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Discussions of queen dispersal and adoption of new queens by own versus alien colonies in Formica rufa played an important role in the early theoretical studies of social evolution (Sturtevant 1938; Williams and Williams 1957). This was inspired by polygyny affecting the genetic relationships among nestmates, which are estimated today by use of genetic markers (Box 3.1). Demography and population dynamics also shape the whole genetic landscape of ant populations, and genetic studies can be used to trace such events. Finally, even long-term processes leave their genetic signatures, and the genome-wide variation patterns support the hypothesis that ants tend to have small effective population sizes and increased genetic loads, resembling vertebrates more than other invertebrates in this respect (Romiguier et al. 2014). Even though this conclusion is general and based only on very few species, it emphasises biological characteristics important for ant populations also in the context of conservation.
Genetic markers are used to estimate the level and distribution of genetic variation in populations and societies. Optimal genetic markers are: (1) not influenced by environment or developmental stages, (2) randomly distributed across the genome, (3) codominant and (4) selectively neutral, but none of the markers in use fulfil all these requirements (Lowe et al. 2004). Genetic markers are used to resolve phylogenetic relationships at different hierarchical levels: among species, among conspecific populations at large geographic scales, among potentially interconnected conspecific populations and among individuals (Avise 2004). In social insects, studies at the first two levels are not different from other organisms: spatial distribution of genetic variability is first assessed and then interpreted based on geological and climatological history. On the other hand, analyses of colony kin structure and spatial population structure are, at least to some extent, idiosyncratic to social insects and need a more detailed introduction (Box 3.2).
In the 1960s, the first molecular polymorphisms employed as genetic markers were enzymes, widely used because of low costs and the ease of use. Allozyme polymorphism is based on the variation in the net charge of enzymes involved in basic metabolism. As only a small fraction of mutations (9%) lead to a change in the net charge of the amino acid chain, allozymes tend to be only weakly polymorphic.
The accumulation of beneficial and harmful mutations in a genome is studied by using analytical methods as well as computer simulation for different modes of reproduction. The modes of reproduction examined are biparental (bisexual, hermaphroditic), uniparental (selfing, automictic, asexual) and mixed (partial selfing, mixture of hermaphroditism and parthenogenesis). It is shown that the rates of accumulation of both beneficial and harmful mutations with weak selection depend on the within-population variance of the number of mutant genes per genome. Analytical formulae for this variance are derived for neutral mutant genes for hermaphroditic, selfing and asexual populations; the neutral variance is largest in a selfing population and smallest in an asexual population. Directional selection reduces the population variance in most cases, whereas recombination partially restores the reduced variance. Therefore, biparental organisms accumulate beneficial mutations at the highest rate and harmful mutations at the lowest rate. Selfing organisms are intermediate between biparental and asexual organisms. Even a limited amount of outcrossing in largely selfing and parthenogenetic organisms markedly affects the accumulation rates. The accumulation of mutations is likely to affect the mean population fitness only in long-term evolution.
Spatial and temporal differentiation in Gerris lacustris and G. odontogaster (Heteroptera, Gerridae) were studied in restricted areas, where one population of each species was subdivided into several subpopulations. The aim of the study was to relate genetic population parameters to ecological population structure studied by mark-recapture methods in the same pond systems. Marked differentiation between the subpopulations and significant enzyme allele frequency changes between subsequent generations were found at several loci. It is suggested that non-selective forces are a sufficient explanation for the observed differences.
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