Biology Reference
In-Depth Information
13
CHAPTER
The 50/500 -100 -1000 -5000
Ne Rules, Actual Population
Size, and Loss of Diversity
to Random Drift
It was noted in Chapter 7 that one of the earlier formal estimates of the
size of an ideal population required to prevent drift and inbreeding is
often referred to as the Franklin/Soulé 50/500 rule: for the average species
an ideal effective population size, Ne, of at least 50 individuals is needed
to prevent inbreeding (or inbreeding depression) and 500 individuals to
prevent random genetic drift (e.g., see Avise 1994; Meffe 1996: 53; Finkeldey
and Hattemer 2007: 123-127). We emphasize that these numbers refer to
the ideal effective population size, not the actual population census size.
In later analyses including the effects of the accumulation of mutations,
Lynch et al. (1995) suggested that Ne sizes should not drop below 100 (and
census sizes less than 1,000), and Lande (1995) recommended Ne should
not decline below 5,000, to prevent population extinction. Lynch et al.
argue that management policies that maintain populations at 100-1,000
census individuals are inadequate, and Schultz and Lynch (1997) suggest
that long-term effective population sizes of a few hundred individuals are
advisable to prevent population extinction.
The use of Ne relative to the above arguments is somewhat problematic
in working with natural populations where a variety of life history and
other historical effects may interact in ways rendering the calculation and
interpretation of Ne diffi cult. However, do the above suggestions for Ne
and actual census size mean that very large expanding populations with
relatively high levels of mixing (relatively high dispersal) will be free from
random drift?
To explore that question, NEWGARDEN trials were conducted with
populations having the following input conditions:
Nine hundred founding bisexual annuals in a central 30 by 30 individual
square with 5 grid units between individuals.
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