Evolution in Action White Animals at White Sands

National Park Service White Sands Department of the Interior White Sands National Monument Evolution in Action: White Animals at White Sands here has been dramatic convergent evolution of light coloration for many of the animals which inhabit the stark Twhite gypsum dunes of White Sands National Monument. A number of different animal groups (including insects, spiders, toads, lizards, mammals) have blanched coloration on the white dunes and dark coloration in the surrounding dark desert soils (see Table 1). The gypsum sands are geologically recent, formed since the end of the last glaciation. Because of the young age of White Sands, we know that animals with lighter coloration have adapted to their environment quickly, as a result of strong natural selection. White Sands is therefore a dramatic showcase for evolution in action. The rapid, convergent evolution of blanched coloration in many species experiencing the same environment allows us to ask several important questions about the role of natural selection in generating biological diversity. Why did light coloration evolve? Natural selection for substrate matching Animals become better adapted to environment and had more offspring Particularly for small diurnal animals, their environment by a process called than the darker lizards. Because the we would expect strong selection for natural selection. Traits that increase light-colored individuals were more substrate matching to avoid being an individual’s reproductive success likely to survive and reproduce, the seen by visual predators. Although by improving its fit to the environment frequency of light-colored lizards no studies have directly measured tend to spread in a population. Light would increase in the next generation. predation intensity on White Sands coloration could evolve through natural After many generations of selection animals, many researchers have shown selection in dune animals if individuals for lighter individuals, the population that well-camouflaged animals are with light-coloration survived better or could eventually become white. less likely to be taken by predators reproduced more than individuals with But why would lighter individuals (Kiltie, 1992). Avian predators, such dark coloration. Imagine a population survive and reproduce better on as roadrunners and shrikes likely of dark colored lizards moving from the the gypsum dunes? There are two represent a significant threat for small brown desert soil into the white dunes. primary hypotheses for why natural animals at White Sands. Camouflage, Assume there is natural variation in selection would favor light individuals: or crypsis, is therefore essential for color in lizard populations, some camouflage and thermoregulation. survival. Even nocturnal animals can individuals being slightly darker and Natural selection for camouflage is the benefit from matching their habitat, some slightly lighter. Also assume that most likely explanation for the light because many nocturnal predators use coloration is heritable, with offspring coloration observed at White Sands vision to find prey illuminated inheriting their color pattern from but the effects of light coloration on Lowe, 1964). Also, predators can their parents. Suppose that the lighter thermoregulation require further benefit from matching their substrate individuals survived better in this new study. as to avoid detection by prey. Natural selection for thermoregulation Another possibility is that lighter coloration has evolved for thermoregulation. Lighter-colored individuals that are active during the day may be better able to regulate body temperature because they reflect more of the intense desert sunlight (Benson, 1933). However, many of the white animals are nocturnal, so regulating absorption of the sun’s rays would not be an important selective factor. Additionally, there is no evidence that temperatures at White Sands are any more intense than in the surrounding dark soil habitat. In Color variation of earless lizard and bleached earless lizard in their environments fact, air and substrate temperatures at White Sands are actually lower for flexible coloration must have been an animal white. The only species than on the dark soils (Hager 2000, more likely to survive, reproduce, for which this appears likely is the Rosenblum unpublished). Hager even and pass their flexible-color genes on white lycosid spider (Table 2), which suggests that lighter colored lizards to their offspring. Bugbee (1942) described as “brown are at a disadvantage because they Third, it is possible that light in basic color but its abdomen usually cannot warm as quickly as their dark coloration did not evolve through appeared as if covered with hoar- soil counterparts. Although substrate natural selection but is simply picked frost.” Because “this white color was temperatures are not unusually high at up from the environment. Color can easily rubbed off when individual White Sands, it is still possible that large be picked up from the environment specimens were handled,” it is heat loads are imposed on animals either from dietary pigments (i.e.: possible that the white color is derived because of the reflective properties of pigments acquired from the food from external substances. However, gypsum. Further study is necessary, cause an animal to become white) it is also possible that the substance but current evidence is most consistent or from direct accumulation (i.e.: that is easily rubbed off is deposited with light coloration as an adaptation gypsum sand on the skin makes an by the spider and is internally for camouflage not thermoregulation. animal appear white). The third derived. Since light coloration possibility, that light color is acquired is unlikely to be derived directly What mechanism produces directly from the environment, from the environment, we must white coloration? is unlikely for most White Sand differentiate between the remaining species. First of all, light coloration two hypotheses: that individual color There are three possible mechanisms is probably not caused by diet. There is fixed or that individuals are able to explain how light coloration is are pigments which are known to to change their color to match their produced in White Sands animals. be acquired from the diet in birds current substrate. These predictions First, light coloration may be fixed for and reptiles. However these dietary have been most thoroughly tested in each individual from birth. For fixed pigments (called carotenoids) result lizards. coloration to evolve, lighter-colored in yellow and red, not blanched, animals must have been more likely coloration. to survive, reproduce, and pass their Second, Benson (1933) argued light-color genes on to their offspring. that it is unlikely for herbivorous Second, individuals may be able to animals to turn white from ingesting change color during their lifetimes certain plants. Plants at White to match their current substrate. Sands are similar to plants present Changing color in response to the in other areas of the Tularosa Basin environment is a type of phenotypic and probably take up substances plasticity and is known to occur in similar to those growing in alkaline other animals such as chameleons soils elsewhere. It is still possible and anoles. For such a mechanism to that direct accumulation of gypsum White Sands collared lizard evolve, individuals with genes coding dust or sand on the skin could turn Coloration in lizards or flexible in the bleached earless in response to temperature. She lizard (H. maculata). He collected collected white and non-white Many species of lizards are able to white lizards from the dunes and individuals from all three lizard quickly change their color in response non-white individuals from areas species which inhabit the gypsum to external and internal conditions, near the dunes. He photographed dunes (H. maculata, S. undulatus including temperature, exposure all animals to record their color and and A. inornata). She exposed the to light, and individual health and then placed them into laboratory lizards to hot and cold temperatures excitatory state (Smith, 1946; Norris cages with either white quartz sand and used a spectrophotometer to 1965). Some lizards, like chameleons or finely ground black cinders. None quantify coloration. All three lizard and anoles, can even change color of the lizards in Bundy’s experiment species did become slightly darker to more closely match their current changed colors to match the ground with colder temperatures. However, substrate. These rapid color changes color in their cages. even when cold, white lizards never are termed physiological plasticity become as dark as dark lizards. and occur as reptiles shift the Therefore rapid color change in distribution of pigments between response to the environment cannot different cell layers. Melanin, the explain observed light coloration pigment responsible for overall (Table 2). body darkness, is located in Rosenblum then tested whether melanophore cells. Melanophore color is fixed throughout a lizard’s cells are distributed several layers lifetime. She collected gravid below the surface of the skin but (pregnant) females from H. also have dendritic processes which maculata and S. undulatus from reach just below the inner surface the gypsum dunes and from nearby of the skin. Reptiles can darken dark soil habitats. She raised the or lighten in color by dispersing or eggs and newborn hatchlings aggregating melanin granules along Bleached earless lizard from

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