue, not least because of the disparity of thoughts which it includes.

With regards to your question one of the characteristics of Israels Radical Enlightenment was exactly support for the abolition of slavery, and a general tendency to discard racist opinions. The most important work was the extremely popular L'Histoire philosophique et politique des établissements et du commerce des Européens dans les deux Indes (The philosophical and political history of the establishment and trade by the Europeans in the Two Indies), which was formally attributed to the rogue clergyman Guillaume Thomas Raynal, but was mostly the work of Denis Diderot and the group of thinkers and writers which were his friends. It was one of the most sold works in the French language according to the archives of the Societe Typographique de Neuchatel. The work is an impressive and comprehensive (4 thick quarto volumes in its later editions) tract against slavery, racism, colonialism and the policies of European imperialism in general. And it inspired later thinkers and many French Revolutionaries, Condorcet and Brissot, and the Abolition movement which came to prominence in the early days of the French Revolution and resulted in the (short-lived) abolition of slavery in the French Empire. Later 19th abolitionist movements were themselves inspired by the thoughts that had risen in the 18th century from abolitionist thinkers. This concerted effort to implement abolition in politics via secular philosophy was decidedly new and particular to the 18th century and the Enlightenment. The idea of abolition was of course not new, the Quakers for example had adhered to it prior to that (and also inspired some Enlightenment thinkers), but the application of it in a universal fashion from a secular standpoint was rather unique to the period.

On the other hand we have the quote in the OP, which expresses a very basic idea which had been inherent in Europe for longer than the Enlightenment. While Voltaire certainly was an Enlightenment thinker, this quote does not appear as an expression of a particular Enlightenment thought, as much as it is an expression of his times, and an example of inherited prejudice which his critical mind did not think to question.

It is true that a "scientific racism" developed later, with race theory as its cornerstone, but that did not happen until the 19th century on ideas from thinkers who were decidedly anti-Enlightenment in their philosophy, like Arthur de Gobineau, and which did not care much for thinkers like Voltaire or Kant in any way, and was extremely unlikely to have been influenced by them.