social evolution

social evolution - How and why do sex ratios vary Sex ratio...

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How and why do sex ratios vary? Sex ratio will be interpreted as referring to the number of females in respect to the number of males in a given population. Sex ratios have been demonstrated to usually present a consistency within a community, in animals the sex ratio being 100:100, that meaning for every 100 males born, 100 females are also born (Grant, 1998). The sex ratio although usually consistent, is not constant; it may alter over the course of a generation’s life or from one generation to the next. Variations between populations and species through space and time do occur (Warren & Beukeboom, 1998). It is how and why this variation from the norm occurs that shall be considered. As mentioned, the accepted standard approximate sex ratio is 1:1, termed the Evolutionary Stable Strategy (ESS) (Hans et al, 1996). Sex determination factors act to affect what the sex of an individual will be, thus if these factors are biased towards either gender, the resulting sex ratio within a population would be skewed (Charnov et al, 1981). In dioecious species, numerous mechanisms exist to cause sex determination; haplodiploidy, paternal genome loss, male/female hetrogamety, polygenic sex determination or even environmental factors (Warren & Beukeboom, 1998). These sex determination factors can alter within a species or between closely related species, even changing if destabilized, leading to the evolution of new mechanisms (Warren & Beukeboom, 1998). During this essay, several different sex ratios may be referred to, each relating to a differing stage; primary sex ratio is at fertilisation, secondary sex ratio is at birth, tertiary sex ratio is upon sexual activity and finally quaternary sex ratio refers to post- reproductive activity (Hardy, 2002). It was Fisher (1930), who first suggested the theoretical explanation of why the two sexes are usually produced in the same number, and thus a basis of standard from which variation can be viewed (Hamilton, 1967). Fisher Page 1
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considered a population in which male births are less common than female births, thus skewing the sex ratio. In this population a newborn male has a statistically better chance of mating than a female, meaning the male would produce more offspring. This is passing the genetic predisposition to produce males onto an increased number of the next generations, thus the male- producing tendencies spread and male births become more common. As the ratio approaches 1:1 the advantage dies away and the sex ratio equalises (Hamilton, 1967). This principle does however have to state that the sex ratio will be in equilibrium as long as within the population as a whole, the total effort spent producing the two sexes are equal (Ellegren et al, 1996). This mechanism has described a theory that acts to maintain the standard sex ratio in equilibrium within a population. Now what shall be considered are the mechanisms that act to cause variation of the sex ratio, the ‘how’s’ and ‘why’s’ that lead to the ratio being skewed. Fisher’s principle highlights a mechanism which would lead to a
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social evolution - How and why do sex ratios vary Sex ratio...

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