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Ch. 22 Notes: Intro to Evolution
Evolution = process by which species arise, change, and become extinct due to natural
forces
Charles Darwin (1859) On the Origin of Species by Means of Natural Selection

Based on his exploratory travels on the HMS Beagle and the works of others
History of Evolutionary Theory

Linnaeus believed organisms vary from simple to complex and placed them on a
scale of increasing complexity = scala naturae
o Believed species were fixed and did not evolve
o Was the father of taxonomy (science of classifying organisms)

Studies of fossils showed some extant (living) organisms could not
be linked to any others that still exist, but resemble extinct
organisms in the fossil record
o Developed binomial nomenclature (the 2-name naming system for
organisms still used today)

Georges-Lois Leclerc (a.k.a. Count Buffon) – was the first to propose the idea
that various environmental influences and the struggle for existence may be
important in “evolution”

Erasmus Darwin – noted many vestigial structures in living things and proposed
evolution may occur, but offered no real evidence or mechanism for it
Other Influences

Cuvier – father of paleontology = study of fossils
o Fossils = Relics or impressions of organisms from the past preserved in
sedimentary rock
o Form when new layers of sand and mud settle to the bottom of seas, lakes,
etc., covering and compressing older layers into rock
o May be deposited in many layers (strata), with older strata being buried
under younger layers
o Understood that extinction had been a common occurrence, as from
stratum to stratum, new species appeared and others disappeared
o Did NOT believe in evolution, but catastrophism = Theory that major
changes in the Earth’s crust are the result of catastrophic events rather than
from gradual processes of change

mass extinctions have occurred periodically in the Earth’s history,
as evidenced in the fossil record

postulated that the region would be repopulated by foreign species
immigrating from surrounding areas or that God created new ones

Jean Baptiste Lamarck (1809)
o Developed a theory of evolution with two parts:

Use and disuse. Those body organs used most to cope with the
environment become larger and stronger while those not used
deteriorate.

Inheritance of acquired characteristics. The modifications an
organism acquired during its lifetime could be passed along to its
offspring. (“The strive for perfection”)

Ex. Giraffes stretched their next over multiple generations
to create the long neck trait

Lyell Hutton (a geologist) proposed that the Earth is ancient (not just a few
thousand years old).

Thomas Malthus (an economist) proposed that human population growth is
limited only by the availability of resources and the competition for these
resources determines survival. Darwin used this to formulate his idea of natural
selection.
The Darwinian Revolution

Clergy man

Famous voyage on the ship: HMS Beagle
o While the ship’s crew surveyed the coast, Darwin spent most of his time
ashore collecting specimens of the exotic and diverse flora and fauna, as
well as making notes about the geography of the land.

Galapagos Islands
o Most animal species on the Galapagos are unique (endemic) to those
islands, but resemble species living on the South American mainland.
o 13 types of finches, although they were similar, were different species.

Realized that they diverged based on food preferences

Shows in the structure of their beaks

Darwin focused on adaption

By the early 1840’s, he realized the importance of his work, but did not publish
the information because he wished to gather more evidence in support of this
theory.

The Origin of Species (1859)
o Convinced readers with logic and evidence.
o Two main points

Descent with modification

Species that are very similar share a common ancestor at a recent
branch point on the phylogenetic tree.
o How Darwin’s Theory of Natural selection and Adaptation was
developed:

Observation 1: All species have such great potential fertility that
their population size would increase exponentially if all individuals
that are born reproduced successfully.

Observation 2: Populations tend to remain stable in size.

Observation 3: Environmental resources are limited.

Inference 1: A struggle for existence will develop when
there are more individuals than the environment can
support, so not all will survive to reproduce.

Observation 4: Individuals of a population vary extensively in
their characteristics; no two individuals are exactly alike.

Observation 5: Much of this variation is heritable.

Inference 2: Some individuals will inherit traits that are
better suited to help them survive in the environment than
others.
o Fitness = reproductive success of an individual
relative to other members of the population

Inference 3: This unequal ability of individuals to survive
and reproduce will lead to a gradual change in a population,
with favorable characteristics accumulating over time.
o Adaptation = any evolved trait that helps an
organism be more suited to its environment
o NATURAL SELECTION is based on the facts that organisms:

Exhibit variation that is heritable

Compete for available resources

Have advantageous traits are more likely to survive and pass on
these traits, while those that don’t tend to die off

Become adapted to conditions as environment changes occur
o Variation and overproduction in populations make natural selection
possible.
Darwin believed in Gradualism = Life did not evolve suddenly by quantum leaps, but
instead by a gradual accumulation of small changes.
Darwin also cited Artificial selection = when humans select which animals or plants are
bred together

Done to select for best qualities

(May lead to decrease in genetic diversity  must be careful!)
.
A Population is the smallest unit that can evolve

Population = A group of interbreeding individuals belonging to a particular
species and sharing a common geographic area.

Individuals do not evolve
Example: Peppered Moths (Biston betularia)

Prior to industrial evolution, the light-colored variant was more common than the
dark-colored (only 10% of population was dark)

Industrial Revolution led to increase in pollution  left dark residue on the tree
trunks they rested on (which were normally light in color)

After this change, dark-colored moths made up a maximum of 94% of the
population

Now with increased pollution control, the dark-colored moths are back to 19%.
Evidence of Evolution
1) Fossil Evidence

Transitional Fossils = represent the intermediate evolutionary forms of
life in transition from one type to another
o Ex. Tiktaalik roseae (the fishapod) respresents the transition from
fish to 4-legged animals (tetrapods)

Approximately 375 million years old
o Ex. Archaeopteryx – an intermediate between dinosaurs and birds

Fossil evidence shows the chronological appearance of the vertebrates as
being sequential: fishes, amphibians, reptiles and then birds and mammals.
2) Biogeography = the geographical distribution of species

Based on present distribution and the fossil record

Similar organisms tend to be found in similar environments

Lack of similar organisms in similar environments points to where a
species originated
3) Comparative Anatomy

Homologous structures = Structures that are similar because of common
ancestry
o Ex. Forelimb of: bat, whale, cat, and human = same bone structures

Analogous structures = structures that have same function but a different
origin
o NOT due to a common ancestor, so CANNOT be used to show
evolutionary relationships


Ex. Bird wing (w/ bones) vs. Butterfly wing (no bones)
Vestigial structures = Reduced structures of marginal or no use to an
organism
o Ex. pelvic and leg bones in snakes
4) Comparative Embryology

All vertebrate embryos possess gill slits
o In fish, the gill slits form gills; in humans, they form the Eustachian
tubes
o New structures are often modified versions of older ones, and can be
seen in embryos
5) Biochemical Evidence

All organisms use the same genetic code  also support common descent

Similarity in DNA and protein structures are studied

The closer two species are taxonomically, the higher the % of common DNA
 this evidence supports common descent.

Even taxonomically distant organisms (bacteria and mammals) have some
proteins in common.
o Ex. cytochrome c
6) Developmental Biology

Many developmental genes are the same among all animals

Ex. Hox Genes (really, Homeobox genes)
o Genes involved in development of the body plan of organisms
Evolution is no longer considered a hypothesis, but a great unifying theory of biology
(Theory = concept that is supported by a large number of observations)
Misconceptions about Evolution
1) Evolution is a theory about how life originated on Earth.

Evolutionists are only concerned about how the diversity of life emerged
after the origin of life
2) There are no transitional fossils

Hard to find!
o Organism has to be preserved (majority are broken down by
decomposition)
o Scientists have to locate and uncover them
3) Evolution proposes life changed as a result of random events; clearly traits are too
complex to have originated “by chance”

Chance does have a role  mutation

Natural selection can only act on traits that exist, cannot form them

Many complex structures take millions of years to form
4) Evolution is not observable or testable, thus it is not “science”

It is both observable and testable
“Nothing in biology makes sense except in the light of evolution.”
– Theodosius Dobzhansky