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Chapter 6
Donna May D. Papa
College of Science
University of Santo Tomas
Change is
constant…
Pre-Darwinian
Evolutionary Ideas
 Before 18th century - origin of species was not
scientific – “Creation Event”
 Irreversible change & mythology
 Xenophanes, Empedocles and Aristotle developed
early ideas about evolution
 Fossils were recognized as former life destroyed
by natural catastrophe
 Archbishop Ussher calculated 4004 bc as date of
life’s creation
 Louis Buffon (1707-1788) – stressed environmental
influences modified animal types
 Jean Baptiste de Lamarck offered first
complete explanation of evolution (1809)
 argued that fossils were remains of extinct
animals
 mechanism was inheritance of acquired
characteristics – transform to produce
evolution
 Lamarck’s concept is transformational;
individuals transform their own traits to evolve
 In contrast, Darwin’s theory is variational or
due to differential survival among offspring
 Principle of Uniformitarianism
 two important principles of Uniformitarianism :
1) Laws of physics and chemistry remain the same
throughout earth’s history
2) Past geological events occurred by natural processes
similar to those observed today

Natural forces acting over long periods could explain
formation of fossil-bearing rocks

Geological changes are natural and without direction;
both concepts made marks on Darwin’s theory
Darwinian Evolutionary
Theory: The Evidence
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Perpetual Change
Common Descent
Multiplication of species
Gradualism
Natural Selection
perpetual change
 Species are not fixed, but
are often changing
(commonly at different
rates)
 Through the generations,
most common forms
change, as shown in the
fossil record
 Fossil - remnant of past life
Geological Time
Sedimentary Rock Layers
 Law of stratigraphy dates oldest layers at the
bottom and youngest at the top
 Time is divided into eons, eras, periods and
epochs (See inside back cover of your books)
Evolutionary Trends
 Fossil record allows observation of evolutionary
change over broad periods of time
 Animal species arise and become repeatedly
extinct.
 Animal survival = 1–10 million years
Horse Evolution Shows
Clear Trend
 From Eocene to Recent periods,
genera and species of horses
were replaced
 Earlier horses - smaller sized and
fewer grinding teeth, and more
toes
 Reduction in toes and increase in
size and numbers of grinding teeth
correlate with environmental
changes
Common descent
 All forms of life had a
common ancestor.
Phylogeny – Evolution
produces a branching
structure and the form,
this takes is called a
phylogeny
Multiplication of species
 New species form by splitting and
transforming older species
 Typically the result of isolation or
separation, and based primarily upon
reproductive isolation
Gradualism
 Evolution is a gradual and slow process (by human standards)
and large, rapid changes are usually not common in the evolution
on a group of organism
 Most large changes over time occur through a series of small
changes
 Large changes tend to be harmful for organisms, although not
always
Natural Selection
Five Observations forming Natural Selection
1.Organisms have a great potential to
reproduce
2. Natural populations normally remain
constant in size, with small
fluctuations
3. Natural resources are limited.
 Therefore, a continuing struggle for
existence occurs among members of
a population
4. All organisms show variation
5. Variation is
heritable.
 Differential
survival and
reproduction
occur among
varying
individuals in a
population, and
over many
generations,
generates new
adaptations and
new species.
Evidence for Perpetual Change
 Seen in the fossil record
 Fossils are deposited in stratified layers
 Dating possible based on position in the
rock
 Fossils tend to illustrate
trends in characteristic
change
Ex. Evolution of horses
Evidence for common descent
Comparison of gill arches of
Different embryos. All shown
Separated from the yolk sac.
Note similarity of embryos.
 Homology – different adaptations of the same structure
from a shared ancestor
 Used to form phylogenies based on shared characteristics
 Used in ontogeny, or the history of development.
 “ontogeny (history of development of an organism throughout its
lifetime) recapitulates “phylogeny”
Evidence for Common
Descent
Homologous structures
 Arm of a human
 Arm of a cat
 Wing of a bat
 Fin of a whale
 Leg of a horse
Multiplication of Species
 Ancestral species
speciate into diverse
forms through isolation
 End result can be
multiple species.
How new species arise
Allopatric speciation
 Speciation due to a geographic barrier
- Separated by an ocean, mountain range or a road
Sympatric speciation
 Speciation in the same place without
separation
Classic Example –
Darwin’s finches
 Adaptive radiation – production of many
ecological species from a common ancestral stock.
gradualism
 A theory of some debate
today
 Fossil record does not
show perfect intergrades from form to form
 Sporting mutations can cause dramatic
changes that may be important to
evolution
Disturbances to
Equilibrium
 Random Genetic Drift
 Nonrandom Mating
 Recurring Mutation
 Migration
 Natural Selection
Nonrandom mating
 Positive assortative mating – Individuals
selecting a particular genotype to mate with
 Inbreeding – mating between close relatives