Local adaptation

More formally, a population is said to be locally adapted if organisms in that population have evolved different phenotypes than other populations of the same species, and local phenotypes have higher fitness in their home environment compared to individuals that originate from other locations in the species range. This is sometimes called 'home site advantage'. A stricter definition of local adaptation requires 'reciprocal home site advantage', where for a pair of populations each out performs the other in its home site. This definition requires that local adaptation result in a fitness trade-off, such that adapting to one environment comes at the cost of poorer performance in a different environment. Before 2004, reciprocal transplants sometimes considered populations locally adapted if the population experienced its highest fitness in its home site vs the foreign site (i.e. compared the same population at multiple sites, vs. multiple populations at the same site). This definition of local adaptation has been largely abandoned after Kawecki and Ebert argued convincingly that populations could be adapted to poor-quality sites but still experience higher fitness if moved to a more benign site (right panel of figure). == Testing for local adaptation ==

Testing for local adaptation requires measuring the fitness of organisms from one population in both their local environment and in foreign environments. This is often done using transplant experiments. Using the stricter definition of reciprocal home site advantage, local adaptation is often tested via reciprocal transplant experiments. In reciprocal transplants, organisms from one population are transplanted into another population, and vice versa, and their fitness is measured (see figure). However, evidence for rapid local adaptation in mobile animals has been gathered through transplant experiments with Trinidadian guppies. == Frequency of local adaptation ==

Several meta-analyses have attempted to quantify how common local adaptation is, and generally reach similar conclusions. Roughly 75% of transplant experiments (mostly with plants) find that local populations outcompete foreign populations at a common site, but less than 50% find the reciprocal home site advantage that defines classic local adaptation. Exotic plants are locally adapted to their invasive range as often and as strongly as native plant are locally adapted, suggesting that local adaptation can evolve relatively rapidly. However, biologists likely test for local adaptation where they expect to find it. Thus these numbers likely reflect local adaptation between obviously differing sites, rather than the probability than any two randomly-selected populations within a species are locally adapted. == Drivers of local adaptation ==

Any component of the environment can drive local adaptation, as long as it affects fitness differently at different sites (creating divergent selection among sites), and does so consistently enough for populations to evolve in response. Seminal examples of local adaptation come from plants that adapted to different elevations or to tolerate heavy metals in soils. Interactions among species (e.g. herbivore-plant interactions) can also drive local adaptation, though do not seem to be as important as abiotic factors, at least for plants in temperate ecosystems. Many examples of local adaptation exist in host-parasite systems as well. For instance, a host may be resistant to a locally-abundant pathogen or parasite, but conspecific hosts from elsewhere where that pathogen is not abundant may have no evolved no such adaptation. == Effects of Gene Flow on Local Adaptation ==

Gene flow can completely prevent local adaptations in populations by increasing the amount of genetic material exchanged which can than lower the frequency of alleles associated with the specific local adaptation. However gene flow can also introduce beneficial alleles to a population, which increases the amount of genetic variation, therefore strengthening the likelihood of local adaptations. Gene flow is the transfer of genetic information from one population to another, mainly through migration of organisms or their genetic material. It is possible for genetic material such as pollen or spores that can travel via wind, water or being brought by an animal, to reach an isolated population. The level of gene flow impacts its effects on local adaptation, high gene flow tends to reduce local adaptation whereas low gene flow can increase local adaptation. A specific local adaptation of the P. biglumis is having a small number of offspring and putting more energy towards defenses against potential intruders, which would help prevent the parasitic wasp from entering the nest. Looking at different local populations with similar levels of gene flow is particularly important because the presence of local adaptations in some populations but not others could suggest factors other than gene flow and selective pressure from parasites are causing the differences. Further, regional populations with varying levels of gene flow allows us to get a better idea of how gene flow at the local population level within these regions contributes to local adaptations at the regional level. The Alps were chosen as the area for the wasp study because the elevation of the mountains separate regional and local populations; resulting in multiple local populations of both host and parasite at different elevations and regions. For example, wasps on the same mountain but at different elevations do interbred so gene flow is occurring between local populations. In addition, there are also more isolated regional populations of both host wasp and parasitic wasp on completely different mountains that do not interbreed with other regional populations. DNA microsatellites, a type of genetic marker, were used to study the differences between local populations, to compare to regional populations, in an attempt to see how gene flow was impacting their genetics. What's very important to note is that gene flow is taking place between wasp populations to the same degree; all local populations in the same region have the same amount of gene flow. Meaning that one host population does not have more exposure to different additional genetic material than another host population at a different elevation. The wasp study found that significant local adaptation only took place in different regional populations, rather than different local populations, for instance higher and lower elevation populations on the same side of the mountain did not have significant differences. But populations in different regions, on the other side of the mountain, a completely different mountain, did have significant differences. Results from the DNA microsatellites showed that the out of the regional wasp populations, the most isolated regional population was the most different from other regional populations. This evidence supports the idea that some level of isolation is needed in order for local adaptations to occur within populations, further supporting the idea that high levels of gene flow do not produce local adaptations. Experimental Evidence Fruit Flies Experimental data suggests limited gene flow will produce the most local adaptations and high gene flow will cause populations to hybridize. There was study done on fruit flies (Drosophila melanogaster) to see if adaptive potential was increased in populations that were previously isolated and then experienced different levels of gene flow, or complete hybridization between two populations of previously isolated fruit flies. Experiments introducing different levels of gene flow and complete hydration of D. melanogaster populations showed that limited gene flow (in comparison to high gene flow or full hybridization) was actually what produced the greatest number of beneficial alleles within the fruit fly population. == See also ==