For a male to put in so much effort to impress a female, you would
hope that it is all worthwhile. Why do females care in the first place about ‘advertisements’
that males have adapted?
Below are some explanations, some of which are still in debate in
the scientific literature.
Females that are picky might get better genes
for their offspring
In this case, females have preferences for males who indicate
their superior genes through more attractive displays. An example of this is
the Gray Tree Frog (Hyla versicolor),
a species of arboreal frog which live in the woodlands of the United States and
South-eastern Canada. In the breeding season, these males give either short
calls or long calls to attract mates. Researchers observed that a large amount
of the time female Gray Tree Frogs purposefully ignored short-calling males and
travelled greater distances in order to find a longer calling male in the
distance. Welch et al (1998) studied
whether males giving long calls were genetically superior to those with small
calls by comparing the fitness of the tadpoles.
The conclusion was that offspring of the long-calling males had a higher
fitness than those of the short-calling males, which was particularly evident
in their ability to feed better as tadpoles.
In this hypothesis, good genes evolve under direct natural
selection, and as a result, the preferences of the opposite sex evolve due to
indirect selection (Ryan, M.J. 1998).
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Gray Tree Frog - Image by Marc Behrendt |
Females that are picky might benefit through
gaining resources
In
this case certain males of the species may provide better resources (such as
food, superior territory and parental care) compared to other males, and
consequentially, they appear more attractive to the female (Freeman 2014). Over
the generations, females that distinguish good providers from bad ones will chose
the one that is beneficial to either her or her young (or both) and males with
better provision of resources survive and reproduce better than others, even if
their trait appears to contradict natural selection.
An example is the
beautiful Blue Tit (Parus caeruleus),
whose carotene colouration is directly related to their ability to find
caterpillars which are a good sense of nutrition for young birds. In an
experiment to distinguish between the good-parent hypothesis (direct signal)
and the good-gene hypothesis (indirect signal), Senar et al (2002) concluded that yellow plumage brightness of father
Tits is directly correlated with their offspring investment, and therefore females
sexually select for these males.
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Blue Tit Male - Image by Silvestr Szabo |
Introducing the Fisherian Runaway/Sexy sons
hypothesis
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Runaway Selection - Image by Mathilde Gordon |
Ok, so I promised I would explain the Austin Powers photo from my
last post. Well, here goes.
Just like the good genes female preference hypothesis, the
hypothesis for runaway selection states that preferences evolve because they
are genetically correlated with male traits which are under direct selection (Ryan,
M.J. 1998).
...Say what?
Basically, female preferences and male traits co-evolve through
genetic correlation. In the good genes selection, male display signifies to the
female a superior genotype, whereas in runaway sexual selection, females prefer
the attractiveness of “sexy sons,” (a.k.a. Austin Powers) which are males that
have a greater mating success, but aren’t necessarily superior in terms of
other aspects of fitness.
It starts with a trait arising in a population through natural
selection. A majority of females favour this trait, based solely on the fact that the male possesses it. It is an arbitrary
preference, based on a completely random whim. Even when this trait is no
longer advantageous, it remains to be sexually selected for. Over the
generations this leads to a runaway selection as a result of a positive
feedback mechanism. Certain males with the right trait are selected for à they reproduce à their offspring are more likely to
possess the trait à females
become more likely to have a preference for that trait à more males with the right trait are
selected for, etc.
There is selection on the females to maintain the preference and
selection on males (by the females because of the preference), and this forms a
continuing loop, as shown in the image above!
In previous blogs I have talked about female choice being the
reason why males evolve such elaborate traits. Although these traits don’t seem
to “follow the rules” of natural selection, the consequential increased mating
success of the males more than compensates for threats to their survivorship. I
hope this last blog has demonstrated a few reasons why females care about these
traits in the first place, and how they come about evolving preferences.
References:
- Freeman, S., Herron, J.C., 2014, Evolutionary Analysis, Pearson Education Limited, Edinburgh
- Ryan, M.J. 1998, ‘Sexual Selection, Receiver Biases, and the Evolution of Sex Differences,’ Science, Vol. 281, No. 5385, pp. 1999-2003
- Senar, J.C., Figuerola, J. and Pascual, J. 2002, ‘Brighter Yellow Blue Tits Make Better Parents,’ Biological Sciences, Vol. 269, No. 1488, pp. 257-261
- Welch, A.M., Semlitsch, R.D. and Gerhard, H.C. 1998, ‘Call Duration as an Indicator of Genetic Quality in Male Gray Tree Frogs,’ Science, Vol. 280, No. 5371, pp. 1928-1930
- http://www.paleoprep.com/Photography%20page/Frogs/01_image.htm, (29th March 2014)
- http://www.photoextract.com/photo/274073.html, (30th March 2014)
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