Biology begins with a curiosity about life that is sometimes at its core an attempt to determine who we are and where we came from on some fundamental level. Though the topic of Charles Darwin and evolution may still ruffle some feathers out in the world, to the biologist, it is a fundamental means of determining the nature of human beings. However, despite efforts like the Human Genome Project, it is still by definition difficult to study human genetics and reproduction for both ethical and practical or time-scale reasons. Discussions like the role of secondary sex characteristics in attraction and how humans respond to stressful, seemingly tenacious circumstances fascinate even the layperson, yet they are difficult to study more than indirectly in human populations. This is where animal breeding studies can fill in some of the gaps in understanding, but there is always a trade-off. The more quickly an animal reproduces, the more generations can be studied in a short length of time, but on the other hand, species that tend to reproduce quickly tend to be very dissimilar to humans. Though not quite the classic fruit fly of so many biology experiments, the guppy is still a relatively simple organism, but it is also very easy to study and interesting in its own right. Research done on guppies may be of only tangential use in answering life’s persistent questions, but it has a high degree of reliability, thanks to the ease of studying relatively large populations over multiple generations. Two of the most fascinating topics for such investigation are the intersections of guppy genetics with predation and with sexual selection.
Sexual selection in captive-bred guppies that are part of research experiments demonstrates many interesting features of guppy genetics and the interaction with guppy behavior. It might at first seem likely that there is a single trait to which female guppies respond in male guppies when seeking a mate, but the reality turns out to be far more complex than that. Indeed, there seem to be many different characteristics involved in sexual selection—the means by which guppies essentially breed themselves for certain traits. For example, Brooks and Endler found that, “There appear to be several male ornaments that females find uniformly attractive and others for which females differ in preference. One consequence is that there is no universally attractive male phenotype” (“Life-History Evolution” 1644). This implies that different traits may be valuable in different environments. Perhaps which trait is phenotypically expressed might depend upon the density of the underwater flora in the environment, so that males with overly decorative tails would not fare well in close quarters, or perhaps the degree of light and shadow in the area might dictate whether males can afford bold coloration without being spotted by predators. Interestingly, as shall be seen later, predation seems to have a very high effect upon guppy genetics.
The work did not end there. In addition, Brooks and Endler also measured female guppies’ level of responsiveness to male sex-linked traits as compared to their degree of discrimination between the various such traits, finding that, “Only responsiveness shows significant additive genetic variation. Variation in responsiveness appears to mask variation in discrimination and some preference functions and may be the most biologically relevant source of phenotypic and genetic variation in mate-choice behavior” (“Life-History Evolution” 1644). To put this plainly, genetically speaking, an individual female guppy’s level responsiveness to the whole collection of male traits seemed to outweigh any particular personal preference for one particular characteristic. Responsive female guppies would respond to male guppies with ornamentation regardless of what ornamentation, in particular, was on display. Perhaps this is why the male guppies have retained the ability to express and one of a number of traits, rather than selecting only one to display. If the variation makes no difference to the female guppies but allows the male guppies to better adapt to the environment over time, taking the whole local population into account, this is clearly an evolutionary advantage. However, one study alone is not enough to confirm this hypothesis; it is important to look at other work, as well.
When two studies find the same results, those results can be viewed with a greater degree of certainty than when one study alone determines the outcome. This is somewhat less true when the studies appear to be two components of a single body of research by the same team, but it is still a good way to validate results. Brooks and Endler’s 2001 research also extended to the topic of exploring the relationship between direct and indirect sexual selection in guppies in a more quantitative way, starting by describing the basic research area quite eloquently:
The ornamentation and displays on which sexual attractiveness and thus mating success are based may be complex and comprise several traits. Predicting the outcome of sexual selection on such complex phenotypes requires an understanding of both the direct operation of selection on each trait and the indirect consequences of selection operating directly on genetically correlated traits. (“Direct and Indirect Sexual Selection” 1002)
Though the direct and indirect aspects of sexual selection had been touched upon tangentially in the previous work of this pair, through the discussion on whether female mate selection was more influenced by the females’ responsiveness in general or by “pickiness” over a certain trait, the approach was not as mathematical as the one taken here.
The more quantitatively based studies have added value to the field of guppy genetics. For example, the second Brooks and Endler study continues, explaining that, “We demonstrate that there is substantial additive genetic variation in almost all measures of male ornamentation and that much of this variation may be Y linked. Attractiveness and mating success are positively correlated at the phenotypic and genetic level” (“Direct and Indirect Sexual Selection” 1002). This shows that it the issue being studied is not merely based on superficial traits but rather goes down to the genetic level. In addition, this fact implies support for the genetic basis behind all studies discussed here, for even those that focused more on the phenotypic characteristics involved in sex selection and reactions to predation are likely indirectly approaching the topic of genetics as well. It is also important to note the difference between laboratory conditions for breeding guppies versus the authentic environment guppies encounter in the wild by addressing studies that investigate this aspect of guppy genetics as well as the others.
Not all experiments on guppy genetics manipulate variables only by means of breeding. Sometimes, nature has done this work for researchers, and this is particularly the case for studies of high-predation versus low-predation environments. These same results could probably be obtained by means of pure breeding experiments as well, and the underlying genetics work remains the same once the field-caught guppies have been studied for a few generations in the lab. In such experiments, the findings seem to be relatively consistent. Naturally, guppies subjected to high-predation environments have a harder time ensuring the survival of their offspring. Given that fish generally do not guard their eggs or protect their young, it can only be expected that when trying to increase the chance of carrying on the genetic line, the approach taken would be one of producing greater numbers of less resource-intensive offspring, rather than, say, the mammalian strategy of doubling down on a few offspring. Indeed, that is exactly what is seen; Reznick’s 1982 work found that, “Field-collected guppies from the [high-predation] localities mature at a smaller size . . . reproduce more frequently, and produce more and smaller offspring than their counterparts from the [low-predation] localities” (1236). This suggests that the guppies are in a sense genetically programmed to respond to the stress of high predation by putting out as many offspring as possible, even if this comes at a cost to the likelihood of success for any one young guppy in particular. In addition, the high-predation guppies were observed to “devote a larger percentage of their body weight to each litter” (Reznick 1236). The guppies were therefore more willing to reproduce even at the cost of reducing their own chances for survival based on body-weight resources, demonstrating what might be called a “live fast and die young” mentality. However, what is less immediately obvious is the relationship between this strategy and the male ornamentation discussed earlier.
It is interesting to note that the same sexual selection traits that help guppies both locate mates and choose among them are costly both in terms of extra expended energy on growth and maintenance and in relation to visibility to predators. Though it is a basic tenet of biology that expensive decoration is most likely when predation is low, such as among island bird populations throughout Oceania, if the goal of high-predation fauna is also to reproduce quickly, it would seem that flamboyant secondary sex characteristics might help facilitate the location of mates. However, it may also be the case that being visible or large is too much of a burden to survival to the males in a high-predation environment. Indeed, what is seen is that the smaller phenotypic expression of the underlying genetics tends to manifest in these situations. In an interesting refinement of earlier work, Reznick and Bryga introduced guppies from an environment where predators preferred large fish into an environment where predators preferred small fish. Though they were expecting that all the introduced fish would subsequently be larger than their control counterparts, once left in the new site for a few generations and then brought back to be bred in the laboratory, what Reznick and Bryga found was quite different: “[M]ales from the introduction site matured at a later age and at a larger size than did males from the control site downstream . . . No differences between localities were observed for female age and size at maturity” (1370). To put this in other words, only the males showed differences in size; the females did not change in response to exposure to the different environmental predators. Given that changes in size based on predation appear to be sex-linked traits for guppies in this experiment, it seems likely there would be some interaction with sexual selection at play here. Though Reznick and Bryga do briefly touch upon this in their discussion of the results, the final conclusions on the topic still seem up for debate.
Contributions of more recent work have helped to confirm the role of the Y-chromosome in determining the traits of male guppies. Though this is always the simplest of explanations for sex-differentiated characteristics, there are also such things as non-Y-linked alleles that nevertheless result in certain phenotypic variations only among males of a species. As Postma determined when looking at differences in color, tail attributes, and body size in adult guppies, these characteristics, “are strongly genetically correlated, both on and off the Y chromosome . . . As predicted, variation in attractiveness was found to be associated with the Y-linked, rather than with the non-Y-linked component of genetic variation in male ornamentation” (2145). Given this information, it seems likely that the slightly puzzling results involving size and predation that Reznick and Bryga found in the study discussed previously are indeed tied to Y-chromosome alleles that are “switched on” by exposure to certain circumstances. Therefore, though predation and sexual selection are two topics related to genetics that are hard to study at the same time, inferences can be drawn about the connection between the two by looking at studies in both areas. Yet there are still relevant questions that remain, such as those regarding how much of a given observed phenomenon is the result of a genetic line of descent versus how much is the simple result of the environment.
Studies on convergent evolution make the “nature versus nurture” debate from the soft sciences look woefully small-minded; when biologists set out to study convergent evolution, it is not mere individuals being studied, but rather whole populations. The question is always one of determining what portion of the morphological similarities between two species or two populations of the same species are due to actual genetic factors based on common lines of descent versus how much simply arises from being subjected to similar environments. In Reznick and Bryga’s even later work, the 1996 follow-up to their 1987 work with a similar title, the researchers stated that, “We document a genetic basis for convergent life-history evolution in guppies from high- and low-predation sites on the north slope of the Northern Range Mountains of Trinidad” (339). This demonstrates that the pressures to which guppies are selected—in essence, breeding them for certain traits—can operate in a way that more closely resembles convergent evolution and parallelism than a strict pattern of similar-looking fish coming from the same populations.
Guppies are no small matter when it comes to genetic studies that investigate the relationship of evolution to phenotypic traits in both captive and wild populations. Through this work, the fascinating balance of sexual selection and the drive for more male ornamentation with the very real need to stay hidden from predators is revealed. All by examining the life of one little fish, concepts such as convergent evolution, parallelism, phenotypic versus genetic traits, Y-linked characteristics, and female choosiness in mating come to the forefront. The life of a guppy might even be said to be a microcosm for the lives of all organisms on planet Earth, for the same pushes and pulls influence every living thing here. In the end, all beings must find a way to stay alive long enough to reproduce, and for many, that means avoiding predation and finding a mate.
Brooks, Robert, and John A. Endler. “Direct and Indirect Sexual Selection and Quantitative Genetics of Male Traits in Guppies (Poecilia reticulata).” Evolution 55.5 (2001): 1002-1015.
Brooks, Robert, and John A. Endler. “Female Guppies Agree to Differ: Phenotypic and Genetic Variation in Mate‐Choice Behavior and the Consequences for Sexual Selection.” Evolution 55.8 (2001): 1644-1655.
Postma, Erik, et al. “Sex‐Dependent Selection Differentially Shapes Genetic Variation On And Off The Guppy Y Chromosome.” Evolution 65.8 (2011): 2145-2156.
Reznick, David. “The Impact of Predation on Life History Evolution in Trinidadian Guppies: Genetic Basis of Observed Life History Patterns.” Evolution (1982): 1236-1250.
Reznick, David N., and Heather Bryga. “Life-History Evolution in Guppies (Poecilia reticulata): 1. Phenotypic and Genetic Changes in an Introduction Experiment.” Evolution (1987): 1370-1385.
Reznick, David N., and Heather A. Bryga. “Life-History Evolution in Guppies (Poecilia reticulata: Poeciliidae). V. Genetic Basis of Parallelism in Life Histories.” American Naturalist (1996): 339-359.