Evolution Intro
change over time. descent with
modification. populations evolve,
not individuals.
Charles Darwin
• (1809-1882) Englishman. Medical school dropout.
Obsessed with nature. Aspiring clergyman. Married
his cousin.
• The 5 year trip that changed Darwin’s life: The
Voyage of the HMS Beagle
▫ collected plants and animals. Noticed they were
suited to diverse environments.
▫ Also found species resembled others around the
world
▫ Influenced by geology (Lyell)
 Earth shaped by slow-acting forces that are still in
work today
▫ The Galapagos. (finches)
• Alfred Russell Wallace. British naturalist that
developed a theory very similar to Darwin’s. (His
ideas were submitted for publication first)
• Darwin publishes On the Origin of Species in 1859. His
ideas convinced scientists of evolution to explain
species diversity.
▫ Through time species accumulate differences; as a
result, descendants differ from their ancestors. In this
way new species arise from existing ones.
▫ Natural Selection is the mechanism for evolution
▫ Descent with modification: species share a common
ancestor and diverged gradually over time
Darwin’s Ideas
• Descent with Modification.
▫ organisms share similar characteristics suggesting common
ancestry.
▫ accumulate adaptations as environment changes, leading to
different species
▫ ‘gaps’ in record filled with fossil evidence
• Natural Selection.
▫ Variation in a population
 certain traits will give a higher probability of surviving to
reproduce
▫ Species produce more offspring than survive.
 lead to accumulation of favorable traits in the population over
generations
 causes organisms to become suited specifically to their
environment
▫ May result in new species forming
Evidence
• Fossils
▫ evolutionary changes over time; “missing links”
▫ compare current (extant) organisms with extinct ones
▫ Use geologic information and carbon dating to estimate age of fossils
• Homology (homologous structures)
▫ similar characteristics (but function differently) resulting from
common ancestry
 forelimb of mammals
 vertebrate embryos
▫ vestigal structures: “leftover” structures from an earlier ancestor
(was important once upon a time)
 pelvic bones in a whale, snake, appendix
Evidence
• Biogeography
▫ geographic distribution of species
▫ influenced by continental drift (plate tectonics)
▫ Convergent evolution (similar environments yield
similar anatomies)
 More evidence supporting organisms adapt to be best
suited to their environment!
• Molecular Evidence
▫ DNA sequences similar from shared genes
▫ relationships between groups of organisms
• microevolution: change in allele frequencies in a
population over generations (small scale)
▫ Result from accumulation of mutations
• Variation: differences among individuals in a
population
▫ only genetically determined phenotypes
(characteristics) will be subject to natural
selection
▫ without variation there is no evolution
The White-Striped Clover…
• Discuss with your group members your answers to this
case study. Think specifically about the answers to the
following….
▫ How does this case study illustrate the process of natural
selection? What is the evidence that it occurred?—Be able
to justify your answers.
▫ What is “selection pressure”? How is it illustrated in each
habitat? How does this affect the survival and reproduction
of the clover?
▫ Thinking more broadly…what influences natural selection
in all organisms?
• Selective pressures
▫ A phenotype (physical traits/behaviors) is selected for
or against depending on the environment in which an
organism lives
▫ Survival benefit or disadvantage
▫ Lead to differential survival and reproduction, driving
natural selection
Examples: climate, food, predators, diseases, humans
Influences on Variation
• Geography (geographic variation)
▫ differences in the genetic composition of populations of the
same species
▫ Affected by environmental differences
• Mutations
▫ change in the nucleotide sequence of DNA
▫ only those present in gametes will be passed to offspring
▫ usually deleterious (harmful)…very few are beneficial
• Chance/random events
▫ chromosomal shifts
▫ recombination during meiosis
Influences on Natural Selection
• Competition = Differential Survival
▫ Individuals with more favorable phenotypes will survive and
reproduce…passing on their traits to subsequent generations
• Genetic Variation and Mutation
▫ Diverse genes allow for survival in changing environments
▫ Variation results from mutation
• Environment
▫ Fluctuations in environment cause specific traits to be favored at
certain times
▫ Human impacts
• Chance and Random Events
▫ ie. Earthquake splits population in two
• Adaptation: inherited characteristics of organisms that enhance
their survival and reproduction in specific environments
• Natural Selection: a process in which individuals that have certain
inherited traits tend to survive and reproduce at higher rates than
others because of those traits.
▫ acts on populations, not individuals
▫ only selects for heritable traits that exist within a population
▫ is specific to the environment in a particular time (flexible)
▫ consistently improves the match between organisms and their
environment
• Fitness: the number of surviving offspring in the next generation
(measure of reproductive success)
• Artificial Selection: human modified traits in a population due to
selective breeding (agriculture, domestic animals)
• Convergent Evolution: independent evolution of similar features in
different lineages (analogous structures)