Adaptive radiation Evolution of individuality Evolutionary genetics Ecological genetics Infectious disease

Adaptive radiation

Team members: Dr. Gayle Ferguson
Collaborators: Dr. Bertus Beaumont, Dr. Rees Kassen, Dr. Bas Ibeling
Key publications: Rainey & Travisano 1998 Nature, Bailey et al 2013 Proc Roy Soc, Flohr et al 2013 PNAS

Our interest concerns the five evolutionary processes (selection, drift, mutation, recombination and migration) and their role in the evolution of diversity. In the past we have used simple populations of the plant-colonizing bacterium Pseudomonas fluorescens (isolate SBW25) to watch in real-time the evolutionary emergence of diversity. Several features of these experimental populations make them powerful tools for this kind of work. Firstly, ecological time and evolutionary time coincide (they typically don't: the generation time of most organisms makes it difficult (or even impossible) to observe evolution during the life time of an individual investigator). Secondly, there is a correspondence between newly emerged niche-specialist genotypes and the morphology of colonies on agar plates. This means that it is possible to track the evolutionary emergence of ecological diversity simply by scoring the frequencies of colony morphology variants on agar plates. The picture below (from Rainey & Travisano 1998) makes this clear.

adaptive radiation plate

The figure shows the diversity apparent (upon plating) that emerges (by mutation) in a broth-filled tube after 5 days incubation at 28 C wihtout shaking. The founding genotype was a single clone of the ancestral type. The diversity manifests as differences in colony morphology. Niche specialization is evident among the different morphs: the smooth (SM) ancestral type (LH colony) colonizes the broth phase; the wrinkly spreader (WS) type (middle) colonises the air-liquid interface; the fuzzy spreader (FS) morph (RH colony) colonizes the bottom of the vial. In essence, this is akin to a minature adaptive radiation: the rapid evolution of ecological diversity from a single ancestral type. The radiation is dependent upon ecological opportunity: no such diversification occurs if the tubes harbouring the bacteria are shaken (shaking destroys the ecological opportunity (vacant niche space) that exists in the static (spatially structured) vials.

The above description of the radiation gives the impression that the ancestral genotype is somehow programmed to deliver three niche-specialist genotypes, but this is a long way from the truth. The diversity that arises -- even during the course of a single week -- is quite staggering and it does so by spontaneous mutation. Even within one niche-specialist class, for example, the WS class, there are literally 100s of different genotypes and many different phenotypes. There are additional morphotypes other than just SM, WS and FS (cart-wheels, fried eggs, gnome hats, super spreaders, etc, etc). Some of the morphs are spectacular (as revealed in these super photos by Jenna Gallie).