Question

The original source of new alleles, upon which selection oper- ates, is mutation, a random event that occurs without regard to selectional value in the organism. Although many model organ- isms have been used to study mutational events in populations, some investigators have developed abiotic molecular models. Soll et al. \((2006 . \text { Genetics } 175: 267-275)\) examined one such model to study the relationship between both deleterious and advantageous mutations and population size in a ligase molecule composed of RNA (a ribozyme). Soll found that the smaller the population of molecules, the more likely it was that not only deleterious mutations but also advantageous mutations would disappear. Why would population size influence the survival of both types of mutations (deleterious and advantageous) in populations?

Short answer

Answer: Population size influences the survival of both deleterious and advantageous mutations in populations because it affects the impact of genetic drift. In small populations, genetic drift may cause the loss of advantageous mutations and the fixation or disappearance of deleterious mutations more often due to random fluctuations in allele frequencies. In larger populations, the impact of genetic drift is reduced, allowing selection to have a greater effect on the fate of mutations.

Step-by-step solution

Understanding mutation in populations

Mutation is a random event that introduces new alleles into a population. These new alleles can be either advantageous, deleterious, or neutral depending upon the impact they have on the organism's fitness. The exercise states that the smaller population of molecules, the more likely it is that mutations of both deleterious and advantageous types would disappear.

Impact of population size on genetic drift

Population size plays an important role in determining the fate of new mutations. In small populations, genetic drift, a random fluctuation in allele frequencies, has a significant impact. Genetic drift can cause the loss of advantageous mutations, or the fixation (increase in frequency until it becomes the only allele) of deleterious mutations. In larger populations, the impact of genetic drift is diluted, and the chance of fixation or loss of mutations is reduced.

Impact of genetic drift on advantageous and deleterious mutations

In small populations, even advantageous mutations may disappear because of genetic drift. This is because their advantage might not be enough to overcome the random fluctuations in allele frequencies within the population. Similarly, deleterious mutations also have an increased chance of disappearing from smaller populations, as they are more prone to get lost due to random fluctuations even before selection can act upon them.

Conclusion

Population size influences the survival of both deleterious and advantageous mutations in populations because it affects the impact of genetic drift. In small populations, genetic drift may cause the loss of advantageous mutations and the fixation or disappearance of deleterious mutations more often due to random fluctuations in allele frequencies. In larger populations, the impact of genetic drift is reduced, allowing selection to have a greater effect on the fate of mutations.

Key concepts

Mutation in Populations

Mutation serves as the cornerstone of genetic variety, fueling evolution by adding fresh genetic scripts into the population's gene pool. These mutations can emerge without clear intent or direction—essentially, they are accidental tweaks to an organism’s DNA. Some changes may have neutral implications, but when they meaningfully impact an organism’s ability to thrive and reproduce, they become pivotal to population genetics.

Consider flipping a coin as a metaphor for mutation. Just as heads or tails can emerge at random, so do mutations present themselves without preference. Small-scale mutations, such as single nucleotide polymorphisms, alter just a tiny fraction of the genetic code, whereas larger-scale mutations can rearrange whole segments of chromosomes, with varying effects on an organism’s fitness.

Genetic Drift

Genetic drift is the fancy term for the serendipitous ebb and flow of allele frequencies in a population, particularly noticeable in smaller gene pools. This phenomenon resembles a bag of mixed candies, where randomly grabbing a handful might give you an uneven representation of flavors—the same can happen with genes.

In every new generation, some alleles may become more common while others fade away, not because of their survival advantage but purely by chance. Think of it as the roulette wheel of genetics, where each spin dishes out different winning numbers, or in this case, alleles. Over time, genetic drift can dramatically remodel the genetic landscape of a population, independent of natural selection.

Allele Frequency

The concept of allele frequency is about counting up the representation of a particular genetic variation within a population. Imagine a beach strewn with pebbles of different colors. The percentage of, say, blue pebbles in the vast sea of stones quantifies their allele frequency.

In terms of genetics, if a large majority of a population carries a specific allele, it has a high frequency. These frequencies are not static but rather shift over time due to mutation, selection, genetic drift, and other evolutionary forces. Monitoring these changes is akin to watching the tides reshape the shoreline, each wave (or generational change) leaving a new pattern in the sand.

Advantageous Mutations

Advantageous mutations are like genetic gold dust; they offer the bearer a leg-up in the survival stakes. They're mutations that fit so well within a particular environment that they enhance an organism's reproductive success. For instance, it's like having the perfect tool for a job while others make do with second-best.

These beneficial mutations can lead to improved traits - perhaps a bird with a sharper beak, better-suited for snagging its prey. If such a mutation occurs, the bird could have more offspring than others, cascading this useful genetic change through the bird's population over generations, a process represented by increased allele frequency.

Deleterious Mutations

On the flip side, nature sometimes rolls the genetic dice and comes up with deleterious mutations—genetic alterations that deduct points from an organism's scorecard of survival or reproduction. It's like downloading a faulty mobile app update that crashes upon opening. These mutations can impair how an organism functions or survives within their environment.

A classic example could be a moth with a mutation that gives it a brightly colored wing pattern, making it an easier target for predators. These harmful genetic changes tend to be weeded out by natural selection, as the individuals carrying them often have fewer offspring. Yet in smaller populations, these mutations sometimes slip through the cracks, either by accidentally becoming fixed or by fading away, courtesy of genetic drift.