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Evolution of Life
Chapter 13
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
Evidence of Evolution
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:
Evidence of Evolution
• Fossil Evidence
• Biogeographical evidence
• Anatomical evidence
• Biochemical evidence
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
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
Absolute Method
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
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
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
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
Process of Evolution
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
Evolutionary Change
• Mutations
• Genetic drift
• Gene flow
• Nonrandom mating
• Natural selection
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
2. Genetic Drift
• Changes in the allele frequencies of a gene pool due to chance
• Greater effects on smaller populations
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
3. Gene Flow
• Movement of alleles between populations
• Occurs when individuals migrate from one population to another
• Keeps gene pools similar
4. Nonrandom Mating
• Occurs when individuals pair up according to phenotype or genotype
• Inbreeding is an example
5. Natural Selection
• Natural Selection
– Process by which populations adapt to their environment
– Charles Darwin explained evolution through natural selection
– Why does this happen????
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
• 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
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
Natural Selection
• Fitness– Measured by the number of fertile
offspring produced by an individual
Natural Selection
• Three Main Types of Natural Selection:
– Stabilizing Selection
– Directional Selection
– Disruptive Selection
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
Stabilizing Selection
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
Directional Selection
Natural Selection
• Disruptive Selection
– Two or more extreme phenotypes are selected
– Two different habitats result in two different
phenotypes in a population
Disruptive Selection
Speciation
Chapter 14
What is a species?
• biological species
• All individuals of 1 species can interbreed
• Offspring are fertileSterile zebra-horse cross: not a
new species
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
Hybrid Animals
• Breeding of two closely related but distinct species
• Human activities
• Unnatural conditions
• Usually sterile
Reproductive Barriers between Species
• Prezygotic isolating mechanisms
• Postzygotic isolating mechanisms
Reproductive isolating mechanisms
• Prezygotic isolating mechanisms– Habitat isolation
– Temporal isolation
– Behavioral isolation
– Mechanical isolation
– Gamete isolation
Reproductive isolating mechanisms
• Postzygotic isolating mechanisms– Zygote mortality
– Hybrid sterility
– F2 fitness
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
Pace of Speciation
• How FAST does evolution occur?
• 2 main mechanisms to explain:– Phyletic Gradualism– Punctuated Equilibrium
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
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
Extinction…• Inevitable • Occurs all the time
– rates have not been steady
• Extinctions typically eliminate various species of organisms– followed by
explosive diversifications of organisms