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Evolution of babysitting in bluebirds

Major transitions in evolution have often involved loss of independence, as discussed last week. Most female bees work to increase their mother’s reproduction, rather than laying eggs themselves. Less extreme examples of helping others reproduce are known in some animals. “Kin selection? favors helping relatives, if the cost of helping is less than the benefit to the one helped, times their relatedness to the helper. This is known as Hamilton’s Rule. As Haldane put it, “I would jump into a river to save two brothers or eight cousins.? “Cost? and “benefit? are measured in number of offspring and “relatedness? is relative to one’s usual competitors. If surrounded by cousins, Hamilton’s Rule would lead to helping only siblings.

For helping behavior to have evolved, there must have been genetic variation in helpfulness. This week’s paper shows that this is still true for western bluebirds in Oregon.

Anne Charmantier (anne.charmantier@cefe.cnrs.fr), Amber Keyser, and Daniel Promislow, of Oxford and the University of Georgia, published “First evidence for heritable variation in cooperative breeding behavior? online in Proceedings of the Royal Society.

1593 bluebirds were observed at nest boxes by volunteers with the Prescott Bluebird Recovery Project. They recorded which birds helped feed or defend babies other than their own and which birds received such help. Only a small fraction of breeding pairs got help. 70% of helpers were sons of the pair helped, 16% were brothers, and 6% were daughters.

Because they had a seven-generation family tree for these birds, they were able to determine whether helping is inherited. It was, with a heritability of 76%. The authors also mention the possibility that inheritance could be cultural rather than genetic, if individuals copy relatives they see helping.

With such high heritability, they note that “rapid microevolutionary changes in the extent of cooperative breeding are possible.? But microevolution in which direction? If the cost of helping is low (because helpers could not have found their own breeding territory), then kin selection favors helping relatives to reproduce. Helping may also enhance individual survival, if those that don’t help are chased away and have nowhere to go (Trends in Ecology and Evolution 17:15-21).

On the other hand, if unoccupied breeding territories are available, then individual reproduction will have a higher payoff than helping. In that case, high heritability could lead to a rapid evolutionary decrease in helping. Rapid evolution back and forth over a few generations may cause little long-term change.


Comments

Funnily enough, I am just re-reading The Selfish Gene and I've just finished the chapter that discusses this kind of trade-off. If I remember correctly, the theory is that rather than constant oscillation between two extremes, the stable state will be a mixed population of helpers and non-helpers. If you can quantitate the relative costs and benefits of helping vs. not helping and of being helped, you should be able to calculate the ratio of different behaviours that will be present in a stable population.

Of course The Selfish Gene is not the most up-to-date text and there have been subsequent advances in this field, as mentioned in Dawkins' footnotes in the edition I'm reading. I might have to read some more about Game Theory to understand this field a little better!

I meant that, as the availability of breeding territories varies (with weather, epidemics, etc.), then the direction of selection could change. I was thinking of rapid short-term evolution of Darwin's finches with wet or dry years, but little long-term trend.

Right you are... sorry about that, I was a little too enthusiastic at the thought of having something to contribute!

Enthusiasm is good. "Selfish gene" is great. I was unclear.

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