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Unit 23 – Indicator Processes and Fisherian Mating Advantages

Difficulty: medium/hard

Time: 6 minutes

The following passage has been adapted from Essays in Animal Behaviour, by Jeffrey R. Lucas and Leigh W. Simmons

In addition to indicators, Fisher (1915, 1930) suggested another major mechanism of sexual selection by female choice, which is now associated with his name. Males with traits preferred by females will have a mating advantage. This advantage is inherited by sons of females with the preference. As genes for preference and trait become associated in offspring, the male trait favoured by female choice will carry the female preference with it. A self-reinforcing positive feedback loop, Fisher’s “runaway process,” can therefore develop, bringing trait and preference to more extreme values (Lande 1981). Although they have sometimes been treated as incompatible alternatives, Fisherian mating advantages (“sexy sons”) and viability-based indicator processes are likely to occur together.

The usefulness of the distinction between Fisherian (sexy sons) and indicator mechanisms was recently questioned (Kokko 2001, Kokko et al., 2002, 2003). The critics suggested that it is a false dichotomy, and that the two processes are opposite endpoints along a continuum, without any qualitative difference between them. A conceptual distinction is useful if it helps us better understand some interesting aspect of the world that might otherwise go unnoticed, unexplained, or misunderstood. Does the distinction between sexy sons and indicator mechanisms provide such help? I think it does, in several ways. Genetic indicator processes may be driven by advantages derived from overall genetic condition, such as relative freedom from deleterious mutations; that is, by genetic mechanisms that involve other and much larger parts of the genome than do Fisherian mating advantages. The latter can be based on genes that, in essence, influence only mating preferences and preferred traits. Genetic indicator processes on the other hand can work without any sexy sons mating advantage. This has been shown in genetic models by using strict monogamy mating rules (Andersson 1986), and by preventing build-up of gametic disequilibrium between genes for male display and female choice (Houle & Kondrashov 2001), which is a crucial component of the Fisherian runaway process. The two mechanisms are qualitatively different also in that indicator mechanisms can maintain female choice in the face of direct costs of choice, whereas Fisherian mating advantages cannot do so.

These important differences also lead to fundamental differences in evolutionary dynamics and outcomes, which cannot be fully understood without a clear distinction between the two kinds of processes (Cameron et al., 2003).

The distinction between sexy sons and indicator mechanisms can also be important for understanding how mate choice translates into various fitness consequences. Many of us seem to be interested in understanding precisely, from a selection point of view, what costs and benefits accrue to offspring from individuals choosing mates with well-developed secondary sex traits. Are the benefits mating advantages (sexy sons), other reproductive advantages, survival advantages, or some combination? The differences between sexy sons and indicator mechanisms concern just these kinds of different consequences, and may therefore be of interest to many. For these reasons it seems to me useful to keep the distinction as clear as possible.

 

1. An example of a trait that develops as a direct result of Fisher’s ‘runaway process’ would be:

  • A      Dark hair
  • B      Mutation-free offspring
  • C      Abnormally small offspring
  • D      Muscular offspring

 

2. Which of the following is an example of a ‘dichotomy’? (2nd paragraph)

  • A      Sound and noise
  • B      Bees and wasps
  • C      Intelligence and stupidity
  • D      Children and adults

 

3. According to the above paggage, Fisherian mating preferences:

  • A      Are only observable in females
  • B      Occur independently of indicator processes
  • C      Leave progeny open to mutations
  • D      Can lead to enhanced secondary sex traits

 

4. Which of the following is not a necessary component of Fisher’s ‘runaway process’?

  • A      Competition
  • B      Monogamy
  • C      Gametic disequilibrium between genes for male display
  • D      Choice
  • Q1: C
    “A self-reinforcing positive feedback loop, Fisher’s “runaway process,” can therefore develop, bringing trait and preference to more extreme values.”
    The sexy sons will have the preferable traits and, in a competitive environment, will be more likely to reproduce than those that do not have the traits. Their sons will also have the traits. In this case the preferable trait is to be small. The small parents have small offspring, which have even smaller offspring. As the population becomes smaller, the preferable traits become more prominent as a result of mating preferences, the resulting cross-over of genes, and competition between sons.
    The answer is C because it is the only one that acknowledges the tendency of the runaway process to bring traits to extreme values.

    2.B
    A dichotomy is the splitting of a whole into two distinct individual, non-overlapping parts which can be clearly defined. The passage gives us an example of a false dichotomy, so you don’t even need to have seen the word before to have a go at this answer.
    False dichotomy: “… the two processes are opposite endpoints along a continuum, without any qualitative difference between them.” This could equally be said about A, C and D – it is very hard to tell where one ends and the other begins. Wasps and Bees are two, often similar looking, but ultimately very different animals.

    Q3:D
    Refer back to the explanation for question 1. One of the main facets of the Fisherian process is the tendency for traits to overdevelop. The phrase ‘secondary sex traits’ is referenced later in the piece; “…offspring from individuals choosing mates with well-developed secondary sex traits.” and is something that should probably be understood for the bio portion Section III of the exam.
    “Fisherian mating advantages (“sexy sons”) and viability-based indicator processes are likely to occur together.” – this is what rules out B. The passage never explicitly says that the preference is only observable in females – the writer just sticks to that gender to facilitate ease of communication. Also in the final paragraph the writer mentions “.. individuals choosing mates…” (individuals, not females), C is wrong for the same reason, it’s not said. Just because indicator mechanisms can result in immunity to disease or mutation doesn’t mean the other leaves them open to it.

    Q4: B
    “This has been shown in genetic models by using strict monogamy mating rules (Andersson 1986), and by preventing build-up of gametic disequilibrium between genes for male display and female choice (Houle & Kondrashov 2001), which is a crucial component of the Fisherian runaway process.” – the point of this experiment was to make sure the runaway process did NOT happen. Monogamy being one of the ways that they tried to ensure it.
    As per Q1, Competition (among males exhibiting the preferred trait) and choice (among females) are also crucial. (though it can just as easily be females competing for male choice).

    Gamsat Sample Questions

    June 3, 2012

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