Effective Population Size Calculation

If a population consists of breeding males and breeding females, the effective population size is given by
 
Where Nm is the actual number of breeding males and Nf is the actual number of breeding females.
Practice Problem : 1
In a population of 500 with a 50:50 sex ratio and all individuals breeding, however 250  females bred with 10 males. Then what will be the effective population size?

Answer:
  Applying the equation

N_m (Number of Breeding Males) = 10
N_f (Number of Breeding Females) = 250
N_E- Effective Population Size = (4 x 10 x 250) / (10 +250)
                                               = 10000 / 260
                                           N_E = 38.5

Exam Questions:
Effective population size for completely monogamous species having 40 males and 10 females would be (CSIR UGC NET JRF, Dec 2006)

(a) 42 (b) 32 (c) 20 (d) 10

Answer:
Effective population size (NE) = 4NmNf/ (Nm+Nf)
Nm- Number of breeding males                                    
Nf -Number of breeding females
Here
Nm-40
Nf -10
(NE) = 4NmNf/ (Nm+Nf)
Effective population size (NE) = (4 x 40 x10) / (40 +10) = 1600/50 =  32