Evolution of Life

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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
  individuals 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 Darwins 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 fertile

Sterile 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