Anthropogenic induced climate change has had a wide-range of effects on many diverse organisms. Most importantly, it has forced the evolution of species. A few of these effects include physiological changes affecting photosynthesis and respiration, altered phenology, and a shift in community composition (Hoffmann 2011). If the pressure is great enough, some of these changes can be rapid, occurring over a few generations. However, some organisms may not possess the ability to adapt to environmental changes fast enough and may go extinct. Therefore, evolution remains an important pathway for natural populations to adapt to environmental changes.



     Natural selection is largely responsible for the documented evolution of species in the context of climate change. For natural selection to act variation must be present in the population and the associated trait(s) must be genetically based and heritable. When confronted with an environmental pressure, the individuals in the population with the highest fitness regarding the trait in question are more likely to survive and pass on the genes responsible for its success to the next generation. The resulting progeny are more adapted to the new environment than its ancestors would have been.



     A changing climate is an environmental pressure that organisms have encountered at one or more times in their evolution. There have been catastrophic environmental changes in the history of the Earth that have wiped out entire families of species. However, most changes in climate are small and occur over a long period of time, enough time for a species to evolve to adapt to the change. In contrast, the anthropogenic increase of atmospheric greenhouse gasses on Earth over the last 100 years has greatly accelerated the rate of mean Earth surface temperature change and has forced the rapid evolution of species and community composition.



    One must exercise caution when observing species change over time as an adaptive response to shifting environmental conditions. Similar to an evolutionary response, phenotypic plasticity can also explain the adaptive changes a species undergoes in a shifting environment. What differs is that evolution results in a genetic change over generations, while phenotypic plasticity is the ability to adjust one’s phenotype in response to the environment. Plasticity can evolve under natural selection although it is unlikely it will lead to any long-term solutions to constant directional change (Hoffmann 2011). Longitudinal studies can be completed to determine ‘evolved’ responses as opposed to ‘adaptive’ responses.



     There are limits on the phenotypic expression of a trait that may affect an organism’s ability to respond to environmental change. Some traits in organisms may be “stretched” far while others have little plastic response. The organisms whose traits reach their upper limits may evolve if the trait(s) is heritable. The rate of evolution will depend on the heritability of the trait(s). However, if the trait(s) is/are not heritable the organism may evolve another way, migrate, or face extinction. Additionally, if the organism does not have the heightened ability to respond plastically it will face the same challenges as mentioned above, but at an earlier time. Generation times can also affect how quickly a species responds to change. ­r-selected species are able to respond quicker to rapid changes in the environment, while K-selected species may evolve slower and could therefore face greater evolutionary pressures.



     A decline in body size in birds (and mammals) has been widely documented as a response to climate change. The mechanism underlying a change in body size in relation to temperature is Bergmann’s Rule, which states body size increases directly with latitude in endotherms. Larger bodied endotherms are found at higher latitudes while smaller bodied species occur at lower latitudes. This distribution is the result of a surface area and volume relationship. As body size increases, surface area increases at a slower rate than it would for smaller bodied species. In effect, larger bodies are more efficient insulators, while smaller bodies are less so.



     Changes in body size could also be influenced by resource availability. Within populations, the most recognized mechanism for a decrease in body size is changes in nutrition (Gardner et al. 2009). A more fragmented habitat leads to less food availability, which could drive the phenotypic response of a smaller body size to cope with the decline in resources. It is recognized that both Bergmann’s Rule and changes in nutrition can force a plastic response in an organism in a few generations.



     Discriminating between a genetic based response and a plastic response has proved difficult. The genetic basis of morphological traits is mostly unknown making direct evidence hard to obtain (Gardner et al. 2009). Additionally, genetic responses may be too slow to detect. Therefore, alternative approaches must be initiated to detect evolutionary adaptation due to climate change.
          
Case Study



     An example of non-genetic approaches to conclude evolution as an explanation for longitudinal changes in trait patterns across gradients includes a study by Gardner et al. (2009). The authors found a shifting latitudinal cline for body size in eight species of passerine birds in southeastern Australia as a result of anthropogenically forced climate change.



     Eight insectivorous bird species were analyzed using four different methods- 1) temporal changes in body size over the past century 2) the presence of latitudinal clines in body size 3) temporal changes in clines as a predicted response to warming 4) ptilochronology to test a non-genetic mechanism in body size: a decline in body size as a result of changes in nutrition.



     After measuring 517 museum skins, a temporal decline in body size was found. This decline occurred over the last 100 years and its magnitude ranged from 1.8 to 3.6 per cent of wing length (an index of body size). Out of the eight species analyzed, four showed a significant reduction in wing length and two species showed a non-significant trend in the same direction.



     The ptilochronology analysis indicated that decline in body size was not due to a change in nutrition associated with a degraded environment.



     Four of the eight species displayed clinal variation in body size associated with latitude, as predicted by Bergmann’s Rule. Because a decline in body size was found, the authors propose that latitudinal clines have shifted. The southern populations of these four bird species are similar to pre-1950 northern populations. The magnitude of this shift is equivalent to a 7-degree change in latitude.



     The presence of geographical clines are a result of natural selection. These distributions demonstrate that species are best adapted to their current environment. Based on the shift seen in bird body sizes as concluded by this study, we can interpret the results as adaptation through evolution. The universal response suggests a large-scale factor, such as temperature, is responsible for the decline in body size, while nutritional changes would result in more variable responses.



     Umina et al. (2005) found a similar shift in the clinal distribution of allele frequencies in relation to temperature in Drosophila melanogaster in southeastern Australia. Southern populations have an allele frequency similar to what the more northern populations had 20 years ago, which corresponds to a 4-7 degree shift in latitude. In addition, the rapid adaptive evolution of bill morphology in Geospiza fortis as a result of drought has been demonstrated. These cases demonstrate selection due to environmental change and of the speed that evolution may occur.



     The shift in bird body-size case study provides an example of how climate change can force the evolution of species across latitudinal gradients. As warming progresses, more species distributions will be changed. As higher latitudes warm, new niches will open up and many populations will move northward. Populations that are not able to adapt or migrate will shrink and/or go extinct. Moreover, a decrease in body size is expected with current species distributions or as species move poleward. Phenotypic plasticity can allow some response to environmental change, but it is not a long-term adaptive solution to rising temperatures. As climate change progresses, we can expect to see more evolved responses to environmental change in a broader range of species.