Kassen 4: Specialist, Generalists, and Diversity

The paradox of diversity

Natural selection should eliminate all but the fittest type

Yet species regularly coexist

The same is true for microbes, where a gram of soil can contain 10² - 10⁶ OTUs (operational taxonomic units)

Resolving the paradox of diversity

Environments are heterogeneous, therefore the conditions of growth vary from place to place

The environment is composed of many niches

Selection eliminates diversity in a niche, but not among niches

Distinct specialist genotypes are favoured in different niches

Niche breadth

The range of habitats or conditions within which a genotype can function

“Generalist” or “specialist” classifications reflect variation in breadth

• Generalists maintain fitness over a wider range of conditions - a single genotype works for a lot of things, leading to low variance

• V_E (generalist) < V_E (specialist)

Evolution of niche breadth

The fittest type is the one that grows the best across all environmental conditions encountered

Niche breadth should evolve to match the extent of environmental variation

• Specialists evolve in an unchanging, uniform environment

• Generalists evolve in variable environments

Divergent selection and specialists

Divergent selection can select for specialists

Divergent selection is the result of selection for distinct optimal phenotypes

Spatial variation in growth conditions across patches of the environment generates divergent selection

In the absence of gene flow, adaptation to each patch will occur without regard to fitness in another patch

Specialization and trade-offs

Genes that improve fitness in one patch are unlikely to improve fitness in another

The direct response to selection (improving fitness in the current patch) will override the indirect response (improving fitness in other patches)

Cost of adaptation

A special type of trade-off

Adaptation to one environment is accompanied by a loss of adaptation (relative to the ancestor) in another environment

Evolution of fitness trade-offs across environments

Trade-offs revealed by reciprocal transplant experiments

• i.e. benthic vs limnetic stickleback

Genetic sources of costs of adaptation

Antagonistic pleiotropy

Mutation accumulation

Antagonistic pleiotropy

Functional interference arising from gene product expressed in a different environment

Example of antagonistic pleiotropy

Cost of antibiotic resistance in nfxB

nfxB is a negative regulator of an efflux pump in Pseudomonas aeruginosa

LOF mutations to nfxB confer resistance to fluoroquinone antibiotics (like ciprofloxacin) because the pump is active

In the absence of drugs, having a constantly “on” pump is energetically costly, so fitness in permissive conditions is low

Mutation accumulation

Mutations neutral in the environment of selection are likely to be deleterious in alternative environments

Example of mutation accumulation

CO₂ exposure in green alga

Growth under high CO₂ did not improve following selection

But three lines evolved at high CO₂ went extinct at ambient CO₂

The evolution of generalists

Should take place in temporally, not spatially, variable environments

• Spatial variation leads to specialization

The geometric mean fitness principle

Fitness is multiplicative, not additive (with each new generation)

Therefore, the geometric mean is more representative than the additive mean

The Geometric Mean Fitness is highest if variation in fitness through time is low

It may be best to hedge one’s bets by decreasing fitness in some years in favour of maintaining fitness over all years

• “Jack of all trades, master of none”

Are generalists jacks of all trades?

Jacks-of-all trades evolve when the founder is very close to the fitness optima

Superior generalists evolve when the founder is very far away

Can diversity be maintained in a variable environment?

Yes, if distinct genotypes have high fitness in different environments

If w₁₁ > w₂₁ and w₂₂ > w₁₂, then selection is balancing or divergent, and different phenotypes are favoured in different environments

But the manner in which an environment varies (in time and space) also matters

Environmental variation in time

Cannot maintain diversity - one type or the other will win out

Spatial environmental variation

Local diversity is maintained, though global diversity is lost

Adaptive radiation in microbes

Pseudomonas fluorescens

Smooth, wrinkly, and fuzzy spreaders all develop within 4 days

Effect of ecological interactions on diversity

Competition and predation modulate diversification