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Evolution of Life
Chapter 13
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Origin of Life
• Age of Planet Earth - 4.6 billion years
• Oldest fossils - 3.5 billion years
• Possible Formation of the First Cells– Inorganic molecules reacted to
form organic molecules– Organic molecules polymerized
to become macromolecules– Plasma membrane formed– Protocells formed
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Evidence of Evolution
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Evidence of Evolution
• Evolution– All the changes that have occurred in living
things since the beginning of life• Due to differential reproductive success
• 4 main areas of evidence that lead us to believe in “some” evolution:
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Evidence of Evolution
• Fossil Evidence
• Biogeographical evidence
• Anatomical evidence
• Biochemical evidence
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1. Fossil Evidence • Fossils
– The remains and traces of past life or any other direct evidence of past life
• Deposited in layers called strata– Each stratum is older than
the one above and younger than the one below
• Transitional fossils– Especially significant– Represent evolutionary
links
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Fossil Evidence - Geological Timescale
• Based on dating of fossil evidence– Relative Dating Method
• Determines the relative order of fossils and strata but not the actual date
– Absolute Method-• Radioactive dating techniques • Assign an actual date to a fossil• Technique based on the half-life of radioactive
isotopes
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Absolute Method
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Fossil Evidence - Mass Extinctions
• Large numbers of species become
extinct in a short period of time
– Remaining species may spread out
and fill habitats left vacant
• Five Major Extinctions have occurred
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2. Biogeographical Evidence
• Biogeography – Study of the distribution of species throughout the
world
• Earth has six biogeographical regions– Each has its own distinctive mix of species
• Barriers prevented evolving species from migrating to other regions
• Continental Drift-– The positions of continents and oceans has shifted
through time– The distribution of fossils and existing species allows
us to determine approximate timeline
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3. Anatomical Evidence• Common descent offers explanation
for anatomical similarities– Homologous Structures
• Same function and same basic structure, indicating a common ancestor
• Ex: human arm and whale forelimb
– Analogous Structures • Same basic function but different
origins• Ex: wing of bird and wing of an insect
– Vestigial Structures• Anatomical structures fully functional
in one group and reduced, nonfunctional in another
• Ex: Modern whales have a pelvic girdle and hind leg bones
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4. Biochemical Evidence
• All organisms use same basic biochemical molecules– DNA– ATP– Identical or nearly identical enzymes
• Many developmental genes are shared• Degree of similarity between DNA base
sequences and amino acid sequences indicates the degree of relatedness
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Process of Evolution
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Process of Evolution
• Occurs at the population level• Evolution – genetic changes within
population – more generations – phenotypic changes
• Microevolution - evolution on a small scale– Gene pool of a population
• All the alleles in all the individuals making up the population
– When the allele frequencies for a population change, microevolution has occurred
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Evolutionary Change
• Mutations
• Genetic drift
• Gene flow
• Nonrandom mating
• Natural selection
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1. Mutations
• Genetic changes that provide the materials for evolutionary change
• Can cause allele frequencies to change in a gene pool – microevolution to occur
• Has the ability to create new alleles in a population
• Happen randomly • Does not have an adverse effect on the
individual’s reproductive state
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2. Genetic Drift
• Changes in the allele frequencies of a gene pool due to chance
• Greater effects on smaller populations
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2. Genetic Drift
• Two main mechanisms– Founder Effect
• Few individuals found a colony
• Their collective genes represent only fraction of the original gene pool
– Bottleneck Effect: • Population is subjected to
near extinction by a disaster
• Only a few genotypes contribute to next generation
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3. Gene Flow
• Movement of alleles between populations
• Occurs when individuals migrate from one population to another
• Keeps gene pools similar
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4. Nonrandom Mating
• Occurs when individuals pair up according to phenotype or genotype
• Inbreeding is an example
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5. Natural Selection
• Natural Selection
– Process by which populations adapt to their environment
– Charles Darwin explained evolution through natural selection
– Why does this happen????
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Charles Darwin and The Origin of Species
• Charles Darwin’s On the Origin of Species by Means of Natural Selection– Published on November 24, 1859
• Argued that contemporary species arose from ancestors– Through a process of “descent with
modification,” with natural selection as the mechanism
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• Darwin made two main points in The Origin of Species:– Organisms inhabiting Earth today
descended from ancestral species– Natural selection was the mechanism
for descent with modification• Basic idea of natural selection is
that:– Organisms can change over
generations– Individuals with certain heritable
traits leave more offspring than others• The result of natural selection is
evolutionary adaptation
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Natural Selection• Evolution by natural selection requires the
following– Variation
• members of a population differ
– Inheritance• differences are inheritable
– Overproduction• populations produce more offspring than the environment
can support (struggle for existence)
– Differential Reproductive Success• better adapted individuals survive to reproduce more
offspring
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Natural Selection
• Fitness– Measured by the number of fertile
offspring produced by an individual
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Natural Selection
• Three Main Types of Natural Selection:
– Stabilizing Selection
– Directional Selection
– Disruptive Selection
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Natural Selection
• Stabilizing Selection
– Occurs when an
intermediate, or
average, phenotype is
favored
– Improves adaptation of
population to a stable
environment
– Extreme phenotypes
are selected against
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Stabilizing Selection
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Natural Selection
• Directional Selection
– One extreme phenotype is favored
– Distribution curve shifts in that
direction
– Can occur when population is
adjusting to a changing
environment
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Directional Selection
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Natural Selection
• Disruptive Selection
– Two or more extreme phenotypes are selected
– Two different habitats result in two different
phenotypes in a population
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Disruptive Selection
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Speciation
Chapter 14
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What is a species?
• biological species
• All individuals of 1 species can interbreed
• Offspring are fertileSterile zebra-horse cross: not a
new species
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Speciation
• Macroevolution
– observed best within the fossil record
• Speciation
– Splitting of one species into two or more
– the transformation of one species into a new species over time
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Hybrid Animals
• Breeding of two closely related but distinct species
• Human activities
• Unnatural conditions
• Usually sterile
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Reproductive Barriers between Species
• Prezygotic isolating mechanisms
• Postzygotic isolating mechanisms
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Reproductive isolating mechanisms
• Prezygotic isolating mechanisms– Habitat isolation
– Temporal isolation
– Behavioral isolation
– Mechanical isolation
– Gamete isolation
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Reproductive isolating mechanisms
• Postzygotic isolating mechanisms– Zygote mortality
– Hybrid sterility
– F2 fitness
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Process of Speciation
• Allopatric Speciation– Geographical barriers separate a
population into two groups
• Sympatric Speciation– Occurs without geographical barriers– Ex: Plants
• Self-reproduction can maintain a new species
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Pace of Speciation
• How FAST does evolution occur?
• 2 main mechanisms to explain:– Phyletic Gradualism– Punctuated Equilibrium
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Pace of Speciation• 1. Phyletic Gradualism
– Change is slow but steady before and after a divergence
– Proposes that speciation occurs after populations become isolated
• Reproductive isolation cannot be detected in fossils
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Pace of Speciation
• 2. Punctuated Equilibrium– Some think species appear
suddenly• Remain essentially unchanged
phenotypically until they undergo extinction
– Long periods of stasis followed by rapid speciation
• Occurs relatively rapidly
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Extinction…• Inevitable • Occurs all the time
– rates have not been steady
• Extinctions typically eliminate various species of organisms– followed by
explosive diversifications of organisms