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During the past century, about 82% of meadows and pastures have been lost in Sweden, with negative consequences for their flora and fauna. An example is the scarce heath butterfly, which is one of the red-listed species threatened by such habitat loss through afforestation. The main aim of this thesis was to study how the scarce heath may be affected by the ongoing loss of habitat and fragmentation. Another aim was to study how the globally peripheral and isolated position of the Swedish populations contributed to the genetic variability and differentiation of the species in Northern Europe. Mark-release-recapture techniques, habitat surveys, breeding experiments and molecular genetic analyses were used to address these questions.

Genetic variability was lower in the Swedish populations than populations in more central parts of its distribution. At a local scale, the occurrence and local abundance of the scarce heath were correlated with patch size and isolation, and the abundance of certain plant species. Populations within a six km² area showed significant genetic differentiation, and small and isolated populations expressed signs of inbreeding depression. Mark-release-recapture studies indicated that mobility between patches was restricted, and the species seemed to disperse primarily in a stepping-stone like fashion. However, the genetic differentiation did not follow an isolation-by-distance pattern. This discrepancy may be due to the relatively recent changes of the landscape. If the ongoing loss of habitat is not stopped and reversed, there is risk of a chain reaction of local extinctions, which may entail collapse of the whole metapopulation.

Multiple mating has been suggested to benefit social insect queens because high genetic variation within colonies might decrease the load imposed by sterile diploid males, enhance resistance to parasites and pathogens, and lead to a more effective division of labour and/or a wider range of tolerable environmental conditions. We tested these hypotheses in the ant Lasius niger with three population samples from Switzerland and Sweden. We found no diploid males in young or mature colonies suggesting a lack of diploid male load. Colonies with multiply-mated queens were not larger nor did they produce more sexuals than colonies with singly-mated queens. We did find a significantly lower frequency of multiple mating among newly mated queens than among the queens heading mature colonies in one population sample (Switzerland 1997). However, this result was not repeated in the other study population, or in the following year in the Swiss population.

Small populations are facing a particular risk of extinction due to a lack of appropriate genetic diversity and associated negative effects, factors dealt with in the discipline of conservation genetics. Many orchid species exhibit characteristics that make them a perfect study object in the scope of conservation genetics. The aim with this thesis was to investigate genetic structure at different levels in two orchid species Gymnadenia conopsea, geographically widespread, although diminishing and G. odoratissima with a long history of being rare. Microsatellite markers, developed in and used in studies of G. conopsea were also used in the study of G. odoratissima.

Populations of G. conopsea expressed high levels of genetic variation and a certain amount of gene flow, although investigated mating pattern in a small population indicated non-random mating among individuals, with the majority of pollen exchange between near neighbours, and noticeable levels of geitonogamous pollinations. Further a pronounced year to year variation in flowering frequency among individuals was found.

It was also discovered that flowering time variants (early and late) within the species G. conopsea were highly differentiated and seem to have had a more ancient historical separation than the separation between the two different species, G. conopsea and G. odoratissima.

Levels of genetic variation in the rare congener, G. odoratissima differed between island and mainland populations where the more numerous island populations expressed larger levels of genetic variation and were less differentiated compared to the few remaining and genetically depauperated mainland populations.

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The social organization of social insect colonies can be broadly defined as monogynous, single reproductive female or polygynous, several reproductive females. Monogynous organization is genetically simple while polygynous is more complex and the two social forms differ commonly in mating and dispersal behaviour as well as in colony foundation. The aim of the thesis was to study how social organization affects spatial genetic structures. Formica wood ants are suitable study organisms as both intra- and interspecific variation in social organization can occur.

A set of microsatellite loci were characterized from a small insert library in the ant Formica exsecta. The microsatellite loci cross amplified among other Formica species. In a population of F. exsecta containing both monogynous and polygynous colonies, genetic differentiation was stronger in the polygynous form at both nuclear and mitochondrial DNA (mtDNA) markers. The data further suggests that female dispersal is restricted in the polygynous form.

Female gene flow between the social forms appears to be absent in the ant F. truncorum as the social forms contained non overlapping mtDNA haplotypes. Strong nuclear genetic differentiation and profound allele frequency differences further suggest that male interform gene flow is weak. The social forms evolve largely independently when found in sympatry though the forms do not represent separate evolutionary lineages within the species.

Spatial intraspecific genetic differentiation was similar in the species F. polyctena and F. rufa though quantitative differences in the level of polygyny existed. In polygynous populations of the ant F. lugubris, female gene flow was restricted but male gene flow was strong and homogenized nuclear allele frequencies.

The great difference in the level of chloroplast variation and its geographic structure among the three main species studied here demonstrates that forest species do not form a homogeneous group. Hazel shows a genetic structure similar to many other thermophilous species and this structure, in combination with fossil evidence, indicates that the post-glacial colonisation of most of Europe originated in a refugium in western France while the Balkan and Italy were colonised from a south-eastern refugium.

In sallow and silver birch the chloroplast DNA variation and its structure does not fit with a scenario of glacial restriction to southern refugia and survival at intermediate latitudes is suggested for both species. The chloroplast DNA variation in silver birch suggests the presence of one western and one eastern European post-glacial colonisation route and limited contribution of southern populations in the colonisation of the rest of Europe. Unique haplotypes by the Ural Mountains indicates the possibility of a separate glacial origin of these populations.

The study of chloroplast DNA in species closely related to sallow and silver birch indicate that extensive hybridisation and cytoplasmic gene flow occurs within both the Salix and Betula genera in Europe. The nuclear and chloroplast phylogenies of 14 Betula species were not in complete agreement with each other or with the classical division of the Betula genus into subgenera or sections. The phylogenetic structure implies that hybridisation has played a role in the evolution of the Betula genus.

This thesis focuses on the chloroplast DNA variation in three forest tree genera: Corylus, Betula and Salix. Chloroplast PCR-RFLP is used to evaluate the post-glacial history of hazel, Corylus avellana, silver birch, Betula pendula and sallow, Salix caprea and to explore the possibility of introgression in the Salix and Betula genera. In addition, the chloroplast matK gene, its flanking regions and the nuclear ADH gene were used to study the phylogenetic relationships within the Betula genus.