Regions of the genome that have been the target of positive selection 
specifically along the human lineage are of special importance in human biology. 
We used high throughput sequencing combined with methods to enrich human genomic 
samples for particular targets to obtain the sequence of 22 chromosomal samples 
at high depth in 40 kb neighborhoods of 49 previously identified 100-400 bp 
elements that show evidence for human accelerated evolution. In addition to 
selection, the pattern of nucleotide substitutions in several of these elements 
suggested an historical bias favoring the conversion of weak (A or T) alleles 
into strong (G or C) alleles. Here we found strong evidence in the derived 
allele frequency spectra of many of these 40 kb regions for ongoing 
weak-to-strong fixation bias. Comparison of the nucleotide composition at 
polymorphic loci to the composition at sites of fixed substitutions additionally 
reveals the signature of historical weak-to-strong fixation bias in a subset of 
these regions. Most of the regions with evidence for historical bias do not also 
have signatures of ongoing bias, suggesting that the evolutionary forces 
generating weak-to-strong bias are not constant over time. To investigate the 
role of selection in shaping these regions, we analyzed the spatial pattern of 
polymorphism in our samples. We found no significant evidence for selective 
sweeps, possibly because the signal of such sweeps has decayed beyond the power 
of our tests to detect them. Together, these results do not rule out functional 
roles for the observed changes in these regions-indeed there is good evidence 
that the first two are functional elements in humans-but they suggest that a 
fixation process (such as biased gene conversion) that is biased at the 
nucleotide level, but is otherwise selectively neutral, could be an important 
evolutionary force at play in them, both historically and at present.