Classics: Effective population size ratio

27 04 2011

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).

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 (Ne:N).

He assessed the effects of fluctuating population size, variance in family size and uneven sex ratios from 102 species. He found that the Ne: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

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



3 responses

1 05 2013

Excellent round up ! shall be most welcome in my up coming exam


27 04 2011
Tom Keen

Interesting. I’ve never heard the term ‘genetic management’ applied to conservation before – I’ve only really heard similar reference to agriculture. How is this applied in practice?

My initial assumption would be that measures taken to reduce extinction risk from demographic and environmental stochasticity would also help to retain a species evolutionary potential? But I’m probably oversimplifying/possibly missed the point…


27 04 2011

Not far off. The idea here is that the absolute number of individuals is sometimes a poor reflection of the population’s genetic diversity because of phenomena such as inbreeding. The implicit assumption behind the MVP size is that if you have a large enough population, you generally account for the genetics as well (but not always – see Traill et al. 2010).


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