As we’ve learned in previous tutorials,
natural selection is the primary mechanism by which biological evolution operates. We typically envision this system as favoring
the organisms who compete the most effectively in order to survive and reproduce. However, natural selection does not always
operate in such a competitive manner. What if it were able to work in order to reward
helpful and selfless behaviors? In nature, there are many examples of animals
helping each other to survive and reproduce, even at the cost of their own welfare. Such behavior is called altruism, though many
scientists debate if true altruism can even exist. But examples of this behavior are clearly
visible across the animal kingdom. How do these behaviors come about? One graduate student in evolutionary biology,
William Hamilton, proposed that helpful behaviors can evolve if they are directed towards kin,
or family members with shared genes. Altruistic behaviors can arise through a process
called kin selection, where individuals help their kin to survive and reproduce at the
cost of their own survival and reproduction. By helping out a relative, the altruistic
individual ensures that some part of their genetic material is passed on to the next generation. This improves what is called the inclusive
fitness of the group, or the increased reproductive success of the extended family. This helps compensate for the altruistic sacrifice,
or evolutionary cost, of the individual, who may hinder or even give up their own ability
to reproduce, for the benefit of the whole group. To see such an example of this phenomenon,
look no further than an ant mound. In ant colonies, the only individual reproducing
is the queen ant. All the female ants are sisters, sharing 75%
of their DNA because of a haplo-diploid pattern of inheritance, which we touched upon in a
previous tutorial. Males are haploid, meaning they have only
one set of chromsomes. This is because they inherit one copy of each
chromosome from their mother, and they have no father. Females are diploid, meaning they have two
sets of chromosomes. This is because they inherit a copy of each
chromosome from both their mother and their father. Since their mothers are also diploid, females
inherit one of two possible sets of genes that come from the mother, and they also inherit
the only set of genes from the father, so this explains the average genetic similarity
of 75% among ant sisters. They will share much less genetic similarity
with their sons, given that the males have half the genetic material. Because of this, females are more related
to their sisters than their own offspring. Many ants will spend their whole lives only
to die in foraging and protecting the nest, so that the queen and her offspring, their
sisters, may live. In this scenario, the benefit towards reproductive
success of the queen would have to outweigh the reproductive cost to individual ants. Hamilton proposed a rule that stipulates under
what conditions altruistic behavior would evolve. According to Hamilton’s Rule, the altruistic
cost of not reproducing is outweighed by the benefit towards the inclusive fitness of the group. This idea can be illustrated mathematically
by the equation: b>c/r, where b is the benefit given to others, either aiding in increased
survival or reproductive success, c is the cost incurred by the altruist, such as losing
the ability to reproduce, and r is the coefficient of the relationship, or how closely related
the individuals are. The closer the blood ties, the higher the
relationship coefficient. Here’s an example. Say that there is a gene that results in an
altruistic behavior, such as sharing food with other members of the group when food
is scarce. If food is shared with members of the group
who are closely related, then the r coefficient would be high. The cost of losing food would be minimized
by the benefit of survival of the close family members. If the close family members also carry this
gene, then their offspring will likely inherit this behavior. In this way, altruistic behaviors can be selected
for and increase over time. Kin selection can lead to the evolution of
some unique family structures. Some experts believe that this contributes
to a theory on the evolution of aging. As individuals age and begin to stop reproducing,
they can invest their efforts in their grandchildren and other relatives. This increases their overall inclusive fitness,
so extension of life after reproduction to care for relatives would be selected for via
kin selection. This explanation as to why life is extended
after reproduction, particularly for females so that they can care for grand-offspring,
is known as the grandmother hypothesis. Kin selection can also be taken to the extreme. In some cooperatively breeding species, especially
birds, offspring can sometimes be recruited as “helpers” to assist in taking care
of their siblings. Occasionally, this can come at their own expense,
hampering their own ability to reproduce. If helpers contribute to the reproductive
success of their siblings, breeders are more likely to prevent the helpers from mating. This parent-offspring conflict can evolve,
where fathers typically disrupt the mating activities of their sons. So that’s a bit about kin selection, which
is a side of natural selection we don’t typically think about, given that it is the
precise opposite of the cutthroat competitiveness we are used to discussing. But given the success of certain altruistic
species, it is clear that nature has found an effective strategy.
