The History of
Evolutionary Thought
Aristotle (384-322 B.C.)
• Ladder of Life
» 2 types of animals
– those w/ blood &
those w/o
• Animals classified by their
way of life
• Plants by structure
• Observation of various
marine life anatomy was
remarkably accurate
• Distinguished whales from
dolphins
Natural Selection Summarized:
Darwin’s theory suggests that in a species:
•
•
•
•
There is a tendency towards overproduction
Variation exists
Variations are inherited
Individuals survive in their environments with
varying degrees of success
• Best adapted, survive and pass favorable
variation on to next generation
• In time, great differences arise, until a new species
evolved from an old species
Jean-Baptiste Lamarck
(1744-1829)
• His name is associated
merely w/ a discredited
theory of heredity – he
died in obscurity &
poverty
• Darwin & Lyell give him
great credit
• Law of use/disuse
• Law of acquired
characteristics
LAMARCK’S THEORY
ACCORDING TO DARWIN…
Alfred Wallace (1823-1913)
• Studied the way
geography limited or
facilitated the extension of
species range
• How ecology influenced
the shaping of adaptations
• In 1858, shared with
Darwin on the Theory of
Evolution by means of
Natural Selection
Charles Darwin (1809-1882)
• Voyaged around the
world 1831-1836
• Wrote On the Origin
of Species which
reveals his ideas on
Evolution by means of
Natural Selection
•Linnaeus was classifying organisms
based on what they looked like.
•This made it difficult to classify
organisms that seemed to share
characteristics with both kingdoms
that Linnaeus proposed, Plants and
Animals.
• For example, fungi including mold
and mushrooms do not move (or do
they?) so they seem to be plants
but, unlike plants…..?????
Lion
Cat
Dog
Man
Catbird
Phylum
Chordata
Chordata
Chordata
Chordata
Chordata
Class
Mammalia
Mammalia
Mammalia
Mammalia
Aves
Order
Carnivora
Carnivora
Carnivora
Primata
Passeriformes
Family
Felicidae
Felicidae
Canidae
Hominidae
Minidae
Genus
Felis
Felis
Canus
Homo
Dymetella
Species
leo
domesticus
carolinensis
sapiens
familiaris
Homologous Structures
• Structures that have different
mature forms in different
organisms but develop from the
same embryonic tissues.
A kind of evolution wherein organisms evolve structures
that have similar (analogous) structures or functions in spite
of their evolutionary ancestors being very dissimilar or
unrelated.
Classifying organisms often
starts at the cellular level
Cladistics (phylogeny)
A system of classification
based on the study of
evolutionary relationships
history of groups of organisms.
An Example of Cladogram Construction for Vertebrates
Trait
Outgroup
(lobedfinned fish)
Frog
Turtle
Kangaroo
Mouse
Human
Dorsal
Nerve
Cord
Yes
Yes
Yes
Yes
Yes
Yes
Legs
NO
Yes
Yes
Yes
Yes
Yes
Nature of
Egg
Requires
water
Requires
water
Hard shell
prevents
drying
Develops
inside the
mother
Develops
inside the
mother
Develops
inside the
mother
Nature of
development
In egg
In egg
In egg
Marsupial
Placental
Placental
Hair
No
No
No
Yes
Yes
reduced
Presence
of pouch
No
No
No
Yes
No
No
Bipedal
posture
No
No
No
Yes
No
Yes
Coevolution
•Mutual evolutionary influence between two species
•Typically evolution of two species totally dependent on each other.
•Exert selective pressure on the other, so they evolve together.
•Extreme example of mutualism.
What is biological fitness???
Evolution of Populations
Populations are the units of evolution
Population
• A group of
individuals of the
same species living
in the same place at
the same time.
Biological definition of a species
• A group of
populations whose
members are
capable of
interbreeding and
producing fertile
offspring.
Population genetics
 An important turning
point for evolutionary
theory the 1920’s.
 Developed in the 1920’s

A field that combines
Darwin’s and Mendel’s
ideas by studying how
populations change over
time.
The Gene Pool
 In studying
evolution at the
population level,
geneticists focus on
the GP
 Total collection of
genes in a
population at any
one time.
The Gene Pool
Sources of Genetic Variation
• Mutations
• Gene Shuffling
• Crossing over
• Sexual reproduction
Single Gene Traits
• The number of
phenotypes produced
for a given trait
depends on how many
genes control the trait
Polygenic traits are controlled by
two or more genes
Natural Selection can affect the
distributions of phenotypes in 3
ways:
• Directional selection
• Disruptive selection
• Stabilizing selection
Directional Selection
• When individuals at one end of the curve have
higher fitness than individuals in the middle or at
the other end.
Disruptive Selection
• When individuals at the upper and lower ends
of the curve have higher fitness than
individuals near the middle.
Stabilizing Selection
• Takes place when
individuals near the
center of a curve have
higher fitness than
individuals at either
end
THE PROCESS OF SPECIATION
Isolating Mechanisms
•Behavioral
•Geographic
•Temporal
Behavioral
Geographical
Temporal
The Hardy–Weinberg principle states:
Both allele and genotype frequencies in a population
remain constant or are in equilibrium from
generation to generation unless…
Disturbing influences happen such as non-random
mating, mutations, selection, limited population size,
random genetic drift and gene flow.
Genetic equilibrium is a basic principle of
population genetics.
Hardy-Weinberg principle is like
a Punnett square for populations,
instead of individuals.
p2 + 2pq + q2 = 1; p + q = 1
p2 + 2pq + q2 = 1; p + q = 1
p2 + 2pq + q2 = 1; p + q = 1