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Topics of this course

Why do most higher organisms reproduce sexually? Why be diploid rather than haploid? Why should there be separate sexes? What factors affect mate choice? How does ecology influence the breeding system? Such questions are the focus of much current research in evolutionary biology. In this course, we will examine explanations for the diversity of breeding systems that we see around us. The course is organized into the following modules.


Diversity of breeding systems

Laurence Loewe
What is sex? What is a breeding system? We will see how various organisms have different solutions for these questions. We will cover concepts like anisogamy, diploidy and haploidy that are important in understanding how bacteria and multi-cellular organisms differ in their evolutionary opportunities. After looking at what makes males male and females female, we will also cover some features of organisms that can do both in one individual. While sex has its advantages, sex is also expensive and we will review the costs and benefits of sex from an evolutionary perspective. The importance of mathematics for this field will be highlighted.

Evolution of inbreeding and outbreeding
Evolution of separate sexes

Deborah Charlesworth
In these 4 lectures the following issues will be discussed in the context of mating system evolution:

  • How selection works to drive evolutionary change (individual selection versus group selection - benefitting the population or species). How selection can lead to consequences harmful to the population or species,
  • How selection can act in indirect ways, not directly on individuals' characteristics such as survival or ability to grow fast, but on characters, such as which individuals to mate with, which can affect the offspring,
  • Trade-offs between different functions.

Lectures 1 and 2 review mating systems, and then deal with changes from inbreeding to outcrossing, including self-incompatibility and its breakdown, discussing empirical data, including how mating systems and self-fertilisation rates in nature can be determined. Theoretical models will be discussed for the kinds of changes observed, including the importance of inbreeding depression and of ecological factors selecting for changes in mating systems.

Lectures 3 and 4 deal with evolution from hermaphroditism to having separate sexes, reviewing the importance of outcrossing and of reallocation of resources in these changes, and considering the genetic changes involved.

Evolution of sex

Laurence Loewe
From a genetic perspective, the main feature of sex is facilitating the regular recombination of genomes. This is important for speeding up adaptive evolution and the removal of deleterious mutations. Various evolutionary scenarios propose circumstances that could have led to the evolution of recombination. An important framework for understanding has been the concept of modifier loci that have no direct effect on fitness but only affect the recombination rate. We will discuss the evolution of recombination in this context.

Sex allocation

Stuart West
Sex allocation is the allocation of resources to male versus female reproduction. It covers a range of questions, such as: (a) should individuals adjust their offspring sex in response to environmental conditions; (b) why do individuals of some species change sex? This is a hugely successful area of evolutionary biology, with theory able to explain the data in a range of organisms, including plants, protozoa, insects, fish, birds and mammals.

The key reading for the lectures is Charnov et al. 1981 Nature 289: 27-33; West & Sheldon 2002 Science 295: 1685-1688. There are no pdfs of the lecture. PDF’s of the majority of relevant papers are available at Stuart West's teaching page.

Evolution of Sex Determination

Brian Charlesworth
Sex can be determined either by environmental factors (such as temperature), or by genetic factors. These range from simple single gene differences between males and female, up to fully developed sex chromosome systems, and bizarre mechanisms such as male haploidy and cytoplasmic female determining factors. We will discuss this great diversity of genetic sex determining systems in the first lecture. In the second lecture, we will examine how evolutionary transitions among different modes of genetic sex determination may occur, and discuss what may cause such transitions. We will also discuss what selective factors may favour environmental versus genetic sex determination.

Evolution of Sex Chromosomes

Brian Charlesworth
The most familiar form of sex determination (as in mammals and Drosophila) involve highly distinct X and Y chromosomes (W and Z with female heterogamety). In such cases, the X and Y fail to cross over along all or part of their length, the Y lacks most of the genes that are carried on the X, and contains a lot of non-coding, repetitive DNA sequence. We discuss evidence that the X and Y evolved from a normal pair of chromosomes with nearly all of their genes in common, initially in response to the evolution of separate sexes. We then discuss the evolutionary forces leading to the loss of functional genes from the Y chromosome. We then examine some model systems for testing for the action of such forces.

Background reading

  • Ridley M (2004) Evolution - Third Edition. Blackwells. Chapter 12.
  • Stearns SC, Hoekstra RF (2005) Evolution : an introduction, 2nd ed. Oxford.
    Chapters 8+10
  • Maynard Smith J (1978) The Evolution of Sex. Cambridge 
    –  still the best text overall, although there has been much work since.
  • Michod RE, Levin Bruce R, editors (1988) The Evolution of Sex: An Examination of Current Ideas (Sinauer)  -  also covers much of the course.
  • Bull JJ (1983) Evolution of sex determining mechanisms. Benjamin/Cummings. 
    – useful for the lecture on sex determination.
  • Current Biology -> 5 Sept 2006 -> Biology of Sex Special. 
    - there is a nice collection of 9 up to date papers on the topic.

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