Spatial isolation is known to enhance speciation – but researchers at the Ludwig-Maximilians-Universitaet (LMU) in Munich have now shown that the opposite is true as well, at least in yeast. New environmental variables can also develop within completely mixed populations.
The idea that speciation depends on choosing which variants best adapt to local environmental conditions is at the core of Charles Darwin’s theory of the origin of species – it is now known to be a central component of biological evolution, and thus of biological diversity. The geographical isolation of populations is often seen as a necessary condition for diverging environmental patterns and the eventual formation of new species. When groups of a particular species are separated by geographical barriers, the favorable mutations that appear in either of them can become locally fixed, as interbreeding between the two groups is prevented. Whether speciation can occur under conditions where gene flow between two groups is possible – such as genetic promiscuity is still possible – remains controversial. To solve this problem, LMU evolutionary biologist Jochen Wolf and his group in collaboration with Simone Immler (University of East Anglia, UK) used baker’s yeast as a model system to explore what happens experimentally at the degree of gene flow between genetically distinct individuals. The population increases gradually.
“The starting point for this project, which has now lasted for 6 years, was a single foundation cell, which gave rise to our indigenous population,” says Wolf. Then we tracked the accumulation of mutations within this group over several generations. Starting with the original ancestor, scientists first chose cells that either float in suspension at the top or sink to the bottom. In this way, they obtained two groups that were adapted to different ‘habitats’ – referred to simply as ‘up’ and ‘down’. Cellocan is associated with differences in cell morphology and their tendency to have multi-cell groups with each other.
After obtaining these genetically disparate populations, researchers set out to mix them in different proportions and monitor their subsequent development. “We first noticed what to expect according to the classical isolation model, when the upper and lower populations were completely separated from each other,” says Wolf. Under these conditions, populations that were “geographically” isolated continued to adapt to the demands of their own domains and rapidly diverged from one another, becoming distinctly distinct over time. For example, upper cells proliferate preferentially by asexual division of cells, and thus grow at a much higher rate than their lower counterparts. Due to the associated decrease in the frequency of mating, cells in the upper compartment also produced fewer sexual spores. “This finding confirms that the effects of selection do not remain constant over the life cycle of an organism. Rather, selection is associated with“ trade-offs. ”In other words, mutations that may be beneficial in one context may be harmful in another.
In the next step, Woolf and colleagues simulated the effects of migration between the two groups. They did this first by adding approximately 1% of the minority population to the dominant portion, then gradually increasing the proportion of the first generation in each subsequent generation until the two cohorts were completely mixed. Theoretical models suggest that shuffling should lead to homogeneity of the gene pool and thus should result in reduced diversity of the mixed population. This effect was actually observed at moderate levels of mixing. Although such mixtures continue to evolve and their members can increase their fitness relative to the ancestral population, the different variants can no longer be clearly distinguished within them.
“But to our surprise, when the populations mixed up completely over time, we found very noticeable differences in the phenotype,” says Wolf. “When the tap is turned on completely, so to speak, one suddenly discovers that the mixtures contain two different variants, specialist and specialist.” A specialist can live well in an upper or lower compartment. This does not apply to the specialist. But it is dividing at a faster rate than a generalist, so it can compensate for its lack of diversity. In Wolf’s view, the emergence of these two classes can be seen as the first step in the speciation process that takes place in the presence of maximum gene flow.
In addition to these phenotypic results, the team distinguished the complete genetic stock of the entire population. These genetic experiments show that adaptation to the upper and lower compartments in the absence of gene flow is accompanied by selection of genetic variants from among those already present in the parent population. In contrast, the emergence of specialized strains in the 50:50 mixture has been attributed to the newly acquired mutations. It is clear that such mutations are not of short supply: “The mutations we see in our replicates are completely independent. We rarely see the same mutation in different samples – yet the phenomenon has been repeatedly observed between generalists and specialists in perfectly mixed populations.”
These findings are of interest in the context of how the population responds to changes in the changing personality and distribution of outlets. “It has always been assumed that gene flow interruption is a prerequisite for adaptive divergence,” says Wolf. “But our study shows that even when populations are highly interconnected, diverse adaptations can nevertheless emerge, so that all available niches can be filled.”
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