Evolution of the sex ratio

Basic problem

Imagine you are a pregnant woman expecting a child. What are the chances that your child will be a girl? It would be a reasonable assumption to say that there is about a 50/50 chance of either sex, but why is this so? Most species have a sex ratio of 1:1, but some species (notably insects and reptiles) deviate markedly from 1:1. Why has a 1:1 ratio evolved so often, and what are the factors that lead to a deviation? This module investigates the evolution of sex ratios and the conditions that can lead to the deviation from an even sex ratio.

General approach

We will first develop a model to simulate a population of males and females. In this population the sex ratio of an offspring is determined by a diploid locus in the mother. We will start the simulations with uniform populations in which all individuals carry the exactly same alleles initially. Then we will introduce mutant alleles that change the sex ratio, and follow the evolutionary fate of these mutant alleles. With this approach we will be able to investigate the evolutionary dynamics of the sex ratio, and find its evolutionary optimum.

What can be learned?

Concepts:

Evolution of the sex ratio
Invasion dynamics
Individual based modelling
Evolutionary optimisation

Method:

Stochastic simulation

Starting point

Download Downloadhandout (PDF, 103 KB) describing the model.
Download and run the DownloadR script (R, 8 KB) for the model.

Interesting questions that you can investigate

What is the evolutionary optimum of the sex ratio in dioecious species, and why is this so?
What happens if the mutant allele of the sex ratio is recessive?
What happens if the mutant sex ratio allele is expressed only when inherited from the father?
What happens if the mutant allele can only deviate the sex ratio by a very small (or very large) amount?
What happens if half of the males are killed before they can reproduce?

Advanced questions:

Implement different cost of producing males and females: what is the effect on the evolutionarily stable sex ratio?
What if a sex distorter allele is located on a sex chromosome? E.g., consider a Y-linked allele that can influence the chance that Y is passed on during the generation of male gametes.
Implement continuous evolution of the sex ratio.
Model the complex life cycle of fig wasps and its effect on the optimal sex ratio.

Glossary

Anisogamy: reproduction by the fusion of two different types of gametes (male and female).
Dioecious species: species that have two sexes each producing only one type of gamete (male/female).
Evolutionary optimisation: evolution of a life history trait (or a combination of traits) towards an optimum that maximizes fitness.

Literature & Weblinks

Hamilton, W.D. (1967) DownloadExtraordinary sex ratios. (PDF, 3.1 MB) Science 156: 477-488
Trivers, R.L. and Willard, D.E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science 170: 90-92
Eric L. Charnov. (1982) Sex allocation. Princeton University Press, Princeton, New Jersey.
Charlat, S. et al. (2007) external pageExtraordinary flux in sex ratio. Science 317: 214.
Morran, L. T. et al. (2011). Running with the Red Queen: external pageHost-Parasite Coevolution Selects for Biparental Sex. Science 333: 216-218. See also the associated external pagePerspective.
CIA World Factbook: external pagecountry by country listing of sex ratios.
external pageSex ratio in Wikipedia