Chapter 16:
Darwin’s
Theory of
Evolution
Evolution: Theory or Fact?
I will present the concepts.
All you need is to understand .
What you believe is up to you.
Early Ideas
❖ Early scientists believed the earth was only a few thousand years old.
❖ Around 1800, geological studies suggested it was much, much older (about 4.5 billion years).
❖ Naturalist Lamarck believed that individuals change and acquire features that help them become more successful.
Prevailing view: If an animal acquired a
trait during its
lifetime, it could
pass it on to its
offspring.
Charles Darwin
❖ What was Charles Darwin’s contribution to science?
❖ Darwin developed a scientific theory of
biological evolution that explains how modern
organisms evolved over long periods of time through
descent from common ancestors.
❖ Evolution - Process of change over time
❖ 1831-1836, Darwin toured the
world by boat, studying and
collecting specimens.
Darwin’s Observations
❖ What three patterns of biodiversity did Darwin note?
❖ As he traveled, Darwin noticed three distinctive patterns of biological
diversity:
1. Species vary globally
2. Species vary locally
3. Species vary over time
Species Vary Globally
Different, yet similar, species inhabit
separate, but similar habitats.
Species Vary Locally
Different, yet related, species often occupy
different habitats within a local area.
Diversity of tortoise
species in the Galapagos – pg. 765, figure 1.2
Finch Beak Tools – pg.
801, figure 4.6
Species Vary Over Time
❖ Some extinct
animals are similar to living species.
❖ Darwin collected fossils, preserved
remains or traces of
ancient organisms
Artificial Selection
❖ Darwin realized that most organisms don’t survive and reproduce.
❖ Which individuals survive and why?
❖ By studying plant and animal breeders, Darwin came up with the idea of artificial selection.
❖ In artificial selection (selective breeding), nature
provides the variation and humans select for the most
desired traits.
The Struggle for Existence
❖ Darwin realized that individuals must compete for resources.
❖ Hypothesized that some variations of traits are better suited to different environments than others.
❖ Fitness – how well an organism can survive and reproduce in an environment.
❖ Adaptation – heritable characteristic that increases an organism’s ability to survive and reproduce.
❖ “Survival of the fittest”
❖ Only those that survive can pass on their genes.
Natural Selection
❖ Darwin realized that nature can act on traits.
❖ Natural selection – nature provides the variation and nature selects the most fit.
❖ Organisms with variations most suited to their
environment survive and leave more offspring.
❖ Acts only on heritable traits
because they are the only ones that can be passed on.
❖ Important: Natural selection does not make organisms
“better” and it doesn’t move in a fixed direction.
Natural Selection pg. 785, Figure
3.2
❖ Under what conditions does natural selection occur?
❖ Natural selection occurs in any situation in which more individuals are born than can survive, there is natural heritable variation, and there is variable fitness among individuals.
❖ If environmental conditions change, some traits that were once adaptive may no longer be useful and
different traits may be selected for instead.
❖ If conditions change faster than a species can adapt to
those changes, the species may become extinct.
Common Descent
❖ What does Darwin’s mechanism for evolution suggest about
living and extinct species?
❖ Every organism alive today is descended from parents who survived and reproduced.
❖ Well-adapted species survive over time.
❖ Over many generations, adaptations could cause
successful species to evolve into new species.
❖ Common descent – living
species are all descended from common ancestors
❖ Descent with modification – living species are descended, with modification, from
common ancestors.
❖ According to the
principle of common descent, all species – living and extinct – are descended from ancient common ancestors.
❖ For evidence, Darwin looked to the fossil record.
❖ Constructed an
evolutionary tree (aka tree of life) to show how descent with
modification could
produce the diversity.
Variation and Mutation
❖ In order for some individuals to be more fit than others, individuals have to be different.
❖ Differences are caused by mutations.
❖ Mutations can be random or caused by mutagens like radiation or chemicals.
❖ Mutations often lead to no change in phenotype (neutral), but sometimes they are positive or negative.
❖ If a mutation is “good” it will stay in the gene pool because it helped the individual survive and reproduce.
❖ If a mutation is “bad” it will likely not stay in the gene pool
because the individual who had it did not survive to pass it
on.
BIG PICTURE
❖ In nature there is variety.
❖ Mutations cause more variety (good or bad).
❖ The more suited (fit) an organism is to its
environment, the more likely it will survive.
❖ Individuals that survive can pass on their traits.
❖ Less fit individuals will die off and cannot pass on their
“less fit” traits.
❖ If an environment changes, what is “fit” may change,
too.
Evidence of Evolution
❖ Evidence that supports evolution comes from:
1. Biogeography
2. Physical Structures
3. Embryology
4. Genetics and Molecular Biology
Biogeography
❖ How does the geographic distribution of species today relate to their evolutionary history?
❖ Patterns in the distribution of living and fossil species tell us how modern organisms evolved from their ancestors.
❖ Biogeography – study of where organisms live now and where their ancestors lived in the past.
❖ Biogeography shows how closely related species
differentiate in slightly different environments.
Biogeography
❖ Biogeography also shows how different species develop similar traits from being in similar environments. (Ex:
flightless birds)
Physical Structures
❖ By Darwin’s time, scientists had noted that all vertebrate limbs had the same basic bone
structure.
❖ By studying physical structures of organisms,
scientists can deduce relationships between
species.
Homologous Structures
❖
Homologous structures – structures that are shared by related species because they were inherited from a common ancestor.
❖
Homo = same, so homologous = same structure
❖
Homologous structures have similar structure but often have different functions.
❖
Shows common ancestry because DNA contains instructions on how to build structures.
❖
Homologous structures evolved from a common ancestor but have changed over time as each organism’s environment
changed.
Homologous = Same structure, different
function
Analogous Structures
❖ Analogous structures – structures that have the same or similar
function, but are built differently.
❖ Opposite idea of
homologous structures.
❖ Examples:
❖
Flippers are used to swim but can be built
differently.
❖
