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EVOLUTION: CHANGE OVER TIME- text pgs571-593
CLUES TO EVOLUTIONARY CHANGE
Studying Evolution:
Scientists define evolution as the change of a species over time and refer to these changes
in living things as biological evolution or organic evolution. The data resulting from the study
of biological evolution help to explain the differences in structure, function, and behavior among
living things and the changes in characteristics in populations throughout time. Scientists work
like detectives trying to find clues to evolutionary change and the relationships involved.
Because of their findings, evolution as change over time is well documented by extensive
evidence from a wide variety of sources. Some of the studies supporting current theories of
organic evolution include fossil records, similarities in skeletal structures, embryo development,
chemical composition among other organisms. Scientists also learned that in sexually
reproducing organisms only changes in the genes of sex cells can become the basis for
evolutionary change and these evolutionary changes may occur over long periods of time, in an
organism’s structure, function, and behavior.
(1) What is evolution?
Interactions Influence Evolution:
Evolution is thought to be the result of many different interactions. Some more important
interactions are listed below and are also discussed separately in this unit.
Important Interactions That Influenced Evolution
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the potential for a species to increase its numbers
the genetic variability of offspring due to mutation and recombination (rearrangement) of genes
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a finite supply of the resources required for life
the ensuing selection by the environment of those offspring better able to survive and leave
offspring
Geologic Time:
Most people are used to thinking of time in terms of years. To understand the evolution
of most organisms, you have to think of time in very large numbers – in millions and billions of
years. Scientists estimate the earth to be more than 4.5 billion years old and life on Earth is
thought by many scientists to have begun as simple, single-celled organisms. The oldest living
thing, a bacteria-like organism, is estimated to be 3.4 billion years old. About a billion years ago
increasingly complex multicellular organisms began to evolve.
To make the study of
evolutionary time easier, scientists developed a time chart that divides geologic time (time from
the beginning of the world to today) into smaller time units called eras.
Fossil Evidence:
Fossils are the remains or traces of organisms that once lived. The study of fossils
provides evidence to support the idea that life changed over time from simple to complex. Fossil
distribution shows hat life began in the sea and then moved to land and indicates that climates and
land surfaces have changed over time. Fossil records provide evidence for the time of origin
(beginning) of various forms of life and proof that 99 percent of the organisms that lived in the
past are now extinct. Extinction means that particular species that once were prevalent on Earth
have now totally disappeared. Extinction does not result in evolution, but is thought to actually
cause it.
Many fossils are found in sedimentary rock. Sedimentary rock is formed from layers of
slowly deposited sediments such as rock particles, silt, and mud, which are usually deposited by
water. After a long period of time and great amounts of heat and pressure, sediments harden into
rock forming visible layers. Skeletons, imprints, shells, bones and other animal and plant remains
become trapped in the sediment layers. When the sediment hardens, the remains become trapped
in the sediment layers. In undisturbed rock layers, fossils found in lower layers are assumed to be
older then fossils found in the upper layers.
Evidence in Structures:
When scientists compare skeletal structures (anatomy) of different vertebrates they see
anatomical similarities indicating that organisms with similar bone structures may have evolved
from a common ancestor population. Organs or structural parts that seem to have a common
evolutionary origin are referred to as homologous structures. For example, the wing of a bat,
the flipper of a whale, and a human arm are homologous structures. Although homologous
structures are similar in structure, their function may not be the same.
Vestigial structures are parts of an animal’s body that are no longer used although they
look like structures that are fully developed and used by other animals. The human appendix is
an example of a vestigial structure. Perhaps some human ancestor used their appendix and, as
evolution continued, humans stopped using this organ. Other vestigial structures include human
ear muscles, and leg bones of a python and porpoise. These structures provide further evidence
of changing structure and function.
Also, the structure of cells and cell organelles from one group of organisms to another
group are found to be basically the same. (Cytology is the study of cell structures.) In terms of
evolution, this is more evidence that different kinds of living things may share a common origin.
(1) What are the three “types” of structures that scientists look at to prove evolution
occurs?
(2) What do these structures show?
Comparing Embryos and Biochemistry:
Other evidence of evolution is the study of embryonic development in different
organisms. Comparisons of early stages of embryonic development show the possibility of
common ancestry and evolutionary relationships.
At early stages, vertebrate embryos, for
example, show gill slits, tails, and two-chambered hearts. See diagram of early embryonic stages.
Similarities in biochemistry (body chemical structure) of living things, such as DNA,
hormones, and enzymes, show a close relationship among various forms of life. Organisms that
are more closely related, like a cat and the lion, have a greater similarity in their protein structure.
Greater differences in cell biochemistry is thought to indicate a lesser evolutionary relationship.