Sex Ratios

Because the ability to pass on one’s genes is dependent on finding a mate, the sex ratio (the relative proportion of males to females) is critical to population biology. When one sex outnumbers the other, as females do below, the rarer sex has a reproductive advantage. Because families with more sons will then have more descendents, alleles that cause parents to over-produce sons will increase in frequency over future generations, bringing the sex ratio back to 1:1.

We explore factors that throw sex ratios out of whack, and the implications of these “wack” sex ratios for population survival, mutation accumulation, and other important aspects of evolution. See Freedberg and Wade (2001, 2004), Freedberg (2002), Freedberg and Debenport (2014), and Butka and Freedberg (2019) for more on sex ratio evolution.

  • Turtles in the bottom row were incubated at 25°C, leaving them with poor coordination and long righting times. Turtles in the middle row were incubated at 30°C and right quickly. Because female turtles must leave the water to build nests every year, they require more agility than males. Sex determination has evolved to produce females at the warm temperatures that benefit them most (Freedberg et al. 2001, Freedberg et al. 2004).