Dr. Sean B. Carroll is an evolutionary developmental biologist and author who has written extensively on the role of genetic mutations and natural selection in evolution. His statement that “mutation is random” refers to the fact that mutations occur randomly and without direction during DNA replication and cell division.
Introduction
Mutations are changes to an organism’s DNA sequence that can lead to changes in physical traits and characteristics. They occur due to mistakes during DNA replication, exposure to radiation or chemicals, errors in recombination, and other random, unirected processes. The mutations that occur in an individual organism are chance events that are independent of selective pressures or the needs of the organism. This randomness is a key component of the evolutionary process.
When Dr. Carroll and other evolutionary biologists say mutation is random, they mean:
- Mutations are not directed toward any particular evolutionary goal or driven by adaptivity. Organisms do not mutate specific genes to try to adapt to their environment.
- The location and types of mutations that occur are unpredictable. Any gene can be mutated, and the types of mutations (e.g. base pair substitutions, indels, etc.) occur by chance.
- The rate of mutation is not adaptive. Mutation rates do not increase when organisms are under selective pressure.
- Most mutations that occur are neutral or deleterious, not beneficial. Beneficial mutations are rare random events.
However, the effects of mutations on an organism’s fitness are decidedly not random. Selection acts on the phenotypic effects of mutations, and populations evolve as certain random mutations are propagated by natural selection while deleterious mutations are removed. So while the process that generates mutations is random, the evolutionary consequences are not.
The randomness of mutation
Mutations arise from a variety of random processes and errors during DNA replication and cell division in organisms. Some examples include:
- DNA replication errors – Mistakes during DNA replication can lead to insertions, deletions, or base pair substitutions in the DNA sequence.
- Errors in recombination – Errors in chromosomal crossover during meiosis can mix up DNA sequences.
- Cosmic radiation – Ionizing radiation from cosmic rays and solar radiation can cause random mutations by damaging DNA.
- Environmental mutagens – Chemicals, toxins, and other mutagenic agents can interact with DNA and cause mutations.
- Replication of transposable elements – Transposons jumping around in the genome can disrupt DNA sequences.
These processes all generate mutations randomly in a molecular game of chance each time a cell divides and copied its DNA. There is no bias toward mutations that may be adaptive or even neutral. Most mutations that arise are deleterious, even lethal, because they disrupt functional parts of the genome. But a few rare random changes turn out to be beneficial and get propagated by selection.
Directed vs. random mutation
It is important to understand that mutation is a random process, even though some mutations can be adaptive. Evolution does not occur through organisms mutating specific genes intentionally in response to selection pressures. In fact, there are no known cellular mechanisms for generating directed, adaptive mutations in response to environmental conditions or selective pressures. (Though some scientists have hypothesized that mechanisms may exist in some instances.)
The directed mutation hypothesis was proposed in the 1980s and suggested that cells could somehow induce mutations specifically in genes where mutations would be beneficial. However, this hypothesis has largely been refuted through later research, and most examples of directed mutation have been found to be consistent with random mutation after all.
There are some documented cases of hypermutation in cells like those of the immune system. But hypermutation simply increases the overall random mutation rate; the mutations are still random.
Why mutation must be random for evolution
Random mutation is an essential component of the evolutionary process. If mutations were not random, but directed toward some end goal, then evolution would be a very different process. Some key reasons why random mutation is important:
- It introduces novel genetic variation into populations for selection to act upon.
- It allows neutral evolution and genetic drift, not just adaptive evolution.
- It enables potentially beneficial mutations to arise by chance.
- It allows accumulation of many small changes over time.
Without random mutations, organisms could only adapt by mutating exactly the genes needed to survive in new conditions. Complex adaptations could not accumulate gradually through many small steps. And neutral evolution would not occur.
Natural selection on random mutations
While mutations arise randomly, natural selection is a non-random process. Selection acts on the phenotypic effects of mutations, sorting random variants based on their impacts on fitness. Beneficial mutations that improve reproductive success tend to be propagated through populations, while deleterious mutations tend to be removed.
Populations evolve through the interplay between these two forces. While mutation proposes random genetic changes, selection disposes based on fitness effects. This combination allows populations to adapt to their environments and change over generations.
So while the generation of mutations is a random process, natural selection filters them and determines the outcome. Selection is the opposite of randomness – it sorts and favors certain variants. This interplay between random mutation and non-random selection drives evolution.
Evidence for random mutation
There is an abundance of evidence from molecular biology demonstrating that mutations arise randomly. Some key evidence includes:
- Observations that mutation rates depend on environmental factors like exposure to radiation and chemicals. These would not affect rates if mutations were directed.
- Most observed mutations have no effect on fitness. They are neutral or nearly neutral.
- Beneficial mutations are rare compared to neutral/harmful mutations.
- Sequences of accumulated mutations in evolutionary lineages show no patterns or directionality.
Molecular studies comparing DNA sequences between lineages reveal that most accumulated mutations are neutral and do not display patterns expected if mutations were directed. And experimentally, inducing mutations through radiation exposure generates random mutations throughout the genome.
Examples demonstrating randomness
Some more specific examples from experimental research that support the randomness of mutations:
- Luria and Delbrück’s fluctuation test – Showed mutation rates remain constant even when environmental conditions change.
- Lederberg and Lederberg’s replica plating – Demonstrated pre-existing mutations, not directed mutations, allow bacterial antibiotic resistance.
- Barry Hall’s experiments – Failed to find evidence for directed mutation in the lac operon of E. coli.
These classic experiments laid the groundwork for understanding mutations as random. More recent genomic studies continue to confirm this understanding.
Implications of random mutation
The randomness of mutation has several important implications for evolutionary theory and processes:
- Adaptation is not always inevitable and may rely on random beneficial mutations arising.
- Complex adaptations require accumulation of multiple mutations over time.
- Many neutral and nearly-neutral mutations fix through genetic drift.
- Deleterious mutations are common.
- Mutation and selection together drive gradual change.
Because mutations are random and undirected, evolution does not have foresight and cannot proceed toward specific goals. The raw material for evolution is whatever random mutations happen to arise in a population at a given time. This places constraints on how quickly and directly adaptation can occur.
Conclusion
Dr. Carroll’s statement that “mutation is random” succinctly summarizes a key tenet of evolutionary biology supported by decades of research. While mutations arise through random accidents and cellular processes, selection then acts upon phenotypic variations caused by mutations in non-random ways. The combination of these two distinct mechanisms drives the overall evolutionary process.
Mutation is the source of novelty and variation, while selection filters this variation. Evolution proceeds based on the random mutations that happen to occur in populations, not any directed or predetermined mutations. The randomness of mutations places important constraints and contingencies on evolutionary trajectories.
Understanding that mutations are random events outside organismal control is critical to explaining observations from molecular biology and how complex adaptations can arise gradually over time. Dr. Carroll’s research has helped demonstrate and explain how evolutionary processes unfold through the interplay between random mutation and selection.
