Evolution By Natural Selection: The Case Of Guppies

Discussion

The Evobeaker® computer simulation program “How the Guppy Got Its Spots” (Simbio Virtual Labs®) was used to conduct the experiment (Biology 112, 2017). It consisted of five study locations (Upper West Stream, Lower West Stream, Upper East Stream, Lower East Stream, Meir Creek) with and without predators (Rivulus, Cichlid A, Cichlid B), one holding cooler and three tanks. Simulations were run for approximately four to eight generations, before and after the removal of predators, and before and after transplantation.

For each simulation, 15 males were randomly sampled and their spot brightness was recorded on a scale from 0-20. Each simulation was replicated 21 times, results were visualised using histograms and relevant two-sample t-tests were performed. To test the hypothesis that male spot brightness is determined by adult predation risk (AH1), we predicted that increased or decreased predation risk would lead to a respective decrease or increase in average male spot brightness over time (P1): male spot brightness was recorded before and 1000 days after the addition of a predator (Cichlid A) to Upper East Stream, and before and 1000 days after the removal of a predator (Cichlid A) from Lower
East Stream.

To test the hypothesis that male spot brightness is determined by sexual selection (AH2), we predicted that average male spot brightness would increase over time in a population free of adult predators (P2): male spot brightness was recorded 800 days after 15 females and 15 dull males (brightness score ≤ seven) were introduced to a predator-free tank. To test the hypothesis that male spot brightness is determined by geographical location (AH3), we predicted that average spot brightness of males transplanted to a new location would obtain an average spot brightness similar to original residents of the same location (P3): male spot brightness in the Upper West Stream and in the Lower East Stream was recorded prior to removal of all fish from Upper West Stream and the subsequent transplantation of all from the Lower East Stream to the Upper West Stream. Male spot brightness in the Upper West Stream was recorded 1000 days after the transplantation event.

We also used P1-P3 to test the hypotheses that male spot brightness is determined by a combination of predation risk, sexual selection and geographical location (AH4), or that male spot brightness is random with respect to predation risk, sexual selection and geographical location (AH5): if two or more of the above predictions are true then AH4 would be supported, but if none of the above predictions are true then AH5 would be supported. 

Discussion

Charles Darwin, in his theory of evolution, put forward a theory on mechanism justifying descent with modification. As per this theory, in case a certain population of organisms exhibits inheritance, variation and differential reproductive success, chances are full that the composition of this population changes significantly from one generation to another (Laland et al., 2014). Guppies (Poecilia reticulata) are aquarium fish that are dramatically patterned. Guppies are found in their natural habitat of mountain streams found in tropical forests in northeasternTrinidad, Margarita Island, Venezuela, and Tobago (Banet et al., 2016). Wild males might not be profligately ornamented; however, they too are impressively beautiful. Spots, stripes and splashes of a range of vibrant colours are worn by them, such as blue, black, orange and yellow (Kodric-Brown, 1985). The famous evolutionary biologist John Endler studied the bewildering colouful patterns of wild guppies of Trinidad in the 1970s, after mystified by the variation among these fishes. He noted that male guppies living in one pool had vivid orange and blue splotches on the sides while those living downstream had only small dots in their tails (Deacon & Magurran, 2016). Further, differences in the distribution of guppy predators were distinct. Endler carried out the research with the hypothesis that the predators present in the aquatic systems are accountable for the variation among the populations. This idea emerged from the fact that that guppy living in the stream with predatory cichlids had less number of spots on their bodies as compared to guppies living with Rivulus (Endler, 1980).

The present laboratory work aimed at challenging learners to design a study that allowed for the documentation of evolution by natural selection in nature. The purpose was to conduct a field exploration on wild guppies and evaluate the variation among males in a number of wild populations. The EvoBeaker simulations used in the work are based on the definitive study carried out by John Endler of natural selection in guppies. The rationale for conducting thiss research would be to evaluate the variation among guppy males in diverse populations.

The results that were obtained from the study showed that adding predators in the stream decreases the average brightness of the male spot significantly. Alternatley,  the average brightness of the male spot increased significantly when the predators were eliminated from the stream. The first hypothesis was thus accepted corresponding to the fact that the male spot brightness is established by risk of adult predation (AH1) . The prediction related to increase or decrease in risk of predation will cause respective reduction or elevation in average brightness of male spot with time (P1) was determined as a true fact. Similary, introducing both males and females in a predator free tank showed a significant rise in the average brightness of male spot in end. Hence, the second hypothesis that explains the determination of brightness of male spot by sexually selecting (AH2) was accepted. Pedicting that average spots brightness in male within a population will rise over time without predators that are adult (P2) was established to be true. In addition, the average brightness of spot in male guppies showed no significant difference both before and after the transplanting factor.

However, the average brightness of spot in males of Lower East Stream after transplanting to Upper West Stream and thereby removing the males from the Upper West Stream previously present there showed significant difference. The hypothesis of spot brightness determined by geographically locating (AH3) was therefore rejected. The prediction explaining that the average brightness of spot in males incorporated to a new region would provide an average brightness of spot same as in original residents of that similar location (P3) was falsified. The fourth hypothetical view was based on the determination of spot brightness in male by combined risk of predation, sexually selecting and locating geographically (AH4). Since two of the predictions out of three were true, AH4 was supported.

The results of the study support the theory of natural selection, more precisely the sexual selection. Sexual selection is a form of natural selection where certain organisms produce offsprings better than them so that they have better chances of getting selected for mating (West-Eberhard et al., 2014). Guppies have adopted the sexual selection theory as they struggle not for existence, but for finding the appropriate mating partner. The result of this struggle between male guppies for possessing females is no or few offspring. Evolutionary pressures push the male guppies to evolve bright for attracting females (Wilson et al., 2014). A brighter the male guppy would imply that chances of a female guppy to choose him as her prefered mate would incease, presenting hima chance to pass the genes to her future generations (Hooker et al., 2016). In water bodies where the risk of predators is high, the evolutionary pressure is to camouflage themselves so that they do not fall prey to the predators. The brightness of body spots is not distinct in such cases as the priority is to remain alive and not to reproduce (Tefertiller et al., 2016). Wright et al., (2017) had tested the model of sex chromosome evolution using genome and transcriptomeresequencing data in the guppy. The results of the study providedvaluable evidence for longstanding models of sex chromosome catalysis, thereby suggesting sexual selection together with sexual conflict in genome evolution.

It can be therefore concluded from the discussion that spots in guppies is a result of natural selection theory proposed by Darwin. More precisely, it supports the theory of sexual selection.

References:

Banet, A. I., Svendsen, J. C., Eng, K. J., & Reznick, D. N. (2016). Linking reproduction, locomotion, and habitat use in the Trinidadian guppy (Poecilia reticulata). Oecologia, 181(1), 87-96.

Deacon, A. E., & Magurran, A. E. (2016). 15 How Behaviour Contributes to the Success of an Invasive Poeciliid Fish: The Trinidadian Guppy (Poecilia reticulata) as a Model Species. Biological Invasions and Animal Behaviour, 266.

Endler, J. A. (1980). Natural selection on color patterns in Poeciliareticulata. Evolution, 34(1), 76-91.

Hooker, D., Kiesewetter, K., Heath, R., Marsh, L., & Bowser, B. (2016). How Color and Size Affect Mate Selection of Poeciliareticulata. Journal of Introductory Biology Investigations, 5(4).

Kodric-Brown, A. (1985). Female preference and sexual selection for male coloration in the guppy (Poeciliareticulata). Behavioral Ecology and Sociobiology, 17(3), 199-205.

Laland, K., Wray, G. A., & Hoekstra, H. E. (2014). Does evolutionary theory need a rethink?. Nature, 514(7521), 161.

Tefertiller, L., Taylor, M., Rose, M., Saunders, G., & Hanna, A. (2016). Studying Male Guppy Mate Selection Behavior. Journal of Introductory Biology Investigations, 5(4).

West-Eberhard, M. J. (2014). Darwin's forgotten idea: The social essence of sexual selection. Neuroscience & Biobehavioral Reviews, 46, 501-508.

Wilson, A. D., Krause, S., James, R., Croft, D. P., Ramnarine, I. W., Borner, K. K., ... & Krause, J. (2014). Dynamic social networks in guppies (Poecilia reticulata). Behavioral ecology and sociobiology, 68(6), 915-925.

Wright, A. E., Darolti, I., Bloch, N. I., Oostra, V., Sandkam, B., Buechel, S. D., ... &Mank, J. E. (2017). Convergent recombination suppression suggests role of sexual selection in guppy sex chromosome formation. Nature Communications, 8, 14251.