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The matrix of natural populations is thought to be dominated by demographic and environmental processes with intraspecific genetic variation and natural selection. Nowadays, molecular genetics has started to investigate population biology on detecting single gene or the combination of multiple genes' effects in natural populations by using genomic tools. Invertebrate is a well-studied group for genetics because it's high abundance and short fertilizing period. For example, some terrestrial isopod, common woodlice, Porcellio scaber, due to their biology with respect to their responses to different temperatures are quite easy to observe, become a good candidate organism in genetic analysis.
There were extensively studied done by using common woodlice. Castaneda, Lardies and Bozinovic investigated the effect of adaptive latitudinal shifts in the thermal physiology of a terrestrial isopod. In this study they examined how Porcellio laevis sampled from various locations responded to different temperatures. They hypothesised that thermal physiology is a crucial factor to the evolutionary success of organisms. By looking at variation in the slaters as the environment shifted, they examined varying latitudes. Samples from a wide variety of environments and locations were collected to minimize biases. They also predicted that the response of the slaters would match their habitats conditions. They collected their samples from Chile only to know more about the heat physiology of P. laevis related to the local temperature conditions. They discovered that recovery time from chill coma in P. laevis exhibits interpopulation variation along a geographic gradient, and low-latitude populations had a lower tolerance to cold compared with high latitude populations (Castaneda et al, 2004). Moreover there was a strong association between temperature required to rollover and increasing latitude (Castaneda et al, 2005). But there were some biases in this experiment, the fact that they acclimatised the slaters before testing may have an effect on their results. The limited sample size (4 populations) examined may also restrict the effectiveness of the results. Moreover, other variables such as food availability or seasonality may also responsible for the differences in cold tolerancability among populations. Lavy et al. proved that starved P. laevis had lower cold tolerance than fed ones (Lavy et al, 1994). A decreased haemolymph osmolality was found in starved animals compared to fed ones. This is assumed to be caused by a combination of the consumption of carbohydrates out of the haemolymph and of protein reserves and the accumulation of body water (Lavy et al, 1994). Bacigalupe and Araya did an experiment to test whether maternal effects & maternal body size can affect offspring's energetic. To achieve that, fifty breeding pairs of Porcellio laevis were set up in the lab. Physiological performance, thermal tolerance and thermal sensitivity were measured in F1 adults. Maternal effects were estimated as: the direct influence of maternal body mass and the variation associated with mothers. Their results showed that maternal body size affects positively offspring long-term metabolism (Bacigalupe & Araya, 2007). Additionally there is an intricate set of relationships among traits and importantly, that offspring exhibited compensational strategies among metabolism, thermal sensitivity and thermal tolerance traits (Bacigalupe & Araya, 2007).
Hanski and Saccheri examined the effects of molecular variation on the growth rate of a butterfly population. In this study, they analysis the influence of molecular-level variation on population growth in a large data set from the well-studied metapopulation of the Glanville fritillary butterfly (Melitaea cinxia) in the Aland Islands in southwest Finland. They tried to find out which particular variants on the molecular level have a dramatic effect in terms of changing the lifecycle of the butterfly. They proved that the allelic composition of the glycolytic enzyme phosphoglucose isomerise (Pgi) has a significant effect on the growth of local populations (Hanski & Saccheri, 2006). Additionally, they concluded that the results on allelic variation in Pgi influencing flight metabolic performance, mating success, and oviposition rate (Hanski & Saccheri, 2006). The strength of the association between growth and PGI was supported by evidence from another six loci which resulted in discovery of no additional effects on population growth by the alleles at those loci (Hanski and Saccheri, 2006). However, despite the strength of their results, it is possible that it is another gene linked to PGI that is having this effect.
Neargarder, Dahlhoff and Rank studied how enzyme polymorphism in a montane leaf beetle (Chrysomela aeneicollis) populations along gradients in environmental temperature can affect the mechanism of local adaptation. They claimed that thermal tolerance describes an organism's ability to maintain functionality at extreme temperatures, and it represents a physiological character that may link allozyme variation with differences in performance and fitness. Their results suggested that over-wintered adults were the most heat-tolerant life stage. Larvae tended to be less tolerant of temperature extremes than adults (Neargarder et al, 2003). Additionally, their results suggested that PGI genotypes differ in tolerance of thermal extremes in beetle populations with 4-4 being the most effective in warmer climates due to being the most stable thermally and 1-1 being the best for subzero conditions (Neargarder et al, 2003). Moreover, there is a trade off between thermal tolerance and energy allocation which has resulted in persistence of this PGI polymorphism (Neargarder et al, 2003).
Many ectothermic insects, especially those found in the Arctic, are quite tolerant of cold in winter. A study done by Coulson & Birkemoe showed that some Arctic invertebrates can recover from long-term cold tolerance after 4 years at below -20°C! They collected soil samples from a High Arctic Cassiope heath site on Svalbard (West Spitsbergen) in midwinter 1995 and transferred directly to -22°C. They found that survival was unexpectedly high in Collembola (Hypogastrura tullbergi and Folsomia quadrioculata) and Nematoda, but there was also isolated survival in two species of mites (Diapterobates notatus and Ameronothrus lineatus. (Coulson and Birkemoe, 2000). They claimed that the unexpectedly extreme cold tolerance has implications for the range of polar habitats available for soil fauna to colonise and for migration between isolated regions (Coulson and Birkemoe, 2000).
All those studies above have been carried out well, with very little to criticize. The only problem may affect the validity of their results would be insufficient sample size they used.