PSYC 3102: Introduction to Behavioral Genetics
Lecture 16
Evolutionary Psychology
5 forces of Human Evolution
1. Natural Selection

Differential reproduction

Adaptation to environment

Heritable traits

Definition: Process of differential reproduction as a function of heritable traits that
adapt organisms to their environment
2. Genetic Drift

Also called Random Genetic Drift

Changes in allele frequencies due to chance and chance alone
3. Mutation

Error in copying DNA

Only mechanism that introduces new genetic material
4. Population Structure

Deals with two phenomena

Mating (who mates with who)

Geography/Mobility (can effect mating)
5. Culture

Transmission of knowledge, behaviors, attitudes, etc. from one generation to the
next or across one generation

Vertical transmission = pass down through generations

Horizontal transmission = through one generation

Ex: use of antibiotics
Natural Selection

Wallace and Darwin

Beak of finch
-- long and narrow = good to get insects
-- short and stubby = good to crack seeds and nuts
“Normal” Environment
Drought Environment
F
r
e
q
u
e
n
c
y
L/N
S/S
Beak
L/N
S/S
Beak
Mean shift

Drought = few insects = S/S finches will be healthier and reproduce more than the
L/N finches – they are more adapted to the drought environment

If characteristics of the beak are heriable, the next generation will have more S/S
genes

No ‘survival of the fittest’

REPRODUCTION
3 Major Modes of Natural Selection
1. Directional Selection

One end of the curve has high fitness, middle has moderate fitness, other end has
low fitness

Ex: Skull and brain sizes in humans (on primate scale); our skulls have increased in
size, more vertical skull, more frontal lobe stuff
Fitness
Phenotypes
2. Stabilizing or Balancing Selection

Individuals near the mean are the most fit, ends decline; it’s good to be average

Ex: Birth weights, low = high infant mortality; high = problems in birthing

Cultural influences! Curve was sharper in the past, modern medical practices makes
a difference!
Fitness
Phenotypes

This is thought to be the most common mode of natural selection

Most species are already fairly well adapted

Don’t want to be extreme, will be less adapted
3. Disruptive Selection

Middle is least fit

Opposite of Balancing Selection

No good human example

Rare

But when it occurs, it is very important

Ex: Butterfly species – Cryptic = blends in with background; Mimic = colorful,
mimics a poisonous species (monarchs and viceroys)
Fitness
Phenotypes

When teaching, learning or reading about Natural Selection – there is the tendency
to deal with only one trait.

But this is an over-simplistic function.

There are many dimensions it is playing on!

Pleiotrophy (one gene influences many traits) – if allele frequencies change, all traits
associated with allele change.
Hypothetical example:
Aggression and Depression

Hypotheses and some data

Serotonin turn over is associated with both aggression and depression

Depressives have low levels

Aggression is associated with low serotonin

Selection for more aggression = more babies, but more depression = more suicide

Many mathematical models view as a slow, gradual process – but usually isn’t a
smooth and uniform transitions
Mathmatical
Model
Actual
Model
Phenotype
Value
Time
Genetic Drift

Changes in allelic frequency by chance and chance alone

Ex: Pop’n of 5 males and 5 females > Randomly mated > Start with 20 alleles ½ A
and ½ a > No selection, just random

Will hit boundary, from there on it will stay the same, the only way to reintroduce
‘a’ is a mutation or if a new individual moves into the population
Freq.
of
A
0

1
2
3
4
5
Generations
6
7
8
9
Population Size – greater drift occurs in small populations, trivial in very large
populations