Fossil Evidence
Fossils
 When organisms die, their soft tissues are either eaten by other animals or they break down (decay).
 Hard remains such as bones, teeth, shells, etc. can be preserved as fossils.
 FOSSILS are preserved remains or evidence of once-living organisms.
Fossil Record
 All of the fossils ever discovered on Earth make up the FOSSIL RECORD.
 The fossil record can help scientists figure out what species that no longer exist looked like when they
were alive.
Fossil Formation
 MINERALIZATION is when the remains of an organism are buried in mud or sand under a body of
water. Over time, the molecules that once formed the remains are replaced by minerals in the water.
 Most mineralized fossils are formed of shell or bone, but wood can also become a mineralized fossil.
 CARBONIZATION occurs when a dead organism is compressed over time and pressure drives off the
organism’s liquids and gases.
 Sometimes organisms make an impression in sand or mud.
 MOLD fossils form an impression in rock. They can only show external features.
 CAST fossils form when impressions are filled with sediments that harden to become rock.
 Sometimes, organisms can leave evidence of their movement or behavior, such as tracks in the mud.
These types of fossils are called TRACE FOSSILS.
 In rare cases, the original tissue of an organism can be preserved.
 Mammoths frozen in ice
 Insects trapped in tree sap
Determining a Fossil’s Age
 THE LAW OF SUPERPOSITION- in an undisturbed rock formation, older layers of rock are below
younger layers of rock.
 RELATIVE-AGE DATING uses the law of superposition to help scientists determine the relative order
species have appeared on Earth over time.
 RELATIVE-AGE DATING uses the law of superposition to help scientists determine the relative order
species have appeared on Earth over time.
 The GEOLOGIC TIME SCALE is a chart that divides Earth’s history into different time units.
 Four eons: Hadean, Archean, Proterozoic, Phanerozoic
 Phanerozoic sub-divided into three eras: Paleozoic, Mesozoic, Cenozoic
 Neither eons nor eras are equal in length (they vary).
 When scientists began developing the geologic time scale, they did not have absolute dating, so they
marked time boundaries using fossils.
 Extinctions occur when the last individual organism of a species dies.
 When many species die off within a few million years of each other, it is called a MASS EXTINCTION.
 The fossil record shows evidence of five mass extinctions during the Phanerozoic eon.
 Extinctions can occur if the environment changes quickly; for example, as a result of a meteorite impact.
 Extinctions can also occur if the environment changes gradually; for example, as a result of the
formation of mountain ranges.
 Extinctions can occur if the environment changes quickly; for example, as a result of a meteorite impact.
 Extinctions can also occur if the environment changes gradually; for example, as a result of the
formation of mountain ranges.
 The fossil record contains clear evidence of the extinction of species over time as well as evidence of the
appearance of many new species.
 The fossil record also contains evidence that species change over time in what appears to be a sequence
 For example, horses descended from organisms from which only fossils remain today.
 BIOLOGICAL EVOLUTION is the change in populations of related organisms over large amounts of
time.
Charles Darwin
Scientists had evidence that showed changes in species over time, but they didn’t know how.
Charles Darwin was one scientist who struggled with this idea.
Darwin was a NATURALIST, a person who studies plants and animals by observing them.
He was not the first scientist to develop a theory about evolution, but his theory is the one best supported
by evidence.
 Darwin’s breakthrough came while on a voyage to the Galapagos Islands (near the equator off the
pacific coast of South America).
 Darwin noticed slight differences in animals living on each of the islands; he later decided that some
varieties were different enough to be classified as different species.
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Darwin’s Theory
 Darwin noticed a relationship between each species and the food sources of the island it lived on.
 The species of tortoise that lived on an island with tall cacti had long necks, while the species of tortoise
on an island with lots of short grass had short necks.
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 Darwin thought all the Galapagos tortoises shared a common ancestor that came to one of the islands
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millions of years ago.
He knew that members of the same species each have slight differences called VARIATIONS.
Darwin didn’t know about heredity, but he realized that variations in populations could help explain how
the different species of Galapagos tortoises and finches evolved.
Darwin knew that food is a LIMITING RESOURCE, so members of a species that live in the same
area compete for food.
If a variation benefited a tortoise, allowing it to compete better for food than other tortoises (more “fit”
to its environment), the tortoise was more likely to live longer, reproduce more, and pass on its
variations to its offspring.
Natural Selection
 NATURAL SELECTION: Individuals within a population that possess variations which help them
survive in their environment tend to live longer and compete better, and reproduce more than
individuals that do not have the beneficial trait.
Adaptations
 An ADAPTATION is an inherited trait that increases an organism’s chance of surviving and
reproducing in its environment.
 Scientists classify adaptations into three categories.
 STRUCTURAL ADAPTATIONS involve color, shape, and other physical characteristics.
 BEHAVIORAL ADAPTATIONS involve the way organisms behave or act.
 FUNCTIONAL ADAPTATIONS involve internal body systems that affect biochemistry.
Environmental Interactions
 A structural adaptation that aids members of a species in blending in with their environment is called
CAMOUFLAGE.
 An adaptation in which one species resembles another species is called MIMICRY.
 The living and nonliving parts of the environment are always changing; species that cannot adapt to such
changes will become extinct.
Natural Selection vs. Artificial Selection
 Darwin’s theory of natural selection predicts that species will develop adaptations, which explains why
we see such diversity of organisms that are perfectly suited to thrive in their environment.
 SELECTIVE BREEDING is the practice of breeding organisms for desired characteristics.
 Darwin realized that natural selection and artificial selection are similar processes.
o In natural selection, nature causes the changes in the species.
o In artificial selection, humans cause the changes in the species.
 Artificial selection explains and supports Darwin’s theory.
Biological Evidence of Evolution Notes
How Species are Related
 Evolution does not occur in a straight line with one species replacing another in a series of orderly steps.
o Living species that are closely related share a common ancestor.
o How closely related two species are depends on when they diverged, or split, from their common
ancestor.
o The fossil record indicates that different species of horses often overlapped with each other.
o One of the main points of debate among biologists today is SPECIATION, the process of one
species splitting into multiple species.
 Phyletic Gradualism
 Many scientists think that natural selection produces new species slowly and
steadily.
 Punctuated Equilibrium
 Other scientists think species exist stably for logn periods oand that change occurs
in short bursts.
Evidence in Anatomy
 Comparative anatomy is the study of similarities and differences among structures of living species.
o Homologous Structures
 Body parts of organisms that are similar in structure and position but different in
function.
 The forelimbs of these species are different sizes, but their placement and structure
suggest common ancestry (they are related).
o Analogous Structures
 Body parts that perform a similar function but differ in structure.
 The wings of these species are similar in function but different in structure, which
suggests they are NOT closely related.
o Vestigial Structures
 Body parts that have lost their original function through evolution.
 The short stubby wings of flightless birds are an example of vestigial structures.
 Penguins
 Ostriches
Evidence of Development
 The science of the development of embryos from fertilization to birth is called embryology.
 All vertebrate embryos exhibit pharyngeal pouches at a certain stage of their development. These
features, which develop into neck and face parts, suggest relatedness.
Evidence in Genetics
 Molecular biology is the study of gene structure and function.
 Differences in DNA are responsible for differences between species. It can tell us how closely related or
distantly related species are.
 Scientists can study relatedness of organisms by comparing genes and proteins among living species.
o Molecular data indicate that whales and porpoises are more closely related to hippopotamuses
than they are to any other living species.
The Study of Evolution Today
 Since Darwin’s discovery, scientists have confirmed, refined, and extended his theory by examining life
through the lens of natural selection.
 Genetic research is a very young science. Almost daily, scientists understanding about life and the
connections between species are being shaped by new information from DNA.
 As new fossils are discovered and added to the fossil record, scientists are able to get a more complete
picture and refine their ideas about the origin and evolution of different species.