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Charles Darwin (1809-1882)
• Not the first to Propose a Theory of
Evolution
– He had the data (so did Wallace)
– Studied geology
– Understood “Artificial” selection
– He was Wealthy
1
The Voyage of the Beagle (1831-36)
• Darwin was a Naturalist
– Career Opportunity
Galapagos Islands
• “New” Volcanic Islands
– Animals are Unique (Why?)
– Some resemble South
American species
– Similar Islands have different
species (Why?)
Change Over Time
• Animals go extinct!
– Causes for extinction?
– Can new ones emerge?
Adaptation to Environment
LaMarckian Theory
• Jean-Baptiste Lamarck (1744-1829)
– Believed that traits were changing according to
biological mechanisms, but..
– Traits were acquired as a result of a parent’s
struggles.
There is a Simpler Explanation
• Offspring are diverse
• They struggle to survive (some die)
• The environment (nature) takes care
of the rest
Thomas Robert Malthus (1766-1834)
• Populations outgrow
resources
• Someone is always
dying
• That’s Nature’s Way
• Stop feeding the
hungry
Thomas Robert Malthus (1766-1834)
• That the increase of population is necessarily
limited by the means of subsistence,
That population does invariably increase
when the means of subsistence increase,
and,
That the superior power of population is
repressed, and the actual population kept
equal to the means of subsistence, by misery
and vice.
Darwin didn’t “discover” as much as
he assembled existing pieces (1859).
Then, he described (mostly
nonhuman) Sex…
…and emotions…
…and child
development.
Summary
• Darwin developed a theory of natural
selection and “species origins” based on
(1) diverse traits and (2) natural
selection for those traits.
• He recognized “traits” as being both
physical features (widely accepted as
adaptations) and behaviors (not so
widely accepted).
Subsequent Reaction
• Darwin was honest about what he didn’t
understand.
– He had no mechanism of inheritance
– Offspring were varied and not “averages” of their
parents
– For almost 50 years, Darwin’s theory was in the
category of “seems to make sense,” but Natural
Selection was not seen as primary driver of
change. The story felt incomplete.
The Modern Synthesis
• Geregor Mendel (1822-1884)
– Working with pea plants
• Reproduction is easier to control
– Described Mendelian Genetics
• Dominant and recessive traits
• Multiple genes could exist in a population
but each individual only carried two
• Wilhelm Johannsen (1857-1927)
– Coined “gene, genotype, phenotype”
– Promoted the significance of M.G.
Chromosomal Replication
• Mitosis and Meiosis
– Walter Fleming (1843-1905)
– August Weismann (1834-1914)
• Offspring come from “germ cells”
not “somatic cells”
– Splitting explains a mechanism for
Mendelian Genetics
• Germ cells do not “learn” from
what the parent does
– Lamarck was wrong!
Mendel and Meiosis (~1910)
• Discoveries of other Mendelian traits.
– Human Examples: Wet (dominant) or dry
(recessive) earwax, Albinism (recessive), Blood
type, several cancers, Huntington's disease,
Lactase persistence (dominant), Sickle-cell disease
• Explained sex-linked traits and pointed to
chromosomes as carriers of the “genes”
• Imperfect copying could lead to mutations
– New genes could be good, most were probably
bad
Population Genetics
• Genes are carried in populations, not just
individuals, and there are many reasons why
more than one (or two) genes can persist.
– Evolution can “tolerate” multiple genes
• The Hardy-Weinberg model explains a way to
predict the proportion of two (Mendelian)
genes in a population during a period of
stability with no selective pressure.
• Sex ratios are balanced by external forces
Microbiology and Symbiosis
• The genetic evidence for common ancestry
Fossil Discoveries
• Evidence for the
speed of change
Human Paleontology
• Is the missing link missing?
– Chimpanzees are our closest relatives (DNA
supports this), though fossils from a specific
common ancestor (6 mya)have not been found.
• There are many examples of human ancestors
The Modern Synthesis
• All the pieces fit and support Darwin’s Natural
Selection theory. The modern synthesis
bridged the gap between the work of
experimental geneticists and naturalists, and
paleontologists.
• It states that:
The Modern Synthesis
• Evolution is consistent with known genetic mechanisms and
the observational evidence from field research.
• Evolution is gradual: small genetic changes regulated by
natural selection accumulate over long periods and many
genes can “drift” through a population at a given time.
Discontinuities amongst species (or other taxa) are
explained as originating gradually through geographical
separation and extinction. Paleontology helps here.
• Natural selection is the main mechanism of change; even
slight advantages are important. The object of selection is
the phenotype in its surrounding environment. The role of
genetic drift (random fluctuations in gene frequency) is
equivocal.
Summary
• For more than a century after Darwin’s
Origin, most psychologists viewed
almost all behaviors – especially human
behaviors - as learned.
• Evolutionary Psychologists regard
behavior as a combination of learned
adaptations that emerge in a lifetime
and inherited adaptations that emerged
long ago.