Lesson Summary:
Evidence for the Theory of Evolution and Macroevolution
Fossils and Ancient Life Fossils are preserved remains or traces of ancient life.
Fossils are the most important source of information about extinct species. An extinct species is
one that has died out.
Most fossils are preserved in sedimentary rock. Sediments build up over time, and bury the
remains and traces of dead organisms.
Scientists who study fossils are called paleontologists.
Dating Earth’s History Relative dating and radiometric dating are used to determine the age of
fossils.
Relative dating establishes the relative age of fossils. Fossils from deeper rock layers are
assumed to be older than fossils from rock layers closer to the surface. Index fossils represent
species that lived for a short period of time but over a wide geographic range. Index fossils can
help determine the relative ages of rock layers and their fossils.
Radiometric dating determines a fossil’s approximate age in years by finding the proportion of
radioactive to nonreactive isotopes in a sample. Radioactive isotopes in fossils and rock layers
decay, or break down, at a steady rate, called a half-life. A half-life is the length of time needed
for half of the radioactive atoms in a sample to decay. A fossil’s age is calculated from the halflife and the amount of remaining radioactive atoms the fossil contains.
Geologic Time Scale The geologic time scale is a time line of Earth’s history based on
relative and absolute dating.
The scale begins with the Precambrian.
Geologic time is divided into four eons: the Hadean, Archean, Proterozoic, and Phanerozoic. The
Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.
Each era is further divided into smaller lengths of time, called periods.
Life on a Changing Planet Climactic, geological, astronomical, and biological processes
have affected the history of life on Earth.
Earth’s climate has changed often in the course of its history. Small temperature shifts can bring
about heat waves and ice ages which have great effects on living things. Plate tectonics is a
theory that Earth’s outermost layer is divided into plates that move. The movement, called
continental drift, has transformed life on Earth through the formation of mountain ranges,
supercontinents, and other geologic features.
The impact of objects from space has affected the global climate.
Biogeography Biogeography is the study of where organisms live now and where they and their
ancestors lived in the past. Two biogeographical patterns are significant to Darwin’s theory:
The first is a pattern in which closely related species differentiate in slightly different climates.
The Galápagos tortoises and finches follow this pattern.
The second is a pattern in which very distantly related species develop similarities in similar
environments. The rheas, ostriches, and emus fall into this pattern.
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The Age of Earth and Fossils
Radioactive dating techniques have confirmed that Earth is ancient—approximately 4.5 billion
years old.
Recent fossil finds document intermediate stages in the evolution of many groups including
whales, birds, and mammals.
Comparing Anatomy and Embryology
Homologous structures are shared by related species and have been inherited from a common
ancestor. Similarities and differences among homologous structures help determine how recently
two groups shared a common ancestor.
 Body parts that share a common function, but neither structure nor common ancestry, are
called analogous structures. Analogous structures do not provide any evidence for
evolutionary descent.
 Homologous structures that are greatly reduced in size or have little to no function are called
vestigial structures.
 Many homologous structures develop in the same order and in similar patterns during the
embryonic, or pre-birth, stages of related groups. These similarities provide further evidence
that the animals share common ancestors.
Genetics and Molecular Biology At the molecular level, the universal genetic code and
homologous molecules such as genes and proteins provide evidence of common descent.
Testing Natural Selection Scientists have designed experiments to test natural selection.
Observations of Galápagos finches confirm that competition and environmental change drive natural
selection.
Isolating Mechanisms Speciation is the formation of new species. For one species to evolve
into two new species, the gene pools of two populations must become separated, or reproductively
isolated. Reproductive isolation occurs when members of two populations do not interbreed and
produce fertile offspring. Reproductive isolation can develop through behavioral, geographic, or
temporal isolation.
Behavioral isolation occurs when populations have different courtship rituals or other behaviors
involved in reproduction.
Geographic isolation occurs when populations are separated by geographic barriers, such as
mountains or rivers.
Temporal isolation occurs when populations reproduce at different times.
Speciation in Darwin’s Finches Peter and Rosemary Grant’s work supports the hypothesis
that speciation in the Galápagos finches was, and still continues to be, a result of the founder effect
and natural selection.
Speciation in Galápagos finches may have occurred in a sequence of events that involved the
founding of a new population, geographic isolation, changes in the gene pool, behavioral
isolation, and ecological competition.
For example, a few finches may have flown from mainland South America to one of the islands.
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There, they survived and reproduced. Some birds may have crossed to a second island, and the
two populations became geographically isolated. Seed sizes on the second island could have
favored birds with larger beaks, so the population on the second island evolved into a population
with larger beaks. Eventually, these large-beaked birds became reproductively isolated and
evolved into a new species.
Timing Lineage Splits: Molecular Clocks A molecular clock uses mutation rates in
DNA to estimate the length of time that two species have been evolving independently.
Molecular clock models assume that neutral mutations, which do not affect phenotype,
accumulate in the DNA of different species at about the same rate.
Two species evolving independently from each other will accumulate different neutral mutations
through time. The more differences between the DNA, the more time has passed since the two
species shared an ancestor.
Gene Duplication New genes evolve through the duplication, and then modification, of existing
genes.
Organisms may carry multiple copies of the same gene. The extra copies of a gene may undergo
mutations.
The mutated gene may have a new function that is different from the original gene. In this way,
new genes evolve.
Multiple copies of a duplicated gene can turn into a gene family.
Developmental Genes and Body Plans Researchers study the relationship between
evolution and embryological development.
Some genes, called Hox genes, control the forms of animals’ bodies.
Small changes in Hox genes during embryological development can produce major changes in
adult organisms.
Some scientists think that changes in Hox genes may contribute to major evolutionary changes.
Speciation and Extinction Macroevolutionary patterns are grand transformations in
anatomy, phylogeny, ecology, and behavior that usually take place in clades larger than a single
species.
If the rate of speciation in a clade is equal to or greater than the rate of extinction, the clade will
continue to exist. If the rate of extinction in a clade is greater than the rate of speciation, the
entire clade will eventually become extinct.
Background extinction is extinction caused by the slow process of natural selection. Mass
extinctions affect huge numbers of species over a relatively short time.
Rate of Evolution Evidence shows that evolution has occurred at different rates for different
organisms at different times.
The idea that evolution occurs slowly and gradually is called gradualism.
In punctuated equilibrium, long periods of little or no change are interrupted by short periods
of rapid change.
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Adaptive Radiation and Convergent Evolution Adaptive radiation is the process in
which a single species evolves into diverse species that live in different ways. Convergent evolution
is the process in which unrelated species come to look alike because they have evolved similar
adaptations in response to similar environments.
Coevolution Coevolution is the process by which two species evolve in response to changes in
each other over time. For example, plants evolved poisons that protected them from insects. In
response, insects evolved ways of protecting themselves from the poisons.
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