One of the reasons for the incredible breadth in the appearance of animals is to help individuals mate with the right species. This is one evolutionary process that causes differences between populations of animals and that can eventually lead to the formation of new species. For example, say there is a population of birds where there are two main kinds of food: small seeds and large seeds. Birds with smaller beaks are better at eating the small seeds and birds with larger beaks are better at eating the large seeds. If you are a bird with a large beak, then you ‘want’ to have babies with another large-beaked individual, as mating with a small-beaked individual would result in a chick with a medium-sized beak that is neither good at eating the small nor the large seeds. Now, let’s say that the large-beaked and small-beaked males are difficult to tell apart. From a female’s point of view, anything that makes these two varieties easier to tell apart will be advantageous. Let’s say a mutation arises which means that large-beaked birds also have red heads. Or say their larger beaks mean that their songs sound different. Any cues that the female is able to pick up on to tell these males apart would be selected for, and could evolve to become signals that males use to directly attract a female.
Over time, as these differences between the two different populations become larger and there is less and less mating between the populations, they will become ‘reproductively isolated’ (i.e. not mating with each other at all) and thus become two new species.
However, in some cases it isn’t clear whether differences between two similar populations or species are due to this process (of ‘sexual selection’) or instead one of genetic drift. Genetic drift is where a population separates (for example, a river splits a population of animals in half). Now, just by chance alone, differences will arise between these two separate populations just because they are only mating within their particular population.
One set of animals where it is not clear how the differences between similar species arose is the lava lizard. Lava lizards live throughout the Galapagos archipelago and there are nine species in total. The different species are separated from each other geographically and have both different colouration and different push-up and head-bob behaviour that they use for communication.
Video credit: David Clark
As part of a larger study, Clark and colleagues investigated why it is that these differences in signals exist between the different species of lava lizard, in a paper published in Animal Behaviour. To do this, they designed robot lizards of two different ‘species’, Microlophus grayii and Microlophus indefatigabilis. The researchers used these robot lizards to simulate interactions between them and real lizards of the same two species. Therefore, a real lizard could either have an interaction with a robot of the same species, or of a different one. The idea here is, if different species have evolved to pay attention to each other’s signals (and thus avoid mating with a different species), then males that are the same species as the robot should respond much more to the robot’s signals than a lizard of the other species. On the other hand, if the different species arose by genetic drift, then we wouldn’t expect for there to have been as much selection on individuals to avoid mating with the other species.
A lava lizard interacts with a robot lizard. Credit: David Clark
You might be wondering why the researchers didn’t test their male robot lizards on real female lizards rather than male ones. Well, in these species the females don’t engage much in active choice of males. Instead, mating is determined more through male-male competition.
The researchers found mixed results: M. grayii treated both robots the same, while M. indefatigabilis responded a lot more to robots that displayed more similarly to themselves (i.e. robots of their own species), responding both faster and then displaying for longer to the robot lizard.
Another example of a lizard reacting to the robot lizard. Credit: David Clark
The researchers then repeated the experiment with a species from the mainland (Microlophus occipitalis) and found that this species very much discriminated against displays made by different species of lizards.
These results would seem to imply that there had been selection on lizards to not mate with other, similar species. However, the researchers also carried out a phylogenetic analysis which showed that the lizards had evolved these traits in isolation from one another and without much effect of sexual selection. So what’s going on? It seems that the lizards have evolved the ability to pay attention to the different behavioural signals of each other (and discriminate against the ‘wrong’ species) without having evolved in the same geographic area as them. Exactly how this happens is still not clear and perhaps just shows more than anything that evolution is complicated!
Clark, D. L., Macedonia, J. M., Rowe, J. W., Stuart, M. A., Kemp, D. J., & Ord, T. J. (2015). Evolution of displays in Galápagos lava lizards: comparative analyses of signallers and robot playbacks to receivers. Animal Behaviour,109, 33-44.