2.2.4: Additional Mechanisms of Evolution
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Unit 2.2.4 - Additional Mechanisms of Evolution
- Please read and watch the following Learning Resources.
- Reading the material for understanding, and taking notes during videos, will take approximately 2 hours.
- Optional Activities are embedded.
- Bolded terms are located at the end of the unit in the Glossary. There is also a Unit Summary at the end of the Unit.
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- Identify the four agents of evolutionary change other than natural selection
- Explain genetic drift, including the bottleneck and founder effects
- Describe gene flow and how it maintains genetic variance in a population
- Explain how each evolutionary force can influence the allele frequencies of a population
In addition to natural selection, there are four other agents of evolutionary change: mutation, genetic drift, gene flow, and non-random mating. These serve to either increase or decrease genetic variation in the population. Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Genetic drift is the impact on a population's genetic variance due to random chance events, such as natural disasters. Gene flow is the exchange of genetic information between populations and serves to keep disparate populations more similar to one another. Nonrandom mating describes the process of "choice" within reproduction based on perceived evolutionary fitness. Each of these additional agents is discussed below in concert with natural selection as none of these forces acts in isolation.
Four Additional Agents of Evolutionary Change
1. Mutation
Species ultimately evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance (Figure \(\PageIndex{1}\)). Without new mutations, genetic variance in a population would lessen over time in stable environmental conditions (stabilizing selection) leading populations to be less able to survive future changes. Mutations can only influence evolution if they are heritable (e.g. carried within the gametes).
Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection as seen in Figure \(\PageIndex{2}\). Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in the genome (e.g. in the somatic cells). Some can have a dramatic effect on a gene and the resulting phenotype.
This 7-minute video provides an overview of DNA mutations and their impact on the proteins they encode and the phenotype of an organism.
Question after watching: If a mutation does not impact an organism's phenotype, and this mutation is found over time to increase in a population, does this constitute evolution? If so, what is the likely cause of this evolution?
2. Genetic Drift
Another way a population’s allele and genotype frequencies can change is genetic drift, which is simply the effect of chance on the population's genetic variation. For example, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).
Genetic Drift vs. Natural Selection
Genetic drift is the opposite of natural selection. The theory of natural selection maintains that some individuals in a population have traits that enable them to survive and produce more offspring, while other individuals have traits that are detrimental and may cause them to die before reproducing. Over successive generations, these selection pressures can change the gene pool and the traits within the population. For example, a big, powerful male gorilla will mate with more females than a small, weak male and therefore more of his genes will be passed on to the next generation. His offspring may continue to dominate the troop and pass on their genes as well. Over time, the selection pressure will cause the allele frequencies in the gorilla population to shift toward large, strong males.
Unlike natural selection, genetic drift describes the effect of chance on populations in the absence of positive or negative selection pressure. Through random sampling or the survival or reproduction of a random sample of individuals within a population, allele frequencies within a population may change. Rather than a male gorilla producing more offspring because he is stronger, he may be the only male available when a female is ready to mate. His genes are passed on to future generations because of chance, not because he was the biggest or the strongest. Genetic drift is the shift of alleles within a population due to chance events that cause random samples of the population to reproduce or not.
Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before leaving any offspring to the next generation, all of its genes (1/10 of the population’s gene pool) will be suddenly lost. In a population of 100, that individual represents only 1 percent of the overall gene pool; therefore, genetic drift has much less impact on the larger population’s genetic structure.
The Bottleneck Effect
Genetic drift can be magnified by natural events, such as a natural disaster that kills a large portion of the population at random. This bottleneck effect occurs when only a few individuals survive and reduce variation in the gene pool of a population (Figure \(\PageIndex{3}\)). The genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population.
Captive breeding programs can fall victim to the bottleneck effect when species are at endangered or threatened levels. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease.
For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression.
This 4.5-minute video describes how genetic drift occurs by chance.
Question after watching: Describe a situation in which a population would undergo the bottleneck effect and explain what impact that would have on the population’s genetic variance.
The Founder Effect
Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind (Figure \(\PageIndex{4}\)). In this situation, it is improbable that those individuals are representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers.
The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners, but rare in most other populations. This was probably due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities, even cancer.
Figure \(\PageIndex{4}\): The founder effect occurs when a portion of the population (i.e. “founders”) separates from the old population to start a new population with different allele frequencies. (Tsaneda; CC-BY-SA 4.0)
This 1-minute video differentiates the founder effect from a bottleneck event, which are similar but different.
Question after watching: 50,000 years ago, a small group of people migrated out of Africa. Their descendants populated Asia, Europe, and the Americas. Which type of event is this: founder or bottleneck?
[Optional Activity] Scientific Method Connection: Testing the Bottleneck Effect
Question: How do natural disasters affect the genetic structure of a population?
Background: When much of a population is suddenly wiped out by an earthquake or hurricane, the individuals that survive the event are usually a random sampling of the original group. As a result, the genetic makeup of the population can change dramatically. This phenomenon is known as the bottleneck effect.
Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each time this experiment is run, the results will vary.
Test the hypothesis: Count out an original parent population of 100 individuals using different colored beads. For example, red, blue, and yellow beads might represent red, blue, and yellow individuals. After recording the number of each individual in the original population, place them all in a bottle with a narrow neck that will only allow a few beads out at a time. Then, pour approximately 33 beads into a bowl. This represents the surviving individuals after a natural disaster kills a majority (67%) of the population. Count the number of the different colored beads in the bowl, and record it. Then, place all of the beads back in the bottle and repeat the experiment four more times.
Analyze the data: Compare the five populations that resulted from the experiment. Do the surviving populations all contain the same number of different colored beads or do they vary? Remember, these populations all came from the same exact parent population.
Form a conclusion: Most likely, the five resulting surviving populations will differ quite dramatically. This is because natural disasters are not selective—they kill and spare individuals at random. Now think about how this might affect a real population. What happens when a hurricane hits the Gulf Coast of the United States? How do the seabirds that live on the beach fare?
Optional Activity \(\PageIndex{1}\)
Do you think genetic drift would happen more quickly on an island or on the mainland? Why?
- Answer
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Genetic drift is likely to occur more rapidly on an island where smaller populations are expected to occur.
3. Gene Flow
Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure \(\PageIndex{5}\)). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the gene structure of the population but can also introduce new genetic variation to populations in different geological locations and habitats.
Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic variation between the two groups. Gene flow strongly acts against speciation, by recombining the gene pools of the groups, thus repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.
For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively linked the population on one side of the highway with the other.
This video takes the examples of the finches in the Galapagos islands and explains how the different sized beaks on each island resulted from the lack of gene flow between the population on each island.
Question after watching: Why does the narrator say that in time the different populations that stop having gene flow can become different species. What step needs to happen for this to occur?
4. Nonrandom Mating
If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice; for example, female robins may prefer males with brighter, more orange breast feathers Figure \(\PageIndex{6}\). Traits that lead to more matings for an individual become selected for by natural selection.
One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.
This 12-minute video makes the connection between DNA, mutations, genetic reshuffling through sexual reproduction, and phenotypes/variability between individuals.
Question after watching: Imagine that the creation of the Panama canal re-opens a route between the two populations of fish presented in the video that had been separated by the landmass. What sort of evolutionary changes would need to happen in either population to lead to specialization?