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Evolution of Populations
Ch 17
• Process of change over time
• A change in the genes!!!!!!!!
Population Genetics
• The science of genetic change in
population.
• Population – all the members of a
species that occupy a particular area at
the same time
• Gene Pool – all the genes in all the
members of a population
Genes and Variation
• Genetics Joins Evolutionary Theory
– Variation is the raw material for natural selection
– Gene pool – consists of all the genes, including all the different alleles
for each gene, that are present in a population
– Relative frequency – the number of times that the allele occurs in a
gene pool, compared with the number of times other alleles for the
same gene occur
– Therefore – evolution is any change in the relative frequency of alleles
in the gene pool of a population over time
• 3 Sources of Genetic Variation
– Mutations
– Genetic Recombination in Sexual Reproduction (Ind assortment and
crossing over)
– Lateral Gene Transfer (conjugation)
• Single-Gene (2 pheno) vs. Polygenic Traits (many pheno/bell curve)
***Natural Selection acts directly on PHENOTYPES not actual
alleles*** some phenotypes are better suited to an environment
than others and they will survive, reproduce and pass on their
genes.
Evolution as Genetic Change in
Populations
How Natural Selection Works – 3 Types
Stabilizing Selection
Individuals with the average form of a trait have the highest fitness
Represents the optimum for most traits
Results in a similar morphology between most members of the species
Directional Selection
Individuals that display a more extreme form of a trait have greater
fitness than individuals with an average form of the trait
A shift in one direction
Peppered moth
Disruptive Selection
Individuals with either extreme variation of a trait have greater fitness
than individuals with the average form of the trait
A shift in both direction, away from the center
Shell color (dark rocks and light sand)
Genetic Drift
• Random changes in the frequency of a gene
in the absence of natural selection 
occurs because of CHANCE
• Occurs efficiently in small populations
because small changes affect more
members
• Two examples:
a. Bottleneck effect
b. Founder effect
Genetic Drift
a. Bottleneck Effect
• Genetic drift (reduction of alleles in a
population) resulting from a disaster that
drastically reduces population size.
– Examples:
1.
2.
Earthquakes
Volcano’s
b. Founder Effect
• Genetic drift resulting from the
colonization of a new location by a small
number of individuals.
• Results in random change of the gene
pool.
• Example:
1. Islands
(first Darwin finch)
Hardy-Weinberg Principle
• Genetic Equilibrium – situation in which allele
frequencies in the gene pool of a population
remain constant
• The concept that the shuffling of genes that
occurs during sexual reproduction, by itself,
cannot change the overall genetic makeup of
a population.
• Shows mathematically and theoretically that
there are situations where evolution DOES
NOT OCCUR
– Seldom achieved in nature
Hardy-Weinberg Principle
• This principle will be maintained in nature
only if ALL five of the following conditions
are met:
1. Very large population
2. Isolation from other populations
3. No net mutations
4. Random mating
5. No natural selection
Hardy-Weinberg Principle
Species
• A group of populations whose individuals have
the potential to interbreed and produce
viable offspring.
Speciation
• The evolution of new species. Species that
occupy an otherwise unoccupied niche face no
competition, they will therefore have a 100%
success rate
Reproductive Isolation
• Any mechanism that impedes two species from
producing fertile and/or viable hybrid offspring
-factor necessary for the formation of a
new species.
• Barriers:
1. Geographic (rivers, mountains)
2. Behavorial - differences in courtship
behavior
3. Temporal - fertile periods (time)
Interpretations of Speciation
• Two theories:
1. Gradualist Model
(Neo-Darwinian):
Slow changes in species
overtime.
2. Punctuated
Equilibrium:
Evolution occurs in
spurts of relatively
rapid change.
Macroevolution
• The origin of taxonomic groups higher
than the species level.
Adaptive Radiation
aka Divergent Evolution
• Emergence of numerous
species from a common
ancestor introduced to new
and diverse environments.
• Example:
Darwin’s Finches
Five steps to the 14 finch
species on the Galapagos
Islands:
1) founders arrive
2) geographic isolation
3) changes in gene pools
4) behavioral isolation
5) competition and
continued evolution
Darwin’s Finches an example of
Adaptive Radiation
Convergent Evolution
• Species from different evolutionary branches
may come to resemble one another if they live
in very similar environments.
• Example:
1. Ostrich (Africa) and Emu (Australia).
2. Sidewinder (Mojave Desert) and
Horned Viper (Middle East Desert)
3. Shark and Dolphin
Coevolution
• Evolutionary change, in which one species act
as a selective force on a second species,
inducing adaptations that in turn act as
selective force on the first species.
• Example:
1. Acacia ants and acacia trees
2. Humming birds and plants with flowers
with long tubes
The Age of the
Earth and Fossils
– Fossils
• Trace the evolution of modern
species from ancient/extinct
ancestors
Radiometric Dating
• Relative dating vs.
– When unstable nuclei
release particles or
• Absolute dating
radiant energy until the
–
nuclei becomes stable
Half-life = The length
of time it takes for
one-half of a sample to
decay to stable form
The Earth is born…
Date
Event
4.6 bya
Earth was born
4 bya
Cooling of Earth, 1st solid rocks formed
on earth’s surface
4-3.8
bya
Volcanic activity & meteorites release
gases that produce earth’s atmosphere
Contained H2O vapor, CO, CO2, H2, N2, NH3, CH4
It did NOT contain oxygen
3.8 bya
Cooling continues, water appears,
beginning of oceans
Earth cool enough for liquid to stay on the ground
3.5 bya
Age of first prokaryotic microfossils
Heterotrophic – obtained nutrients from organic “soup”
Anaerobic – able to live in oxygen-free environment
3.4 bya
Appearance of 1st autotrophs
Organic soup begins to run out
Photosynthesis begins – using H2S instead of water
2.2 bya
Introduction of oxygen into the
atmosphere
More modern form of photosynthesis
appeared
Used H2O instead of H2S
Caused earth to cool as they converted CO2  O2
Led to aerobic respiration
Ozone layer protection
1.6-1.1
bya
1st eukaryotic cells evolved
Nucleus contains DNA, have membrane bound
organelles, etc.
Increased the speed of evolution
Increased genetic variation
1700s
Spontaneous Generation (abiogenesis) –
idea that life comes from non-life
Disproven through Redi, Spallanzani, & Pastuer
1953
Miller & Urey
mix methane, water, ammonia, and
hydrogen with energy (sun & lightning)
Primordial Soup
Amino acids & other organic compounds are produced
as by-products
Provide glimpse at how molecules (proteins) may 1st
have formed on the Earth
Sexual reproduction evolved
Multicellular organisms arose
Additional Info
The Age of the Earth and
Fossils
– The Age of the Earth
• Earth had to be old enough for these proposed
changes to occur
• Earth is ~4.5 byo (determined by radioactive
dating)
History of Earth (24hours)
12:00am – Earth is formed
5:00am – Prokaryotes appear
4:00pm – Eukaryotes appear
10:00pm – Invasion of land
11:59:30pm – Humans appear
Miller/Urey Experiment and
Primordial Soup
• Oparin and Haldane: hypothesized that the early
atmosphere was composed of: ammonia (NH3), hydrogen gas
(H2), water vapor (H20), and compounds made of carbon and
hydrogen, like methane (CH4). They thought that at high
temperatures, simple organic compounds (like amino acids)
could form. When earth cooled, and lakes and oceans
formed, theses simple compounds could be found in the
water and enter complex chemical reactions fueled by
lightning and ultraviolet violet radiation resulting in
macromolecules essential to life like proteins.
• Miller and Urey: 1953 set up an apparatus to test Oparin’s
hypotheses. Their experiment produced a variety of organic
compounds, including amino acids
EndosymbioticTheory
• What is the theory of
endosymbiosis? Theory that large
prokaryotic, unicellular organisms
engulfed (ate) smaller prokaryotic,
unicellular organisms. Engulfed
prokaryotes eventually gave rise to
modern mitochondria and
chloroplasts.
• What evidence supports the
hypothesis that mitochondria and
chloroplasts were once free living
prokaryotic cells? Replicate
independently and replicate like
prokaryotes (binary fission), have
their own DNA and their own
ribosomes (also similar to DNA and
ribosomes of prokaryotes)