Classics: Effective Population Size Ratio

Here’s another concise Conservation Classic highlighted in our upcoming book chapter (see previous entries on this book). Today’s entry comes from a colleague of mine, Dick Frankham, who has literally written the book on conservation genetics. I’ve published with Dick a few times – absolutely lovely chap who really knows his field more than almost any other. It is a great pleasure to include one of his seminal works as a Conservation Classic.

This entry is highly related to our work on minimum viable population size, and the controversial SAFE index (more on that later).

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Although it had long been recognized that inbreeding and loss of genetic diversity were accentuated in small, isolated populations (Charlesworth & Charlesworth, 1987), genetic hazards were generally considered to be of less consequence to extinction risk than demographic and environmental stochasticity. Frankham (1995) helped overturn this viewpoint, using a meta-analysis to draw together comprehensive evidence on the ratio of genetically effective to actual population size (N_e:N).

He assessed the effects of fluctuating population size, variance in family size and uneven sex ratios from 102 species. He found that the N_e:N ratio averaged only 10 % in wild populations, signalling that even apparently large and ‘secure’ populations might, in reality, already be suffering from inbreeding depression and thereby require genetic management to lower their extinction risk and retain evolutionary potential.

CJA Bradshaw

Frankham, R. (1995). Effective population size/adult population size ratios in wildlife: a review Genetical Research, 66 (02) 95-107. doi: 10.1017/S0016672300034455