8.2: Sources of Evidence for Evolution
pg. 332 - 340
Key Terms: fossil record, transitional fossil, vestigial structure,
biogeography, homologous structures, analogous structures,
embryology.
Darwin was not the first person to conclude that life had changed
over time. He was the first to publish (The Origins of Species) his
theory, and support it with evidence (biologists, geologists,
geographers, and paleontologists), the theory of evolution through
natural selection.
Fossils: Evidence for the History of Life
Sedimentary rock with fossils showed evidence of species that
lived in the past. Burgess Shale fossil beds in British Columbia
sow fossil records dating back to the Cambrian period, 500 million
years ago. Some of the fossils found are early ancestors of
common animals of today.
Fossil records – are the remains and traces of past life that are
found in sedimentary rock; it reveals the history of life on Earth
and the kinds of organisms that were alive in the past.
Figure 8.5: This geological time scale illustrates the approximate appearance in the fossil
record if various organisms during Earth’s 4.6 billion year history.
Evidence from the Fossil Record
1. Fossils found in young, shallower layers of rock (closer to the
surface) are much more similar to species alive today, than
fossils of older, deeper layers.
2. Fossils appear in chronological order in the rock layers.
3. Not all organisms appear in the fossil record at the same time.
(Fishes, amphibians, reptiles, mammals, birds, order of
vertebrate evolution).
*changes were slow and took millions of years
Figure 8.6: Paleontologists have used fossils to trace the evolution of the modern
camel. The Paleocene, Eocene, Oligocene, and Miocene epochs are subdivisions of
the Cenozoic era.
Evidence from Transitional Fossils
Transitional fossils have helped scientists to better understand the
evolutionary processes and relationships between groups of
organisms, linking the past with the present.
Transitional fossils – is a fossil that shows intermediary links
between groups of organisms and shares characteristics common to
two separate groups.
Fossil of whales from 36 to 55 million years ago linked present day
whales to terrestrial ancestors. Whales which had tiny hind limbs;
Basilosaurus and Dorudon, but led aquatic lives, were useless and
reduced forms of structures functional in terrestrial organisms.
These features are called vestigial structures, and can be found in
modern whales (Baleen Whales).
e.g.: Whales and transitional forms.
The link between; Pakicetus, Ambulocetus, Rodhocetus and
modern whales is supported by Ambulocetus which is the
transitional fossil, linking Pakicetus (terrestrial) to Rodhocetus
(aquatic).
Vestigial structure – is a structure that is a reduced version of a
structure that was functional in the organism’s ancestors.
Figure 8.7: Fossil evidence suggests that modern toothed whales evolved from a
terrestrial ancestor, Pakicetus attocki. Basilosaurus and Dorudon, not shown in this
illustration, appear more recently in the fossil record, after the appearance of Rodhocetus
Archaeopteryx: A Transitional Fossil
Archaeopteryx showed a transitional stage in the fossil record
because this species had characteristics of both reptiles (teeth,
claws, bony tail, and scales) and birds (feathers).
The Archaeopteryx is a link between the Atrociraptor (dinosaur)
and modern birds.
Evidence from Biogeography
Darwin’s and Wallace’s observations used to support theories were
based on biogeography. They hypothesized that species evolve in
one location and then spread to other regions.
a) Geographically close environments (desert and forest habitats of
South America) are more likely to be populated by related species
than are locations that are geographically separate but
environmentally similar (deserts in Africa and Australia).
b) Animals found on islands often closely resemble animals found on
the closest continent. Therefore the animals on the island have
evolved from the mainland animals, and the population becoming
adapted over time to the island environment.
c) Fossils of the same species can be found on the coastline of
neighbouring continents. Therefore the continents are not in a fixed
location, but are slowly moving away from each other. (Gondwana,
510 million years ago: South America, Africa, India, Antarctica,
and Australia of today)
d) Closely related species are almost never found in exactly the same
location or habitat.
Biogeography – is the study of the past and present geographical
distribution of species populations.
Figure 8.8: As the southern supercontinent Gondwana broke apart about 150 million
years ago, the land masses that became the current continents of Africa, Australia, South
America, and Antarctica were isolated from each other
Learning Check: questions 7 – 12
pg. 336
Evidence from Anatomy
Homologous structures – are structures that have similar structural
elements and origin but have a different function.
Analogous structures – are structures of organisms that do not
have a common evolutionary origin but perform similar functions.
Figure 8.9: These vertebrates have the same basic arrangement of bones (as indicated by
the colours), but the bones have different uses.
Evidence from Embryology
Embryos of different organisms, exhibit similar stages of
embryonic development, can be used to determine evolutionary
relationships.
Similarities between embryos in related groups (such as vertebrates)
point to a common ancestral origin.
Embryology – is the study of early, pre-birth stages of an
organism’s development.
Figure 8.10: Similarities in the embryos of fish, birds, and mammals provide evidence of
evolution of species from a common ancestor.
Evidence from DNA
Evolutionary relationships between species are reflected in their
DNA. Scientists can determine how closely related two organisms
are by comparing their DNA sequences. The more similar the
sequences are the greater the opportunity that they came from a
common ancestor. Darwin’s theory of natural selection is
supported by genetic evidence and our understanding of genetic
inheritance.
Figure 8.11: DNA evidence supports conclusions about relationships and common
ancestry provided by other areas of evidence.
Review Questions: questions 1 – 17
pg. 340
Activity 8.2: Homologies of Hair
1–3
pg. 337
Study Guide: Evidence for Evolution: Fossils and Biogeography
(8.2) questions 1 – 3, SG. 123 – 124
More Evidence for Evolution: Anatomy, Embryology, and DNA
(8.2) questions 1 – 4, SG. 125 – 126
Bringing It All Together, SG. 127
Practice Test questions 1 – 14, SG. 128 – 129