Human Behavioral Biology – 03 – Behavioral Evolution II

December 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.

The last class introduced the requirements for evolution to occur and the Three Building Blocks for passing on genes.

Sapolsky starts off the third class by discussing various evolved animal behaviors and how they can be explained using the Three Building Blocks.

Babies and Alpha-males

We start with a seeming ‘universal’, that babies are always so darn cute!

Why does this happen? Is it a means to reduce aggression? Probably not, since there are lots of species that practice infanticide.

Why does this happen? How could they kill such cute, adorable babies? It turns out that infanticide usually happens when males are killing the offspring of other males.

When does that happen? When the length of time that you could be alpha-male is shorter than the time it takes females to rear children, and therefore they aren’t ovulating. (There are exceptions, however, such as when the babies are close relatives of the alpha-male.)

So, some alpha-males (e.g. in lions) have evolved the behavior of killing cute babies because (1) it takes out competing genes, and (2) it gets the females ovulating again!

Some Female Behaviors

There are also some very interesting female evolved behaviors.

The females of some species are known to miscarry at the mere presence of a new male. They are also more likely to risk death to protect their child when they are older and less likely to give birth again.

Females are also sometimes able to fool males into thinking that a baby is theirs!

There are also examples of polyandry, where one woman will ‘marry’ a set of brothers.

Females have even evolved the ability to adjust the frequency of male or female births. Because male fetuses are more expensive to ‘grow’, there is a direct correlation between sex ratio fluctuation (frequency of male vs female babies) and the social context (middle of a drought, an abundance of food, etc.). Therefore, in good economic times there are more males, and in bad economic times there are more females.

Imprinted Genes

Sapolsky takes a short amount of time to discuss imprinted genes, which are genes that differ based on which parent you get it from.

How can the sex of the parent make a difference? There is a mechanism called methylation, i.e. gene silencing.

The scientist David Hames was able to show examples of when this happens. For example, if the genes for fetal growth come from the father, they are promoted, otherwise they are slowed. Likewise, if the genes for suckling come from the father then there will be more suckling than if it comes from the mother.

This is something that only happens in tournament species, not pair-bonding species!

So where do humans fit in? Are our genes influence by which parent they came from? As with the tournament—pair-bonding split, humans sit somewhere in the middle.

A Fourth Building Block for Passing On Genes

At this point, Sapolsky takes the time to discuss what is in fact a fourth Building Block for passing on genes, called Group Selection.

Now, this is not the ‘group selection’—the “animals behave for the good of the species”—that came from the 60’s and was rightly trashed. This is a different form of group selection.

4. Group Selection

True group selection only happens under one of two very special circumstances.

A. The Founder Effect

The Founder Effect occurs when there is an accidental forced jump-start in cooperation like in the following example:

  1. Take an initial population of animals.
  2. Split off a small number of the population (e.g. an island separates into a small and large chunk).
  3. The smaller island will suffer from increased inbreeding.
  4. Increased inbreeding results in (1) an increased number of relatives within the population and therefore cooperation between the animals, and (2) and increased frequency of mutations and potential for advantageous traits (i.e. increased rate of evolution).
  5. Eventually, the smaller population is re-incorporated with the larger population, but they are now at an advantage—they are stronger and more cooperative!
  6. Eventually, the cooperation of the smaller populations “crystalizes outward” and what used to be Kin Selection causing cooperation is turned into Reciprocal Altruism!

B. General Group Advantages

There is also a more generalized way to have group selection.

Imagine having a batch of chickens. ‘A’ chickens are much more aggressive and lay many eggs. ‘B’ chickens are more passive and lay less eggs.

What happens when you mix some of these chickens together?

  • A + B > A lays lots of eggs
  • A + A + A + A + A > All get stressed out by each other; very few eggs!
  • B + B + B + B + B > All are happy and lay many eggs!

So, when individuals are put together A will lay many more eggs than B.

But, if you put a group of them together then the B chickens will do much better than the A chickens!

What we’re seeing here is multilevel selection. Sometimes selection occurs entirely on the basis of an individual gene, but other times it depends on the organism or group level.

Two Problems with Applying this to Humans

Before we can really get into how humans fit into all of the above, there are two problems that must be addressed:

(1) Taking a small number of animal traits and applying them to humans

We must avoid saying, “Look, it happens in these animals, so it’s natural and we must do it too!”

(2) We have all these great models… then humans have all these quirks and don’t fit in!

Humans are a very complicated animal. Even though we’ve made some nice models and frameworks for understanding behavior through evolution, it will be challenging to apply them to humans.

We will delve more into applying this stuff to human behavior in the second half of the course.

Criticisms of Behavioral Evolution

The class finishes off with reviewing some of the criticisms of the framework of behavioral evolution.

There are four critiques: the three key features of evolution (1) heritability, (2) adaptiveness, and (3) gradualism cannot explain all behavior, and that there are (4) political issues with behavioral evolution.


The heritability of behavior has been attacked by many fields.

“How do you figure out what behavior has a genetic influence? Show me the genes!


Behavioral evolution has been criticized for the “adaptationist fallacy”, that selection implied adaptation. Critics claim that behavioral evolution is merely making up stories to explain adaptiveness, and that the “best story wins!”

In other words, “Show me the experimental evidence!”

S.J. Gould was big on this one. He introduced the idea of spandrels.

Spandrels are traits that occur as a side-effect of some other adaptation. They are the by-product and are not selected for, like the human chin, which is entirely a byproduct of the necessary skeletal structure that humans have.


Gould was also a big critic of gradualism. Maybe there are long periods of nothing, until a sudden change occurs, what he called “punctuated equilibrium”?

For example, the size and structure of shells for various sea life did not change gradually. When mapped on a graph it looks more like stairs than a gradually rising line.

Political Critique

Some have criticized behavioral evolution for its emphasis on hierarchies, aggression, and male vs female dominance. All of these things have big political implications!

It has been noted that all of the original founders of behavioral evolution where white, American males. What does this imply for all their models? Have they been used justify e.g. male dominance? Is it possible that their worldview impacted their research and findings? Could they have been systematically biased in their approaches and conclusion?

Sapolsky just introduces these criticisms, with plans to address them in the following lecture.

In the next class: Address the challenges to heritability, adaptiveness, and gradualism; introduce molecular genetics, proteins, amino acids, and DNA; show examples of genetic mutations and their effects; introduce the idea of environmental influences on genetic expression.