LOCAL ADAPTATION OR ENVIRONMENTAL INDUCTION? CAUSES OF POPULATION DIFFERENTIATION IN ALPINE AMPHIBIANS

Transcription

1 biota 2/ /2/01 17:28 Page 31 MIAUD, MERILÄ Biota 2/1, LOCAL ADAPTATION OR ENVIRONMENTAL INDUCTION? CAUSES OF POPULATION DIFFERENTIATION IN ALPINE AMPHIBIANS Claude MIAUD 1 & Juha MERILÄ 2 1 University of Savoie, UMR CNRS 5553 Laboratory of Biology of Alpine Populations, F Le Bourget du Lac, France claude.miaud@univ-savoie.fr 2 Department of Population Biology, Evolutionary Biology Centre, University of Uppsala, Norbyvägen 18d, SE Uppsala, Sweden juha.merila@ebc.uu.se Abstract Many amphibian species have wide altitudinal and latitudinal distribution ranges, and hence, different populations of the same species may face very different environmental constraints and selection pressures. Although the potential differences in selective regimes along altitudinal and latitudinal gradients are still largely unknown, a number of studies have documented conspicuous phenotypic differentiation along these gradients. We reviewed the evidence for among population genetic differentiation (past adaptation) and within population genetic variation (potential for future adaptation) in different life history traits in amphibians, focusing mainly in studies conducted with the Common Frog, Rana temporaria, and with the Alpine Newt, Triturus alpestris. Except for larval traits, the evidence for genetic basis of population differentiation, as well as for heritability of different traits, is limited. This is especially true for traits expressed only in adults, such as age of maturity, fecundity and longevity, and traits relating to tolerance to different types of environmental stresses (e.g. nitrate, UV-B, pesticide tolerances). We suggest that due to the relative ease in which the genetic basis of most traits in amphibians can be studied, they might provide good model systems to study possible consequences of, and adaptation to, environmental changes expected to occur in forthcoming decades. Key words: altitude, amphibians, environmental gradient, heritability, latitude, local adaptation, phenotypic variation, Rana temporaria, Triturus alpestris Received 15 November 2000; accepted 1 March 2001

2 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ INTRODUCTION A central thesis in evolutionary biology is that natural selection acting on heritable phenotypic variation will result in adaptation and differentiation among local populations inhabiting environments differing in their selective regimes (Linhart & Grant 1996). Hence, to understand the processes of evolution and local adaptation, we need to understand the factors and processes generating and maintaining genetic variation, how this variation is translated to the phenotypic level, and how phenotypic expression of genetic variation in a given trait will interact with other traits and the environment to determine fitness (Stearns 1989). These fundamental questions also are important in the realm of applied ecology: anthropogenic influences such as elevated CO 2 -levels (Dixon et al. 1999, Fitter et al. 2000), increased ultraviolet-b (UV-B) radiation reaching the earth s surface due to ozone depletion (Blaustein et al. 1999, Madronich et al. 1998), and shifts in global climate (e.g. Hulme et al. 1999) are predicted to lead to rapid environmental changes in the forthcoming decades (Chambers 1998, Marchettini et al. 1999). Therefore, knowledge about the genetic basis of traits likely to be under selection in changing environments will enable us to predict population and ecosystem responses to these environmental changes (Kareiva et al. 1993, Bürger & Lynch 1997, Mousseau et al. 2000). For a given trait, the mean value of the trait in a population (the mean phenotype) is determined by genetic factors, environmental influences, and interactions between the two (genotype-environment interactions; Falconer & Mackay 1996). Thus, when discussing potential for adaptive differentiation among populations, the first step is to establish to what degree the observed differences among populations are of genetic versus environmental origin (e.g. Berven & Gill 1983, Conover & Schultz 1995). One way to separate between genetic differentiation and direct environmental induction as a cause of population differences in life-history traits is to perform common garden experiments, in which individuals from contrasting environments are reared in a common laboratory environment (Conover & Schultz 1995; Figure 1). Another possibility is to conduct reciprocal transplant experiments in which individuals from different populations are swapped to the environment of other populations and their performance is compared with those individuals remaining in the population of origin (Figure 1). The expectation from such experiments is that if genetic adaptation has taken place, the differences observed in nature will persist also in the common environment (or in the locality of transplantation), whereas if environmental induction is the principal cause of population differences in the wild, these differences will vanish when the organisms are reared in a common environment (Figure 1; Conover & Schultz 1995). Although the answer to the question whether variation among populations and species is mainly environmental or genetic is crucial (Berven & Gill

3 biota 2/ /2/01 17:28 Page 33 MIAUD, MERILÄ Biota 2/1, Figure legends Figure 1: Schematic presentation of the logic of (a) common garden and (b) reciprocal transplant experiments. The size of the circles represents the mean phenotype in the two populations inhabiting environments 1 and 2, respectively, and the circles at the ends of the arrows indicate the mean phenotype in the new environment. For both (a) and (b), two possible outcomes are pictured: one in which the initially observed difference is genetically determined, another where the difference is entirely of environmental origin. For more complex outcomes, see Conover & Schultz (1995). 1983), it does not necessarily give us cues as to whether the populations would be able to respond evolutionarily to further changes in their environment. In other words, although there must have been genetic variation within at least one of the populations for two populations to have diverged genetically, any future adaptation and evolution in a given trait in these populations depends on - apart from selection acting on the trait in question - whether there is (still) heritable genetic variation in the population (Falconer & Mackay 1996), and how the expression of this variation is influenced by environmental conditions (Hoffman & Merilä 1999). To this end, to predict potential for future adaptation we need some knowledge about the proportion of the phenotypic variation in the population that is attributable to the additive effects of genes, i.e. about the trait s heritability (h2; Falconer & Mackay 1996). The aim of this review is two-fold. First, we review what is known today about the (i) levels of heritable variation in different life history traits within amphibian populations, and (ii) about the relative importance of genetic versus environmental determinants of life history trait differentiation among amphibian populations in the Alps. In our treatment, we will focus primarily on life-history trait variation in two model organisms: the Common Frog Rana temporaria and the Alpine Newt Triturus alpestris. When no data are available from these two species, information from the Wood Frog Rana sylvatica - a vicariate species of Common Frog in North America - will be included. Second, being ectothermic and relatively poor disperses (e.g. Ward et al. 1992), amphibians are potentially very sensitive to changes in levels of abiotic stresses (e.g. temperature, length of growth season) predicted to occur over large areas in forth coming decades (e.g. Beebee 1995, Meyer et al. 1998, Hulme et al. 1999,

4 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ Houlahan et al. 2000). To this end, we will reflect upon the question how the altitudinal and anthropogenic gradients in the Alps could provide a suitable model system to study biological responses to predicted environmental changes, and how the data on alpine amphibians could be utilised in this context. The determinants of life history trait variation in the Common Frog and the Alpine Newt As the starting point for our treatment, we will first briefly summarise what is known about phenotypic variation in adult life history traits within and among different populations of the Common Frog and Alpine Newt. The Common Frog The Common Frog is the most widely distributed, and one of the most abundant anuran species of Europe (Grossenbacher 1994, Gasc et al. 1997). It occurs from northern Spain to the Barents Sea in the north, being the only amphibian species with a distribution reaching the North Cape (Norway). To the south, it is increasingly restricted to mountainous regions where populations can be found at altitudes above m (Gasc et al. 1997). Adult characteristics (viz. age structure, mean body size, growth patterns) have been described from several populations (reviewed in Miaud et al. 1999). Table 1 summarises the general comparisons between sexes and populations differing in their activity seasons (the activity period decreases with increasing altitude and latitude). Both sexes show similar trends in age, body size and growth with respect to decreasing activity period: age at maturity is delayed; life-span is prolonged; and adult body is increased (Table 1). The growth coefficient increases when asymptotic size decreases, i.e. in populations in which the asymptotic length is large (short activity period), the growth coefficient tends to be low (Miaud et al. 1999, 2000). The Alpine Newt The range of the Alpine Newt is centred on the middle of Europe (Grossenbacher 1994, Gasc et al., 1997). The northern range limit is in Denmark, and runs eastward to Romania and Bulgaria via the Carpathian and Balkan Mountains (Gasc et al. 1997). As with the Common Frog, the Alpine Newt occupies a very broad altitudinal range from sea level in the Netherlands to 2500 m in the Alps. Data on age, body length and growth are now available from several lowland populations, and from the French and Austrian Alps (reviewed in Miaud et al. 2000). A general comparison of main life history features between sexes and populations differing in their activity period is shown in Table 1. Considering the comparison among sexes or populations, the patterns are remarkably similar to those observed in the Common Frog (Table 1), although some details differ (e.g. of age at maturity; Miaud et al. 1999, 2000). Nevertheless, what was said above in the case of the Common Frog also applies to the Alpine Newt.

5 biota 2/ /2/01 17:28 Page 35 MIAUD, MERILÄ Biota 2/1, (a) Comparisons between sexes within populations Trait Rana temporaria Triturus alpestris Age at maturity males < females similar Longevity similar similar Body size at maturity males < females males < females Mean body size males < females males < females Asymptotic size1 males < females males < females Growth coefficient males < females males < females Growth rate during the juvenile phase males < females males < females Adult survival rate similar similar (b) Comparisons between populations short activity period: Age at maturity increases increases Longevity increases increases Body size at maturity increases increases Mean body size increases increases Asymptotic size 1 increases increases Growth coefficient 1 decreases decreases Adult survival rate increases increases 1 Parameters derived from Von Bertanlanffy (1938) growth model Table 1: Comparison of adult life history traits in the Common frog Rana temporaria and the Alpine newt Triturus alpestris. Comparison between (a) sexes and (b) populations from Europe varying in altitude or latitude. Data from Miaud et al. (1999, 2000). In summary, both species show similar life history trait responses to decreasing activity period (i.e. to increasing altitude and latitude). These responses could be caused either by: 1) parallel evolution in response to same environmental factors; 2) shared environmental influences; or 3) some combination of (1) and (2). In order to differentiate between these alternatives, it is necessary to investigate to what degree variation in these adult life history traits, as well as other traits, are of genetic and environmental origin. Timing of reproduction Within population variation Timing of reproduction is potentially a trait of great importance for individual fitness, but we are not aware of any amphibian study which would have investigated the genetic basis of this trait. Among population variation There is a great deal of variation among different Common Frog populations in the timing of breeding (e.g. Koskela 1973, Elmberg 1990, Guyétant et al. 1995). We are not aware of any studies which investigated a possible genetic component to these differences in the Common Frog and the Alpine Newt. However, using a reciprocal transplant experiment, Berven (1982a) investigated the possibility of a genetic component to population divergence in the breeding time between two Wood

6 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ Frog populations. Marked juveniles were transplanted reciprocally between a highland (1100 m) and lowland (0 m) population in the Shenandoah Mountains, Virginia, USA, during three successive years. Recaptures in the following breeding seasons showed that there was no difference in timing of breeding between transplants and residents: the timing of breeding was solely environmentally determined. This remains to be tested in Common Frog and Alpine Newt. However, longterm data from a number of vertebrate species (Forchhammer et al. 1998), including amphibians (Beebee 1995, Forchhammer et al. 1998), show marked changes in breeding during recent decades. Whether these changes reflect phenotypic plasticity or genetically based evolution in response to changing climate remains largely unexplored (but see: Przybylo et al. 2000). Fecundity Within population variation A positive correlation between body size and egg number is well-documented in amphibians (Kaplan and Salthe 1979), including Common Frog (e.g. Joly 1991, Elmberg 1991, Martin & Miaud 1999) and Alpine Newt (C. Miaud, unpublished data). However, there are no data available on heritability of clutch size, neither from the two focal species nor from any other amphibian species. Among population variation Despite the fact that a number of studies have documented amongpopulation variation in clutch size of the Common Frog (e.g. Kozlowska 1971, Cummins 1986, reviewed by Joly 1991), next to nothing is known about the relative importance of genes and environment underlying this variation. The same is true in the case of the Alpine Newt. Berven (1982a) compared the fecundity of reciprocally transplanted juvenile Wood Frogs when they started to reproduce. Transplanted females showed similar fecundity to their congeners in the parental population, suggesting a genetic component to fecundity/clutch size. However, the possibility of maternal/carry over effects (Falconer & MacKay 1996, Rossiter 1996) on fecundity, as demonstrated in other taxa (e.g. Gustafsson & Schluter 1993), cannot be strictly excluded. Egg size Within population variation Although the relationship between female body size and egg size is well documented in the Common Frog (e.g. Kozlowska 1971, Beattie 1977, Cummins 1986, Augert & Joly 1993, Martin & Miaud 1999) and in the Alpine Newt (e.g. Brand & Grossenbacher 1979, C. Miaud, unpublished data), no information is available on the genetic basis of egg size variation in these species. Among population variation As in many other amphibians (Kaplan 1998), there is extensive among-population variation in egg size in Common Frogs (e.g. Cummins 1986, Laugen et al. 2001). For example, Martin & Miaud (1999) compared egg size in three populations (at 326,

7 biota 2/ /2/01 17:28 Page 37 MIAUD, MERILÄ Biota 2/1, , and 2450 m) in the northwestern part of the Alps, and found that egg size increased with increasing altitude, corroborating earlier findings by Kozlowska (1971), Brant & Grossenbacher (1979), and Holms (1982). Also, in the Alpine Newt, egg size increases from low-to-highland populations (C. Miaud, unpublished data). Whether these patterns result from phenotypic plasticity or genetic divergence is not known for Common Frog and Alpine Newt. In reciprocal juvenile transplant experiments on the Wood Frog (Berven 1982a), egg size and in particular the relationship between this trait and female body size were functions of the parental population, and not the environment in which the transplanted individuals matured. Nevertheless, by virtue of the environmental effect on body size (see below), this trait is indirectly influenced by the environment (Berven 1982a). Rate of embryonic development Within population variation Despite the relative ease with which a genetic component of the rates of embryonic development could be assessed, there appears to be no published study which has estimated heritability of this trait. However, there is evidence for the genetic basis of embryonic developmental rates in Swedish Common Frog populations from a common garden experiment (M. Pahkala, A. Laurila & J. Merilä, unpublished data). However, these results are based on a full-sib design, and maternal effects as a cause of observed variation cannot be ruled out. Among population variation A number of studies have compared developmental rates of eggs from Common Frog populations originating from different altitudes using the common garden approach (1500 vs m, Angelier & Angelier 1968; 200 m vs m, Kozlowska 1971; 50 vs. 800 m, Beattie 1977; 100 vs. 500 m, Holms 1982; 326 vs vs. 2450m, Martin & Miaud 1999). These studies confirmed a faster development of high altitude eggs over those from lower altitudes (except Holms 1982), suggesting a genetic capacity for faster egg development in high as compared to low altitudes. These results support counter-gradient selection hypothesis (Levis 1968); eggs with faster development are selected at high altitudes possibly because of their need to develop faster (shorter growing season) and/or to counteract the negative effects of a colder climate on developmental rates (Conover & Schlultz 1995). In the Alpine Newt, Brand and Grossenbacher (1979) found that eggs from highland populations had a faster developmental rate (and better tolerance to low temperature) than eggs from lowland populations. Hence, it appears that in both species, populations inhabiting higher altitudes have evolved a capacity for faster development than those inhabiting lower altitudes. These data are corroborated by latitudinal comparisons in the Common Frog: embryos from northern Sweden seem to be capable of faster development than those from southern Sweden when reared in a constant temperature (A. T. Laugen, A. Laurila

8 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ and J. Merilä, unpublished data). Larval developmental and growth rate Within population variation There is evidence that larval developmental rates - or age at metamorphosis - is under genetic control in the Common Frog. In a lowland population, Plytycz et al. (1984) demonstrated a genetic control of the larval phase duration with tadpoles obtained from controlled matings (North Carolina II design; Kearsey & Pooni 1996). Merilä et al. (2000a) used a full-sib design and failed to find evidence for genetic variation in developmental rates in two Swedish populations, but a low number of families was used. However, a halfsib design (North Carolina I design; Kearsey & Pooni 1996) applied to individuals from one of these populations revealed a significant additive genetic component to developmental rate (Merilä et al. 2000b). In the Wood Frog, Berven (1987) conducted experimental crosses with adults of two populations, obtaining fulland half-sib families for both a lowland and mountain population. In each of the populations, developmental rate was heritable. Among population variation A fair number of studies have investigated altitudinal and latitudinal variation in larval traits of the Common Frog (Table 2). Using a common garden approach, Aebli (1966) compared the developmental rates of Common Frog larvae among four lowland (below 900 m) and highland (above 1800 m) populations, and found that the larval period in a constant temperature experiment increased in linear fashion from 59 days (highland) to 153 days (lowland). Holm (1982) reared tadpoles from two populations - situated at 100 and 500 m, respectively - at constant temperature in the laboratory, and found no effect of population of origin on age at metamorphosis. In a similar experiment, Brand and Grossenbacher (1979) found that tadpoles from 2550 m developed at a faster rate and metamorphosed at a larger size than those from 525 m. Tadpoles obtained from controlled matings of Common Frog originating from two populations at 55 N and 68 N in Scandinavia were reared at two temperatures and three food levels (Merilä et al. 2000a). Low temperature and low food level led to lowered growth rates and delayed metamorphosis, while high temperature and high food level increased growth rates and accelerated metamorphosis. Tadpoles from the north metamorphosed earlier with a higher growth rate than tadpoles from the south. The clinal nature of this differentiation was later confirmed in a larger experiment involving six different populations along a latitudinal gradient across Scandinavia (Laugen et al. 2001). Hence, Common Frogs from northern Sweden (Merilä et al. 2000a) and high elevation in the Alps (see above; Martin & Miaud, unpublished data) have a genetic capacity to complete their larval development faster than Frogs from southern Sweden and lowland populations. Similar results have been obtained in the Wood Frog: when tadpoles were

9 biota 2/ /2/01 17:28 Page 39 MIAUD, MERILÄ Biota 2/1, Trait Direction of difference Reference Growth rate northern > southern Merilä et al. 2000a northern > southern Johanssen et al northern > southern Laugen et al Age at metamorphosis low > high altitude Aebli 1966 low > high altitude Brand and Grossenbacher 1979 high = low altitude Holm 1982 southern > northern Merilä et al. 2000a southern > northern Johanssen et al southern > northern Laugen et al low > high altitude Martin & Miaud, unpublished Size at metamorphosis high > low altitude Brand & Grossenbacher 1979 northern > southern Merilä et al. 2000a northern > southern Johanssen et al midlatitude > southern = northern Laugen et al Table 2: Comparison of larval life history traits among different Common Frog Rana temporaria populations. reciprocally transplanted between two populations, the same duration of the tadpole phase was observed in transplants and residents but mountain to lowland transplants grew faster and were larger (Berven 1982b). However, reciprocal crosses between elevations were also performed (Berven 1982b) to separate non-genetic maternal effects associated with egg size differences between mountain and lowland populations from true genetic differences in larval traits. Environmental effects accounted for 88% of the variation in the length of the larval period, while only an insignificant (1%) part of the variation was explained by genetic origin. A significant maternal effect on developmental time also was demonstrated. Hence, as to genetic basis of the differentiation in larval developmental rates, the data from different studies is conflicting. Reciprocal crosses remain to be conducted in the Common Frog, and no data on larval development in different populations of Alpine Newt is yet available. Size at metamorphosis Within population variation Again, surprisingly little information is available about the genetic basis of metamorphic size in the Common Frog, and no information in this respect exists for the Alpine Newt. Merilä et al. (2000a) found marginally significant family effects on size at metamorphosis in the Common Frog, but since this was a full-sib design, these effects could result from maternal influences. Merilä et al. (2000b) did not find any evidence for genetic variation in metamorphic size using a half-sib design. Both of these studies were based on a very small number of families, and larger designs are clearly required to reach any confidence about the genetic basis of metamorphic size. In Berven s (1987) experiments, size at metamorphosis was strongly heritable (h2 = 0.66, S.E. = 0.31) among mountain larvae while

10 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ h2 was not significant among the lowland larvae. There was, however, a strong female component in the lowland population, indicating that a significant proportion of the variation in larval size was female-dependent (even if maternal effects through egg size were not significant determinants of larval body size, Berven 1987). Among population variation Egg masses of the Common Frog collected in 14 populations varying in altitude (from 525 m to 2550 m in the Alps) and reared in the laboratory showed that tadpoles from the highland populations metamorphosed at larger sizes than tadpoles from lowland populations (Brand & Grossenbacher 1979). However, using a common garden experiment, Merilä et al. (2000a) found that metamorphic size did not differ between one northern and one southern Swedish population, but after accounting for differences in age at metamorphosis, metamorphs from northern Sweden were larger than those from southern Sweden (Merilä et al. 2000a). An extended study along the same latitudinal gradient revealed that size at metamorphosis was a positive function of latitude up to the mid-part of Sweden, and negative thereafter (Laugen et al. 2001). Hence, the relationship between season length and size at metamorphosis may be more complex than previously anticipated. Finally, a significant genetic component (20%) to the inter-elevation variation in size at metamorphosis was reported in the Wood Frog (Berven 1982b). No data on larval size in different populations is yet available for the Alpine Newt. Age and size at maturity Within population variation We are not aware of any study that has examined sources of within population variation in age and size at maturity in the Common Frog or in the Alpine Newt. However, since metamorphic size is often correlated with size at maturity in other species (Smith 1987 Semlitsch et al. 1988, Berven 1990, Amézquida & Lüddecke 1999), it is possible that a genetic component to these traits also exists in Common Frogs and Alpine Newts. Among population variation Augert & Joly (1993) found differences in age and size at maturity between two neighbouring Common Frog populations. Because professional fishermen mixed eggs and tadpoles from these two populations each year, the differences in age and size at maturity are likely to be due to differences in local environmental conditions (i.e. phenotypic plasticity). On the contrary, in a reciprocal transplant study of juvenile Wood Frogs between populations at different altitudes, Berven (1982a) found that transplants matured at ages and sizes intermediate to residents, suggesting at least partial genetic control of these traits. We are not aware of any study which would have checked whether this would also apply among altitudinally or latitudinally separated Common Frog populations. In the Alpine Newt, Schabetsberger & Goldschmid (1994) found that larvae caught from a highland lake (1643

11 biota 2/ /2/01 17:28 Page 41 MIAUD, MERILÄ Biota 2/1, m) and reared in the laboratory reached the size of mature animals within 2-3 years, while this size was not reached before 10 years of age in the field. This suggests that a major part of the variation in size at maturity among Alpine Newt populations in different altitudes may result from phenotypic plasticity, but it does not disprove the possibility of a genetic component. In conclusion, it is clear that more studies focusing on the genetic vs. environmental determination of age and size at maturity are needed in these amphibian species. Other traits We have focused on major life history traits which might be targets of varying natural selection in different habitats. These traits also may be important in terms of adaptation to changing environmental conditions. However, we should also point out that there is a whole suite of other traits of potential relevance to adaptation to environmental changes. These include, for example, tolerance to low ph, high levels of ultraviolet (UV-B) radiation and high concentrations of environmental pollutants, such as nitrates and pesticides. Also, changes in hydroperiod length and predation pressure as a consequence of climate change (e.g. Griffiths 1997) may render plastic responses to pond dessication (e.g. Laurila & Kujasalo 1999) and predation risk (e.g. Lardner 2000) important for further adaptation. To this end, studies of the genetic basis of stress responses and adaptation to local differences in levels of these stressors would be most illuminating. As to the physiochemical stressors, we are aware of only two studies addressing these issues in the Common Frog. First, Pahkala et al. (2001a) compared the combined effects of low ph and high levels of UV-B radiation between southern and northern Swedish Common Frog populations in a laboratory experiment. They found synergistic negative effects of UV-B radiation and low ph (see also: Long et al. 1995; Pahkala et al. 2001b), only in the northern population. This suggests that the negative effects of UV- B on embryonic development of the Common Frog may be expressed only in the presence of other stress factors, and furthermore, in some but not in all populations. Second, in an attempt to test possible adaptation to varying background levels of nitrate, Johansson et al. (2001) compared nitrate tolerance between two northern Swedish and two southern Swedish populations of Common Frogs. Their results suggest that although all populations were highly tolerant to nitrate, the southern Swedish populations tolerated high concentrations better than the northern Swedish populations. This difference is consistent with the interpretation that the southern Swedish tadpoles might have adapted to higher background levels of nitrates in their habitat. Similar comparisons using other types of stressors, such as pesticides (e.g. Bridges and Semlitsch 2000), remain to be conducted. As to the dessication and predation risk, some data are available from the Common Frog: at least in the lowland populations studied so far, Common Frog tadpoles appear able to speed

12 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ up their development (at the cost of smaller size at metamorphosis) when facing a pond drying risk (Laurila & Kujasalo 1999, Loman 1999, Merilä et al. 2000b). However, it is unclear whether this apparently adaptive capacity is also present at high elevations or high latitudes, where the larvae seem to have been already selected to maximise their developmental rates. Likewise, predator induced (adaptive) morphological changes occur in many lowland populations of Common Frog (Laurila 2000, Lardner 2000), but since these responses are costly, it is not clear whether they could also be expected to have evolved at high altitudes and high latitudes where the costs of induced defences might be even higher than in the lowland populations studied so far. Furthermore, there are no studies focusing on the question of the genetic basis of plastic responses in R. temporaria (but see Merilä et al. 2000b) which would be required to evaluate whether these traits can evolve in response to selection. Conclusions We have reviewed the currently available data on life history trait variation in two model organisms - the Common Frog and the Alpine Newt - and propose here some possible directions for future studies. First, although the reviews above suggest that local adaptation has taken place at least in the case of the most thoroughly studied traits, and estimates of genetic variability are available for a few traits, studies on the genetic and environmental determinants of variation (both within and among populations) are typically conducted on the aquatic stages (i.e. egg and larval development). Even if these traits are important determinants of individual fitness, it is clear that information on the traits expressed in terrestrial phases (juveniles and adults) are also of critical importance. For instance, it is possible that selective events taking place during terrestrial phases are more important for population processes than those taking place at larval stages. We also note that nothing is known about the genetics of behavioural traits, such as male calling characters, dispersal behaviour, or habitat choice (but see: Laurila 2000), despite the fact that Common Frog populations inhabiting different environments have been assumed to have diverged in mating behaviour (Elmberg & Lundberg 1991). The same applies for tolerance to different environmental stressors - evidence is accumulating that different amphibian populations are differently sensitive to the same environmental stressors (e.g. Bridges & Semlitsch 2000, Johanssen et al , Pahkala et al. 2001a, b), and understanding the causes and consequences of this variation poses a challenge for further research. In particular, we propose that testing how populations from different altitudes may differ in their responses to different stressors might provide insights to the question how forthcoming environmental changes will impact upon amphibian populations, and what kind of constraints (ecological and genetic) there might be for evolution of adaptive responses.

13 biota 2/ /2/01 17:28 Page 43 MIAUD, MERILÄ Biota 2/1, Figure 2: (a) Schematic diagram of the female life cycle for an amphibian with both aquatic and terrestrial phases, each (egg [N1], larvae [N2], juvenile [N3- N4], and adult [N5]) with agespecific probabilities of survival (S) and population size (N). Reproduction starts with N5, fecundity (F), Juvenile survival (S3) and adult survival (S4) are independent of age, and there is no senescence. (b) The corresponding transition matrix of this amphibian life cycle. N1 is the size of the population at time t1 and N2 at time t2. The population growth depends on (, the leading eigenvalue of the average matrix M. If ( = 1, the population is stationary, decreases if ( < 1 and increases if ( > 1. Sensitivity analyses on the different parameters (F, S1 to 4) allows identification of factors that most affect ( (i.e. population growth). One potentially fruitful avenue for future research would involve more in-depth study of the impact of life history variation on population dynamics. In demographic ecology, matrix projection models (e.g. Leslie matrix, Ferriere et al. 1996) can be used to determine the effects of the different phases in the life cycle on the population growth rate (Fig. 2a, b). Here, a central index that quantifies population growth or decline is the leading eigenvalue ( of the average matrix (Fig. 2b). If ( > 1, matrix theory predicts that, regardless of the initial population size and structure, the population will grow at a geometric rate given by (. The calculation of stage-specific sensitivities or elasticities allows identification of factors affecting population growth and persistence (Caswell 1978, Gaillard et al. 1998, Mills et al. 1999). Also, since ( is considered as one of the best estimate of fitness (McGraw & Caswell 1996), knowledge about the relative importance of different life history traits on ( (i.e. fitness) is thus clearly informative about the adaptive nature of life history trait variation among different populations. Hence, comparative analyses of selection on life history traits, evaluation of the genetic potential for evolutionary change, and studies on the impact of traits on demographic properties of a population would all benefit from such quantification of a trait s impact on the population growth rate (Benton & Grant 1999). This approach would be most interesting to apply to different populations along altitudinal and latitudinal clines, since it would give us cues as to what would be the most vulnerable lifestages in a given population, and whether effects of various stressors would differ among different populations. For example, in Alpine Newt, the strong delay in age at maturity in highland populations increases the role of adult survival in population

14 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ dynamics. Hence, we can hypothesise that factors promoting adult survival (e.g. body size) will be selected more strongly in highland than in lowland populations, whereas the opposite may be true for the traits impacting larval survival. Another (related) direction concerns a quantitative genetics approach: when selection operates on a single trait, the response to selection is roughly proportional to the additive genetic variance of the trait. If the same genes influence the expression of different traits, then an evolutionary change in one trait will lead to changes in correlated traits (Falconer & Mackay 1996). This can impede the evolutionary process when there is a conflict in the fitness consequences of selection operating on correlated traits. Consequently, understanding the constraints on the evolution of systems of complex characters requires information on the magnitude and direction of genetic correlation between characters. Quantitative-genetic methodology provides a useful, but data demanding, means of elucidating these issues (Mousseau et al. 2000). However, the development of new statistical methods and access to hypervariable markers (e.g. Berlin et al. 2000, Rowe & Beebee 2000) to infer degree of genetic relatedness among individuals will facilitate the estimation of heritability of quantitative traits even in wild populations (Ritland 2000a, b). Finally, selection by multiple factors on multiple traits, or parts of traits (e.g. life history components such as body size, fecundity, and survival) often gives rise to counterintuitive results (Sinervo 2000). One possibility would be to start with suites of traits with known interrelated adaptive functions. Path analysis gives explicit predictions of what form the correlations would take and then allows measurement of natural selection (Scheiner et al. 2000). This method consists of building an analytical model around a specific set of causal relationships among traits that determine fitness. Sinervo (2000) conducted a five-year study (i.e. five generations) on the lizard Uta stansburiana with controlled matings in nature. Clutch size and egg-mass were significantly heritable and a negative genetic correlation was found between the traits. A path analytic model describing the causal relationship between factors for year, postlaying mass, clutch size, and egg mass allowed him to address the question of how natural selection acts on the genetically based trade-off between egg-mass and fecundity. A pattern of multivariate selection was determined to act on the trade-off: a disruptive selection acted on variation in clutch size (tended to pull the population average clutch size to extreme values) and stabilizing selection acted on egg-mass (limited variation in the population). Such analyses could also greatly improve our knowledge of selection acting on amphibian life history traits along altitudinal / latitudinal gradients. In conclusion, we have reviewed the evidence for a genetic basis for population differentiation (past adaptation) of amphibian populations along altitudinal and latitudinal gradients in

15 biota 2/ /2/01 17:28 Page 45 MIAUD, MERILÄ Biota 2/1, Europe, as well as what is known about the genetic basis of within population variation in different life history traits (potential for future adaptation). Given the relative ease with which the genetic basis of among and within population variation of amphibians (at least in larvae) can be studied, and because altitudinal and latitudinal gradients mimic in many ways (e.g. temperature, hydroperiod length, predator fauna, season length, etc.) the environmental changes caused by climatic change, they could provide suitable models for research evaluating effects of global change on animal populations. To this end, we suggest that it is also time to promote development of interactions and collaborative projects between people with interests on the interface between ecology, genetics, ecotoxicology and evolutionary biology. Acknowledgements We thank Anssi Laurila and an anonymous referee for insightful suggestions for the manuscript. Our research is supported by the French National Research Council (CNRS) and the Swedish Natural Sciences Research Council (NFR). REFERENCES AEBLI, H. 1966: Rassenunterschiede in beuzug auf entwicklungsgeswindigkeit und geslechtsdifferezierung bei Rana temporaria in des kantos Glarus (Schweitz). Rev. Suisse Zool. 73: AMEZQUITA, A. & L(DDECKE, H. 1999: Correlates of intrapopulational variation in size at metamorphosis of high-andean frog Hyla labialis. Herpetologica 55: ANGELIER, E. & ANGELIER, M.-L. 1968: Observations sur le développement embryonnaire et larvaire de Rana temporaria L. (Batracien, Anoure). Annales de Limnologie 4: AUGERT, D. & JOLY, P. 1993: Plasticity of age at maturity between two neighbouring populations of the common frog (Rana temporaria L.). Canadian Journal of Zoology. 71: BEATTIE, R.C., 1977: Studies on the biology of the common frog Rana temporaria (Linnaeus) with particular reference to altitude. Unpublished PhD thesis, University of Durham, U.K. BEEBEE, T.J.C. 1995: Amphibian breeding and climate. Nature 374: BENTON, T.G. & GRANT, A. 1999: Elasticity analysis as an important tool in evolutionary and population ecology. Trends Ecol. Evol. 14: BERLIN, S., MERIL(, J. & ELLEGREN, H. 2000: Isolation and characterisation of polymorphic microsatellite loci in the common frog Rana temporaria. Molecular Ecology 9, in press. BERVEN, K.A. 1982a: The genetic basis of altitudinal variation in the wood frog Rana sylvatica. I. An experimental analysis of life history traits. Evolution 36:

16 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ BERVEN, K.A. 1982b: The genetic basis of altitudinal variation in the wood frog Rana sylvatica. II. An experimental analysis of larval development. Oecologia 52: BERVEN, K.A. 1987: The heritable basis of variation in larval developmental patterns within populations of the wood frog (Rana sylvatica). Evolution 41: BERVEN, K. A. 1990: Factors affecting population fluctuations in larval and adult stages of the wood frog (Rana sylvatica). Ecology 71: BERVEN, K.A. & GILL, D.E. 1983: Interpreting geographic variation in life-history traits. American Zoologist 23: BLAUSTEIN, A.R., HAYS, J.B., HOFFMAN, P.D., CHIVERS, D.P., KIESECKER, J.M., LEONARD, W.P., MARCO, A., OLSON, D.H., REASER, J.K. & ANTHONY, R. 1999: DNA repair and resistance to UV-B radiation in western spotted frogs. Ecol. Appl. 9: BRAND, M. & GROSSENBACHER, K, 1979: Untersuchungen zur Entwicklungsgeschwindigkeit der Larven von Triturus a. alpestris (LAURENTI 1768), Bufo b. bufo (LINNAEUS 1758) und Rana t. temporaria (LINNAEUS, 1758) aus Populationen verschiedener Höhenstufen in den Schweizer Alpen. Unpublished Ph.D. thesis, University of Bern, Switzerland, 260 p. BRIDGES, C.M. & SEMLITSCH, R.D. 2000: Variation in pesticide tolerance of tadpoles among and within species of Ranidae and patterns of amphibian decline. Conservation Biology 14: B(RGER, R. & LYNCH, M. 1997: Adaptation and extinction in changing environments. In Environmental stress, adaptation and evolution. Edited by R. Bijlsma and V. Loeschcke. Springer Verlag, Berlin. pp CASWELL, H. 1978: A general formula for the sensitivy of population growth ratee to changes in life history parameters. Ther. Pop. Biol. 14: CHAMBERS, F. 1998: Global warming : New perspectives from palaeoecology and solar science. Geography 83: CONOVER, D. O. & SCHULTZ, E. T Phenotypic similarity and the evolutionary significance of countergradient variation. Trends Ecol. Evol. 10: CUMMINS, C.P. 1986: Temporal and spatial variation in egg size and fecundity in Rana temporaria. Journal of Animal Ecology 55: DIXON, R.K., SMITH, J.B., BROWN, S., MASERA, O., MATA, L.J. & BUKSHA, I. 1999: Simulations of forest system response and feedbacks to global change: experiences and results from the US Country Studies Program. Ecol. Model. 122: ELMBERG, J. 1990: Long-term survival, length of the breeding season and operational sex ration in a boreal population of the common frog Rana temporaria L. Canadian Journal of Zoology 68:

17 biota 2/ /2/01 17:28 Page 47 MIAUD, MERILÄ Biota 2/1, ELMBERG, J. & LUNDBERG, P. 1991: Intraspecific variation in calling time allocation and energy reserves in breeding male common frogs Rana temporaria. Annales Zoologici Fennici 28: FITTER, A.H., HEINEMEYER, A. & STADDON, P.L., 2000: The impact of elevated CO2 and global climate change on arbuscular mycorrhizas: a mycocentric approach. New Physiologist. 147: FALCONER, D.S. & MACKAY, T.M.C 1996: Introduction to quantitative genetics. 3rd Ed. Longman Sci. And Tech., Harlow, U.K. FERRIERE, R., SARRAZIN, F., LEGENDRE, S. & BARON, J-P. 1996: Matrix population models applied to viability analysis and conservation: theory and practice using the ULM software. Acta Oecologica 17: GASC, J-P, CABELA, A, CRNOBRNJA-ISAILOVI_, J., DOLMEN, D., GROSSEN- BACHER, K., HAFFNER, P., LESCURE, J., MARTENS, H., MARTINEZ RICA, J.P., MAURIN, H., OLIVEIRA, M.E., SOFIANIDOU, T.S., VEITH, M. & ZUIDERWIJK, A. (eds.), 1997: Atlas of Amphibians and Reptiles in Europe. Societas Europaea Herpetologica & Museum National d Histoire Naturelle (IEGB/SPN), Paris: 496 p. GRIFFITHS, R.A., 1997: Temporary ponds as amphibian habitat. Aquatic Conserv. Mar. Freshw. Ecos. 7: GROSSENBACHER, K. 1994: Herpétofaune de l Arc Alpin et des régions montagneuses. Bull. Soc. Herpétol. Fr : GAILLARD, J-M., FESTA-BLANCHET, M. & YOCCOZ, N.G. 1998: Population dynamics of large herbivores: variable recruitment with constant adult survival. Trends Ecol. Evol. 13: GUSTAFSSON, L. & SCHLUTER, D. 1993: Maternal inheritance of condition and clutch size in the collared flycatcher. Evolution 47: GUYETANT, R., MIAUD, C., BATTESTI, Y. & NELVA, A. 1995: Caractéristiques de la reproduction chez la Grenouille rousse Rana temporaria L. (Amphibia, Anura) en altitude (Massif de la Vanoise, Alpes du Nord, France). Bull. Soc. Herp. Fr : HOFFMANN, A. A. & MERIL(, J. 1999: Heritable variation and evolution under favourable and unfavorable conditions. Trends Ecol. Evol. 14: HOLMS, P. 1982: Altitudinal comparisons in the ecology and reproduction of the common frog (Rana temporaria L.). Trans. Nat. Hist. Soc. Northumberland 49: HOULAHAN, J.E., FINDLAY, C.S., SCHMIDT, B.R., MEYER, A.H. & KUZMIN, S.L., 2000: Quantitative evidence for global amphibian population declines. Nature 404: 752 HULME, M., BARROW, E. M., ARNELL, N. W., HARRISON, P. A., JOHNS, T. C. & DOWING, T. E. 1999: Relative impacts of human-induced climate change and natural climate variability. Nature 397: JOHANSSEN, M., R(S(NEN, K. & MERIL(, J. 2001, in press: Comparison of nitrate tolerance between different populations of the common frog, Rana temporaria. Aquatic Toxicology.

18 biota 2/ /2/01 17:28 Page Biota 2/1, 2000 MIAUD, MERILÄ JOLY, P. 1991: Variation in size and fecundity between neighbouring populations in the common frog, Rana temporaria. Alytes 9: KAREIVA, P.M., KINGSOLVER, J.G. & HUEY, R.B. (eds) 1993: Biotic interactions and global change. Sinauer, Sunderland, MA. KAPLAN, R. H., 1998: Maternal effects, developmental plasticity, and life history evolution. pp in T. E. Mousseau and C. W. Fox ed. Maternal effects as adaptations. Oxford University Press, New York. KAPLAN, R. H. & SALTHE, S. N. 1979: The allometry of reproduction: an empirical view in salamanders. American Naturalist 113: KEARSEY, M. J. & POONI, H.S. 1996: The genetical analysis of quantitative traits. Chapman & Hall, London. KOSKELA, P. 1973: Duration of the larval stage, growth and migration in Rana temporaria L. in two ponds in northern Finland in relation to environmental factors. Annales Zoologici Fennici 10: KOZLOWSKA, M. 1971: Differences in the reproductive biology of mountain and lowland common frogs, Rana temporaria L. Acta Biologica Cracoviensia 14: LARDNER, B. 2000: Morphological and life hsitory responses to predators in larvae of seven anurans. Oikos 88: LAUGEN, T. A., LAURILA, A., R(S(NEN, K. & MERIL(, J. 2001: Latitudinal counter gradient variation in common frogs, Rana temporaria. Submitted to where? LAURILA, A. 2000: Responses to predator chemical cues and local variation in antipredator behaviour of Rana temporaria tadpoles. Oikos 88: LAURILA, A. & KUJASALO, J. 1999: Habitat duration, predation risk and phenotypic plasticity in common frog (Rana temporaria) tadpoles. Journal of Animal Ecology 68: LEVINS, R., 1968: Evolution in changing environment. Princeton Univ. Press, Princeton. LINHART, Y. B. & GRANT, M. C. 1996: Evolutionary significance of local genetic differentiation in plants. Annu. Rev. Ecol. Syst. 27: LOMAN, J. 1999: Early metamorphosis in common frog Rana temporaria tadpoles at risk of drying and experimental demonstration. Amphibia- Reptilia 20: LONG, L.E., SAYLOR, L.S. & SOULE, M.E. 1995: ph/uv-b synergism in amphibians. Conservation Biology 9: MADRONICH, S., McKENZIE, R. L., BJ(RN, L. O. & CALDWELL, M. M. 1998: Changes in biologically active ultraviolet radiation reaching the Earth s surface. J. Photochem. Biol. 46: MARCHETTINI, N., MOCENNI, C. & NICCOLUCCI, V. 1999: Modelling the effect of global warming on an aquatic ecosystem in central Italy. Ecosyst. Sustain. Development 2: MARTIN, R. & MIAUD, C. 1999: Reproductive investment and duration of the