3. Pitcher\'s stichwort, an annual plant listed as threatened or endangered in s

Question

3. Pitcher's stichwort, an annual plant listed as threatened or endangered in several states, is restricted to limestone glades and undergoes extreme variation with climatic variables. Annual censuses of population numbers over a three-year time period begin with a population high of 3,000 individuals in year 1, low of 20 individuals in year 2, and recovery of 3,000 individuals in year 3. Based on these three years of data, calculate the overall effective population size at the end of year 3. Explain how change in population size over time affects heterozygosity in the population.

Explanation / Answer

Please note that for calculating the effective population size you need to know the number of males and females. Effective population size is calculated using the formula :

Ne = (4 * Nf * Nm) / (Nf + Nm) where, Ne = effective population size; Nm = number of males; Nf = number of females

Now, if census count is given as 'P'; then we consider Nm = Nf = P/2

In such a case, Ne = P

So, effective population size at the end of third year = 3000

Lower the effective population size, faster is the loss of heterozygosity. According to Hardy Weinberg's equilibrium, if two heterozygous individuals are crossed, then the number of heterozygous progeny will be 2n

Suppose the cross is Aa X Aa

Then, "AA:Aa:aa" in the progeny will be 1:2:1 (where, 'Aa' is 2)

Let initial heterozygosity be Ho

Then, final heterozygosity, Ht = (1- 1/2Ne)t Ho

Here, t = number of generations; so, this formula also proves that heterozygosity is inversely proportional to Ne.

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