When I was a telecom engineer there was a particular transistor that we would routinely change on any circuit board we were working on. It suffered from a condition known as thermal runaway. A drop in resistance across the collector-base junction caused a rise in current that generated heat which reduces resistance in semiconductors which increases the current even more, creating more heat and so on until the transistor self-destructed.
(For geeks - this is why field-effect transistors became the choice for amps)
Thermal runaway is an example of a positive feedback loop - in this case an unhelpful one. A more constructive example of positive feedback is responsible for some of the most interesting features in the natural world.
In the history of life, survival is only a means to an end. It is reproduction that transmits successful genes to future generations. A body that is well equipped to live to a great age but never attracts a mate is unsuccessful from the perspective of evolution. It was this observation that led Darwin to propose the theory of sexual selection.
In 1982 Malte Andersson carried out a neat experiment on a group of 36 male widowbirds in Kenya. The widowbird is about the size of a sparrow but in breeding plumage the male has a spectacular tail 18 inches long. Andersson divided the birds randomly into group of four. In each group he cut the tail feathers of one bird so that it was left with 5 1/2 inches. He then glued the extra feathers to one of the other birds in each group so that they ended up with extra long tails. The third bird was left untouched and as a control, the fourth bird had his feather cut and glued back on again in order to eliminate the possible effects of human interference.
He then returned all 36 to their territories and left them to go about the task of attracting mates and breeding. When the number of viable nests in each territory were counted a very clear pattern emerged. The birds with the longest tails had attracted four times as many females as those with the shortened tails. The other two males in each group had intermediate results.
Clearly the extraordinary plumage of the widowbird is being driven by the preferences of females - preferences that are under the control of nervous systems that are built by genes.
Now here is where the positive feedback loop comes in. A male with a long tail is likely to have a father with a long tail as well as a mother who likes males with long tails. So when a female chooses a male with a long tail they are also choosing a genome that causes a female to desire a male with a long tail.
Remember that females have all the same genes as men and vice-versa. The difference is in which genes are expressed.
Looking at it from the perspective of the female if she has the genes for preferring a long tail then the chances are her mother also had those genes. It follows then that her father had a long tail and so she too has the genes both for long tails and for preferring long tails.
In both male and female bodies these two genes travel together in a connection that geneticists call "linkage disequilibrium". As genes are shuffled by meiosis from one generation to the next they are likely to get shuffled together.
In Andersson's experiment the females tended to choose males who had tails much longer than ever occurs in the real world. The reason that some males have not evolved 23 inch tails which would guarantee them the pick of females is there is a high price to pay for elaborate ornaments. Difficulty in flying and avoiding predators has to be balanced against female preference. On the one hand there is the sexullay selected optimum and on the other a utilitarian optimum. The tail length of male widowbirds is a compromise between the two.
Sexual selection is the driving force behind some of the most remarkable features in the natural world. The human brain may be the most impressive example of all.
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