"Frequency dependence limits
divergent evolution by favouring
rare immigrants over residents"
Bolnick DI, Stutz WE
"Frequency dependence limits divergent evolution by favouring rare immigrants over residents"
Divergent Evolution
Divergent Evolution

One species, two habitats

Different environmental challenges

Given enough separation and time, different adaptations will accumulate

Eventually speciation

Allopatry
Parapatry
Negative
Frequency-Dependent
Selection

Trait giving access to a resource

Common variant

Rare variant

Many Individuals → high competition

Few individuals →
low competition

High selective pressure

Low selective pressure

Negative frequency-dependent selection:
Rarity grants an advantage

Background

Divergent selection has been studied between populations

Frequency-dependent selection has been studied within populations

Background

Divergent selection has been studied between populations

Frequency-dependent selection has been studied within populations

Question

What happens when they act simultaneously?

Do they oppose or enhance each other?

Methods

The study considered two parapatric
populations of Gasterosteus aculeatus,
over six weeks of experiment

https://phasegenomics.com/threespine-stickleback-genome-upgraded-proximo-hi-c/

To study
migration effects,
the animals
were split in
cages in a precise
ratio

Tested parameters

~ Amount of parasites per host in relation to their MHCIIb
~ Relative size of the individuals
~ Sex-biased mortality

Results
Overall survival rate
Rare MHC variants fought parasites better

Migrants should be less adapted against local parasites,
and indeed they have more parasites.
But locally rare variants had better survival rates!

A rare size meant a better survival rate

"Bigger animals eat bigger prey"
Both bigger and smaller fared better.
Rarity, not high mass, correlated with survival!

No sex-biased mortality was observed

Also no predator-driven mortality was observed,
as the cages kept them away

This is evidence that
frequency-dependent
selection favors rare
migrants over locals!

https://www.warrenphotographic.co.uk

Migration
and frequency-dependent selection
seem to reduce gene variability
between populations

Migration
and frequency-dependent selection
seem to reduce gene variability
between populations

But there's more at play...

... as factoring in also directional selection gives interesting results
Discussion

1) Definining species as a collection of individuals able to have fertile prole

2) Does selection constrain or enhance evolution?

Bibliography

THANK YOU!

Bolnick DI, Stutz WE. Frequency dependence limits divergent evolution by favouring rare immigrants over residents. Nature. 2017 Jun 8;546(7657):285-288. doi:10.1038/nature22351

Ingram T, Stutz WE, Bolnick D. Does Intraspecific Size Variation in a Predator Affect Its Diet Diversity and Top-Down Control of Prey? PLoS ONE 6(6):e20782 doi:10.1371/journal.pone.0020782