Fixation of detrimental alleles

If drift can lead to the loss of beneficial alleles, it should come as no surprise that it can also lead to fixation of deleterious ones. In fact, we can use the same formula we've been using (equation (1)) if we simply remember that for an allele to be deleterious $s$ will be negative. So we end up with

$$P_1(p) = \frac{1 - e^{2N_esp}}{1 - e^{2N_es}} \quad .$$ (2)

One implication of equation (2) that should not be surprising by now is that evan a deleterious allele can become fixed. Consider our two example populations again, an ideal population of size 100 ($N_e = 100$) and a population with 1 male and 99 females ($N_e = 4$). Remember, the probability of fixation for a newly arisen allele allele with no effect on fitness is $1/2N = 5 \times 10^{-3}$ (Table 1).⁷

Table 1: Fixation probabilities for a deleterious mutation as a function of effective population size and selection coefficient for a newly arisen mutant ($p=0.01$).

	$N_e$
$s$	4	100
0.001	$5 \times 10^{-3}$	$5 \times 10^{-3}$
0.01	$5 \times 10^{-3}$	$2 \times 10^{-5}$
0.1	$3 \times 10^{-3}$	$2 \times 10^{-10}$