Evolution
Evolution - A process of change through time.
Theory of Evolution
• Suggests that existing forms of life on earth have evolved from earlier forms over long periods of time.
• Evolution accounts for the differences in structures, function, and behavior among life forms as well as
changes that occur in populations over many generations.
(I) Evidence of Evolution
Observations supporting the theory of organic evolution can be made through the study of:
1. Geologic record
2. Comparative Cytology, Biochemistry, Anatomy, and Embryology.
(A) Geologic Record
1. Earth is between 4.5 to 5 billion years old (age determined by radioactive dating)
2. Fossils - are the remains of traces of organisms that no longer exist.
Fossils have been preserved in ice, sedimentary rock, amber, and tar.
3. Other fossils have been formed from petrification, a process by which the
tissues are gradually replaced by minerals that produce a stone replica of the original material.
4. Imprints, casts, and molds of organisms or parts of organisms are frequently
found in sedimentary rock.
5. In undisturbed layers of sedimentary rock, the lower layers contain older fossils while the
upper layers contain younger fossils.
The Fossil (Geologic) Record
Trends found in the fossil record
 one-celled  multicellular organisms
 cells with no nuclei  cells with nuclei
 water dwellers  land dwellers
 simple multicellular organisms  complex multicellular
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(B) Comparative Anatomy
• Evidence supports that similarities of basic structures exist between different organisms.
• Homologous structures have the same basic structure but different functions
• The presence of such homologous structures suggest that these organisms have
evolved from a common ancestor
Vestigial Structures – structures that organisms have that no longer have a function
Ex) appendix, tailbone, whale and snake hind limbs
(C) Comparative Embryology
Although certain adult organisms may be different from each other, a comparison of the early stages of
their embryonic development may show similarities that suggest a common ancestor.
(D) Comparative Cytology
• All living things are made up of cells.
• Cell organelles including the cell membrane, ribosomes, and mitochondria are structurally and
functionally similar in most organisms.
(E) Comparative Biochemistry
All living things contain similar biochemical compounds.
Examples would include: structure and function of DNA, RNA and proteins (including enzymes) are
similar in all organisms.
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(II) Theories of Evolution
Attempts to explain the similarities and differences among species.
Adaptations are a major component to these theories. Adaptations are features, which make a species
better suited to live and reproduce in its environment.
(A) Lamark
The evolutionary theory of Jean-Baptiste Lamark
was based on the principle of:
1. Use and Disuse
2. Inheritance of acquired traits
Principle of Use and Disuse
- For an organism, new structures appeared in the course of evolution because they were needed.
Structures that were present and were used became better developed and increased in size; structures
that were not used decreased in size and eventually disappeared.
Ex: muscles of an athlete
Inheritance of Acquired Traits
Useful characteristics acquired by an individual during its lifetime can be transmitted to its offspring.
These acquired traits results in species that are better adapted to their environment.
Ex: a giraffe’s neck became longer as a result of stretching to reach higher branches. This acquired trait
was then passed down to the offspring.
(B) Weisman
• August Weisman did not agree with Lamark’s theory of acquired traits.
• In a series of experiments, Weisman removed the tails of mice
• The mating of these tailless mice produced offspring with tails of normal length.
• Weisman removed the tails of these mice and allowed them to mate.
• Again, offspring were produced with tails of normal length.
• The acquired condition of “taillessness” was not inherited.
**(C) Darwin 1809-1882
Charles Darwin devised a theory of evolution based on variation and natural selection.
Included in his theory were five main ideas:
1. Overpopulation
2. Competition
3. Survival of the Fittest
4. Reproduction
5. Time/Speciation
** Darwin could not explain where variations came from
**Natural Selection **
• Natural selection is the evolutionary process, which selects the variation(s) of organisms best suited for
a particular environment.
• Natural selection and its evolutionary consequences provide a scientific explanation for the fossil
record of ancient life, as well as for the molecular and structural similarities observed among the diverse
species of living organisms.
• The relationship between organisms or species can be estimated from the similarity of their DNA
sequences; this similarity often closely matches organisms' or species' classification based on anatomical
similarities.
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DNA Sequence Similarities in Some Primates
The graphic above shows that:
1.All of these primates had a common ancestor, the ancestral primate.
2.The human and chimpanzee have the closest evolutionary relationship as their DNA is the most
similar.
Overpopulation
Within a population, there are more offspring produced in each generation than can possibly survive.
Competition
Natural resources; like food, water, and space available to a population are limited. Because there are
many organisms with similar nutritional requirements, there must be competition between them for the
resources needed to survive.
Survival of the Fittest
* Variations among members of a population make some of them better adapted to the environment than
others.
• It is generally the best adapted individuals that will survive.
• The environment is the agent of natural selection determining which species will survive.
What is a selecting agent ? A part of the environment that determines which traits are the most useful
ex) food availability, climate, etc.
• The individuals who survive are the ones best adapted to exist in their
environment due to the possession of variations that best suit them to their environment. This genetic
variability within a species is chiefly due to mutation and genetic recombination. The variation of
organisms within a species increases the likelihood that at least some members of the species will
survive under changed environmental conditions.
Reproduction
Individuals that survive and then reproduce, pass these variations to their offspring.
Time/Speciation
The development of a new species occurs as variations or adaptations accumulate in a population over
many generations.
Ex: caveman  present man
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Modern Examples of Natural Selection
1. Peppered moth: (Industrial Melanism)
• Two varieties of peppered moth existed, a light colored and a dark colored one
• As industrialization and coal burning increased, the environment in England
where these moths lived became dirtier
• The dark colored variety of the moth blended into the trees and increased in numbers, while the light
colored moth was less adapted and decreased in numbers.
2. Insect resistance to insecticides:
• Insecticides kill insects not resistant to the insecticide, while insects resistant to the insecticide live to
reproduce. The insecticide acts as a selecting agent.
3. Bacterial resistance to antibiotics:
• Bacteria not resistant to an antibiotic are killed by it, while resistant bacteria live to reproduce. The
antibiotic is a selecting agent for these bacteria.
(III) Evolution and Extinction
Evolution does not necessarily mean long-term progress is going to go in a certain direction.
Evolutionary changes often appear to be like the growth of a bush. Some branches survive from the
beginning with little or no change, many die out altogether, and others branch out repeatedly, sometimes
giving rise to more complex organisms.
* Extinction of a species occurs when the environment changes and the adaptive characteristics of a
species are insufficient to allow its survival.
• The fossil record indicates that many organisms that lived long ago are extinct.
• Extinction of a species is common; most of the species that have lived on earth no longer exist.
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(IV) Modern Theories of Evolution
The modern theory of evolution includes both Darwin’s ideas of variation and natural selection and the
current knowledge of the sources of variations.
A. Sources of Variations
1. • Crossing over/independent assortment during sexual reproduction.
• Mutations are random changes in the genes or DNA of sex cells may result in new gene combinations
creating variation in the offspring formed from these.
• Only mutations that occur in sex cells can be passed on to the offspring. Mutations, which occur in
other cells, cannot be passed on to offspring.
An Example of Variation Driving Natural Selection
Natural selection favors longer necks - better chance to get higher leaves. Favored characteristics are
passed on to next generation.
Original group exhibits variation in neck length. After many generations, the group is still variable, but
shows a general increase in neck length.
• The variation of organisms within a species increases the likelihood that at least some members of the
species will survive under changed environmental conditions.
• The great diversity of organisms is the result of billions of years of selection for favorable variations
that has filled available niches of our planet with life forms.
Adaptive Radiation – the divergence of a single population into groups having different traits
1. new species are formed
2. new species fill available environmental niches, the role the species plays in the
environment
Ex) Darwin’s finches
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B. Geographic Isolation
1. Gene frequency- the % of organisms in a population that carry an allele.
2. Isolation of a population increases the chances for speciation (the development of a new
species) by separating a small group of organisms from
the main population with its large gene pool (inheritable traits).
3. Changes in gene frequency are more likely to occur in small populations than
in large ones.
4. Geographic isolation of a population is caused by natural barriers like mountains, large bodies
of water, and deserts
C. Reproductive Isolation
If the isolated population becomes so different from the main population that
members of the two cannot interbreed and produce fertile offspring, then they have become two
distinct species.
Evolution is Likely to occur if…
1) There is LOTS of genetic variation
ex. in species that reproduce sexually; high mutation rates (bacteria)
2) The environment is CHANGING (unstable)
Evolution is NOT likely if these conditions are not met!
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