Table 1.

How evolutionary forces affect sex chromosomes and autosomes

Evolutionary force	Predicted effect	Observations and comments	Key references
Mutation	Mutation rate should be higher in  chromosomes that spend more time  in males than in females.	Confirmed in mammals and in  birds. Equivocal in flies.	47 – 56
	(Y > Z > A* >W)	Effect strongest in mammals with  long generation times.
Genetic drift	Drift will have larger effect on Y and Z  than on A because Y and Z have  smaller effective population sizes.	Generally confirmed.	47, 57–62
		The relative effects of drift on X  versus A and Z versus A will  depend on reproductive skew  (see text).
Selection A. SA** selection	SA variance should be greatest on X and  Z. Alleles benefiting females but  deleterious to males should be  abundant on X. Those benefiting  males at expense to females should be  less common on X.	SA selection appears pervasive.  Predictions generally  confirmed in laboratory  experiments with flies.	63 – 69
B. Dominance and  selection	Negative selection should be more  efficient weeding out deleterious  X-linked alleles if recessive. Positive  selection should also be more  efficient fixing beneficial recessive  alleles.	Fly studies confirm that negative  selection is more efficient on  X.	47, 70–72
		The relative efficiency of positive  selection of the X versus A is  lineage dependent.
C. Additional  effects	Recombination rates may differ  between X and A.	Recombination rate differences  will also affect efficiency of  positive and negative selection.	47, 73, 74
Genomic conflict  (meiotic drive)	Drive should be more common on X  than A.	Generally confirmed, but there  may be an ascertainment bias.	16–19, 21,  75–78

^*A stands for autosome.

^**SA = sexually antagonistic