Evolution Unit
Study Sheet
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Section 1. Darwin:
1. In 1831, Charles Darwin, a young Englishman, began a trip around the world. At
every stop along the way, Darwin carefully observed and documented the many new
plant and animal species he had never seen before. He wondered why these new
organisms were so different from the ones he had known back in England.
2. A species is a group of organisms that are so closely related that they can mate
and produce offspring.
3. Mutations cause genetic change. If the change allows an organism to better
survive in its environment, that change is passed on to future generations. If the
change reduces the organism’s chances of survival, the organism will not survive.
4. Darwin hypothesized that species of organisms gradually changed over many
generations as they adapted to new conditions. (Examples: The Galapagos finches
and tortoises)
5. Darwin’s observations led him to propose that changes in organisms over time occur
through natural selection. Natural selection occurs because:
 Organisms produce many more offspring than could possibly survive.
 There is variation (differences) among the offspring
 There is competition among the offspring for resources (food, water, shelter, etc.)
 The offspring that are best adapted survive.
6. Only traits controlled by genes can change over time as a result of natural
selection.
7. If a group of individuals becomes isolated (separated) from the rest of its species
long enough to evolve different traits, a new species can form. The evolution of a
new species as a result of separation is called geographic isolation.
8. Darwin’s observations led him to develop one of the most important scientific
theories of all time: the theory of evolution through natural selection.
9. Evolution is an explanation of the way in which present-day organisms have
been produced through changes that have taken place in the genetic make up of
populations.
Section 2. The Fossil Record
1. A fossil is the preserved remains or traces of an organism that lived in the past.
2. Very few fossils are of complete organisms. The formation of a fossil is a rare
event. The conditions have to be just right for a fossil to form. Most fossils form
when organisms die and particles of sand and rock carried by the water of a river,
lake, or ocean bury them. These particles are called sediments. The sediments
harden, preserving all or parts of the organism.
3. Layers of hardened sediment become sedimentary rock.
4. Some fossils are preserved as mineral deposits or petrified, and some are imprints or
molds and casts that preserve the shape and size of the original organism. Some
fossils are not preserved in sediments but are preserved in ice, evergreen tree sap
called amber, or even tar!
5. Scientists have several ways to estimate the age of a fossil. These procedures are
called dating methods. One method involves “reading” the layers of sediment to
determine the oldest and youngest fossils. This is called relative dating. Absolute
dating involves determining the amount of radioactive elements that are present in a
fossil. The amount that is left tells you how old the fossil is. This method is called
absolute dating.
6. Fossils help scientists understand how extinct organisms – those that no longer live
on Earth - looked when they were alive and how organisms may have changed over
time.
7. The millions of fossils that scientists have collected are called the fossil record.
This record is incomplete because many organisms die without leaving fossils behind.
Nevertheless, the fossil record has given science a lot of important information
about past life on Earth. (The study of fossils is called paleontology.)
8. The Geologic Time Scale is a chart or “calendar” that shows when the major groups
of organisms lived over Earth’s 4.6 billion year history.
9. Scientists believe that changes in some organisms have occurred gradually at some
times and rapidly at other times. (Gradualism vs. Punctuated Equilibria)
Section 3. Other Evidences of Evolution
1. Scientist compare body structures, development of organisms before birth, and
DNA sequences of base pairs to show the evolutionary relationships among
organisms.
2. Homologous structures are structures that are similar form but different in use
such as the flipper of a whale, the arm of a human, the leg of a dog, and the wing of a
bird.
3. Homologous structures can show of how species are related and how they
evolved from a common ancestor.
4. Scientists use similarities in early developmental stages as evidence that species
are related and share a common ancestor. For example, the presence of gill slits
and a tail in the embryos of all animals with backbones would show that all
vertebrates share a common ancestor.
5. If related species have similar body structures and similar embryological
development scientists assume that they have inherited many of the same genes
from a common ancestor. Scientists compare DNA and protein sequences to
determine more precisely how species are related.
6. A branching tree (cladogram) is a diagram that shows how scientists think
different groups of organisms are related.