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
  1. Genetic Drift
  • Also called Random Genetic Drift
  • Changes in allele frequencies due to chance and chance alone
  1. Mutation
  • Error in copying DNA
  • Only mechanism that introduces new genetic material

  1. Population Structure
  • Deals with two phenomena
  • Mating (who mates with who)
  • Geography/Mobility (can effect mating)
  1. 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

  • 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
  1. 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!
  • 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
  1. 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)
  • 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

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
  • Population Size – greater drift occurs in small populations, trivial in very large populations