In this exercise we will examine how drift, migration, and selection all interac

Question

In this exercise we will examine how drift, migration, and selection all interact to shape allele frequencies (and lead to the evolution of) populations. In order to do this, we will be using data from a study conducted on fur color in the highly endangered Rio Negro tuco-tuco (Ctenomys rionegrensis), a medium-sized subterranean rodent. Their current range is highly fragmented due to plantations (See figure 1). When scientists cataloged their fur color, they noticed huge inter-population variation. They also noticed relatively low within population variation (see below). The question you will be asking is: are the allele frequencies for fur color fixed in these populations due to strong natural selection, or due to drift? Read all information before starting the homework! Figure 1. Map showing the distribution of Rio Negro tuco-tuco populations (reprinted form Wlasiuk et al 2003). Each circle represents a different population. Phenotype prevalence is shown on the map as well. You will need to determine how dirft and migration are influencing the genetic similarities (or differences) among these populations. The degree to which the populations are different from each other is known as “genetic differentiation”, as is known as an Fst value.

Basic information: Scientists went out and sampled individuals from the populations. Population size can be very difficult to estimate, so scientists often assume all populations are approximately the same size (as we are doing here). When they directly sampled one of the populations, they found that there are approximately 30 breeding females and 20 breeding males (males have a higher mortality rate than females) in the populations (remember that we are assuming that all populations exhibit the same patterns). Also the scientists were able to use capture/recapture methods to determine that the proportion of migrants (migration rate: m) in any given population per generation is 0.01. Scientists assume that migration is equal among all the populations (this might be somewhat unrealistic since migration might be more difficult among populations that are separated a longer distance, but for the purposes of this assignment we will assume all migration is equal). When they actively looked at the difference in survival among the different phenotypes they found that survival of the agouti phenotype was approximately 63%. The survival of the black-backed phenotype was approximately 62.8% and survival of the melanistic phenotype was approximately 62.6%.

QUESTIONS:

1. How genetically differentiated are the populations? (For reference of comparison, a value of 0.3 or larger would be considered very differentiated, a value close to 0 would be not differentiated). (3 points)

2. What do these results suggest regarding gene flow among the populations? Given only these results, do you suspect that the alleles are fixed due to selection or drift? Why (this is just a guess, but you must justify your answer) ? (2 points)

3. In order to follow up with the gene flow analysis, you decide to directly measure selection on the fur color by using the survival estimates relating fur coloration. What is selection on the different phenotypes? Given this new information, and using a quantitative explanation, is drift or selection responsible for fixing the allele frequencies? (hint: you will need to use an equation that relates selection and drift, and you will need to do it for ALL 3 alleles) (3 points)

4. The melanistic phenotype has the lowest fitness of the 3 phenotypes, yet it still exists in relatively high proportion in the various populations. Why? (1 point)

5. The Rio Negro tuco-tuco has both fragmented populations and small population sizes. How do these interact to create issues for allele fixation? Why might this be an issue for conservation purposes? (1 point)

BRAZIL 10 Km URUGUAY Mafalda Abrojal Nuevo Berlín Rio Negro Las Cañas Guarida Chaparei melanic dark-backed agouti Arrayanes Tabaré

Explanation / Answer

1. Genetic differentiation (Fst value) is calculated by the formula

Fst = 1 / 4Nem+1

where, Ne = number of individuals in the population. Here, it is 30 + 20 =50

m = Migration rate. Here, it is given 0.01

Therefore, Fst = 1 / 4*50*0.01 + 1 = 0.33

Hence, the population is very differentiated.

2. Gene flow is the mechanism by which genetic diversity takes place in the population due to migration. The given populations has undergone migration, hence gene flow is the driving force for genetic diversity in this population..

Natural selection occurs in large populations whereas genetic drift occurs in small populations. Here the population size is small consisting of just 50 individuals. Hence the possibility is there the the populations is fixed to genetic drift rather than natural selection.

3. Fur color in rodents is an example of natural selection with adaptation. Following gene flow and genetic drift, natural selection for fur color phenotype took place which gave the particular population residing there a selective advantage.

Hardy Weinberg equilibrium equation states p2+2pq+q2 = 1

where p = frequency of allele AA

q = frequency of allele aa

pq = frequency of allele Aa

Also, p+q should be 1 which is not the case here. Hence, this population is not in Hardy Weinberg equilibrium.

4. Melanism is a type of adaptation for the population. Melanistic rodents may have a selective advantage that they are better camouflaged and hence these still exist in the population.

5. Small fragmented populations are generally a result of bottleneck effect. They reduce the genetic diversity, heterozygosity and may lead to loss of alleles or fixation of alleles. They also bring a change in allelic frequencies.

Small and fragmented populations are likely to undergo inbreeding depression which brings about homozygosity in alleles. Such populations are more vulneration for extinction and is a major concern for conservation of such species.

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