In most animal species, but in only a small percentage of plant species (7.6% according to Renner and Ricklef), individuals are unisexual and sex ratio is defined as the ratio of male to female individuals. In plants, there are several types of breeding systems, distributed on a continuous scale from dioecy to hermaphroditism. The categories are (1) dioecy, in which individuals bear only male or only female flowers throughout their life spans; (2) monoecy, in which individual plants bear separate male (staminate) and female (pistillate) flowers; and (3) hermaphroditism or cosexuality, in which individual plants bear bisexual (perfect) flowers. Transitions between these three forms exist, such as gynodioecy (in which individuals within a single population produce either female or bisexual flowers) and androdioecy (populations include both male and hermaphroditic individuals).
Theoretical models suggest that four factors influence the evolutionary outcome ofselection on breeding system:
(1) the cost of reproduction in term of available resources;
(2) the existence of a tradeoff between allocation to male and female fitness, and the shape of the curve describing that tradeoff; (3) the probability of self-fertilization; and (4) inbreeding depression.
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