Does sexual selection protect stressed populations from extinction?

Darwin was the first to recognise the importance of sexual selection as a powerful force acting when individuals of one sex (usually males) compete for reproductive success. Darwin must have realised that a world without sexual selection would be very much more sombre than we are accustomed to. The myriad of sexually-selected sights, sounds and smells across the natural world would be gone. Perhaps even the accelerated evolution of human intelligence, and my capacity to (attempt to) communicate these thoughts to you, would not have happened without sexual selection.

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The experimentally evolved model Tribolium castaneum: a small brown beetle that will answer questions about the importance of sexual selection for population genetic health.

We know quite a lot about how specific traits evolved through sexual selection. We also recognise big variations in the intensity and results of sexual selection between different species. However, we understand very little about the consequences of sexual selection at the level of the population, and how sexual selection shapes a genome. These gaps in our knowledge are the focus of my Leverhulme Fellowship, which will address two questions. First, does a history of sexual selection improve a population’s genetic health? Second, how far and wide across the genome does sexual selection act?

Achieving reproductive success depends upon many interacting condition and health traits so, in theory, reproduction in the face of high sexual selection should influence the whole genome, ruthlessly filtering out any deleterious, or even neutral, mutations, and promoting advantageous alleles through those individuals that reproductively succeed. As a result, this process should improve the genetic health of a population, a relevant consideration at a time when many natural populations face unprecedented pressure.

To test these ideas, my genetic model is the flour beetle Tribolium castaneum, a global pest. We have established lines of this beetle that have experimentally evolved under conditions where the only variable is the level of sexual selection. There are three general regimes:

  1. No sexual selection, where each generation is created with a series of monogamous pairs
  2. Low sexual selection, where each male has plenty of female partners, meaning low male:male competition and low opportunity for mate choice by females
  3. High sexual selection, where many males must fight for the few available females, and females enjoy wide choice of mates.

Now, after more than six years, and over seventy-five generations, the lines have evolved and are ready to be screened.

A good measure of a population’s underlying genetic health is how it copes under inbreeding, when the underlying mutational load in a population becomes exposed, resulting in a reduction in population fitness known as ‘inbreeding depression’. If sexual selection acts across the genome, and ‘reproduction of the fittest’ purges deleterious forms of genes that make up mutational load, then we expect lineages with histories of high sexual selection to resist extinction. After twelve generations of sib-sib inbreeding, and many extinctions, the results so far are extremely encouraging.

In parallel with these inbreeding-to-extinction assays, I will also be measuring how far and wide sexual selection impacts on the genome, using some of the excellent molecular analysis facilities at the Norwich Research Park. Because all the lines derive from one ancestral population, we can make meaningful comparisons between lineages exposed to high or low sexual selection, and use the sequenced T. castaneum genome to measure which specific genes have changed.

Professor Matthew Gage
School of Biological Sciences, University of East Anglia