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Patterns of Evolution /
Descent with Modification
Chapter 23 / 19
Impacts, Issues Video
Measuring Time
Evolutionary Theories
• Widely used to interpret the past and present, and even to
predict the future
• Reveal connections between the geological record, fossil
record, and organism diversity
Geological Discoveries
• Similar rock layers throughout world
• Certain layers contain fossils
• Deeper layers contain simpler fossils than shallow layers
• Some fossils seem to be related to known species
Fossils
• Recognizable evidence of ancient life
• What do fossils tell us?
– Each species is a mosaic of ancestral and novel traits
– All species that ever evolved are related to one another by way of
descent
– The fossil record indicates that there have been great changes in the
kinds of organisms on Earth at different points in time
• Few individuals have fossilized, and even fewer have been discovered
• The fossil record is biased in favor of species that
– Existed for a long time
– Were abundant and widespread
– Had hard parts
Radiometric Dating
Radiometric dating
How Rocks and Fossils
Are Dated
• Sedimentary strata reveal the relative ages of fossils
• The absolute ages of fossils can be determined by radiometric dating
• A “parent” isotope decays to a “daughter” isotope at a constant rate
• Each isotope has a known half-life, the time required for half the parent
isotope to decay
• Radiocarbon dating can be used to date fossils up to 75,000 years old
• For older fossils, some isotopes can be used to date volcanic rock layers
above and below the fossil
Geologic Time Scale
Geologic time scale
Changing Land Masses
Drifting continents
Plate Margins
Plate margins
Evidence of Movement
• Wegener cited evidence from glacial deposits and fossils
• Magnetic orientations in ancient rocks do not align with the
magnetic poles
• Discovery of seafloor spreading provided a possible mechanism
Geologic Forces
Geologic forces
19th Century - New Theories
• Scientists attempt to reconcile evidence of change with traditional belief in
a single creation event
– Georger Cuvier
• Paleontologist
• speculated that each boundary between strata represents a
catastrophe that destroyed many species
19th Century - New Theories
– James Hutton and Charles Lyell
• geologists
• perceived that changes in Earth’s surface can result from slow,
continuous actions still operating today
• Lyell further proposed that the mechanisms of change are constant
over time
• This view strongly influenced Darwin’s thinking
19th Century - New Theories
– Lamarck
• hypothesized that species evolve through use and disuse of
body parts and the inheritance of acquired characteristics
• The mechanisms he proposed are unsupported by evidence
DARWIN
Galapagos Finches
• Darwin observed finches with a variety of lifestyles and body
forms
• On his return, he learned that there were 13 species
• He attempted to correlate variations in their traits with
environmental challenges
Galapagos Finches
Finches of the Galapagos Islands
Darwin’s Theory
A population can change over time when individuals differ in one or
more heritable traits that are responsible for differences in the ability
to survive and reproduce.
Darwin’s Focus on Adaptation
• In reassessing his observations, Darwin perceived adaptation to the
environment and the origin of new species as closely related processes
Ideas from
The Origin of Species
• Darwin explained three broad observations about life
– The unity of life
– The diversity of life
– The match between organisms and their environment
Descent with Modification
• The phrase refers to the view that all organisms are related through
descent from an ancestor that lived in the remote past
Artificial Selection, Natural Selection, and
Adaptation
• Darwin noted that humans have modified other species by selecting and
breeding individuals with desired traits, a process called artificial selection
• Darwin argued that a similar process occurs in nature
Cabbage
Selection for
apical (tip) bud
Brussels
sprouts Selection for
axillary (side)
buds
Selection Broccoli
for flowers
and stems
Selection
for stems
Selection
for leaves
Kale
Wild mustard
Kohlrabi
• Darwin drew two inferences from two observations
– Observation #1: Members of a population often vary in their inherited
traits
– Inference #1: Individuals whose inherited traits give them a
higher probability of surviving and reproducing in a given
environment tend to leave more offspring than other individuals
» Survival of the Fittest
– Observation #2: All species can produce more offspring than the
environment can support, and many of these offspring fail to survive
and reproduce
– Inference #2: This unequal ability of individuals to survive and
reproduce will lead to the accumulation of favorable traits in the
population over generations
• Darwin was influenced by Thomas Malthus, who noted the potential for
human population to increase faster than food supplies and other
resources
• If some heritable traits are advantageous, these will accumulate in a
population over time, and this will increase the frequency of individuals
with these traits
• This process explains the match between organisms and their environment
Natural Selection: A Summary
• Individuals with certain heritable traits survive and reproduce at a higher
rate than other individuals
• Over time, natural selection increases the match between organisms and
their environment
• If an environment changes over time, natural selection may result in
adaptation to these new conditions and may give rise to new species
• Note that individuals do not evolve; populations evolve over time
• Natural selection can only increase or decrease heritable traits that vary in a
population
• Adaptations vary with different environments
Natural Selection
• A difference in the survival and reproductive success of different
phenotypes
• Acts directly on phenotypes and indirectly on genotypes
Variation in Populations
• All individuals have the same genes that specify the same
assortment of traits
• Most genes occur in different forms (alleles) that produce
different phenotypes
• Some phenotypes compete better than others
Change over Time
• Over time, the alleles that produce the most successful
phenotypes will increase in the population
• Less successful alleles will become less common
• Change leads to increased fitness
– Increased adaptation to environment
Comparative Morphology
• Study of similarities and differences in body plans of major groups
• Puzzling patterns:
– Animals as different as whales and bats have similar bones in
forelimbs
– Some parts seem to have no function
• Guiding principle:
– When it comes to introducing change in morphology, evolution
tends to follow the path of least resistance
Comparative Morphology
Comparative pelvic anatomy
Morphological Divergence
• Change from body form of
a common ancestor
• Produces homologous
structures
Morphological Convergence
• Individuals of different lineages evolve in similar ways under similar
environmental pressures
• Produces analogous structures that serve similar functions
Morphological Convergence
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel
Comparative Development
• Each animal or plant proceeds through a series of changes in
form
• Similarities in these stages may be clues to evolutionary
relationships
• Mutations that disrupt a key stage of development are selected
against
Similar Vertebrate Embryos
FISH
REPTILE
BIRD
MAMMAL
Pharyngeal
arches
Post-anal
tail
Chick embryo (LM)
Human embryo
Altering Developmental Programs
• Some mutations shift a step in a way that natural selection favors
• Small changes at key steps may bring about major differences
• Insertion of transposons or gene mutations
Proportional Changes in Skull
Mutation and proportional changes
Comparative Biochemistry
• Kinds and numbers of biochemical traits that species share is a
clue to how closely they are related
• Can compare DNA, RNA, or proteins
• More similarity means species are more closely related
Comparing Proteins
• Compare amino acid sequence of proteins produced by the same gene
• Human cytochrome c (a protein)
– Cytochrome c functions in electron transport
– Deficits in this vital protein would be lethal
• Identical amino acids in chimpanzee protein
• Chicken protein differs by 18 amino acids
• Yeast protein differs by 56
Sequence Conservation
Cytochrome C comparison
Nucleic Acid Comparison
• Use single-stranded DNA or RNA
• Hybrid molecules are created, then heated
• The more heat required to break hybrid, the more closely related
the species