Table of Contents
Mutation and recombination are the two main sources of the genetic variation on which natural selection and other evolutionary factors can act. In the context of the Synthetic Theory of Evolution, they explain where new hereditary differences come from and how they are combined into new genotypes.
Mutation: Creation of New Genetic Variants
What biologists mean by “mutation”
In evolutionary biology, “mutation” usually refers to any heritable change in the genetic material (DNA, or RNA in some viruses) that can be passed to the next cell generation or to offspring. It does not include short‑term, non‑heritable changes of the phenotype (these belong in “Modification”).
Key points:
- Mutations arise spontaneously (e.g., replication errors) or are induced by environmental influences.
- Most mutations are rare events, but in large populations and over many generations they accumulate continuously.
- Mutations are random with respect to their effects on fitness: they do not occur because an organism “needs” them.
The detailed biochemical causes and types of mutations (such as point mutations, chromosomal mutations, etc.) are dealt with in Genetics; here the focus is their evolutionary role.
Mutation as a source of new alleles
At the level of populations, the unit that matters is often the allele—a particular version of a gene.
- A mutation can create a new allele from an existing one.
- Once present, the new allele can:
- Increase in frequency (if advantageous, or by chance),
- Decrease in frequency (if disadvantageous, or by chance),
- Be maintained in the population (e.g., by balance between selection and mutation, or by drift).
If the mutation rate for a particular locus (gene location) is $u$ per generation, then in a large population each generation a fraction $u$ of alleles at this locus will be converted into new variants. Even if $u$ is very small (e.g., $10^{-6}$), over thousands of generations this continuously supplies new variation.
Distribution of fitness effects
Mutations differ strongly in their consequences for survival and reproduction (“fitness”):
- Deleterious mutations: reduce fitness. These are the majority; severe ones are quickly removed by selection.
- Neutral mutations: have no detectable effect on fitness in a given environment. They can accumulate by genetic drift and are important in molecular evolution.
- Beneficial mutations: improve fitness in a particular environment. These are rare, but essential for adaptive evolution.
For the Synthetic Theory, this leads to two key ideas:
- Mutation–selection balance
Even if selection removes harmful alleles, mutation keeps reintroducing them. For a recessive deleterious allele with mutation rate $u$ and selection coefficient $s$, the equilibrium frequency $q$ (very approximately) satisfies:
$$ q \approx \sqrt{\frac{u}{s}} $$
This shows how continuous mutation can maintain low frequencies of harmful alleles in a population. - Mutation as the ultimate source of adaptation
All adaptive traits must ultimately trace back to mutations that once introduced the underlying genetic changes, even if those changes were later reshaped and combined by recombination and selection.
Mutation rate and evolutionary tempo
The rate of mutation is crucial for how fast evolution can proceed:
- Too low: very little new variation; adaptation may be slow and constrained.
- Very high: many deleterious mutations overwhelm selection, which can reduce mean fitness (mutational load) or even threaten population survival.
Different species and genomes show different typical mutation rates. For example:
- RNA viruses: very high mutation rates, rapid evolutionary change.
- Large multicellular eukaryotes: lower mutation rates per base per generation, but large genomes and many individuals mean that overall many new mutations still occur every generation.
In the Synthetic Theory, mutation rate is often treated as a parameter that, together with selection, drift, and recombination, shapes genetic change over time.
Recombination: New Combinations of Existing Variation
What recombination is (in evolutionary terms)
“Recombination” broadly means processes that rearrange existing genetic variation, creating new combinations of alleles. It does not usually create new alleles itself, but shuffles those already present.
Important forms of recombination for evolution are:
- Crossing-over during meiosis in sexually reproducing eukaryotes.
- Independent assortment of chromosomes in meiosis.
- Genetic exchange in prokaryotes (e.g., conjugation, transformation, transduction), which also reshuffles DNA.
Details of the cellular mechanisms are discussed elsewhere; here, the focus is again on evolutionary consequences.
Recombination and genetic linkage
Genes that are located close to each other on a chromosome are linked; they tend to be inherited together. Recombination breaks this linkage:
- High recombination between two loci $\Rightarrow$ they are shuffled more independently.
- Low recombination (tight linkage) $\Rightarrow$ alleles at these loci tend to travel together as a haplotype.
From an evolutionary viewpoint:
- Recombination can create new haplotypes by re‑mixing alleles at linked loci.
- It reduces “linkage disequilibrium” (non-random association of alleles at different loci), unless new disequilibrium is created by selection or other processes.
Generation of genotypic diversity
Even without new mutations, recombination can produce many different genotypes in a population:
- Consider two loci with alleles $A/a$ and $B/b$.
- Without recombination, haplotypes AB and ab may dominate.
- With recombination, new combinations Ab and aB can appear and spread.
As more loci are involved, the number of possible combinations grows enormously. This is especially impactful for quantitative traits influenced by many genes, where recombination continually reshuffles polygenic variation into new phenotypes on which selection can act.
Evolutionary advantages and costs of recombination
Recombination has both benefits and drawbacks from an evolutionary perspective.
Potential benefits:
- Bringing together beneficial alleles
Two beneficial mutations that arise in different individuals (e.g., $A$ and $B$) can be combined into the same genotype (AB) by recombination. This can speed up adaptation in large populations. - Breaking up harmful combinations
Recombination can separate beneficial alleles from deleterious backgrounds. For example, if a favorable mutation arises on a chromosome carrying several harmful alleles, recombination allows that favorable allele to escape and spread more efficiently. - Increased diversity in changing environments
If the environment changes frequently, recombination helps generate diverse genotypes, some of which may be better suited to new conditions.
Potential costs:
- Breaking up successful gene combinations
Recombination can also destroy favorable combinations that selection has built up. - Exposure of hidden deleterious alleles
By reshuffling genomes, recombination can expose harmful recessive alleles in homozygous combinations.
The Synthetic Theory recognizes that the balance of these effects can influence the evolution of mating systems (e.g., sexual vs. asexual reproduction, selfing vs. outcrossing) and patterns of recombination rates across genomes.
Recombination and genetic hitchhiking
When selection strongly favors a beneficial allele at one locus, linked alleles at nearby loci can “hitchhike” along:
- With low recombination: large regions of the chromosome can ride along with the beneficial allele, reducing variation around it.
- With higher recombination: the hitchhiking region is narrower because recombination breaks up the association.
Thus recombination shapes the genomic “footprints” of selection and influences how widespread the side effects of strong selection are.
Interaction of Mutation and Recombination in Evolution
Complementary roles
In the Synthetic Theory of Evolution, mutation and recombination are seen as complementary:
- Mutation introduces new alleles (novel sequence changes).
- Recombination creates new combinations of these alleles.
Taken together, they determine the supply of genetic variation:
- In predominantly asexual populations, evolution depends very directly on the sequence and effects of single mutations (“clonal interference” when multiple beneficial mutations compete).
- In sexual populations, beneficial mutations that arise in different individuals can be combined by recombination, potentially speeding adaptation and reducing interference.
Standing variation vs. new mutation
Adaptation can proceed from:
- Standing genetic variation: alleles already present in the population, rearranged and redistributed by recombination when the environment changes.
- New mutation: alleles that arise after environmental change.
Recombination is particularly important for making full use of standing variation; mutation is crucial when completely new solutions are required or previous variation is insufficient.
Role in speciation and divergence
Differences in mutation and recombination patterns can contribute to the formation of new species:
- Local mutational input: populations in different environments accumulate different mutations over time, which are then redistributed by recombination within each population.
- Changes in recombination rate or pattern: if recombination between diverging populations is reduced (e.g., by chromosomal rearrangements or behavioral barriers), beneficial gene combinations adapted to local conditions are preserved and can diverge further.
Summary
- Mutation and recombination are the central generators of genetic variation in the Synthetic Theory of Evolution.
- Mutation produces new alleles; recombination reshuffles them into new genotypes.
- Most mutations are neutral or deleterious, but rare beneficial mutations, especially when combined by recombination, underpin adaptive evolution.
- The rates and patterns of mutation and recombination strongly influence evolutionary tempo, genetic diversity, and the genomic signatures of selection and speciation.