Sci-Day 10: Coevolution

Happy Sci-Day, fans! Today, I want to talk about an interesting phenomenon called 'Coevolution'.

Coevolution is when an evolutionary change in one species causes evolutionary change in another, which itself causes further change in the first species. Essentially, this is a sort of positive feedback - the cycle continues to build on itself until it is interrupted (perhaps by the extinction of one of the two species).

There two main categories of Coevolution: mutualism, and antagonistic coevolution. Mutualism is when two species each use each other as a resource, and evolutionary change that facilitates interaction between them is beneficial to both (benefits outweigh the costs). Antagonistic coevolution is when evolutionary change in one species decreases fitness in another species, inducing a change in that species that then decreases fitness of the first.

Many examples of mutualism exist in nature - one great example is plant-pollinator relationships. One such example is the genus of tropical plants Glochidion and the moth genus Epicephalia. Each species of Glochidion is pollinated exclusively by a single Epicephalia species. Moths actively pollinate flowers and deposit their eggs into approximately 20 flowers. Their larvae then develop in those flowers and consume some fraction of the seeds that result from pollination. Thus, both species rely on each other for reproduction and could not survive without each other (Blumenstiel, 2016).

Antagonistic coevolution has two common types - predator-prey coevolution and parasite-host coevolution. In predator-prey coevolution, the prey species evolve some adaptation to help defend themselves against predators, such as longer legs to run faster, or thick armor to protect from claws and jaws. This reduces the fitness of the predators, and they evolve adaptations to overcome or bypass these defenses. Examples of possible responses to the aforementioned prey examples would include evolving good camouflage so they could catch the prey before it could run, and a stronger set of weapons to punch through the armor. Parasite-host coevolution happens when parasites evolve adaptations to better infect/reproduce inside their hosts, and the hosts evolve adaptations such as changes in the immune system to better resist infection. A commonly used term for antagonistic coevolution is an "evolutionary arms race" - if this is occurring, we should see some association between the level of defense and level of predation of the organisms suspected to have such a history.

Unfortunately, it can be quite difficult to determine whether or not such patterns are occurring in long-extinct species such as those from Hell Creek. This is partially because we cannot observe the animals interacting with each other in life, instead relying on evidence such as tooth marks on fossil bones and overall body proportions.

One example of this uncertainty is the idea that Tyrannosaurids and Ceratopsians were engaged in predator-prey coevolution. For a long time, it was thought that Ceratopsians had evolved frills and horns as greater measures of defense, and the trend towards larger jaws and subsequently more powerful bites was the evolutionary response. However, analyzing phylogenies of these two groups, there does not seem to be a strong overall trend in increased frill and horn size to match increasing bite strength in contemporary Tyrannosaurs. Now, the most widely accepted theory to explain the diversity in structure of Ceratopsian frills and horns is that it was driven by sexual selection and species recognition. In fact, evidence of injuries from Triceratops horns have only been found on the skulls of Triceratops, which is evidence of non-fatal intraspecific combat in this species (Farke et al., 2009).

Coevolution may actually be an idea that Dinosaur Battlegrounds could help paleontologists investigate. As more formations and paleoecosystems are added, it would be possible to look at the functional differences of different lineages through time (and since restorations and variables influencing the simulation are all based on scientific data, their functionality can be safely considered as the most accurate picture of that species at a given time). Then, one could actually alter the variables of an earlier species, such as prey, to more closely resemble those of a derived relative, and look for any changes in the fitness of species that it regularly interacts with. This could provide some level of evidence for a coevolutionary relationship, given enough replication with a large enough phylogenetic sampling.

Well, I hope this has given you a bit more information on coevolution! It's a very interesting topic that truly shows how all organisms are interrelated, and all species have their role to play in the environment!

Acknowledgements:
Blumenstiel, Justin. 2016. Coevolution lecture [Evolutionary Biology class at KU].
Farke, A. A.; Wolff, E. D. S.; Tanke, D. H.; Sereno, Paul. 2009. Sereno, Paul, ed. "Evidence of Combat in Triceratops. PLoS ONE 4 (1): e4252.