How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles - PubMed (original) (raw)

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How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles

Carrie L Morjan et al. Mol Ecol. 2004 Jun.

Abstract

The traditional view that species are held together through gene flow has been challenged by observations that migration is too restricted among populations of many species to prevent local divergence. However, only very low levels of gene flow are necessary to permit the spread of highly advantageous alleles, providing an alternative means by which low-migration species might be held together. We re-evaluate these arguments given the recent and wide availability of indirect estimates of gene flow. Our literature review of F(ST) values for a broad range of taxa suggests that gene flow in many taxa is considerably greater than suspected from earlier studies and often is sufficiently high to homogenize even neutral alleles. However, there are numerous species from essentially all organismal groups that lack sufficient gene flow to prevent divergence. Crude estimates on the strength of selection on phenotypic traits and effect sizes of quantitative trait loci (QTL) suggest that selection coefficients for leading QTL underlying phenotypic traits may be high enough to permit their rapid spread across populations. Thus, species may evolve collectively at major loci through the spread of favourable alleles, while simultaneously differentiating at other loci due to drift and local selection.

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Figures

Fig. 1

Fig. 1

Number of generations for an advantageous allele of selective advantage s to spread across a species range. The data are based on Table 1b provided by Slatkin (1976), which are results from a stepping-stone simulation model assuming that 20 steps are required for an allele to spread across 20 populations (n = 500 per population) in a linear arrangement.

Fig. 2

Fig. 2

Number of generations for a single advantageous allele of selective advantage s to spread across a species range. The data are based on Table 2 provided by Cherry & Wakely (2003), which are results of simulations based on Wright’s island model of equal exchange of migrants among populations.

Fig. 3

Fig. 3

Estimates of mean and median gene flow (± range and SE, n in parentheses) as evaluated by (A) _F̃_ST (see footnote for Table 1) and (B) Nem for common taxonomic groups of animals using allozymes and nuclear data.

Fig. 4

Fig. 4

Frequency histogram of (A) _F̃_ST (see footnote for Table 1) and (B) migration rate (Nem) for total nuclear data from plants and animals. Data for Nem are binned for Nem = 0.01, 0.1, 0.25, 0.5, 1, and binned into 1-unit intervals thereafter.

Fig. 5

Fig. 5

Distribution of 133 linear selection gradients and 96 linear selection differentials for 172 traits from experimentally manipulated or disturbed populations. Data with values < 1 are binned into 0.1 unit intervals, and binned into 1-unit intervals thereafter.

Fig. 6

Fig. 6

Distributions for the estimated strength of selection (s) for leading QTLs underlying phenotypic traits in animals as measured by (A) percentage of difference in parental means (%D) for 26 traits, and (B) percentage of variance explained (PVE) for 79 traits. s was calculated by multiplying either %D or PVE for a QTL by the average selection gradient for phenotypic traits (0.13) and halving for diploidy. Interspecies differences are indicated by stippled bars, and intraspecies differences are indicated by white bars.

Fig. 7

Fig. 7

Distributions for the estimated strength of selection (s) for minor QTL underlying phenotypic traits in animals as measured by (A) percentage of difference in parental means (%D) for 26 traits, and (B) percentage of variance explained (PVE) for 79 traits. s was calculated by multiplying either %D or PVE for a QTL by the average selection gradient for phenotypic traits (0.13) and halving for diploidy. Interspecies differences are indicated by stippled bars, and intraspecies differences are indicated by white bars.

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