Goals: define kin selection and inclusive fitness. Learn how Hamilton's formula (br-c>0) can explain when altruism can evolve. Consider haplodiploidy in the insect order Hymenoptera, and its possible importance to the evolution of altruism.
Lab Manual Chapter: based on the material in this lecture, you should be able to answer questions 5-11 in chapter XX of your lab manual.
The Lecture:
Remember from the previous lecture that altruism refers to actions of one individual (an altruist) that increase the survival and reproduction of another individual, the recipient of altruism, and decrease the survival and reproduction of the altruist. Such behavior can not be explained through individual level natural selection. In this lecture we will see how it can be explained through another form of natural selection, called kin selection.
Kin selection refers to the evolution of traits because they are passed on by the relatives (the kin) of individuals who express the traits. The main kind of trait that is thought to evolve through kin selection is altruism. The way this is proposed to occur is as follows. Suppose altruism is a genetic trait that some individuals will express but for which other individuals can carry the alleles but not express them. Suppose an altruistic individual helps another individual to reproduce. If that individual (the recipient of the altruist's help) is kin to the altruist, that means it is a genetic relative, likely to carry the same alleles, and therefore likely to carry the allele for altruism. So the allele for altruism can be reproduced by the individual who receives help if that individual is related to the altruist. It may be reproduced so much that it increases in the population, even though the altruist does NOT reproduce it very much.
Altruism can evolve, therefore, not because it increases the survival and reproduction of the individual who expresses the trait of altruism (since this individual has decreased reproduction) but because it is reproduced by kin of that individual who have, but do not express, the alleles for altruism. Like other traits, altruism will evolve if it is passed from generation to generation more than are alternative alleles for non-altruism. However, we can't describe how altruism will evolve based on the survival and reproduction of the individuals who express altruism, so our usual measure of relative fitness does NOT work to explain the evolution of altruism. Instead, we need to consider something called "inclusive fitness."
Inclusive fitness refers to the degree to which a trait is passed from generation to generation. The trait can be passed from generation to generation directly, by reproduction by individuals who express the trait, and also indirectly, when individuals who express the trait help (are altruistic toward) individuals who carry the alleles for the trait, and who reproduce more because they receive help from the altruistic individuals who express the trait. So both ways in which a trait like altruism can be passed on must be considered to evaluate its inclusive fitness.
An evolutionary biologist named W.D. Hamilton described the conditions under which an allele for altruism will be passed on more than an allele for non-altruism -- that is, the conditions under which altruism will have higher inclusive fitness than does non-altruism, and under which altruism will, therefore, evolve. These conditions are typically presented as a formula (called Hamilton's formula) which is:
br-c>0
In this formula, the terms b, r, and c mean the following:
The term c in the formula tells us the degree to which an allele for non-altruism will be passed on, since it tells how much more a non-altruist reproduces than does an altruist.
So if br is greater than c, it means that the allele for altruism is being passed on more than the allele for non-altruism, so altruism has higher inclusive fitness and will evolve. If br is greater than c, then br-c>0, so the formula br-c>0 tells us when altruism will evolve.
We're not going to plug numbers in and solve this formula. The numbers we'd need are very hard to measure exactly -- how do we tell how many more offspring a recipient of altruism is having than is a non-altruist? What we will do with the formula, instead, is to try to interpret it to predict or explain situations in which altruism evolves. We can consider factors that would make b or r larger. Such factors will make altruism more likely to evolve. We can also consider factors that would make c larger. Such factors will make altruism less likely to evolve.
Various aspects of the ecology of species can affect values of b and c. If resources such as food or good nesting sites are scarce, for example, individuals may not be able to reproduce very much without help, so c would be low (non-altruists would not have many offspring because of scarce resources) and b would be high (individuals who receive help could have many more offspring than they could without help.) r can be affected by ecological factors that determine how likely relatives are to meet each other. If individuals who contact each other are not likely to be related, then r, the relatedness between individuals who interact with each other, will be low and altruism will not evolve.
r can also be affected by the genetic system of a species. In particular, in the group of organisms that have some of the highest levels of altruism -- the bees, ants, and wasps -- the genetic system causes a higher level of relatedness between sisters than we see in most groups. Here's how this works:
Wasps, bees, and ants belong to an order of insects called the Hymenoptera. Insects in the Hymenoptera have a genetic system called haplodiploidy. Haplodiploidy means that males are haploid and females are diploid. The way this occurs is that the eggs produced by a female can develop if they are fertilized by sperm or if they are not fertilized. When eggs are fertilized, they are diploid; these develop into females. When eggs are not fertilized, they are haploid; these develop into males.
Let's compare the relatedness of sisters in diploid species with the relatedness of sisters in haplodiploid species. To determine relatedness, we consider two halves of the genome -- the half inherited from the mother and the half inherited from the father. In a species where males and females are both diploid (like us), the independent assortment of alleles will cause about half of the alleles the sisters inherit from their mother to be the same. So in the half of the genome inherited from the mother, about half will be the same, so 1/4 of the alleles will be the same through the mother. Similarly, independent assortments of alleles will also cause about half of the alleles the sisters inherit from their father to be the same. So in the half of the genome inherited through the father, about half will be the same. So 1/4 of the alleles will be the same through the father. The total relatedness, r, we get by adding the proportion of alleles the same through the mother to the proportion of alleles the same through the father. This is 1/4 + 1/4 which equals 1/2.
Now consider a haplodiploid species. What is the relatedness between sisters? Through their mother, it works just like it works for a diploid species (because their mother is female, and therefore diploid): in the half of the genome inherited from the mother, about half the alleles will be the same, so 1/4 of their alleles are the same through the mother. But on their father's side, there's a difference. Their father is male, and is haploid. This means he produces his sperm cells through mitosis -- each sperm cell has exactly the same chromosomes are in every other cell, and each sperm call has exactly the same chromosomes as each other sperm cell. So ALL of the alleles inherited from the father are the same between the two sisters. This means the half of the genome inherited through the father is the same between the sisters, so the proportion of the whole genome that is the same because of inheritence through the father is 1/2. So the total relatedness, r, equals 1/4 (from the mother) plus 1/2 (from the father) which equals 3/4.
The point to all that math and genetics is that full sisters in haplodiploid species have higher relatedness, r, than do siblings in diploid species. This probably helps to explain why altruism where females help sisters -- as when the worker bees, which are sterile females, help the queen bee -- has evolved so often in the ants, bees, and wasps.