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Evidence of Evolution
Voyage of the Beagle
 Charles Darwin’s observations on a voyage
around the world led to new ideas about species
Voyage of the Beagle
Darwin, Wallace, and Natural Selection
 In 1858, Charles Darwin and Alfred Wallace
independently proposed a new theory, that
natural selection can bring about evolution
Descent with Modification
 Darwin compared the modern armadillo with the
extinct glyptodont
Variations in Traits
 Darwin observed that variations in traits
influence an individual’s ability to secure
resources – to survive and reproduce
Theory of Natural Selection
 Natural selection
• The differential in survival and reproduction
among individuals of a population that vary in
details of their shared traits
• Can lead to increased fitness
 Fitness
• An individual’s adaptation to an environment,
measured by its relative genetic contribution to
future generations
Fossil Evidence
 Fossils
• Physical evidence of life in the distant past
 Found in stacked layers of sedimentary rock
• Younger fossils in more recently deposited layers
• Older fossils underneath, in older layers
Interpreting the Fossil Record
 The fossil record is incomplete
 Favors species with hard parts, dense
populations with wide distribution, and that
persisted a long time
Plate Tectonics Theory
 Movements of Earth’s tectonic plates rafted land
masses to new positions
 Pangea: First ancient supercontinent
• Gondwana (later southern supercontinent)
 Movements had profound impacts on the
directions of life’s evolution
Evidence of Drifting Continents
 Evidence for plate tectonics theory
Distribution of global land masses
Global fossil distribution
Magnetic rocks
Seafloor spreading from mid-oceanic ridges
Drifting Continents
Biogeographical Evidence
Morphological Divergence
 Homologous structures: Similar body parts that
became modified differently in different lineages
 Evidence of descent from a common ancestor
Comparative Morphological Evidence
Homologous Structures
Analogous Structures
DNA, RNA, and Proteins
 Comparisons of DNA, RNA, and proteins reveal
and clarify evolutionary relationships
Processes of Evolution
Rise of the Super Rats
Populations Evolve
 Population
• Individuals of the same species in the same area
• Generally the same number and kinds of genes
for the same traits
 Gene pool
• All the genes of a population
Variation in Alleles
 Individuals who inherit different combinations of
alleles vary in details of one or more traits
 Mutations are the original source of new alleles
• Lethal mutations result in death
• Neutral mutations neither help nor hurt
Phenotypic Variation in Populations
 Changes in allele frequencies of a population
Natural selection
Genetic drift
Gene flow
Natural Selection
 Natural selection
• Differential survival and reproduction among
individuals of a population that show variations in
details of their shared traits (alleles)
 Allele frequencies
• Maintained by stabilizing selection
• Shifted by directional or disruptive selection
Modes of Natural Selection
Peppered Moth
Pocket Mice
Stabilizing Selection: Birth Weight
Sexual Selection
Balanced Polymorphism
Genetic Drift
 Genetic drift
• Random change in a population’s allele
frequencies over time, due to chance
• Can lead to loss of genetic diversity
 Most pronounced in small or inbred populations
• Bottleneck: Drastic reduction in population
• Founder effect: Small founding group
Gene Flow
 Gene flow
• Movement of alleles into or out of a population by
immigration or emigration
• Helps keep populations of same species similar
 Counters processes that cause populations to
diverge (mutation, natural selection, genetic drift)
Gene Flow Between Oak Populations
Reproductive Isolation
 Individuals of a sexually reproducing species
can produce fertile offspring, but are
reproductively isolated
 Reproductive isolating mechanisms evolve when
gene flow between populations stops
 Divergences may lead to new species
Mechanical Isolation
Behavioral Isolation
Allopatric Speciation
 A geographic barrier stops gene flow between
two or more populations of a species
• Example: Isolated continents or archipelagos
 Genetic divergence and reproductive isolation
give rise to new species
Allopatric Speciations
An Isolated Archipelago
Patterns of Macroevolution
 Coevolution
• Close ecological interactions cause two species
to act as agents of selection upon one another
 Extinction
• Irrevocable loss of species
• Mass extinctions and recoveries have occurred
several times in the history of life
• Most species that ever existed are now extinct
Adaptation to What?
 Evolutionary adaptation
• Heritable traits that improve an individual’s
chance of surviving and reproducing (under
conditions that prevailed when genes evolved)