Basic information: Scientists went out and sampled individuals from the populati

Question

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%.

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)

Explanation / Answer

1. How genetically differentiated are the populations?

ANS- Generally, there is significant differentiation among the populations in this complex. The level of gene flow between populations was estimated to be only 0·3303 individuals per generation, indicating that there is a low migration rate between populations. The coefficient of genetic differentiation between populations was 0·6022, of which only 41?% contributed to the differentiation between populations within the defined species. This finding is corroborated by analysis showing that 67·83?% of the total variation was partitioned between populations, of which, 50·60?% was between the defined species. The inter-specific genetic distances were 0·2286 on average, which were higher than the intra-specific genetic distances with a mean value. Populations and individuals of the C. balansae complex appear to have declined dramatically during recent decades. The two principal threats to cycads are (a) severe habitat loss and (b) selective removal from the wild for trade or utilization due to their popularity among horticulturalists. Accordingly, it is essential to take action to preserve the species and their habitats, which includes setting up nature reserves and protection stations, education for cycad protection and strengthening the co-operation between governments in this region on wild cycad conservation management.

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

ANS- The pronounced spatial genetic structure suggests that it is necessary to preserve as many populations as possible and that it is not appropriate to preserve only part of populations, since the low genetic diversity in populations implies a reduced ability to survive changing environments and increased susceptibility to diseases in the long term. If methods to enhance genetic diversity, such as transplant programs and artificial pollination between populations, are required to promote gene exchange and species survival, genetic contamination must be avoided between genetic units. The genetic divergence between populations or accumulated by a single population from an ancestral group can be quantified using different measures. None of them can be considered better than others in all respects. The type of marker(s) or gene(s) analysed, the temporal scale considered and the level of variation are differentially affecting the quality of an index, and standardization for comparisons across groups of populations or species is important.

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?

ANS- Several measures of population differentiation can be computed and their comparison may provide useful insights into the evolutionary history of populations. Fst and related indices are moment estimators that measure the degree of population differentiation. Fst computed from multi?allelic markers usually under?estimates population differentiation; some alternative indices do not show this behaviour. Measures of differentiation can be used to define conservation units.

Differentiation measures can be used both to estimate demographic parameters and to detect genomic regions under selection. The Lewontin–Krakauer and related tests are based on the idea that loci showing particularly high or low values of population differentiation have been shaped by selective processes. Demographic parameters can be estimated from measures of population differentiation or using likelihood/Bayesian methods.

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?

ANS - Declining populations are predicted to possess low levels of genetic diversity because of the high probability of inbreeding within populations driven by random genetic drift. In this species, given both small population number and size, genetic diversity is likely to be reduced. As long-term population viability is associated with the levels of genetic, the erosion of genetic diversity would eventually harm the health of populations and species.

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?

ANS - These populations are subject to an increased risk of local extinction because they are more vulnerable to demographic, genetic, and environmental stochasticity. Considering that natural areas have been becoming progressively more fragmented and smaller, understanding the genetic structure and evolutionary dynamics of small populations is critical. Ctenomys lami has 26 karyotypes distributed in a small area. The observed isolation by distance pattern and a clinal genetic variation suggest a stepping-stone population model. The results did not indicate genetic structuring associated with distinct karyotypes. However, mitochondrial and nuclear molecular markers demonstrated the existence of 2 demes, which are not completely isolated but are probably reinforced by a geographical barrier. The lack of knowledge about ecological and genetic structure has contributed to difficulties in developing conservation strategies for C. lami. Also, the high chromosomal polymorphism of this species associated with the progressive human disturbance of its limited geographical territory could lead to a great loss of genetic diversity and the evolutionary process in which this species is involved. In order to provide information for the development of appropriate conservation strategies for C. lami.

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