Ecological-evolutionary feedback in evolved lineages of Pseudomonas fluorescens : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Environmental Microbiology at Massey University, Albany, New Zealand
Multicellularity cannot proceed to subsequent stages without the evolution of
collectives. Thus, it becomes essential to understand the evolution of cooperation before the
evolution of multicellularity. However, understanding the evolution of cooperation presents a
problem. This is because natural selection rewards selfish behavior; yet, in nature, cooperation
is apparently common. Here arises a conflict between individual and collective interest and thus
raises questions concerning the evolution of cooperative behavior. Assortment between
cooperating types has been identified as the underlying mechanism behind theories for the
evolution of cooperation. However, these theories assume the environment to be stable and fail
to acknowledge interactions are density and frequency dependent (two components of the
environment), capable of generating co-evolutionary interactions. In this regard the feedback
between ecology and evolution (eco-evo feedback) is of likely importance.
In a previous experiment, a rudimentary life cycle was established in model bacterial
populations where lineages were repeatedly cycled between a cellulose producing, group living
cooperator type, termed WS, and solitary, free living cheater types, termed SM. The results of
the experiment showed that collective level fitness increased in evolved lineages compared to
baseline lineages. I believe that the eco-evo feedback is likely to have occurred on the WS-SM
interactions and that this is responsible for the increased fitness of the evolved lineages.
The aim of this thesis was to identify the presence of the feedback in evolved lineages. I
compared the evolutionary dynamics of frequency dependent interactions between WS and SM,
and population dynamics due to density dependent factors on the interactions between WS and
SM. I also report the joint influence of evolutionary and population dynamic patterns via ecospace
diagrams of the ancestor and evolved lineages.
The results showed that the interactions between WS and SM are both frequency and
density dependent and the joint influence of the above two factors reveals the presence of an
eco-evo feedback. The nature of the feedback is suggested to be a reduced transition capacity of
SM to switch to WS in evolved lineages. The tendency of evolved SM to produce few WS
suggest a strategy on the part of the SM to save the metabolic cost of production of cellulose, by
WS, and to trade-off this cost with an increase in fitness of the evolved lineages.