To me, "in an evolutionary light" suggests a particular light that results when you shine pure light from a light source through a particular color filter. Viewing something in an evolutionary light doesn't differ in an operational sense from viewing it in a creationist light or in a postmodernist everything-is-random light—or in a different evolutionary light (Lamarckian, say, instead of Darwinian). In each case, you apply a particular color filter to your light source and then look at your subject through that filtered light.
"In the light of evolution" suggests something different: Evolution is itself a light source where there wasn't one before; and rather than offering a contrasting view to that which is available from competing light sources, it makes visible what was previously in darkness—or at the very least casts light on what previously had been insufficiently illuminated.
The distinction in effect between "in the light of X" and "in an X light" undoubtedly has more to do with cultural intonations than with the objective meanings of the two phrases, although the bare difference between "the light" and "a ... light" does enlist and deploy an inherent difference between definite and indefinite articles to some purpose.
Looking at historical uses of the phrase "in the light of X," we find various examples that suggest a grandeur in that phrase that "in an X light" lacks. For example, John Guillim, A Display of Heraldrie (1611) writes:
The most wise and provident God, before the creation of his other works, did first create the Light, to teach man to lay the first foundation of all his actions in the light of true knowledge, thereby to direct his waies aright, and that his doings bee not reprooved as Works of darknesse : especially sith God would not suffer the Night it selfe to be so wrapt in darknesse, but that the Moone and Starres should somewhat illuminate it.
If we reframe "in the light of true knowledge" as "in a truly knowledgeable light," much of the power of the original seeps out.
Likewise, Francis Rous, The Arte of Happines (1619):
Let us endeavour carefully to walke in the light of Grace, which will bring us to the full Revelation of the yet inaccessible light of Glorie; where Happinesse shall at once bee fully knowne, and fully enjoyed.
This exhortation pales somewhat if we recast it with indefinite articles:
Let us endeavour carefully to walke in a Grace-given light, which will bring us to the full Revelation of a yet inaccessible glorious light; where Happinesse shall at once bee fully knowne, and fully enjoyed.
And again, Alexandre Grosse, Sweet And Soule-Perswading Inducements Leading Unto Christ (1632):
Christ is not to them a plaine and pleasant way ; they cannot according to the Apostles phrase walke in Christ ; by believing Christs Promises, by obeying Christs Precepts, by conforming themselves to Christs Example, with fulnesse and chearfulnesse, as a Travellor walks fully and joyfully in a plain and pleasant path : they cannot walk in the light of Christ, as a man walks joyfully in the light of the Sun ; in the strength of Christ, as the Rider moves speedily in the strength of the Horse which carries him , as the Ship rows in the strength of the Wind which drives him, and the Prophet walked to Mount Horeb in the strength of the Barley cake which fed him ;
The difference between "they cannot walk in the light of Christ, as a man walks joyfully in the light of the Sun" and "they cannot walk in a Christly light, as a man walks joyfully in a solar light" is quite large, and not merely because "in a solar light" sounds like a ridiculously roundabout way of saying "in sunlight."
Such is the baggage that "in the light of X" has acquired over centuries of sonorous use. It may indeed be too strong a formulation for many applications: "in the light of contrast" versus "in a contrasting light," for example, or "in the light of caution" versus "in a cautionary light." Whether evolution is up to the mark is a matter for each author to decide.