Biology 320Name:
In-Class Assignment 3 – Altruism and the Evolution of Eusociality
Earlier we discussed altruism, inclusive fitness and the effects of kin selection. Now we want to investigate the
role of altruism in the evolution of eusocial organisms such as ants, bees, wasps and my personal favorite, naked
mole rats! Eusocial systems are characterized by three factors: 1) overlap in generations between parents and
offspring, 2) cooperative brood care, and 3) castes (i.e. classes of individuals) that are non-reproductive. In
eusocial species, especially eusocial insects, the worker castes typically do not reproduce, but rather spend their
lives caring for the colony, including the eggs and the young produced by the queen. Eusociality is in essence an
extreme form of altruism. As with sexual selection, Darwin recognized that eusocial insects (and other animals)
posed a severe challenge for his theory of evolution by natural selection.
Let’s first consider the evolution of eusociality in some Hymenoptera (ants, bees and wasps). A single ant colony
may contain millions of individuals, making up several castes that perform different roles (workers, soldiers, and
reproducers). In the hymenoptera, an unusual system of sex determination, called haplodiploidy, has evolved. In
this system, males are haploid and females are diploid. How is this possible? Simple. Males develop from
unfertilized eggs, and females develop from fertilized eggs. In other words, males contain only chromosomes from
their mother, whereas females contain chromosomes from both their mother and father. It has been
hypothesized that the existence of haplodiploidy in Hymenoptera may have predisposed many of them to
eusociality. Below you will determine the relatedness of colony members to see if this hypothesis can explain the
evolution of eusociality.
Part 1. Coefficient of Relatedness
Given the following diagram of haplodiploidy, calculate the coefficient of relatedness for each of the colony
member pairs listed.
Sisters to sisters =
Sisters to brothers =
Females to offspring =
Arrows describe paths by which
genes can be identical by descent.
Given the relatedness of sisters you calculated above, would females maximize their inclusive fitness by acting as
workers (i.e., caring for their sisters that the queen produces), or reproducing on their own? Why?
Given their relatedness to sisters versus brothers, would females workers in an ant colony be more likely to care
for male or female young? Why?
Can you imagine how the results of haplodiploidy might lead to the evolution of a eusocial system in these
insects? As good as your arguments may sound, reality is not so simple. It turns out that the evolution of
eusociality is likely based on ecological factors as much (or more) than it is on genetic factors. Given the
constraints imposed by the haplodiploid system, coupled with the costs of nest building and care of larvae,
eusocial systems are likely an effective way to ensure that the majority of your genes are passed on to future
generations. The costs of building a nest and providing parental care make it very challenging for female
hymenoptera to breed on their own. In such cases, cooperating with a colony of siblings is likely a more effective
way to increase the inclusive fitness of an individual female.
Part 2: Playing by the Rules
In this exercise we want to investigate how the phenomenon of reciprocal altruism might have evolved in various
animal species. As our example we will consider vampire bats. Populations of these bats roost in social units of 1012 adult females and a number of dependent offspring. Additionally, members of these units will move from one
group to another, creating wide variation in the degree of association between individuals in a population.
Vampire bats forage at night in search of blood meals that they take primarily from horses and cattle. Individuals
will typically starve to death if they are unsuccessful at finding a blood meal for three or more days. However,
altruistic behavior is common among vampire bats. Individuals returning to the nest are commonly observed
regurgitating blood to unsuccessful foragers, sharing their meal to help prevent their neighbor form starving.
Observations and studies indicate that this altruistic behavior is reciprocal, with those individuals that share blood
meals also accepting blood meals from other individuals when they themselves are unsuccessful at foraging.
You job in this portion of the assignment is to imagine the rules that would make such a system work and evolve.
Realize that this behavior must be controlled genetically (i.e., altruism alleles), and that reciprocal altruism can
only work when all individuals in a population cooperate. Cheating will bring the whole system down, as “cheater”
alleles would soon rise to high frequency due a low cost, high benefit situation.
Design a set of rules/conditions that must be met for reciprocal altruism to evolve in a population, keeping in
mind the following: inclusive fitness (system must still improve an individuals fitness, so Br-C > 0), cheating must
be discouraged, and of course, the name of the game is for each individual to promote their own genes.
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