Biological Bases of Behavior III:
Some Notes on Heredity and Evolutionary Psychology
Heredity & Behavior
- Are the sayings true: like father, like son, or, like mother, like daughter?
- Does craziness run in families?
- Is biology destiny?
Basic Principles of Genetics
Chromosomes = strands of DNA (deoxyribonucleic acid) molecules that carry genetic information.
Genes = segments of DNA that serve as the key functional unit in hereditary transmission. Each gene contains the blueprint or template for the production of a protein.
Human beings have 23 pairs of chromosomes: 22 pairs plus for men an X & Y chromosome and for women two X chromosomes.
Family studies: Examine blood relatives to see how much they resemble one another on a trait
Twin studies: Monozygotic (identical) twins [share 100% of same genes] vs. dizygotic (fraternal) twins [share ca. 50% of same genes] (see chart on right)
Adoption studies: adopted children compared to adoptive parents vs. birth parents
Example: Schizophrenic (SCZ) Disorders
Note: A concordance rate means the odds (expressed as a percentage) that, if one member of a population has a factor/disorder/trait, another member will have that factor/disorder/trait
- Random pair in general population ≤ 1% concordance rate
- First cousins = 2% concordance rate
- Siblings = 9% concordance rate
- Identical twins = 40-50% concordance rate
Why is this evidence that SCZ cannot be merely an hereditary disorder, that is, that SCZ is not due solely to genetic influences?
Genetic Mapping (= Genome Wide Association Studies, GWAS)
- In the years since 2010 the cost of completing a complete sequencing or map of an individual's genome (all the genes belonging to a particular individual) has dropped dramatically (e.g., from $2.7 billion in the 1990s --> $100 million in 2001 --> $10 million in 2007 --> $10,000 in 2011 --> about $1,000 today).
- As a result, scientists now study issues like the genetic causes of mental disorder, cognitive abilities, etc. by studying many thousands of individuals. Such research is called a "Genome Wide Association Study" or GWAS.
- The original expectation for GWASs was that scientists would identify a handful of genes associated with different mental conditions, e.g., a couple of genes causing schizophrenia, bipolar disorder, overall intelligence level, etc.
- Some individual genes were found to be associated with specific disorders, e.g, cystic fibrosis, Huntington's disease, forms of muscular dystrophy.
- HOWEVER, the results of GWASs show that the heritability of most behavioral traits and mental conditions results from the small effects of dozens, hundreds, or even thousands of individual genes as they have an interactive effect, i.e., polygenetic determinism (see below).
The Interplay of Heredity & Environment
1. Traits associated with genes are usually not associated with just a single gene but with a network, group, or set of multiple genes = polygenetic determinism
2. We tend not to inherit a disorder genetically, but a genetic vulnerability to or disposition for disorder (called a "diathesis" in medicine) which must be triggered or induced by experiences within the environment. Thus, diathesis + high environmental stress = disorder.
3. Epigenetics = study of heritable changes in how genes express themselves that do NOT involve modifications to the DNA sequence. Recent findings show that certain environmental effects such as high stress or various chemicals in the environment can lead to the silencing of genes or reducing how well they can cause things to happen (in biology, often resulting from a process called DNA methylation). Some of these changes can be passed on to the next generation.
The Evolutionary Basis of Behavior
- Is monogamy natural?
- Are humans doomed as a species because our minds were fundamentally shaped by the demands of the Stone Age, but we now live in a world of germ warfare, atomic weapons, and the potential to kill the planet?
- Are men programmed or destined to be unfaithful to their wives?
- Are women less interested in sex than men are?
- Is it natural that men mostly look for beautiful women while women mostly look for men who are (potentially) rich or influential?
Some of these questions may sound like something to be argued on Oprah. But, they are some of the kinds of questions which are often suggested by aspects of the relatively new theory of evolutionary psychology.
Charles Darwin's Theory (1859: On the Origin of Species by Means of Natural Selection)
- Various physical characteristics or traits emerge due to random genetic changes. These traits can be passed down to subsequent generations.
- Fitness: a characteristic which makes an organism more likely to have reproductive success (that is, have offspring) compared to the average member of a population. BY THE WAY, Darwin did not talk about the "survival of the fittest" but believed in the survival of the "more fit".
- Natural selection: the process by which hereditary characteristics tend to survive or be passed on (that is, "selected" out or "chosen" by nature) because such characteristics make the organism either
- More likely to survive to have offspring and pass the characteristic to a new generation, or
- More likely to attract a mate so that offspring result and the characteristic is passed to a new generation
Modern Refinements = Evolutionary Psychology
- Adaptation: A characteristic which is inherited and increases an organism's overall ability to cope with an environment is an adaptation. Adaptations can, over time, loose their original function(s), e.g., human taste preference for high caloric food is not adaptive in the contemporary world. But, it may have made sense many thousands of years earlier.
- Behaviors As Adaptive Traits: It is not just physical characteristics, but also behavioral traits or tendencies which can serve as adaptations.
- Example: Caution of rats when approaching food never consumed before: they only consume a little bit. If poisonous, they learn not to eat this again.
- Example: Grasshopper or other insect buries self partly and covers self with sand to blend into environment
- Other examples
- Acting shy or timidly as way of avoiding predators
- Trading sex for material goods
- Trading sex for male displays of power or elaborate plumage