Adaptations To Climatic Conditions In Invertebrates Biology Essay

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Organisms inhabit in different geological regions due to the degree of inheritance from their evolutionary ancestors and the adaptation processes they experience throughout their life. They adapt in various areas according to the availability of both biotic and abiotic factors (Forsman 1999). The limitation in either of them is considered as selection pressure to the survivability and heritability of an organism. For example, the increased number of natural enemies may bring catastrophic consequences on a group of vulnerable tree-frogs. Biotic and abiotic factors are interrelated and combined to create the complex ecosystem. The physiology, ecology and evolution of an organism can be altered through changes in either of them after a long period of time which eventually will differentiate the populations (natural selection). Among all abiotic factors, temperature is considered as the most influential attribute (Bacigalupe & Araya, 2007). It affects almost all aspects of ectothermic organisms and their underlying physical and physiological mechanisms including biochemical, metabolic, locomotion and life-history traits (Lavy et al., 1996). The relationship between organisms and their living environment, and the link with their underlying genetic composition is perhaps one of the most fascinating genetic topics that attract many evolutionary scientists (Endler 1986).

MAIN

Evolutionary genetics is the study of genetics which focus not only on the underlying genetic variation but also the traits organisms may adopt that cause evolution. Various experiments from different angles have been carried out in order to understand the evolution process through adaption of an organism. Countless studies have been done using isopods and woodlouse is one of the best candidates among them because they are by far the most successful group of terrestrial crustaceans and distributed in a relatively broad range in a high abundance. Warburg (1973) tested the response of isopods Armadillidium vulgare (A. vulgare) and Verticalis arizonicus (V. arizonicus) towards temperature, humidity and light. He found that A. Vulgare became more active at high temperature whereas the degree of movement in V. arizonicus was negatively related with increased temperature. But the activeness of both organisms showed no significant differences when either the light or humidity was altered. Thus, temperatures played a superior role that determines isopod species' behaviour under certain circumstances. The underlying reasons why they had different responses under different situation was unknown at that time. With the development of modern techniques and the resources needed to conduct a genetic research become available, E. Luis and colleagues (2005) performed an experiment which explained the mystery behind Warburg's test. They collected the common woodlouse: Porcellio Laevis (P. Laevis) from different thermal conditions across a broad latitudinal range in Chile and the time it takes for P. Laevis to recover from chill coma is applied to show differences in cold tolerability. When the temperature is low, the internal ice formation will disequilibrate the ionic level, interrupting the nerve cells' excitability then causing the organism to be motionless (Hosler et al., 2000; Sinclair et al., 2004). They hypothesised that thermal physiology is a crucial factor to the evolutionary success of organisms since almost all metabolic activities such as movement speed, digestion rate, and reaction rate are suppressed in cold temperature. They discovered that recovery time from chill coma in P. laevis exhibits inter-population variation along a geographic gradient, and low-latitude populations had a lower tolerance to chill coma compared with high latitudinal populations (Castaneda et al, 2005). However, only one population sampled from each site may not be enough to reflect the real differences as more replicas from each location should be used and compared to eliminate any inter-population biases such as inbreeding and migration among subpopulations (Endler 1986).

Other variables such as food availability and seasonally may also contribute negatively to the accuracy of the recovery time (Forsman 1999). To eliminate the biases occurred in Luis's experiment, Lavy and colleagues (1996) tested the effects of prolonged starvation on the body composition of P. scaber. They discovered that starved individuals showed lower cold tolerance than fed ones and a significant decrease in heamolymph osmolarity was detected in starved ones compared to fed ones. A potential drawback of the study is that only few populations were sued which reduced the statistical power of the test. They then further hypothesized that the decreased cold tolerance in starved slaters may be caused by the decrease in reserves needed to produce the cryoprotectant which is used to protect biological tissues from freezing damage due to internal ice formation (Lavy et al., 1996). In fact, most organisms that live in Antarctica and Arctic, including insects, amphibian and reptiles produce cryoprotectant in assisting their cold tolerability from freezing damage during extreme winter conditions (Peterson 1984). This superior cold tolerability enables low-latitudinal populations to have better survivability during cold seasons (Gilbert et al., 2001). Nevertheless since slaters are susceptible to subzero temperature, how they adapt in extreme winter conditions becomes an intriguing question. Tanaka and Udagawa (1993) exercised an experiment and tried to find out varies factors that may help P. scaber to survive under subzero environment in a subnivean environment. They discovered that the lower lethal temperature can cause 50% mortality ranged from -1.37 ℃ in August to -4.58℃ in December with the absolute limit of cold tolerability (extreme cooling point) was maintained at around -7 ℃ across the year. They conducted three explanations how P. scaber can survive in freezing conditions: 1) food contents may act as efficient ice nucleators (Tanaka and Udagawa 1993); 2) P. scaber hibernate in sheltered habitats beneath logs and rocks and 3) the presence of snow cover can act as a thermal buffer since its low thermal conductivity (Danks 1978). The results suggested that P. scaber are able to adjust themselves against harsh environment even the condition is not favoured. Bacigalupe and Araya (2007) tested whether maternal effects maternal body mass and the variation associated with mothers) and body size can affect offspring's energetic using P. laevis. By measuring the physiological performance, thermal tolerance and thermal sensitivity in F1 adults they concluded that maternal body size have a positive impact on offspring long-term metabolism (Bacigalupe & Araya, 2007). However, potential pregnancy among experimented female P. laevis does contribute negatively to the accuracy of the maternal body mass since pregnant individuals will undoubtedly weight heavier than unpregnant ones. Understanding the cold tolerability and other symbolic behaviours can help us to further acknowledge the relationship between their phenotypic and physiological behaviours even the effects to the next generations. However, apart from the phenotypic behaviours that scientists have worked out, it is ultimately the underlying genotypic study that reveals the hidden molecular information about the organism on an evolutionary sense (Thompson 2005).

Thanks to the combination of modern genetic tools and progressively study in evolutionary genetics, scientists can now go beyond the phenotypic level to reveal increasingly underlying genotypic facts. Mccluskey and colleagues (1993) studied the relationship between behavioural responses to temperature and genotype at a PGI locus (with two allele types: F & S) using P. laevis. They discovered that individuals with a PGI-S allele favour "cool" conditions and the PGI-F carrying individuals prefer a near-freezing (0 ℃) environment. Another experiment achieved by Hanski and Saccheri (2006) indicates that the differences in PGI variants can alter the flight stability and cold tolerability in Colias butterflies. Additionally, Hoffman (1981) conducted similar results from experiment about the aquatic poikilotherm Metridium senile. All of the above experiments have shown that among all abiotic factors, temperature is perhaps the most essential one affecting slaters' physiology, ecology and evolution. Nevertheless, not all experiments conducted are limited to slaters as the only target species. Neargarder and colleagues (2003) did an experiment to examine the relationship between variation in thermal tolerance and differences in PGI genotype in a montane leaf beetles (Chrysomela aeneicollis) collected from Sierra Nevada. Their results supported the experiment conducted from Colias that different PGI genotypes have different thermal tolerance (Neargarder et al., 2003). More specifically, for C. aeneicollis, 4-4 genotypes were less tolerant of chronic exposure to thermal extremes than 1-1 and 1-4 genotypes (Neargarder et al, 2003). Apart from the temperature influences on organisms' physiology and phenotypic behaviours, there are other factors that play considerable roles as well. Frati and colleagues (1991) examined the allozyme variation in metal-exposed natural populations of Orchesella cincta (O. cincta) in the Netherlands. A significant homogeneity was detected and 18 loci were monomorphic at all sites. Another experiment performed by Knigge and Kohler (1999) indicated that a metal-resistant P. scaber population did not evolve in the lead-contaminated site due to the inability to synthesize stress proteins in response to lead contamination.

CONCLUSION

Both the biotic and abiotic factors restraint the development of almost all organisms. Individuals either die out under intolerable natural selection or go against it to evolve a better survivability that will increase their fitness and more importantly, heritability. Many genetic studies are being conducted to reveal more and more underlying genetic information. But among all, it is the genotypic researches that draw most attention from scientists because they are the key to open the mystery micro-genetic world and lead us beyond unknown.

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