Goals: Introduce the concept of natural selection occurring at different levels. Discuss group selection, and why it is unlikely. Consider traits that were proposed to have evolved through group selection, and the importance of finding other explanations for them.
Related Textbook Material: Freeman and Herron (2001) Chapter 10
Lab Manual Questions over this material are in Lab Manual Chapter XIX
The Lecture:
In this lecture, we're going to consider selection at what are called different "levels." To understand what levels of selection are, first consider the way we've defined natural selection up to this point. Natural selection, as we've considered it so far, occurs because some individuals have higher fitness than do other individuals. As a result, those traits that give some individuals higher fitness are the ones that are likely to be passed from generation to generation the most, and to evolve. We can refer to natural selection that occurs this way as individual level natural selection because the evolution of traits occurs because of differences among individuals. So individual level natural selection is just natural selection occurring the way we've been defining it all semester. We're just adding the words "individual level" because we're going to consider some other levels at which natural selection can occur so we're going to need to be able to distinguish them. Sometimes, for short, people call individual level natural selection just "individual selection."
What we're going to ask, in this lecture, is whether traits can evolve because of differences characteristics like survival and reproduction between something other than individuals. You have actually already seen an example of this in the lecture on species selection. Remember that species selection refers to the process through which traits become common throughout all of life because species that have them either have a high rate of speciation or a low rate of extinction. Having a low rate of extinction can be thought of as how well a species survives; having a high rate of speciation can be though of as how well a species reproduces. So this is selection at the species level -- traits become more or less common across all of life because of differences in the speciation and extinction rates of the species with those traits.
An important difference between species selection and other levels of selection such as individual selection is that species selection does not explain how a trait evolves within a species. Suppose, for example, that there are many species of mammal with small body size because small mammals have a higher speciation rate. This explains why so many species have the trait of small size, but does NOT explain why small size evolved in the first place. It only explains what happens once small body size has evolved (for some other reason, such as individual selection.)
For the rest of this lecture we will consider whether traits can evolve within a species because of selection occurring at levels other than the individual level.
V.C. Wynne-Edwards, in the early 1960's, proposed that traits could evolve through what he called group selection. Group selection is defined as the evolution of traits because they decrease the chance of a population going extinct. This is selection at the population level; the idea is that the traits that would evolve through group selection are traits that increase population survival (rather than individual survival, as is proposed for individual selection.)
NOTE that I have been telling you all semester that traits do NOT evolve because they increase group (population) survival! It turns out that Wynne-Edwards was mostly wrong -- in general, we're going to see that group selection is unlikely. However, we're going to consider group selection because historically, it took Wynne-Edwards proposing the group selection hypothesis to make people realize that it really doesn't generally occur. It turned out that before Wynne- Edwards discussed group selection, many evolutionary biologists were sloppy about considering whether a trait was good for the individuals who had it or good for the population or species (this kind of sloppiness is what I've been trying to get you to avoid.) As a result, there were traits that people hadn't studied carefully because it seemed obvious that they were good for species. Once Wynne-Edwards proposed group selection, other biologists realized that it does not work, and then realized that there were traits whose evolution had to be explained, because the advantages people had proposed for them were advantages to species. So even though Wynne-Edwards turned out to be wrong, his work turned out to be extremely important because it led to so much further study. As a result, his name is well known among evolutionary biologists -- isn't it nice to know you can get famous by being wrong?
So let's consider what Wynne-Edwards proposed, why it doesn't generally work, and then, most importantly, how we can explain the evolution of some traits that he though evolved through group selection because he thought individual level natural selection couldn't explain them.
Wynne-Edwards thought that, through group selection, traits that were bad for population survival would lead to population extinction, so that the only populations to survive would be populations of individuals who did not necessarily have traits that were best for their own survival and reproduction, but which were good for the survival and reproduction of the population. He described a number of traits for which it appeared that individuals were doing things that would decrease their own fitness but would increase the population's chance of survival. Two of these traits are:
Once we realize how unlikely group selection is, however, we have to explain the evolution of the traits Wynne-Edwards proposed would evolve through group selection. The group selection explanation for decreased reproduction and for altruism is apparently wrong. So how DID they evolve?
It turns out that we can explain decreased reproduction through individual level natural selection. Wynne-Edwards noted that individuals reproduce less than they physiologically could. However, it turns out that individuals who reproduce less than the physiological maximum each time they reproduce may have more offspring in their total life. Reproduction takes time and energy. If individuals had a large number of offspring at a time, it's likely that not all the offspring would survive -- the parents might not have energy to care for them. Further, caring for all those offspring could decrease the energy for survival or future reproduction in the parents -- they might not live long enough to have more offspring, or might not have enough energy to produce more offspring later. So we don't need to consider group selection to explain the degree of reproduction of individuals, rather, we need to consider their opportunities for future reproduction and what level of reproduction will give the most offspring, on average, during a lifetime.
In contrast, we can not explain all the observed cases of altruism through individual level natural selection. Through individual level natural selection, we would expect that the trait of worker bees of becoming sterile and helping the queen bee reproduce should die out -- it should have zero fitness since workers do not reproduce. There are, however, many species of insect that show this kind of trait. Further, in many social birds and mammals there are individuals who may never reproduce, and who help other individuals to reproduce. This can not be explained by individual selection. Group selection does not work, so group selection won't explain it. We are going to have to consider another form of selection, called kin selection, to explain it.
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. Kin selection occurs 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 (a genetic relative) to the
altruist, it is likely to carry the same alleles as the altruist, and is
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, both 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. 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:
Now that we have the terms defined, let's see why the formula br-c>0
explains when altruism will have higher inclusive fitness
than non-altruism. When we multiply b by r, what we're doing is multiplying
the number of extra offspring the recipient has,
who can inherit the allele for altruism if the recipient has it, by
the chance that the recipient actually does have the allele for
altruism. To be passed from generation to generation, the allele for
altruism must occur in the recipient and must be passed on.
The higher the chance that the recipient has the allele, the more offspring
will inherit the allele. The more offspring the recipient
has, the more offspring there are who may have the allele. So br tells
us the degree to which the allele for altruism is passed on
by telling us the number of offspring who may inherit it and the chance
that they really can inherit it.
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.