Skin, the largest organ in the body, serves as the first line of defense against microorganisms, trauma, and ultraviolet radiation. Pigmentation is a complicated trait and a result of strong selective pressures influenced by various alleles at numerous loci. The varying pigmentation within humans and lizards alike regulate the effects of UV radiation.
It has long been understood that when an organism is left exposed to ultraviolet radiation (UVR), the organs and tissues can be damaged. Defense mechanisms against distinct UVR wavelengths have encouraged various organisms to evolve to survive and thrive.
Unlike humans who contain both innate and manufactured protective agents for UVR, the melanin concentration found in many animals, like lizards, is essential for protection from the sun’s harmful rays (Porter 1967). Many herpetologists believe that lizards containing a black peritoneum decrease the impact of sunlight damage on sperm and eggs. The peritoneum acts as a membrane lining of cavities within the body including the abdomen, abdominal organs, and pelvic cavities.
Species of many diurnal lizards, such as the desert iguana, contain black peritoneums – those that are densely pigmented (Hyman 1992). Contrary to their nocturnal counterparts who lack dark pigmentation, concentrated melanin acts as a preventative measure from penetration of UVR.
Numerous experiments allowed scientists to recognize that skin, muscles, and black peritoneum (in particular) were all imperative in thwarting the effects of ultraviolet radiation (Gates 2011). Michael Mares, author of Encyclopedia of Deserts, notes that most lizards found in desert areas are diurnal containing peritoneal linings while nocturnal lizards are absent of the dark lining.
One could assume that the differences in the peritoneum were due to evolutionary adaptions, particularly in the diurnal lizards that would be forced to experience the maximum force of ultraviolet radiation. The nocturnal lizards, although still experiencing UVR, are not exposed to the same magnitude of radiation as the diurnal lizards. Mares also highlights that this adaptation evolved to protect internal organs (as mentioned previously) but more specifically the ovaries and testes which are responsible for the reproduction of the species (Mares 2017). Without this protective pigmented membrane, the eggs and sperm of the species would be damaged thereby reducing the rate of increase.
Interestingly, lizards can lighten their skin which Mares believes to have coevolved with the adaption of the black peritoneum. The lightning process occurs during thermoregulation to allow lizards to radiate heat over their body during high temperatures. This depigmentation process, without the black peritoneum, would risk the integrity of the lizard’s internal organs. The evolutionary adaptation of the black peritoneum, even with the lizard’s compromised depigmented skin, protects the organism from solar radiation (Mares 2017)
Gates also records that many diurnal vertebrates, not just lizards, contain the pigmented membrane that protects the gonads (reproductive organs). He references the work of Warren Porter back in 1967 where he observed the melanin contained within the peritoneum of the Ulta stansburiana and how it varied throughout the lizard (Gates 2011). Porter became initially interested in the pigmented peritoneum once he discovered its defensive nature to the central nervous system and gonads from UVR. He found it puzzling that only diurnal vertebrates had black peritoneums while nocturnal vertebrates were left without such pigmentation. Porter goes on to reference work completed by Bruce Collette in 1960 on the species of Anolis. These findings demonstrated a positive correlation between the amount of melanin present in the peritoneum and sun exposure (Porter 1967). An experiment in the removal of the black peritoneum in the Uta stansburiana and Cuemidophorous Tigris lizards showed an increase in transmission of UVR indicating that the harmful rays were most certainly penetrating the body of the reptiles (Porter 1967). These findings further prove that without the black peritoneum the sperm and eggs would be at risk.
Hypothesis: the lizards lacking the black peritoneum will have lower reproduction rates than those with the black peritoneum.
To test if the black peritoneum seen in several lizard species reduces sunlight damage to sperm and eggs one would first have to re-create a dessert-like environment within a laboratory setting. Various species of lizards with and without black peritoneum would be chosen for this experiment. Each species would contain an equal number of male and female lizards to match for mating purposes. The mating schedules of the lizards would have to be determined to ensure prime mating conditions. The lizards would be placed in an enclosure with portable UV lights or Fluorescent lamps that emit UV radiation to mimic the conditions found in a desert. After mating and egg production I would compare the number of successful hatchings from lizards with and without black peritoneums. The results from this experiment would presumably show that the lizards lacking the pigmented peritoneum experienced UVR damage to sperm and eggs thereby lowering the number of hatchings.
It is widely understood that environmental changes over time influence genotypic and phenotypic frequencies within an organism. An organism must adapt to the new environment or be selected out – killed off. Within some of the world’s oldest deserts, the Sahara and the Namib, one could predict that lizards would be more developed – physiologically and behaviorally due to the intense environment (Brown 1974). As mentioned previously the dark pigmented peritoneum, along with skin and muscles prevents the absorption of harmful ultraviolet rays produced by the sun. The intensity of the sun in the desert over time would force the desert lizard to develop this specialized lining to protect the body from damaging the tissues and organs. Species of lizards that are nocturnal or not exposed to extreme concentrations of UVR would not require the black peritoneum (Hyman 1992). Nocturnal lizards in particular would not be exposed to the same deathly solar rays as their diurnal associates and therefore could survive without the pigmented peritoneum (Mares 2017).
Most organisms experience natural selection when genetic factors influence the survival and fecundity of the individual. Organisms that are faced with similar physical environmental stresses will evolve similar adaptions.
The work performed by Marshall in 2015 describes the convergence of 5 subspecies of the Aegean wall lizard on their islands. Although the lizards varied in coloration, each subspecies’ coloration was selected to optimally match the local environment for that island (Marshall et.al 2015). Marshall desired to compare each lizard subspecies’ ability to camouflage on their local islands versus a neighboring island. Through experimentation, it was determined that selection drove the subspecies to be camouflaged against their local rocks (Marshall et.al 2015).
Well before the work completed by Marshall on the coloration of Aegean wall lizard, Kettlewell described the color variation of the Biston betularia. He analyzed the selective elimination of predation by studying industrial melanism as a result of evolutionary change. After collecting moths from a museum in Birmingham, England Kettlewell noticed that the moths varied in color (Kettlewell, 1956). He inferred that “something” was forcing the moths to morph over time. The driver of this change was the increased pollution during the early industrial age which in turn fueled the rise of bird predation. The black smog emitted from the factories covered the trees where the moths would gather – those that could blend into the trees were not detectable by the birds (Kettlewell, 1956). Kettlewell tested this theory by releasing moths in both the polluted city of Birmingham and another English city (not polluted). The only moths that he was able to be re-collected were those that had evolved the black coloration.
Holling performed a similar experiment where he pinned moths to observe which species of B. betularia moth would be eaten by predators (Kettlewell, 1956). He found that the Carbonaria (all black moths) could blend into the background of the tree – indicating that the black moths survived while the Insularia (white moths) were preyed upon (Kettlewell, 1956). Kettlewell recognized that the species of moth evolved in 2 regions responding to the environmental change as well as predation by birds. Kettlewell showed that moths with coloration not concealed by their environment had increased predation by studying camouflage efficiency and the moth’s survival (Kettlewell, 1956).
Marshall details the reasons for which the Aegean wall lizards have adapted to the local camouflage provided by the environment. Each subspecies of lizard is preyed upon by birds – especially when they are sunbathing on rocks during the day (Marshall et.al 2015). They have evolved to develop coloration on the dorsal portion of the body to mimic the rocks to go undetected by their avian predators. The volume of avian predators also influenced the degree of camouflage each subspecies required. For example, the P.e. Maxentius has 2 avian predator species as compared to the P.e. myogenesis which has 5 avian predator species – these differences in the number of species would unequivocally vary the degree of camouflage of the subspecies (Marshall et.al 2015).
It is widely believed that predation serves as a major force of color variation amongst organisms. The experiments performed above by Kettlewell and Marshall both indicate that predation caused the evolutionary color change within the moths and lizards respectively. The results of Marshall’s experiment mentioned above describe how adaptation to the avian predators within each subspecies drives divergence and influences ecological speciation due to isolation (Marshall et al. 2015).
Natural selection has allowed animals to varying in color not only to camouflage themselves from predation but also for thermoregulation and protection from ultraviolet light (Stevens 2016). Many diurnal lizards living in the desert depigment through a lightning process to allow heat to radiate over their body during high temperatures (Mares 2017). The term phenotypic plasticity has been used to describe an animal’s ability to change color to resemble their local environment or a specific background- like rocks in the case of the Aegean wall lizard (Stevens 2016). After understanding that several drivers influence color variation in an organism, we can now determine how these affect isolated populations on various Islands.
The Aegean wall lizard described by Marshall has 5 different subspecies each being found isolated on 5 different islands. To examine the drivers influencing coloration in these isolated subspecies, I would follow an approach detailed in Takahashi’s study of Ischura senegalensis in 2017. Here he compared population divergence of the neutral loci retrieved from next-generation sequencing. The evolutionary forces manipulating allele frequencies are divergent selection, balancing selection, and gene flow (Takahashi). Statistical analysis should be used to determine the drivers affecting each subspecies of lizard where the Fst values of each would determine if divergent selection or balancing selection were prevalent in the subspecies (Takahashi 2017). Takahashi found that together balancing selection and divergent selection were both acting on the color locus of the I. senegalensis.
Marshall notes that the variability of each island is responsible for both phenotypic and genotypic variation of the lizards. Although this had not been tested, in another similar experiment Mclean desired to determine the driver of phenotypic variation within the Australian tawny dragon lizard. She found that allele frequencies varied geographically, which could explain the variation in colors of the subspecies of Aegean wall lizard (Mclean et. al 2015). Various measurements of topographic, rock, and vegetation cover were performed in the 2015 experiment where the vegetation cover was found to be the most important predictor of morph frequencies (Mclean et. al 2015). Following this procedure, measurements of rock and vegetation present on each island of the 5 subspecies of Aegean wall lizard would help to predict the most important factor affecting the coloration of the lizard. Like Mclean, I would expect to find a similar association between divergence in allele frequencies and divergence in environmental variables (the rocks and vegetation)