Chapter 12: Humans, Human Brain, and Evolution
PART I: Evolution of Humans
- evolutionary psychology: which attempts to apply the principles of adaptation and
selection to human behavior
- the discovery of evolutionary theory is generally credited to both charles Darwin and
Alfred Russel Wallace
- Wallace wrote the essay “on the tendency of varieties to depart indefinitely from the
original type”. And sent it to darwin.
Historical Theory of Evolution
- the historical theory of evolution can be summarized by 3 terms: variation, inheri-
tance, and differential reproduction.
- Natural selection: requires that all individuals are unique and that characteristics that
give an animal a reproductive advantage will result in the magnification of these traits in
- Adaption: when a trait results in a reproductive advantage and is selected for. For a
trait to be an adaption it must be inherited from one generation to the next.
- In darwins second book on evolution “The Descent of Man” he looks at sexual selec-
tion: competition among individuals for reproduction that determine a gene’s fate.
There are 2 types of sexual selection:
- 1. Intersexual Selection: one sex chooses a mate from among members of the
other sex on the basis of specific traits. For human men, the choice appears to
be most influenced by facits such as attraveness and youth. For human women,
the choice is also influenced by physical factors such as height and muscularity.
But the mans resouces also comer into play.
- 2. Intrasexual Selection: members of the same sex comoete for partners of
the opposite sex. Males might compete with each other by becoming more mus-
cular or acquiring and displaying resoucreses (such as wealth), whereas fe-
males might compete with each other by enhancing their appearance and
- Historical thery of reolution: term used to refer to the original theory of evolution, be-
caue of the changes to the theory that have occurred suince its inception. - Examples genes and mutation.
- Modern synthetic theory of evolution: the current version of the evolution theory.
- the modern synthesis combines info from such diverse areas as molecular biology and
- It is based on what is known about genes, DNA, Chromosomes, and population biolo-
- Genes made up of DNA, assort in pairs and are located ln chromosomes, which are
stings of genes.
- Humans have 23 pairs of chromosomes, and it is estimated that we have between
26,000 and 40,000 genes which are located on these chromosomes.
- Bipedal gait: walking upright.
- Genotype: the entirelty of your genetic composition, and barring exposure to certain
chemicald or radiatio, your genotype is invariant during your lifetime.
- Phenotype: the interaction of your genotype with the environment in wich you develop.
- Regardless of your phenotype, only your genotype willl be passed on to your children.
- Genes do not make traints or diseases, they makeproteins.
- Different forms of genes are called alleles.
- The rules of gene expression was fust shown by gregor mendel in experiments in plant
hybridization, written in 1885.
- As genes assort in pairs, dominant genes are always expressed whenever they are
present, in contrast recessive genes are expressed only when there are no dominant
- Homozygous: when both alleles are the same
- Heterozygous: when the 2 alleles are different.
- Polygenic: when multiple genes (often on different chromosomes) affect a trait.
- Genetic variations may come about by random mutation and/or recombination of DNA.
- Mutation occurs when there is a change Imo the genotype due to an error in the repli-
cation of DNA.
- Replication: when DNA makes copies of itself.
- The rate of mutation is slow, and most mutations are harmful to the organism, resulting
in reduced ability to survive. - This variation, whatever the cause, mutation or recombination, results in differing abili-
ty to adapt to the environment.
- Modern Synthesis:
- 1. The central tenet of the modern synthesis is that certain environments select
certain phenotypes, and phenotypes are an expression of the genotype interact-
ing with the environment.
- 2. Although genes occur at the level of the individual, evolutionary change oc-
curs at the level of populations.
- Genetic drift: the tendency for isolated populations to depart from the original
genetic composition of the population, also can produce evolutionary change.
This drift away from the original genetic composition of the population is most
likely the result of extensive inbredding (offspring resulting from related parents)
and adaption to the isolated environment. Geneflow is the movement of genes
through a population that results from mating. Geneflow can be profound when
there is extensive inbreeding.
- 3. Species represent different gene pools, rather than fundamentally unique
groups. Species are judged by their genotype, not their phenotype.
- There are 3 major differences between the modern synthetic theory of evolution and
the historical evolutionary theory.
- 1. Modern theory recognizes that traits are the result of genes that are inherited
from one’s parents and interact with the environment.
- 2. The recognition that there are mechanisms other than natural selection that
can effect evolutionary change.
- 3. There is the recognition that what we can species are only differences in
gene pools of a population, not totally distcint or new organisms that arrived ful-
ly formed from nowhere.
Evolution of the Nervous System
- Endocast: a mold of the inside of the skull.
- From studying the brai