Darwin’s Postulates & Firemouth Cichlids

Why cichlids?

Cichlid fish are one of the most biologically diverse groups of vertebrates on the planet. This diversity (as well as population size, reproduction rate, etc) allows scientists to study the cichlid evolution, and the role of natural selection, more closely than other populations. Many aspects of cichlid characteristics have undergone (and are currently undergoing) selective pressures. Cichlid evolution can be found at multiple levels, including behavioral changes, molecular adaptations, size and coloration variation.

Screen Shot 2014-07-24 at 10.42.50 AMThorichthys meeki is a species of cichlid commonly known as the “firemouth”, because of its bright red-orange coloration on the jaw. This specific coloration is unique to the firemouth, and is used in mating, competition and defense, and is therefore strongly affected by selective  pressures. In general, male firemouth cichlids have prominent  jawline coloration (10). The female Firemouth cichlids have specific molecular adaptations that that allow them to see the male coloration very clearly (6).

In the case of the firemouth cichlids, jawline coloration in males (and the subsequent female response) plays an important role in the evolution of the species due to the pressures of sexual selection. Sexual selection has been shown to be a mechanism of significant biological change for firemouth cichlids, even if the environment stays very stable (9), and research confirms that male coloration corresponds to sexual selection by the females (8). The females tend to mate with the more dominant males, so any characteristic that improves one’s chance of winning in a male-male competition scenario will be selected for. In any cichlid population, males who display the same color will compete more intensely. In firemouth cichlid populations, all of the males have a distinguishing red color, so male-male competition is very strong (4).

firemouthIt has been shown that smaller, less dominant males have physiological and behavioral changes during any social interaction with a more dominant cichlid. (3).  Many times, cichlids will engage in “pre-fight behaviors” (7) and  whichever male is smaller and duller will back down (5).    The coloration allows the more dominant males to emerge victorious from a fight with other males, without even having to engage in actual physical contact. The firemouth cichlids open their mouths and expose more of the red jawline. This behavior enlarges the head considerably when seen from the front and the side. In the event that there is no definitive pre-fight winner, they will engage in the more dangerous physical form of fighting. (1) In many cases, coloration is directly correlated to body size, which also plays a role in determining the winner of the conflict. (2)

In addition to behavioral changes, firemouth cichlids have specific molecular adaptations, especially related to the genes responsible for color perception. Under light levels that magnify or negate the effects of the color, the females respond accordingly. If the females notice no significant difference in male coloration, mating is random. If the male coloration differences are evident, the females prefer the more colorful male. (11)

How do we know if a population is evolving?

In order to confirm that any characteristic is evolving in a population, Darwin came up with four postulates. If confirmed, we can confirm that evolution is indeed occurring within the population. In order to test how rapidly the firemouth cichlids are evolving, actual data can be compared to the following postulates:

1. The traits within a population vary from individual to individual.
2. The variation must be heritable in some manner; not controlled completely by the environment.
3. Survival and reproduction differ among individuals
4. Individuals with more favorable traits are better adapted to the environment, and therefore pass more genetic information to the next generation.

All four of Darwin’s postulates can be confirmed within the firemouth cichlid population, which would confirm that evolution is occurring within the firemouth population. In fact, studies have shown that evolution is taking place at a very high rate in many cichlid populations, especially in the firemouth cichlids (7, 8, 9).

tmeeki_smlVariation within the population is necessary for evolutionary change to occur, which is Darwin’s first postulate. The Cichlids do have variation in the coloring of the ventral surface, and this has been confirmed by a number of different studies.  Color variation is one of the predominant features in most cichlid populations,  including firemouth cichlids. (1, 2, 3, 4, 5, 8). In my research, I will be analyzing the firemouth’s body to determine what percentage of the body is red. I will also analyze the color itself and compare to a standard range of colors, from light red to dark red. Because the variation is inherent in the population, the actual effect of these variations is addressed by the next three postulates

Darwin’s second postulate, that variation is heritable, can be observed by the increase of various “dominant” traits over successive generations. In firemouth cichlids, most of this is due to competition between males and sexual selection by the females (2, 7, 8).

The third postulate that Darwin proposed was that there are differing survival and reproductive rates among the individuals within the population. This has been confirmed by various studies done on the long-term evolutionary change in coloration due to natural and sexual selection in firemouth cichlids (1, 4, 8, 9).

Screen Shot 2014-07-26 at 3.06.55 PMDarwin’s fourth postulate is essentially a confirmation of the effects of  the first three postulates. Based on the previous three postulates, scientists are able to document heritable variation within the population and differential mating success and survival. From these observations, the evolutionary change of the firemouth cichlid population can be determined.

The firemouth cichlid population is a good example of evolution in action. With a confirmation of all four of Darwin’s postulates, the factual nature of firemouth evolutionary change is impossible to deny. These changes are primarily due to male-male competition (via fighting), sexual selection by the females, and various other behavioral, morphological and molecular changes.


Works Cited

(1) Baerends, G. P., and J. M. Baerends-Van Roon. 1950. An Introduction to the Study of the Ethology of the Cichlid Fishes. Behaviour. Supplement.

(2) Beeching, S. C., and Pike, R. E. 2010. Ontogenetic Color Change in the Firemouth Cichlid, Thorichthys Meeki. The American Society of Ichthyologists and Herpetologists.

(3) Chen, C., and Fernald, R. D. 2011. Visual Information Alone Changes Behavior and Physiology during Social Interactions in a Cichlid Fish (Astatotilapia burtoni). PLoS ONE.

(4) Dijkstra, P. D., and Groothuis, T. G. G. 2011. Male-Male Competition as a Force in Evolutionary Diversification: Evidence in Haplochromine Cichlid Fish. International Journal of Evolutionary Biology.

(5) Enquist, M. Leimar, O., Ljungberg, T., Mallner, Y., and Segerdhal, N. 1990. A test of the sequential assessment game:fighting in the cichlid fish Nannacara annomala. The Association for the Study of Animal Behavior.

(6) Evans, M. R., and Norris. K. 1996. The importance of carotenoids in signaling during aggressive interactions between male firemouth cichlids (Cichlasoma meeki). Behavioral Ecology

(7) Kodric-Brown, A. 1998. Sexual Dichromatism and Temporary Color Changes in the Reproduction of Fishes. American Zoology.

(8) Maan, M. E., Hofker, K. D., van Alphen, J. J. M., and Seehausen, Ole. 2006. Sensory Drive in Cichlid Speciation. The American Naturalist.

(9) Nagl S., Tichy, H., Mayer, W. E.,Takezaki, N., Takahata, N. and Klein, J. 2000. The origin and age of haplochromine shes in Lake Victoria, East Africa. The Royal Society.

(10) Neil, S. J. 1984. Color Pattern Variability and Behavioral Correlates in the Firemouth Cichlid, Cichlasoma meeki. Copeia.

(11) Sugawara, T. Terai, Y., and Okada, N. 2002. Natural Selection of the Rhodopsin Gene During the Adaptive Radiation of East African Great Lakes Cichlid Fishes. Society for Molecular Biology and Evolution.


This was a research project done in Evolutionary Biology (BIO405W) during the spring 2013 semester



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