Wired for Culture: Origins of the Human Social Mind by Mark Pagel Page A
Authors: Mark Pagel
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years, comes to so little.
This rule of two turns out to have a simple explanation that tells us it could not be any other way. For every offspring of a female, a male has been involved (the rule of two becomes the “rule of one” if we have in mind asexual species that reproduce on their own). If a male and female produce two surviving offspring before they die, those two replace this male and female. If this male and female were to leave behind even just three offspring, and each of these three in turn produced three that survived to reproduce, and so on, the numbers of this species would increase without end. Consider just a population of fifty males and fifty females in which each of these females produced three surviving offspring. In the first generation, the 50 females would produce 150 surviving offspring (three each), increasing the population size by 50 once both the parents had died. These 150 would in turn become 225 when each of the 75 females in this generation left behind three offspring. It is easy to see that the world would quickly become covered in layers of rats, or rabbits, and even only slightly less quickly in a layer of elephants if they could break the rule of two. If oaks and chestnuts could leave more than two, our world could become forests of these great trees. The capacity of common bacteria to reproduce is so great that if their growth went unchecked we would in a matter of days (or less time) all be standing up to our waists in a mat of bacteria that carpeted the entire world.
We learn three lessons from this. One is that for most species the difference between the numbers of offspring they produce and the numbers that survive is so large that it is not much of an exaggeration to say that all offspring ever born die before they get a chance to reproduce. Such startlingly high levels of mortality are the same as saying that competition for survival is fierce. It is this competition that lies behind the nineteenth-century philosopher Herbert Spencer’s summary of Darwin’s evolution by natural selection as “survival of the fittest.” Those few of us who have survived are well adapted: we are the rare descendants of a long line of other rare survivors who were our ancestors. The genes in our bodies and those of every other organism are those that have survived for millions or even in some cases billions of years because they were good at producing successful vehicles , while uncountably greater numbers have died trying. This means we can expect the genes we see today to be very good at promoting their interests, and they will do so by means of the ways they vary the bodies they produce. But even with all this fine-tuning, the average female still produces just two surviving offspring.
The second thing we learn is that different organisms have adopted different tactics for trying to break through the two barrier. Some—like oak trees and rabbits—go all out. Others, like elephants and whales, show more restraint, but put more effort into each offspring.
The third thing we learn is that all those different ways of producing offspring, some as rabbits, others as trees, are just different but approximately equally good ways of making vehicles for transporting genes into the next generation. All that time, bulk, energy, and trillions of individual cells required to make an adult elephant yield the same number of surviving offspring averaged over long periods of time as a rabbit, or even a single-celled yeast (of which there are, technically, not males and females, but two mating types called α and a ). Nearly every cell that resides inside a complex organism like a tree or ourselves never sees the light of day, laboring away instead to propel a small number of others into the future. It is even starker than this. The egg of a female and the sperm of a male are single cells. We could say that the trillions of cells that make up our bodies spend a lifetime devoted to seeing just two of their
This rule of two turns out to have a simple explanation that tells us it could not be any other way. For every offspring of a female, a male has been involved (the rule of two becomes the “rule of one” if we have in mind asexual species that reproduce on their own). If a male and female produce two surviving offspring before they die, those two replace this male and female. If this male and female were to leave behind even just three offspring, and each of these three in turn produced three that survived to reproduce, and so on, the numbers of this species would increase without end. Consider just a population of fifty males and fifty females in which each of these females produced three surviving offspring. In the first generation, the 50 females would produce 150 surviving offspring (three each), increasing the population size by 50 once both the parents had died. These 150 would in turn become 225 when each of the 75 females in this generation left behind three offspring. It is easy to see that the world would quickly become covered in layers of rats, or rabbits, and even only slightly less quickly in a layer of elephants if they could break the rule of two. If oaks and chestnuts could leave more than two, our world could become forests of these great trees. The capacity of common bacteria to reproduce is so great that if their growth went unchecked we would in a matter of days (or less time) all be standing up to our waists in a mat of bacteria that carpeted the entire world.
We learn three lessons from this. One is that for most species the difference between the numbers of offspring they produce and the numbers that survive is so large that it is not much of an exaggeration to say that all offspring ever born die before they get a chance to reproduce. Such startlingly high levels of mortality are the same as saying that competition for survival is fierce. It is this competition that lies behind the nineteenth-century philosopher Herbert Spencer’s summary of Darwin’s evolution by natural selection as “survival of the fittest.” Those few of us who have survived are well adapted: we are the rare descendants of a long line of other rare survivors who were our ancestors. The genes in our bodies and those of every other organism are those that have survived for millions or even in some cases billions of years because they were good at producing successful vehicles , while uncountably greater numbers have died trying. This means we can expect the genes we see today to be very good at promoting their interests, and they will do so by means of the ways they vary the bodies they produce. But even with all this fine-tuning, the average female still produces just two surviving offspring.
The second thing we learn is that different organisms have adopted different tactics for trying to break through the two barrier. Some—like oak trees and rabbits—go all out. Others, like elephants and whales, show more restraint, but put more effort into each offspring.
The third thing we learn is that all those different ways of producing offspring, some as rabbits, others as trees, are just different but approximately equally good ways of making vehicles for transporting genes into the next generation. All that time, bulk, energy, and trillions of individual cells required to make an adult elephant yield the same number of surviving offspring averaged over long periods of time as a rabbit, or even a single-celled yeast (of which there are, technically, not males and females, but two mating types called α and a ). Nearly every cell that resides inside a complex organism like a tree or ourselves never sees the light of day, laboring away instead to propel a small number of others into the future. It is even starker than this. The egg of a female and the sperm of a male are single cells. We could say that the trillions of cells that make up our bodies spend a lifetime devoted to seeing just two of their
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