In population genetics, fitness measures the reproductive success of a genotype or phenotype relative to others in the population.
Fitness determines which alleles are more likely to persist through generations and can include multiple components, such as survival, mating success, fertility, or any combination affecting contribution to the next generation.
Importantly, fitness depends on both the organisms’ biology and the environment.
Fitness landscapes
A populations’ fitness can be viewed as a dynamic landscape, you may have already come across fitness landscapes, first proposed by Sewall Wright in 1932.
In a fitness landscape genotypes correspond to points in sequence space, and their fitness determines the height of the landscape.
The landscape itself can shift, where the same genotype can be beneficial in one environment but harmful or neutral in another.
We could imagine, for example, that being a great swimmer has little to no reproductive benefit in the event of a sudden and wide spread drought.
Sequence space
Sequence space is the set of all possible genetic sequences, in other words, every change that could theoretically occur in a genome.
Not all changes are equally likely or biologically viable. Evolutionary forces determine which paths through sequence space are explored.
Constrained vs Unconstrained forces
Evolutionary forces can be broadly divided into deterministic forces, (which are guided by fitness and the environment) and stochastic forces (governed by randomness).
In this post we’ll think of these as constrained and unconstrained.
Unconstrained forces
Unconstrained forces are not guided by fitness, they allow populations to explore sequence space randomly:
Genetic drift: Where chance alone can alter allele frequencies in a population (see the previous post for a recap).
Mutation: Introduces novel genotypes.
Recombination: Creates novel combinations of alleles.
These random explorations of sequence space can lead to advantageous traits, depending on the environment.
Constrained forces
Under constrained forces, populations have less freedom to roam through sequence space. They are imposed by fitness and environmental pressures, guiding populations towards genotypes more likely to survive and reproduce.
Examples include:
Purifying selection: Also called negative selection, where deleterious alleles are removed from the population. It is evident in highly conserved genes or gene regions where changes are most harmful, such as conserved structural regions of functional proteins.
Balancing selection: Where diversity is maintained at a particular loci, for example at a host parasite-interface, where multiple alleles might be favoured to resist diverse parasite populations. Balancing selection can also include heterozygote advantage, where individuals with two different alleles have higher fitness than either homozygote at the same locus. Preserving variation keeps multiple peaks accessible in sequence space.
Episodic selection: Short-term shifts in selective pressures. This could occur during a sudden but temporary environmental change that pushes populations towards certain genotypes.
Background selection: Removes linked deleterious alleles and can shape diversity in other areas of the genome that aren’t under direct selection. As nearby neutral or slightly beneficial alleles are physically linked on a chromosome they are removed, reducing the size of sequence space.
Together, constrained and unconstrained evolutionary forces influence populations through time.
The interplay of free exploration and constraint ultimately determines the evolutionary trajectories of populations, shaping genetic variation over time.