Human Behavioral Biology – 02 – Behavioral Evolution I

November 2011

in Self-awareness

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This post is part of a series summarizing Human Behavioral Biology, taught by Robert Sapolsky at Stanford University in 2010. Lecture videos are available for free online.

Evolution Happens

For starters, evolution most certainly happens – there are changes in species and speciation over time.

It is a common misconception that Darwin discovered evolution. In fact, Darwin found a mechanism for evolution: natural selection.

As an aside, Alfred Wallace also discovered evolution by natural selection independently, but for whatever reasons he has been historically screwed out of recognition.

Requirements for the Evolution of Populations

Four things must be present:

  1. Traits that are heritable (a large focus for this course)
  2. Variability among traits
  3. Some versions of traits that are more adaptive than others
  4. Mutation (i.e. new traits)

How does this apply to behavior? First a caution…

Evolution Doesn’t Have Feelings

It is easy to use semantics like, “What would this organism want to do?” when this actually means “How would evolution sculpt this organism?”

When we talk about what evolution does and wants to do it’s easy to envision a primordial bacterial thinking “Hmm, if I do this then I’ll be more likely to survive…” but this is not how it works. Evolution sculpts reality because of the four requirements above.

Do not fall into the trap of giving animals anthropomorphic thoughts – thinking that only humans are capable of. They are actually carved by evolution without their awareness.

So again, how does this apply to behavior?

“For the Good of the Species”

The worse urban myth of evolution is that, “Animals behave for the good of the species”.

This idea comes from Wynne-Edwards who came up with “group selection”. This idea is WRONG!

Sometimes animal behavior appears to be for the good of the species, but this is accidental because:

Animals behave for passing on as many of their genes as possible.

That is, animals act so as to maximizing their genetic spread.

How do they achieve this?

Three Building Blocks for Passing on Genes

1. Individual Selection

“A chicken is an eggs way of making another egg.”

Individual Selection deals with individual self-reproduction. This is similar to the idea of “selfish genes” from Dawkins.

This plays itself out in several realms:

  1. Adaptive/survival ability – ex. speed, muscle metabolism, senses.
  2. Sexual selection – ex. bright blue wings, colourful fish.

Adaptive ability is straightforward, but sexual selection needs a little explaining. These are random sexual preferences evolved by animals of a species for whatever reason that will influence an animal’s ability to reproduce.

You even sometimes get a balance in nature where the survival ability and the sexual attractiveness of an animal are forced to balance against each other, as in the example of a colourful fish: he is more attractive to females and to prey.

Overall, you want to maximize the number of times you yourself get to reproduce.

2. Kin Selection

“I would lay down my life for two brothers or eight cousins.” – J.B.S. Haldane

Kin Selection is inclusive fitness that comes from helping out relatives.

You share many genes with close relatives; the closer the relation, the more genes are shared. Therefore, if the ‘goal’ is to maximize gene spread, you have an incentive to increase the reproductive abilities of kin and relatives – sometimes even at the expense of your own reproductive abilities!

In many animals it has been observed that the better the cooperation between several animals, the more likely they are to be related.

In primates, for example, it has been shown that there is a strong sense of relation between animals in the group. Everyone knows who’s baby belongs to who, who is related to who, etc.

The result is that evolution selects for organisms that cooperate with their relatives.

3. Reciprocal Altruism

“Many hands make the task lighter.”

Reciprocal altruism is what happens when you have various individuals that have an incentive to cooperate.

In a simple rock-paper-scissors scenario, you can have several different animals, each with a vulnerability, and each with a strength.

A study with three bacteria in a rock-paper-scissors scenario shows that equilibrium was quickly reached, where they were all able to live side-by-side in peaceful cooperation, though a ‘stale mate’ or ‘truce’ might be more accurate. (Remember, bacteria can’t actually ‘think’!)

Then there are cooperation scenarios, such as cooperative hunters who hunt in packs, regardless of their relation to each other.

But such cooperation has conditions. You have to make sure people aren’t getting more than they are putting in! It must be reciprocal.

  1. You must be good at detecting cheaters.
  2. You must cheat when it’s advantageous for you.

Which opens up the world of Game Theory.

Game Theory

Sapolsky goes into a fair amount of detail about various theories, I won’t get into all of it here, suffice to say that Game Theory tries to mathematically formalize strategies for cooperation and reciprocal altruism. Specifically, when to cooperate and when to defect.

The basic Tit-for-Tat strategy is that by default all parties cooperate, and if one of them ‘defects’ (ex. takes more food than is fair) then the others punish them momentarily (ex. withholding food) until everyone returns to cooperating.

A key idea to take out of the topic of game theory is that reciprocal altruism can get very complicated. There are many instances where it seems someone is ‘getting away’ with too much, but this is because the reciprocation can happen in many different ways and on many different levels.

For example, in a lion pride there may be one lion that is always the first to run away when the pride is attacked. Why does this happen? Why do the other lions let him get away with it?

But perhaps he eats less food than others? Or perhaps he reciprocates in some other way?

Another humorous example: within naked mole rat colonies there are often several mole rats that sit around, do nothing, eat a disproportionate amount of food, and get fat. Why do the other mole rats let them get away with this?

It turns out that if you wait long enough you’ll see these very same ‘lazy’ mole rats using their fat bodies to plug the entrances to the colony during rainy season, thereby putting their asses on the line, quite literally, for the colony.

In other worse, sometimes there is role diversification.

Tournament vs. Pair-bonding Species

Sapolsky finishes off the class with a discussion of tournament vs. pair-bonding species.

Imagine you were given the skulls of a male and female mammal. What could you tell just by their relative sizes?

It turns out you could tell a lot.


  • Relative sizes of male and female skulls.

If it’s a Tournament Species:

  • Skull size: male >>> female
  • Aggression: high ***key!
  • Trait variability: higher
  • Female wants: strength, size
  • Lifespan: lower
  • Parental behavior: males have little to no involvement
  • Frequency of twins: less likely (mother is less likely to be able to care for them)
  • Frequency of cheating/abandoning: very high
If it’s a Pair-bonding Species:
  • Skull size: male = female
  • Aggression: lower
  • Trait variability: lower
  • Female wants: paternal behavior, competent males
  • Lifespan: higher
  • Parental behavior: high males involvement
  • Frequency of twins: more likely (because they could be taken care of)
  • Frequency of cheating/abandoning: low; women are more likely to abandon kids

The obvious question is, “Where do humans fit in?” And the answer is that it’s complicated. We humans seem to be horribly confused about what type of species we are.

We sit somewhere in the middle of the two above types, taking elements from both.

In the next class: Discuss additional evolved animal behaviors, introduce a controversial fourth building block called Group Selection, and various criticisms of behavioral evolution.