Fredric J. Janzen

Visualizing and quantifying natural selection

Modern methods of analysis are enabling researchers to study natural selection at a new level of detail. Multivariate statistical techniques can Identify specific targets of selection and provide parameter estimates that fit into equations for evolutionary change. A more Intuitive understanding of the form of selection can be provided through graphical representation of selection surfaces. Combinations of quantitative and visual analyses are providing researchers with new insights into the details of natural selection in the wild.

Environmental Sex Determination in Reptiles: Ecology, Evolution, and Experimental Design

Sex-determining mechanisms in reptiles can be divided into two convenient classifications: genotypic (GSD) and environmental (ESD). While a number of types of GSD have been identified in a wide variety of reptilian taxa, the expression of ESD in the form of temperature-dependent sex determination (TSD) in three of the five major reptilian lineages has drawn considerable attention to this area of research. Increasing interest in sex-determining mechanisms in reptiles has resulted in many data, but much of this information is scattered throughout the literature and consequently difficult to interpret. It is known, however, that distinct sex chromosomes are absent in the tuatara and crocodilians, rare in amphisbaenians (worm lizards) and turtles, and common in lizards and snakes (but less than 20% of all species of living reptiles have been karyotyped). With less than 2 percent of all reptilian species examined, TSD apparently is absent in the tuatara, amphisbaenians and snakes; rare in lizards, frequent in turtles, and ubiquitous in crocodilians. Despite considerable inter-and intranspecific variation in the threshold temperature (temperature producing a 1:1 sex ratio) of gonadal sex determination, this variation cannot confidently be assigned a genetic basis owing to uncontrolled environmental factors or to differences in experimental protocol among studies. Laboratory studies have identified the critical period of development during which gonadal sex determination occurs for at least a dozen species. There are striking similarities in this period among the major taxa with TSD. Examination of TSD in the field indicates that sex ratios of hatchlings are affectd by location of the nests, because some nests produce both sexes whereas the majority produce only one sex. Still, more information is needed on how TSD operates under natural conditions in order to fully understand its ecological and conservation implications. TSD may be the ancestral sex-determining condition in reptiles, but this result remains tentative. Physiological investigations of TSD have clarified the roles of steroid hormones, various enzymes, and H-Y antigen in sexual differentiation, whereas molecular studies have identified several plausible candidates for sex-determining genes in species with TSD. This areas of research promises to elucidate the mechanism of TSD in reptiles and will have obvious implications for understanding the basis of sex determination in other vertebrates. Experimental and comparative investigations of the potential adaptive significance of TSD appear equally promising, although much work remains to be performed. The distribution of TSD within and among the major reptilian lineages may be related to the life span of individuals of a species and to the biogeography of these species. Answers to many of the questions and tests concerning TSD in reptiles would be facilitated by controlling the conditions of incubation, by standardizing the experimental design, and by depositing voucher specimens in accessible collections after completion of the study. Goals for future research are discussed.... pure observation of the normal developmental processes proves absolutely unsuited to elucidate the question how the alternative between the two possible sexes is decided, i.e., how the sex is determined. Only experiments can possibly reveal what part is played by hereditary factors and which are the physiological realizators to stimulate the morphological differentiation (Witschi, 1929). In the absence of experimental data, it would be mere speculation to attempt a statements as to nature of the sex-determining factors and the sex-differentiating mechanisms at work in the turtle embryo (Risley, 1933).

LOGISTIC REGRESSION FOR EMPIRICAL STUDIES OF MULTIVARIATE SELECTION

Understanding the mechanics of adaptive evolution requires not only knowing the quantitative genetic bases of the traits of interest but also obtaining accurate measures of the strengths and modes of selection acting on these traits. Most recent empirical studies of multivariate selection have employed multiple linear regression to obtain estimates of the strength of selection. We reconsider the motivation for this approach, paying special attention to the effects of nonnormal traits and fitness measures. We apply an alternative statistical method, logistic regression, to estimate the strength of selection on multiple phenotypic traits. First, we argue that the logistic regression model is more suitable than linear regression for analyzing data from selection studies with dichotomous fitness outcomes. Subsequently, we show that estimates of selection obtained from the logistic regression analyses can be transformed easily to values that directly plug into equations describing adaptive microevolutionary change. Finally, we apply this methodology to two published datasets to demonstrate its utility. Because most statistical packages now provide options to conduct logistic regression analyses, we suggest that this approach should be widely adopted as an analytical tool for empirical studies of multivariate selection.

IMPACT OF NEST-SITE SELECTION ON NEST SUCCESS AND NEST TEMPERATURE IN NATURAL AND DISTURBED HABITATS

Nest-site selection behavior is a maternal effect that contributes to offspring survival and variation in offspring phenotypes that are subject to natural selection. We investigated nest-site selection and its consequences in the snapping turtle, Chelydra ser- pentina, in northwestern Illinois. We evaluated nest-site selection at both the microhabitat and habitat patch levels. Turtles selected nest sites with shorter vegetation, more open sand, and fewer cacti than random locations. These microhabitat characteristics described sandy patches where both nest density and success were higher compared to grassy patches in 1999. We subsequently investigated nest-site selection within two discrete subdivisions of the study area that varied in the degree of human disturbance to determine if nesting behavior, nest success, or nest temperatures were affected. The tendency to nest in sandy patches was much stronger at the natural site due to habitat modifications at the residential site that have blurred the distinction between sandy and grassy patches. Additionally, the residential site had a high density of nests within 5 m of houses and a fence (both areas with disturbed habitat similar to sandy patches), compared to the overall density. Thus, nest success associated with sandy patches may be compromised at the residential site; an ecological trap may result in lower nest success in areas with preferred microhabitat char- acteristics. Despite a similar basis for nest-site selection in terms of microhabitat charac- teristics at both sites, nest temperatures were correlated with microhabitat characteristics used to select nest sites only at the natural site. Nest temperatures at the residential site were instead correlated only with the percentage overstory vegetation cover and therefore averaged 2 8C lower than at the natural site, a temperature difference that influenced offspring sex. The higher percentage overstory vegetation cover at the residential site was due to human alterations of the habitat, and may serve to extend the ecological trap biasing the sex ratio of this population. This study illustrates the importance of (1) nest-site selection as a substantive maternal effect, (2) understanding habitat use during crucial life-history events, and (3) the potential for human disturbance to modify offspring phenotypes and negatively impact nest success despite adaptive nesting behavior.

VEGETATIONAL COVER PREDICTS THE SEX RATIO OF HATCHLING TURTLES IN NATURAL NESTS

I monitored natural nests of an Illinois population of the western painted turtle (Chrysemys picta beMlN), a species with temperature-dependent sex determination, for 4 yr to investigate the relationships between vegetational cover at the nest site and hatchling sex ratio. Most nests produced hatchlings all of one sex (66% of 116 nests). Hatchling sex ratio within nests was predictably related to the nesting beach and to the quantity of solar exposure at the nest mainly from the south. The North beach tended to produce more all- male nests and an overall male-biased sex ratio than did the South beach. Within each beach, nests with more vegetational cover on the southern and perhaps western aspects produced more males than did more exposed nests. However, the relationship between solar exposure and nest sex ratio was absent during two atypically cool summers. This study suggests a possible mechanism by which female turtles choose the thermal environment of nests and, hence, the sex ratio of their offspring: they may assess vegetational cover on the nest site at oviposition.

Exploring the evolution of environmental sex determination, especially in reptiles

Environmental sex determination has been documented in a variety of organisms for many decades and the adaptive significance of this unusual sex‐determining mechanism has been clarified empirically in most cases. In contrast, temperature‐dependent sex determination (TSD) in amniote vertebrates, first noted 40 years ago in a lizard, has defied a general satisfactory evolutionary explanation despite considerable research effort. After briefly reviewing relevant theory and prior empirical work, we draw attention to recent comparative analyses that illuminate the evolutionary history of TSD in amniote vertebrates and point to clear avenues for future research on this challenging topic. To that end, we then highlight the latest empirical findings in lizards and turtles, as well as promising experimental results from a model organism, that portend an exciting future of progress in finally elucidating the evolutionary cause(s) and significance of TSD.

Genetic Effects of a Persistent Bottleneck on a Natural Population of Ornate Box Turtles (Terrapene ornata)

Human activities in the past few hundred years have caused enormous impacts on many ecosystems, greatly accelerating the rate of population decline and extinction. In addition to habitat alteration and destruction, the loss of genetic diversity due to reduced population size has become a major conservation issue for many imperiled species. However, the genetic effects of persistent population bottlenecks can be very different for long-lived and short-lived species when considering the time scale of centuries. To investigate the genetic effects of persistent population bottlenecks on long-lived species, we use microsatellite markers to assess the level of genetic diversity of a small ornate box turtle population that has experienced a persistent bottleneck in the past century, and compare it to a large relatively undisturbed population. The genetic signature of a recent bottleneck is detected by examining the deviation from mutation-drift equilibrium in the small population, but the bottleneck had little effect on its level of genetic diversity. Computer simulations combined with information on population structure suggest that an effective population size of 300, which results in a census population size of 700, would be required for the small population to maintain 90% of the average number of alleles per locus in the next 200 years. The life history of long-lived species could mask the accelerated rate of genetic drift, making population recovery a relatively slow process. Statistical analysis of genetic data and empirical-based computer simulations can be important tools to facilitate conservation planning.

Pattern does not equal process: Exactly when is sex environmentally determined?

Of prime importance in evolutionary biology are the description of pattern and explanations of process. Frequently, however, multiple processes can explain a given pattern. Such cases require experimental protocols or research criteria to distinguish among alternatives so pattern can be critically assigned to process. Noteworthy examples of this approach include evaluating adaptations and identifying character displacement (Gould and Lewontin 1979; Schluter and McPhail 1992). The field of vertebrate sex determination similarly requires such criteria. The sex of organisms is determined by two distinct mechanisms. In genotypic sex determination (GSD), sex is determined at conception by genes usually contained in sex chromosomes. In environmental sex determination (ESD), sex is determined permanently after fertilization by environmental factors (Bull 1983). In ESD (unlike in GSD), there is little if any genetic difference between the sexes (Solari 1994), so sex cannot be predicted by zygotic genotype (Bull 1983). In many ESD vertebrates, sex is determined after fertilization by incubation temperature (TSD). Though ESD’s biological significance seems clear for various taxa (Bull 1983; Conover 1984; Michaud et al. 1999), TSD evolution in vertebrates remains unexplained (Shine 1999). To complicate matters, temperature can influence sex ratios in a multitude of ways other than TSD.

Molecular systematics, phylogeography, and the effects of pleistocene glaciation in the painted turtle (Chrysemys picta) complex

Abstract.— The painted turtle, Chrysemys picta, is currently recognized as a continentally distributed polytypic species, ranging across North America from southern Canada to extreme northern Mexico. We analyzed variation in the rapidly evolving mitochondrial control region (CR) in 241 turtles from 117 localities across this range to examine whether the painted turtle represents a continentally distributed species based on molecular analysis. We found strong support for the novel hypothesis that C. p. dorsalis is the sister group to all remaining Chrysemys, with the remaining Chrysemys falling into a single, extremely wide‐ranging and genetically undifferentiated species. Given our goal of an evolu‐tionarily accurate taxonomy, we propose that two evolutionary lineages be recognized as species within Chrysemys: C. dorsalis (Agassiz 1857) in the southern Mississippi drainage region, and C. picta (Schneider 1783) from the rest of the range of the genus. Neither molecular nor recent morphological analyses argue for the hybrid origin of C. p. marginata as previously proposed. Within C. picta, we find evidence of at least two independent range expansions into previously glaciated regions of North America, one into New England and the other into the upper Midwest. We further find evidence of a massive extinction/recolonization event across the Great Plains/Rocky Mountain region encompassing over half the continental United States. The timing and extent of this colonization is consistent with a recently proposed regional aridification as the Laurentide ice sheets receded approximately 14,000 years ago, and we tentatively propose this paleoclimatological event as a major factor shaping genetic variation in Chrysemys.

Experimental analysis of an early life-history stage: avian predation selects for larger body size of hatchling turtles

One common life‐history pattern involves an elevated rate and nonrandom distribution of neonatal mortality. However, the mechanisms causing this pattern and the specific traits that confer a survival benefit are not always evident. We conducted a manipulative field experiment using red‐eared slider turtles to test the hypothesis that diurnal avian predators are a primary cause of size‐specific neonatal mortality. Body size was a significant predictor of recapturing hatchlings alive and of finding hatchlings dead under natural conditions, but was unimportant when diurnal predators were excluded from the field site. Overall recapture rates also more than doubled when predators were excluded compared to natural conditions (72.4 vs. 34.9%). We conclude that birds are an important cause of size‐specific mortality of recently emerged hatchling turtles and that ‘bigger is better’ in this system, which has important implications for life‐history evolution in organisms that experience size‐specific neonatal mortality.