What is science illustration?
'Science illustration' is very much what it sounds like- a depiction with a scientific subject or purpose, intended to communicate information. In some form it has existed as long as science has, and it has changed in ways that mirror the changes within both art and science over the centuries. Early woodcuttings of animals with hints of real anatomy ornamented with fantastically imaginative artistic liberties were succeeded by Darwin's sketches, Audobon's birds, Blaschka glass models and more. Sculpture, engravings, graphite, ink, watercolor, gouache, colored pencil, markers, oil paint, acrylics, glasswork, digital illustration, and animation are just some of the mediums commonly utilized in science illustration today.
I haven't heard of science illustration before; where is it still being used?
Everywhere! Science illustration is extremely pervasive today, and so easy to take for granted that it's almost invisible to us. It's in textbooks, field guides, pamphlets, magazines, instruction manuals, scientific publications, museums, zoos, aquariums, botanical gardens, parks, and historical sites; on posters, plaques, and websites, to name a few places.
Doesn't photography make science-based illustration irrelevant?
The short answer is that illustrations can do things that photographs can't.
If you can call up a mental image of the solar system, the structure of DNA, plate tectonics, a developing chicken embryo or a Tyrannosaurus, it is because of illustration. Illustration can show difficult-to-photograph behaviors and processes, from exactly the right angle and with exactly the right lighting necessary to discern exactly what is happening, something that can be incredibly difficult to achieve when photographing a subject that isn't posable. Illustration can communicate inner workings, the impossibly large and the indescribably tiny, the long ago and the distant future.
Illustration isn't just applicable to unphotographable subjects, though. Think back to your high school dissection lab, and imagine the only references you had to make sense of the frog glop in front of you were photos of the same frog glop, instead of illustrations with more defined shapes and colors that helped guide your eye into seeing the underlying structures. It can emphasize important aspects of a subject, and downplay or entirely do away with features that distract from or obscure information essential to the purpose of the picture (as a direct result illustrations can often remain coherent at smaller sizes than the clearest photos of the same subjects). It can portray a 'typical' subject when no two specimens are quite alike. A photograph can show you a thing, as it is. An illustration can show you what makes it that thing, instead of something else entirely. It can give you the ability to visually summarize a subject, instead of being forced to portray the whole stream-of-consciousness reality of it. This ability makes illustration invaluable to science.
Why should I hire a science illustrator when there are so many graphic designers everywhere who could do this project?
There are a lot of incredible graphic artists out there. So many in fact that you could almost certainly hire a very gifted one for the kind of low rates necessitated by such a competitive field. You could easily come away with a very beautiful and eye catching piece at the end of the project, done by someone with an excellent sense of design. If that is all you want from your finished piece, then there is no reason to not potentially save some money by hiring a graphic artist.
However, if your final piece needs to be accurate, a graphic designer might not be your best choice. With science illustrators, you are not just paying for their talents, skills, and sense of aesthetics, you are also paying for their attention to detail and their understanding of your subject. Science illustrators enter into your contract with a considerable body of knowledge to build on, and understand that learning enough about your particular subject to be able to render the important parts accurately is part of the job. You are paying for trained observation, a highly specialized skill set, and an artist who won't sacrifice function for style. If your final piece needs to be functional as well as beautiful, you may find that a science illustrator is far and away the less costly choice once corrections need to be made.