Unequal sex ratio

Ne with unequal sex ratios in a breeding population

    In many species, a very small number of individuals of one sex or the other do most or almost all of the breeding. Recall that under these conditions, Ne = (4)(Nm)(Nf) / (Nm + Nf). As the sex ratio increase, the effective population number (Ne) falls rapidly as a fraction of the population census count (Nc). In the extreme case, where a single individual of either sex is the only breeding individual, Ne = 4, no matter how large the census count. A notable example is Elephant Seals (Mirounga angustirostris), where a single alpha male has sole reproductive access to a so-called 'harem' comprising 90% of more of the breeding females. The reciprocal example is the hive of eusocial insects, where a single female "queen" has access to several hundred "drones" and is the parent of 10s of 1,000s of workers, all of whom are half-sisters.

    HOMEWORK: In the table above, the total population size
Nc drops as the number of males Nm decreases. Repeat the calculation, and adjust Nf so that total Nc = 2,000. Compare the numbers and graphs for the two calculations.

Figure & Text material © 2021 by Steven M. Carr