Had a chat with professor of evolutionary ecology Tom Tregenza today about a grant that I am
struggling with writing about bacterial speciation. This is a topic that I am really excited about, perhaps because I am a natural history enthusiast more than a hardcore molecular microbiologist. I have written an ‘armchair’ opinion paper about the topic, arguing that speciation in bacteria is not mediated by the separation of populations by geographic barriers (‘allopatric speciation’), as is the case in the vast majority of animals. Although bacteria do show spatial population structure (‘populations that are distant are genetically more dissimilar than populations living closer by’), dispersal is not frequent enough for these differences to become very distinct. Put simply: there does not seem to be the bacterial equivalent of an Australian marsupial.
So what barriers prevent the exchange of genes between different ecological types, allowing them to take off on their own evolutionary trajectory towards something that might classify as a distinct species? High quality data are now accumulating in a variety of bacterial and archaeal systems that support the hypothesis that bacterial speciation is ‘sympatric’, with gene flow between incipient species being inhibited by ecological barriers. Whether it is sufficient for different ‘ecotypes’ to thrive in (slightly) different times or places in the environment is enough to prevent gene flow between them is not known. Additional mechanisms, such as genetic differences (e.g. restriction enzymes) or in the timing of competence (the state wherein bacteria take up DNA from the environment) may be important as well. ‘Microallopatry’ where two species occupying different spots in the same landscape is a relatively simple scenario, as a single genetic difference can determine a difference in niche preference and thereby also the probability of encounter and genetic exchange between types. If microallopatry is not enough (for instance because DNA of the different types will float freely between cells themselves occupying distinct spatial niches), additional barriers are required. However, this would necessitate a simultaneous change in at least one additional gene, making this scenario less likely to occur.
All well and good, but I actually started typing this post because I wanted to link to a really nice website highlighting the work of Tom and his colleagues (including my old pal Thor Veen) on field crickets: Wild Crickets! Using over hundred CCTV cameras in a Spanish meadow, researchers are able to track every movement of a population of crickets in their natural habitat. (They have recorded hundreds of thousands of hours of footage and so work is ongoing to have computers do (part of) the image analysis.) Tom etal argue that lab experiments are all nice and well, but are not a substitute for field observations. They give the example of a lab experiment where cricket males that sing more get more matings. However, in the field these males are perhaps more likely to be eaten by birds, turning any conclusions on their evolutionary success upside down.The same goes of course for the study of bacteria: for some types of research, you can get away with lifting a bug outside of its soil habitat and placing it in a flask of nutrient broth hundred times richer than it is likely to experience whilst shaking it 180 times per minute. To answer other questions, such as those related to speciation, it is crucial to operate in ‘natural’ parameter space. Anyway, the site highlights a number of cool insights into big evolutionary questions that the wild cricket system has given, check it out.
“Female 6C having a rare old time with Male C5”