Breeding effective population size Nb

The number of individuals found in a genetic neighborhood defined by the variance in gamete dispersal.

Deme The largest area or collection of individuals where mating is (on average) random.

Genetic neighborhood An area or subunit of a population within which mating is random. Isolation by distance Decrease in the probability of mating and dispersal of gametes as physical distance increases.

In a classic study, Schaal (1980) estimated the breeding effective population size in Texas bluebonnets (Lupinus texensis). This species is pollinated by bumblebees and occurs in large continuous populations that cover many hectares (see color images on the text web page in the section for Chapter 3). An experimental population was constructed using 91 plants with known genotypes at the phospho-glucose isomerase-1 enzyme locus (Pgi-1). Mating distances were estimated from seven central plants (homozygous for the Pgi-1 fast allele) by genotyping progeny from plants without the Pgi-1 fast allele in the experimental population. Since gametes are also dispersed in seeds every generation, the passive dispersal of seeds was tracked as well. Dispersal distances were very limited, with gametes moving via pollen an average of 1.82 m and seeds moving an average of 0.58 m. The genetic neighborhood size for the experimental population of L. texensis was estimated as 6.3 m2, containing a breeding effective population size of 95.4 individual plants.

The term deme, the largest area or collection of individuals where mating is (on average) random, is often applied to continuous populations and is closely connected with the concepts of breeding effective population size and genetic neighborhoods. The bond orbital in chemistry serves as a metaphor for the deme in population genetics. A bond orbital describes the probability that an electron will be found at some location in space. Similarly, a deme describes the probability that an average individual will move its gametes some distance in space by mating or dispersal in a generation. Members of the same deme are considered able to mate at random whereas members of two different demes have a low probability (< 5%) of mating and are therefore members of separate populations. It is important to distinguish demes, or genetically defined population demarcations, from geographically defined populations. From the perspective of effective population size and predicting the behavior of genetic variation, deme is a more useful definition for populations than geographic or spatial definitions.

Effective population sizes of different genomes

Plastid genomes (mitochondria and chloroplasts) are an example where the effective population size is lower than that of the nuclear genome of the same organism. The effective population size of plastid genomes is reduced by two independent factors. First, these genomes are haploid (one copy of the genome per plastid) compared to the two homologous copies of each diploid nuclear chromosome. In addition, most plastids are inherited by offspring from one parent only via the gamete cytoplasm (uniparental inheritance). In species with two equally frequent sexes, uniparental inheritance causes plastid genomes to have half the effective population size of genomes inherited from all possible parents. These two factors combined cause animal mitochondrial genomes and plant mitochondrial and chloroplast genomes to have one-quarter of the effective size of the diploid, biparentally inherited nuclear autosomes. Thus, loci in these genomes experience a higher rate of genetic drift compared to loci in the nuclear genome. Genetic marker studies frequently take advantage of this fact, using plastid genome marker loci to study phenomena such as recent population divergence due to genetic drift where nuclear marker loci would show much less divergence because of a larger effective population size.

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