Population Biology and Evolution

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Center for Population Biology

Until now we have discussed evolution as a change in the characteristics of a population of organisms, but behind that phenotypic change is genetic change. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Using the ABO blood type system as an example, the frequency of one of the alleles, I A , is the number of copies of that allele divided by all the copies of the ABO gene in the population.

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The I B and I 0 alleles made up A change in this frequency over time would constitute evolution in the population. The allele frequency within a given population can change depending on environmental factors; therefore, certain alleles become more widespread than others during the process of natural selection. Because many of those offspring will also carry the beneficial allele, and often the corresponding phenotype, they will have more offspring of their own that also carry the allele, thus, perpetuating the cycle. Over time, the allele will spread throughout the population.

Some alleles will quickly become fixed in this way, meaning that every individual of the population will carry the allele, while detrimental mutations may be swiftly eliminated if derived from a dominant allele from the gene pool. The gene pool is the sum of all the alleles in a population. Sometimes, allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies.

This phenomenon is called genetic drift. Natural selection and genetic drift usually occur simultaneously in populations and are not isolated events.

Exponential and logistic growth in populations - Ecology - Khan Academy

It is hard to determine which process dominates because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence. An event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, is called the founder effect. Natural selection, random drift, and founder effects can lead to significant changes in the genome of a population. In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg stated the principle of equilibrium to describe the genetic makeup of a population.

The Hardy-Weinberg principle assumes conditions with no mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population; while no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes. Working under this theory, population geneticists represent different alleles as different variables in their mathematical models. In other words, all the p alleles and all the q alleles make up all of the alleles for that locus that are found in the population.

If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes. Since each individual carries two alleles per gene, if the allele frequencies p and q are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypes if two alleles are drawn at random from the gene pool.

In other words, the frequency of pp individuals is simply p 2 ; the frequency of pq individuals is 2pq; and the frequency of qq individuals is q 2. When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined from the Hardy-Weinberg equation.

If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play. In plants, violet flower color V is dominant over white v. How many plants would you expect to have violet flowers, and how many would have white flowers?

In theory, if a population is at equilibrium—that is, there are no evolutionary forces acting upon it—generation after generation would have the same gene pool and genetic structure, and these equations would all hold true all of the time. Of course, even Hardy and Weinberg recognized that no natural population is immune to evolution.

Populations in nature are constantly changing in genetic makeup due to drift, mutation, possibly migration, and selection. As a result, the only way to determine the exact distribution of phenotypes in a population is to go out and count them. But the Hardy-Weinberg principle gives scientists a mathematical baseline of a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play. If the frequencies of alleles or genotypes deviate from the value expected from the Hardy-Weinberg equation, then the population is evolving.

The distribution of phenotypes in this litter of kittens illustrates population variation. Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. Understanding the sources of a phenotypic variation in a population is important for determining how a population will evolve in response to different evolutionary pressures. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against.

Acquired traits, for the most part, are not heritable. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child.

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Evolutionary & Population Biology (IBED-EPB)

Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The diversity of alleles and genotypes within a population is called genetic variance.

Evolution, Ecology & Population Biology

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.

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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. Changes in allele frequencies that are identified in a population can shed light on how it is evolving.

In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances. The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father.

Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure , or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure. By chance, 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.

Click for a larger image. Genetic drift in a population can lead to the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele B are dominant over rabbits with the white coat color allele b. In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of. Only half of the individuals reproduce, resulting in a second generation with p and q values of. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color.

As a result, in the third generation the recessive b allele is lost. Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. A chance event or catastrophe can reduce the genetic variability within a population.

Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. 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. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect.

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 is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations.

Watch this short video to learn more about the founder and bottleneck effects. Note that the video has no audio. 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 the original population 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.