| dc.description.abstract | Ongoing global change has made understanding the factors that affect adaptation
and survival of populations in the context of changing environments a central problem in
evolutionary biology. Special focus has been given to the probability of survival through
genetic adaptation to lethal environments; a process termed evolutionary rescue. Many
studies of this process, both theoretical and empirical, have been carried out over the last
two decades. As a result, we now understand how a number of factors may affect the
probability of population survival. However, two factors that are known to affect
evolutionary responses, mode of reproduction and interspecific interaction, have received
limited attention.
The main aim of my work was to investigate whether and how mode of
reproduction and negative interspecies interactions (competition) affect the probability of
evolutionary rescue. To achieve this goal, I set up a series of selection experiments, by
propagating populations of unicellular alga Chlamydomonas reinhardtii in various stressful
conditions, and monitored their survival and fitness. To investigate the effect of sex in these
experiments, I manipulated mode of reproduction, by constructing the experimental
populations allowed to reproduce either only sexually or asexually or both. To investigate
the effect of competition, I manipulated the presence of the competitor(s) in the
experimental populations, by cultivating them either in presence or absence of the
competitor.
I first tested the effect of rate of environmental deterioration and mode of
reproduction on extinction dynamics and evolutionary rescue of the experimental
populations. I found positive correlation between the rate of extinctions and the rate of
environmental deterioration. The experiment revealed an interaction between mode of
reproduction and the rate of deterioration, manifested through significantly reduced
extinction rate of sexual populations relative to asexual populations in environment
deteriorating at intermediate rate.
I then investigated the effect of sex and competition on the probability of
evolutionary rescue, by propagating the experimental populations in environment
deteriorating in a simple way (the change comprising a single abiotic factor) and complex
way (the change of both abiotic and biotic factors). I found the negative effect of
competition on the probability of evolutionary rescue, and beneficial effect of sex in both
types of environmental deterioration, reflected in higher number of rescued populations
relative to asexual group.
I then tested whether phylogenetic relatedness between a competitor and the focal
species and the extent of their ecological similarity affect the likelihood of evolutionary
rescue, by subjecting the experimental populations to the presence of 10 different
competitors, isolated from two different types of habitats, and each being positioned on a
different branch of the phylogenetic tree of Chlamydomonas genus. The probability of
evolutionary rescue was contingent on the identity of a competitor species, but the results
showed no significant effects of phylogenetic relatedness and ecological similarity.
Finally, I investigated which experimental factors could potentially select for the
long-term maintenance of sex, by subjecting the experimental populations to different
types of selective environments (directional and fluctuating change of abiotic factors, the
presence of the competitor) and monitoring the frequency of sex over the course of time.
No selective environment significantly increased the rate of sex in the experimental
populations. In contrast, I found reduction in frequency of sex in the populations subjected
to fluctuating environmental change.
My results demonstrate that both mode of reproduction and competition affect the
probability of evolutionary rescue, which is generally positively affected by sex and
negatively affected by competition. However, these general effects may be altered by other
factors, namely mode of environmental change and the identity of the competitor species. | en |
