Arnav Agarwal 2011
Evolution II
Module 1: Introduction to Social Behaviors
Introduction
- Although organisms evolved to maximize reproductive success and survival, many cases of
altruism exist:
o Most honey bees don’t reproduce or have reproductive organs; they spend their lives
serving the colony, raising the Queen’s eggs and defending against predators
o Belding’s ground squirrels will frequently give alarm calls, warning others to flee and run
from a nearby predator but alerting the predator of its (whistleblower’s) exact location
o Humans helping each other out and not solely focusing on individual survival and
reproduction
- If evolution acts on genes and those genes that contribute positively to an individual’s fitness
will get replicated more often and increase in frequency, how does altruistic behavior work?
o “Selfish gene” concept: natural selection will favor the genes and gene complexes that
best serve their own interest (ie: replication)
Table of Social Behaviors
- Four kinds of social behaviors that individuals engage in:
Effect on actor’s well-being
+ -
Effect on recipient’s + Cooperation Altruism
well-being
- Selfishness Spite
- Cooperation: if action helps both actor and recipient
- Selfishness: if action helps actor and hurts recipient
- Altruism: if action hurts actor and helps recipient
- Spite: if action hurts both actor and recipient
Cooperation = personal gain
- Sometimes, cooperation can be better than selfishness
- For example, the cost of teaching a new player how to play a game may be less than the benefit
of being on a better team and winning games -> the unselfish behavior is more rewarding
- Therefore, increasing the fitness of others can sometimes improve your own fitness prospects.
The good of the helping gene
- Common misconception: group selection
- Adaptations aren’t for the good of the group or the species; they are for the good of the gene Arnav Agarwal 2011
- Group success simply translates to better success for the gene
Good of the group?
- Group foraging: doesn’t having other hungry individuals around make it more competitive?
o Not necessarily
o Easier to find food with more individuals looking for it
o Although others might take some of your food when you find some, you will have
opportunities to take food from others when they find it first as well
o Vigilance against surprise attacks: as heads are normally down when foraging for food,
predators can attack; having one’s head raised and looking around is important, but this
is a tradeoff as one can’t forage and have their head raised
When foraging in groups, the tradeoff is reduced: while most forage, a few scan
for predators, and after a quick scan, they can forage while others raise their
heads and look around
Head jerks by individuals vs. by the entire flock
- Result of group foraging as group gets larger: lower rate of head jerks by individuals per minute,
higher rate by the entire flock
- Even though every individual is looking up less often (therefore able to find more food as well),
the group is being more vigilant than the individual alone
- More time spent on eating, less time spent on foraging; therefore, all individuals benefit as long
as there is enough food and every individual benefits from others’ vigilance Arnav Agarwal 2011
Can selection be good for the group and bad for the gene?
- The vigilance vs. foraging example shows that selection favors the good of the group, only as a
side-effect of favoring the good of the individual
- For the good of the actual gene (which is the important aspect, not the group): has to be some
benefit to the helpful gene, not just the group
- Therefore, selection cannot be good for the group and bad for the gene
o More effectively shown in the study of altruism
Problem of altruism
- In a group of altruists, individuals are regularly helping one another, so the group will thrive
- However, as altruism is a decrease in fitness for the individual and an increase for the recipient
- Altruism leads to fitness decreases whereas a selfish individual’s actions will not
Lemmings
- Small rodents that live in the Far North, that are believed to commit suicide when their
populations become too large
- Myth: self-sacrificial act is apparently altruistic as enough food is now present for the remaining
individuals to survive
- However, good-of-the-group thinking doesn’t apply; if there is one gene for selfish restraint and
one for altruistic suicide, the altruistic gene will die as the individuals will sacrifice themselves,
and the selfish restraint gene will live on and reproduce, passing on its genes to the next
generation
o Unless the genes of the altruistic individuals are reproduced more than the genes of the
selfish restraint individuals, the selfish restraint genes will inevitably live on Arnav Agarwal 2011
Module 2: Inclusive Fitness
Introduction
- How are seemingly altruistic behaviors, where they give up their own reproductive
opportunities to help others survive and reproduce, come to be favored by selection?
o Eg: eusocial hymenoptera (described earlier with honeybees)
Includes all ants, some bees and some wasps
Most individuals spend their lives serving the colony without reproducing
Inclusive Fitness
- W.D. Hamilton, 1960’s: genes for altruism could be successful if they helped identical copies of
themselves
- An individual’s fitness is measured by the number of genes left in the next generation
- Hamilton’s new definition to fitness (two-pronged):
o Direct fitness is an individual’s genetic contribution through its personal reproduction
o Indirect fitness is an individual’s genetic contribution through the reproduction of close
genetic relatives
- Hamilton’s theory: one can increase one’s fitness by helping kin successfully raise offspring,
sometimes even when it has a negative impact on direct fitness
o This is the case with eusocial hymenoptera and similar behaviors
- Inclusive fitness = direct fitness + indirect fitness
o Natural selection can, as a result, favor not only behaviors that increase an individual’s
own reproductive success, but also behaviors that increase the reproductive success of
close genetic kin
o Shared genes between relatives can drive the evolution of altruism Arnav Agarwal 2011
Hamilton’s Rule
- An inequality that predicts when altruistic behavior is favored, based on inclusive fitness
- Prediction: altruistic behavior will be favored when rB > C, or:
o (degree of relatedness)(reproductive benefit to recipient) > (reproductive cost to actor)
- In other words: the reproductive benefit to the recipients (B) multiplied by the probability that
the recipients actually have identical copies of the same gene, or coefficient of relatedness (r),
must be greater than the reproductive cost to the actor (C)
Coefficient of Relatedness, r
- Relatedness: probability that the actor and recipient share the gene in question (leads its bearer
to be helpful)
- Dependent on the way the individuals inherit their genes
- Two copies of every gene (one from each parent), but these aren’t always identical; they can be
the same or different. The gene passed on to one’s offspring and the genes that were passed on
to one’s siblings from one’s parents, will also not necessarily be the same as the ones received
by you
Probability of identical genes
- Parents: 50/50 chance of inheriting a gene one’s mother has, 50/50 chance of inheriting a gene
one’s father has
o 0.5 relatedness to each parent (r=0
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