Mimicry is when one species has changed over time via Natural Selection to look like another species. Three commonly defined forms of mimicry are:
- Batesian mimicry, named after Henry Walter Bates, a 19th century Natural Historian, where one species is poisonous or otherwise dangerous to a predator, and another species takes evolutionary advantage of that by looking like it but not actually being poisonous;
- Müllerian mimicry, named after Fritz Müller who worked at about the same time as Bates, where two species that are poisonous or otherwise bad for predators evolve to look like each other thus shortening the learning curve for the predators; and
- Agressive mimicry, not named after anyone, in which a predator mimics its prey to better sneak up on or ambush it.
There is not necessarily a clear distinction between Batesian and Müllerian mimicry in all cases. Also, although the statements above are generally considered true (they could be from a glossary of a biology textbook) they are misleading, because the relationship is not always between predator and prey. For instance, a European Cuckoo looks like a bird eating raptor, and thus scares off the host birds in who’s nest they wish to lay eggs. There is no predator eating something and there is no poison, but it is considered Batesian mimicry because one species is imitating another and thus fooling a third into reacting by aversion or avoidance with the expectation of something nasty happening. In general, it is better to think of inter- and intra-species relationships that are potentially shaped by Natural Selection in more abstract and general terms. Putting it this way, we could define the first two types of mimicry as selection operating on one species (the “mimic) to appear to the senses of a “dupe” be a member of a third species (the “model”) with whom interaction will confer a negative effect on fitness on the “dupe” and whereby the mimicry will confer a positive effect on fitness of the “mimic.” In the case of pure Batesian mimicry, the model and the mimic are distinct, while in Müllerian, the model and the mimic share the role. The effect on the model may be positive or it may not be.
There is a bit of an irony working here. In order for mimicry to work there has to be either a deeply built-in genetically determined avoidance mechanism (something that says “orange butterflies are always poisonous”), which is highly unlikely for various reasons, or there has to be learning on the part of the predator. The predator must learn by trial and error that certain species taste bad, are poisonous, fight back, or whatever. This means, however, that in order for mimicry to work, the strategy of mimicry also has to be vulnerable, because just as the mimic can dupe the dupe into learning the wrong thing, the dupe can smarten up. Taking this a step further, species that can learn stuff may end up, depending on evolutionary context, becoming a species that is really good at learning stuff.
Another consequence of the dupe learning to avoid the model and mimic morph is that both the model and the mimic are influenced in their fitness by numbers. If there are exactly two individuals, a model and a mimic, that will ever be encountered by a predator (and eaten on the spot) and if the predator (the dupe) learns in one trial to never eat that kind of thing again, then there is a 50–50 chance for either mimic or model of being eaten first. Note that if the mimic is eaten first, then the model may well be eaten because the predator did not learn to avoid food that looks like these particular individuals. If, however, there are a gazillion models and only a few mimics, then the mimics will do pretty well because most predators it encounters will have already learned to avoid individuals that look like model or mimic.
Also, consider each node of the mimicry triangle in relation to the selective forces working on it, in relation to all the variables. For in stance, a model species might suffer a fitness consequence if there are too many mimics, because predators may actually learn to like the morphotype that has been conferred on it by Natural Selection. Mimics would not gain as much if they were too common as well, but that sets up a tension between the benefits of reproducing a lot and the cost of being common. Also, one might ask why mimics don’t just make their own poison or other nasty bits. There must be a cost of doing so that is being avoided.
There is a case that I’ve written up here of a bird mimic (one of the afore mentioned cuckoos) that has two morphs mimicking a predator; the idea here is that polymorphism (having several different appearances) helps with this problem of being too common, or of the dupe learning too much. Also, the dupe species seem to do a lot of social learning, which can really mess up your strategy if you are a cuckoo. Go have a look.
Agressive mimicry is a totally different thing that we won’t talk about here except to mention that humans are one species that does this, and that it is a key feature in the biologically underrated novels by Mary Doria Russell, Children of God
and The Sparrow
. In those novels, there are two intelligent life forms on another planet, and one is a predator the other prey. They look a lot alike, and this is attributed to agressive mimicry by the predaceous species.
By the way, Henry Walter Bates was an interesting guy. His BD dates are 1825–1892, so he overlapped a great deal with Darwin (1809–1882). Bates worked with A.R. Wallace in the Amazon. Wallace, who is often viewed as under appreciated in the blinding light of Charles Darwin, lost all of his Amazon biological material in a shipwreck. Bates, on the other hand, supplied the scientific community with samples representing over 14,000 species, most insects. He discovered about 8,000 new species.
” and subsequent to the storm of debate that arose from that it has been looked at more carefully and a little bit verified (see 
… that I found hit home very closely like maybe Henry was me reincarnated and then transported back through time so his, er, our timeline would cross. This is not surprising since Henry and I have had overlapping interests in science for several decades, so his novel references a sense of understanding of the landscape, the kind of thing a geologist or archaeologist achieves either over time or because of an innate capacity. One of his characters is such an archaeologist. Another overlap is our experience observing academic culture. We tend to breed within (“we” meaning academics, not Henry and me specifically), and sometimes we form teams where thinking, understanding, and explaining are done as a compound organism. Also, and this may be too much of an inside reference, Book 1 at least is pretty much Cenozoic, which is cool.
, 
, and editor of Nature’s Futures column, which is anthologized 
.
