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Evolution
Theory of Evolution
• Theory – a well supported testable explanation of phenomenon
occurring in the natural world.
• Evolution – the process by which modern organisms changed over
time from ancient common ancestors.
• Microevolution – change in allele frequency in populations over
generations.
• Macroevolution – large scale change, such as the formation of new
species.
Summary: Theory of Evolution
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Species are different due to variation in their genes
(variation results from random mutation).
Some individuals are better suited for survival, and will
leave more offspring (natural selection or “survival of the
fittest”).
Over time, change within species leads to the replacement
of old species by new species as less successful species
becomes extinct.
There is clear evidence from fossils, anatomy, physiology,
DNA, and embryology that the species now on Earth have
evolved from ancestors that are now extinct.
3 Patterns of Biological Diversity
• Species – group of similar organisms that can breed and
produce fertile offspring.
• 1. Species vary globally – different yet ecologically similar animals
are found in different yet similar environments.
• 2. Species vary locally – different yet related species occupy
different habitats in one area.
• 3. Species vary over time – fossils of extinct species are similar to
current species.
Artificial Selection
• Artificial Selection – nature provides variation (variety) in
organisms’ traits, but humans choose to breed those
organisms that have the most useful traits.
• Example: humans breed cows that produce the most milk.
• Example: humans breed trees that create the most fruit.
Natural Selection
Darwin proposed a mechanism for evolution that he called
natural selection.
Individuals whose characteristics are well-suited to
their environment survive and reproduce.
• By surviving, these attributes can be passed onto their
children, causing an increase of these traits in the species
population, thus causing a gradual change in the
characteristics of the population.
Individuals whose characteristics are not well-suited to
their environment die, or leave few offspring.
Because natural selection favors a certain trait over
others, more individuals in the population carry the
genes for that trait.
AKA: “survival of the fittest”
Parameters of Evolution by NS
• 1. Struggle for existence – more organisms are produced than
can survive.
• Competition – individuals or groups of organisms compete
for similar resources (territory, mates, food, water, etc.) in
the same environment.
• 2. Variation and adaptation – some variations are better
suited.
• Adaptation – heritable characteristic that increases an
organism’s ability to survive and reproduce.
• 3. Survival of the fittest – individuals with adaptations that are
well suited to their environment survive and reproduce.
• Fitness – measure of how well an organism
survives/reproduces.
Common Descent
• Common descent – all species (living and extinct) descended
from a common ancestor.
• Over many generations, adaptations caused a successful
species to evolve into a new species.
• The fossil record provides evidence for this “descent with
modification”.
Patterns of Evolution
• Divergent evolution – single species or group of species evolve over a
short period of time into different forms living in different ways due
to a change in environment that makes new resources available.
• Aka adaptive radiations
• Ex. Dinosaurs, Darwin’s finches.
• Convergent evolution – similar structures are produced in distantly
related organisms.
• Ex. Mammals that feed on ants/termites evolved independently 5
times.
• Coevolution – 2 species respond to changes in each other over time.
• Neither can survive without the other.
Evidence for Evolution includes:
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Geographic distribution of species
Fossils
Anatomy (homologous structures)
Physiology (analogous structures)
Embryology
Universal genetic code
Biochemical homology
1. Geographic Distribution of Species
Species of animals on different continents had similar structures
and behaviors.
Darwin theorized that animals on each continent were living under
similar ecological conditions and were exposed to natural selection
in a similar way.
Similar selection pressures caused animals to evolve common
features.
2. Fossils
• A fossil is the preserved or mineralized remains (bone, tooth,
shell) or trace of an organism that lived long ago.
• Fossils show evidence that support the ancestry between species.
• Fossils trace the evolution of modern species from extinct
ancestors.
• Example: Fossils have shown that whales evolved from four legged
land mammals; 1990’s transitional forms of whales found.
3. Anatomy
Evolutionary relationships can be viewed by studying and
comparing anatomy.
 Scientists view the anatomy of limbs and see common
similarities.
 Over course of evolution, vertebrates moved into environments
causing different survival needs.
• Homologous structures – parts of different organisms (that are
often quite dissimilar) that developed from the same ancestral
body parts.
• Forelimbs of whales, bats, crocodiles, and chickens have similar
anatomy but are modified for different functions – common
ancestry.
• Are similar in structure but differ in function!
Homologous structures
-differ in function, similar in structure
4. Physiology
• Analogous structures -- structures that are similar in
appearance and function but have different origins and usually
different internal structures.
• Examples: a bat’s wing and a insect’s wing--both are wings and
both are used for flight, but a bat has bones and an insect does
not.
Analagous Structures
-differ in structure, similar in function
• Vestigial structures – any body structure that has reduced or no
body function.
• Examples: A human’s appendix; a whale’s pelvis.
• As species adapt to environments the change in form and behavior and
continue to inherit these structures as part of the body even though
they have no function.
5. Embryology
Embryology shows links between different species.
 Embryos of different species in early development are
indistinguishable from each other.
 Common cells and tissues develop in similar patterns in all
vertebrates.
 Illustrates descent from a common ancestor.
6. Universal Genetic Code
• Genetic code – mRNA codons specify particular amino acids.
• The genetic code of all organisms on Earth (bacteria, yeast,
fruit flies, humans) is the same!
• Example: the AUG codon always codes for the amino acid
methionine.
7. Biochemical Homology
Similar DNA, RNA, and amino acid sequences amongst species
in same taxonomic group.
 Remember comparing your insulin gene DNA and amino acid
sequences to that of a cow?
Allele Frequencies
• Population – mating group of organisms of the same species.
• Gene pool – all genes (and their alleles) present in a
population.
• Allele frequency - # of times allele occurs in a population.
• Changes as population evolves over time.
• Natural selection operates on individuals, but causes a change in
the allele frequency.
Mutations:
-Sources of Genetic Variation
• The main source of genetic variations in populations
is mutations!!!!
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These mutations occur randomly.
Not all mutations affect an organism’s fitness.
Only heritable mutations matter for evolution.
Neutral mutations don’t change phenotypes.
• Other sources of variation include:
1.Genetic recombination during crossing over and
independent assortment in meiosis.
NS and Phenotype
• An organism’s genotype and environmental conditions makes
up its phenotype.
• Natural selection operates on variation in organisms’
phenotypes.
• Higher fitness = phenotypes better suited for the environment.
Types of Selection in
Populations
• When NS on polygenic traits affects the
fitness of phenotypes, it leads to
selection:
• Directional Selection – organisms at one
end of the curve have a higher fitness
than those in the middle or at the other
end.
• Stabilizing Selection – organisms in the
center have highest fitness.
• Disruptive Selection – organisms at the
ends of curve have highest fitness.
Other Source of Changes in Allele
Frequencies
• Genetic drift – change in allele frequency that occurs in small
populations due to random chance.
• Genetic bottleneck – change in allele frequency following a
dramatic reduction in population size.
• Founder effect – change in allele frequency following
migration of a small subgroup out of the population to start a
new population.
Macroevolution and Speciation
• Species – population of organisms that can interbreed.
• Speciation – evolution/formation of a new species.
When environments change, the process of evolution enables
some species to adapt to new conditions and thrive while some
species fail to adapt and become extinct.
• Niche - combination of an organism’s profession and place
where it lives.
• No 2 species can occupy the same niche in the same location for
a long period of time!
• The more efficient species will survive and reproduce driving the
other to extinction.
Isolating Mechanisms of Speciation
• Reproductive isolation occurs when 2
populations can’t interbreed - causes speciation!
• Once reproductive isolation occurs, natural selection increases
the differences between the separated populations.
1. Behavioral isolation – different courtship.
2. Ecological/habitat isolation – can only mate in specific or
preferred habitats.
3. Mechanical isolation – no sperm is transferred.
4. Gametic isolation – no fertilization of egg occurs.
5. Temporal isolation – reproduce at different times.
• Geographic isolation – population becomes
divided (isolated) by a physical barrier.
Rates of Speciation
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Gradualism – slow, steady change leading to
new species.
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Punctuated equilibrium – brief periods of
rapid change leads to the formation of new
species.
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Rapid change occurs when a small population
is isolated from the rest of the population or
migrates.