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Evolution
Darwin’s Theory of Natural Selection and Evidence for
Evolution
What is Evolution?
• The change in species over time
• Generally happens very slowly (difficult to observe), but can also happen
more quickly.
• Common belief prior to 1800s was that organisms were placed on the Earth
in a fixed form and did not change
• Jean-Baptiste Lamarck (early 1800s) proposed theory of inheritance of
acquired characteristics = the environment can lead to a change an organism
and that change can then be inherited by offspring = NOT TRUE
Development of Darwin’s Theory
• Charles Darwin was a naturalist who took a trip around the world in 1831
• On his trip around the world, he found that the Galapagos Islands (located off the coast of
Equador) had tortoises, finches and iguanas similar to those on other islands, yet also
unique
• Finches all resembled one from S. America, but each was specialized
• Both land and sea iguanas co-existed on the islands
• The tortoises were much larger than those found in other places
• Video = Galapagos Islands
• Darwin determined that the organisms from S. America migrated to Islands and, as a
population, changed over time to fit new home = evolution
After his Trip…
• Darwin continued his studies after his trip on the Beagle
• He determined that only some animals live long enough to produce offspring due to
struggle for existence = competition
• Compete for: 1) food and water, 2) escape from predators, 3) ability to find shelter, 4) mates
• He performed pigeon-breeding experiments to prove his theory
• Breeders used artificial selection to breed birds with desired traits
• Today, human breeders choose the individuals that will parent the next generation this way. (dogs, cows,
etc.)
• Selection = differential survival and reproduction of genotypes
• Darwin asked “Does nature work the same way??”
Darwin’s Theory
• 1850 – Charles Darwin published his book called On the Origin of Species
by Natural Selection
• Evolution occurs when organisms with favorable variations survive,
reproduce and pass on variations = populations, not individual organisms
• The environment is key = presents challenges that individuals with particular
traits can better overcome – thus the environment “selects” which organisms
will survive and reproduce more often.
Main Ideas of Theory of Natural Selection
• Nature has a tendency toward overpopulation of offspring.
• Individuals exhibit slight genetic variations.
• Individuals compete and those with favorable variations (called adaptations) are more likely
to survive and pass on variations
• Offspring of survivors gradually make up larger proportion of the population and the
alleles for successful traits increase in frequency.
• This process makes organisms better suited to their environment. The environment
determines the direction of the genetic change.
• Video: Evolution in the Galapagos
Evidence for Evolution #1 – Organism
Comparisons
• Anatomical Comparisons provide evidence
• Homologous structures – structures that share a common ancestry.
• Modified versions of structures of ancestors
• Examples: Limbs of humans, wings of birds and fins of dolphins
• Analogous Structures
• Structures that have closely related function but do not come from the same ancestor
• Examples: Wings of bats, birds and butterflies
Evidence for Evolution #1 – Organism
Comparisons
• Vestigial Structures provide evidence = remnants of an organism’s evolutionary
past.
• Structures without function can be found in living things.
• Humans = appendix, wisdom teeth, tailbone
• Some snakes have remnants of legs, flightless birds have wings.
• Cave dwelling crayfish are blind, but have eyes.
• Developmental patterns also show evolutionary relationships
• Embryos of humans and all other vertebrates are similar.
• In later stages, the human embryo develops fine coat of fur.
Evidence for Evolution #2 – Fossil Record
• Fossil Record/Transitional forms of organisms link new species to old.
• Video: How do we know evolution happens?
• Darwin predicted fossil record would find transitional species between older &
newer species and many examples of transitional species have been found.
• Fossils are any traces of dead organisms
• Death and burial must occur under very specific conditions to leave an intact fossil
(sediment – ocean floor, mud, tar pits, etc.)
• Calcium in bones or shell is replaced with harder minerals
• Fossils only typically show bones and thus do not give the entire picture of the organism.
Evidence for Evolution #2 – Fossil Record
• The age of fossils can be determined two ways
• Superposition or the principle that the lower strata or layer of rock are older than the strata
at the top = determines relative age of fossils
• Measuring the amount of radioactive decay in the fossil dates fossils = determines absolute
age of fossils
• A radioactive atom changes to a different atom to obtain a more stable combination of protons and
neutrons.
• Scientists can determine how old a fossil is by looking at the amount of Carbon-14 or other isotopes
with longer half lives
• The technique is called radioactive or radiometric dating.
Evidence for Evolution #3 – Biogeography
• The study of the location of organisms around the world.
• The Galapagos Islands’ organisms are similar to those found in South
America, yet had differences that allowed them better survival on the islands.
• Animals in Australia are similar to those found elsewhere in the world, but
often are marsupials = one explanation is that they evolved in isolation.
Evidence for Evolution #4 – DNA and
Proteins
• Genes develop more alterations in nucleotide sequences over time.
• Closely related species should have more similarities in nucleotide sequences
than organisms that are distantly related.
• Amino acid sequences in proteins should also be similar:
• Scientists looked at the amino acids in the protein cytochrome c (necessary for cellular
respiration).
• Human and chimpanzee cytochrome c are identical in all 104 amino acids.
Scientists/Specific Examples that support
Darwin
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E.B. Ford/Kettlewell – Peppered Moths change from light to dark
Bacteria and Antibiotics = Bacteria are becoming resistant
Insects are becoming resistant to pesticides
Most evolution cannot be seen in a lab = too slow
Modern understanding of evolution is based on
two aspects
• 1) Darwin’s theory of Natural Selection
• 2) Principles of genetics have identified two main sources of variations
• Mutations provide new alleles/trait variations
• Recombination of alleles in meiosis provides new gene combinations
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Crossing over in prophase I
Independent assortment in metaphase I
• Determined through knowledge of genes and DNA
• Evolution occurs in populations, not individuals
• Individuals learn and change behavior to adapt, but they do not evolve.
• Evolution/Nat Selection operates in populations over many generations.
Patterns of Evolution
• Divergent Evolution - Species diversity when introduced to new environments.
• Species that were once all similar in ancestral species become more and more distinct as
species begin to adapt to different environments. (results in homologous structures)
• Humans are speeding up this type of evolution with artificial selection
• Convergent Radiation - Distantly related species evolve similar features
• The pattern of evolution in which distantly related organisms evolve similar traits, but will
never be the same (results in analogous structures)
• Different starting points and different niches
• Face similar selection pressures due to similar environments
Coevolution
• The process by which the interactions between two species cause them to
evolve jointly
• Example: pollinating insects and flowers
• Other Examples?