Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Individuals within a single population can vary widely in their phenotype e.g. in their body shape. These differences are an important source of biodiversity and they can precede evolutionary divergence within a population. In this thesis we use the biological invasion of the zebra mussels into Swedish lakes to investigate which processes create or maintain phenotypic diversity within populations of the two native fish species perch and roach and the mussel itself. Both fishes have specially adapted body shapes that depend on whether they feed in the near-shore or open-water habitat of lakes. This habitat-specific divergence was more pronounced in lakes with zebra mussels, probably because resources in both habitats were in higher supply due to the mussels’ effects on the lakes. Divergence in perch body shapes between habitats was also higher in lakes with a higher water clarity, suggesting that visual conditions can affect the resource use and thus also the expression of a habitat-specific body shape. When investigating the diversity of body shapes in the mussel itself we found that mussels from one lake changed their shell shape when exposed to different predators: fish predators induced a more elongated shell shape while crayfish predators induced a rounder shell. These specific shell shapes probably serve as two alternative predator defenses protecting the mussel from predation. We conclude that the availability and use of distinct resources is an important source of diversity within populations. Abiotic conditions can play a previously underappreciated role by promoting or impairing the use of the distinct resources thus affecting the divergence. The diversity of shell shapes we found in the zebra mussels complements our study by demonstrating that not only consumer responses to resources but also resources’ responses to predators can generate phenotypic diversity.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Atlantic horseshoe crabs (Limulus polyphemus) have endured decades of intense harvest pressure. Genetics studies have shown evidence of distinct sub-groups spanning the coast, although few fine-scale studies have been done to delineate these groups on a local level. Massachusetts lies directly between two of these sub-groups. With documented differences in prosomal widths of horseshoe crabs from either side of Cape Cod, it is possible that Cape Cod is a barrier to gene flow and that there are two distinct genetic groups within Massachusetts. Regulations currently consider all horseshoe crabs to be of one stock. I examined 6 microsatellite loci from 193 horseshoe crabs collected from 7 locations across Massachusetts between 5 May and 24 June 2010. I also analyzed the prosomal widths of 324 horseshoe crabs from 8 locations across Massachusetts. Data analysis revealed low divergence with a G′ST of 0.005 (95% CI −0.004–0.013) and a G″ST of 0.015 (95% CI −0.014–0.045). Wellfleet Bay showed evidence of divergence from all other sites except Buzzards Bay. Isolation by distance is apparent via the Atlantic Ocean. Phenotypic variation in the prosomal widths of horseshoe crabs shows greater divergence among sites than neutral markers and indicates the presence of additive genetic effects. Low divergence and high heterozygosity indicate that although documented population declines have occurred, effective population size (Ne) is still large enough to maintain allele frequencies. With isolation by distance, divergence is likely to increase over time if populations remain low. Phenotypic divergence shows the possibility of local adaptation and that the implementation of management units (MUs) to the north and south of Cape Cod would be recommended as a conservative measure.