Consequences of within population variation have recently attracted an increased interest in evolutionary ecology research. Theoretical models suggest important population-level consequences, but many of these predictions still remain to be tested. These issues are important for a deepened understanding of population performances and persistence, especially in a world characterized by rapid fragmentation of natural habitats and other environmental changes.

I review theoretical models of consequences from intra population genetic and phenotypic variation. I find that more variable populations are predicted to be characterized by broader resource use, reduced intraspecific competition, reduced vulnerability to environmental changes, more stable population dynamics, higher invasive potential, enhanced colonization and establishment success, larger distribution ranges, higher evolvability, higher productivity, faster population growth rate, decreased extinction risk, and higher speciation rate, compared with less variable populations.

To test some of these predictions I performed experiments and compared how different degree of colour polymorphism influences predation risk and establishment success in small groups. My comparisons of predation risk in mono- and polymorphic artificial prey populations showed that the risk of being eaten by birds does not only depend on the coloration of the individual prey item itself, but also on the coloration of the other members of the group. Two experiments on establishment success in small founder groups of

Tetrix subulata pygmy grasshoppers with different degree of colour morph diversity show that establishment success increases with higher degree of diversity, both under controlled conditions in outdoor enclosures and in the wild. These findings may be important for re-stocking of declining populations or re-introductions of locally extinct populations in conservation biology projects.

I report on remarkably rapid evolutionary shifts in colour morph frequencies in response to the changed environmental conditions in replicated natural populations of pygmy grasshoppers in fire ravaged areas. This finding

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illustrates the high adaptive potential in a polymorphic species, and indicates the importance of preserved within-species diversity for evolutionary rescue.

Finally, I review if theoretical predictions are supported by other published empirical tests and find strong support for the predictions that more variable groups benefit from reduced vulnerability to environmental changes, reduced population fluctuations and extinction risk, larger distribution ranges, and higher colonization or establishment success.

In conclusion, my thesis illustrates how within-population variation influences ecological and evolutionary performances of populations both in the short and long term. As such, it emphasizes the need for conservation of biodiversity also within populations.

According to theory, genetic diversity can be maintained by environmental variation and the degree of genetic and phenotypic polymorphism may enhance the ability of populations to endure stress imposed by changing environments. In my thesis I used colour polymorphic pygmy grasshoppers (Tetrix subulata) as a model system to explore how environmental variation influenced genetic diversity. I compared population colour morph frequencies between populations in burnt and non-burnt areas and performed experiments to investigate to what extent colour patterns in these insects are determined by genes and influenced by phenotypic plasticity in response to environmental effects experienced during development. My results showed that the frequency of black individuals on average was much higher in recently fire ravaged areas, a condition known as fire melanism. The highest proportion of black individuals was reached within the first year after a fire. After the initial increase, the proportion of black individuals declined again and the distribution among alternative colour morphs became more even. Data for individuals raised in captivity revealed a high correspondence between maternal and offspring colour patterns, indicating a strong genetic influence on colour. Additional experiments demonstrated that the development of colour patterns in pygmy grasshoppers was not influenced by burnt material or high population densities, two environmental cues associated with post fire environments.

To test if reduced competition among alternative colour morphs may contribute to the maintenance of colour pattern polymorphism in these insects I examined if average survival was higher in diverse compared to homogeneous groups of individuals. I found that survival increased with colour pattern diversity, presumably due to reduced competition among alternative colour morphs. Relaxation of competition may explain why the distribution among alternative colour patterns changed and became more even after the initial evolution of fire melanism. My results demonstrate that environmental change may cause extremely rapid and reversible evolution, indicate that fluctuating selection may preserve genetic variation and support the notion that polymorphism may increase average individual success and enable populations to withstand environmental change.