Download Chapter 15 Evolution: Evidence and Theory

Chapter 15 Evolution: Evidence and Theory
The Fossil Record
- Fossil evidence shows a long history of life on Earth.
- A fossil is a trace of long-dead organisms.
o They are often found in layers of sedimentary rock, which is formed when
sediment, such as dust, sand, or mud is deposited by wind or water.
 Sedimentary fossils usually develop from the hard body parts of an
organism.
 A type of fossil called a mold is an imprint in rock in the shape of
the organism.
 Some molds are filled with hard minerals, forming a cast, which is
a rock-like model of the organism.
Distribution of Fossils
- Nicolaus Steno made an important contribution toward modern understanding of
Earth’s geological and biological history. He proposed the law of superposition,
which states that successive layers of rock or soil were deposited on top of one
another by wind or water.
o The lowest stratum (layer) is the oldest layer and the top stratum is the
most recent.
 Using Steno’s law, observers could establish the relative age of a
fossil and the fossil’s absolute age could be estimated from the
amount of sediment deposited above the fossil.
Succession of Forms
- Fossils data shows that species of organisms existed for a while, and then
disappeared, or became extinct.
o There were 5 mass extinctions; periods in which many different species
died.
Biogeography
- The study of the geographical distribution of fossils and of living organisms is
called biogeography.
o A comparison of recently formed fossil types with types of living
organisms in the same geographic area shows that new organisms arise
areas where similar forms already lived.
Theories of Evolution
- The word evolution refers to an orderly succession of changes.
Lamarck’s Explanation
- He proposed that similar species descended from common ancestor, thus living
species were descended from similar extinct species evident in the fossil record.
o He cataloged an extensive collection of invertebrates and related them to
living animals based on similar appearances.
-
To explain how species changed, he hypothesized that acquired traits were passed
onto offspring.
o An acquired trait is one that is note determined by genes.
 It comes about as a result of behavior over a lifetime.
 If it’s favorable, it will be preserved and then passed on to the
organism’s offspring.
Modern Evolutionary Thought
- In the mid-1800s, Charles Darwin and Alfred Wallace independently proposed
the hypothesis that species were modified by natural selection.
o Natural Selection – organisms best suited to their environment reproduce
more successfully than other organisms.
 Thus, over time, organisms with favorable traits increase within a
population.
 Population – interbreeding single-species group.
Charles Darwin
- Darwin struggled in school. His dad was a rich doctor that wanted him to become
either a doctor or a minister.
- Darwin liked the outdoors. At the age of 16, he was sent to study medicine in
Scotland. He hated surgery (because it was done without anesthetics) and skipped
lectures to collect biological specimens.
- He completed a degree in theology but spent most of his time with friends who
were also interested in the natural sciences.
- In 1831, one of his teachers recommended him to be an unofficial naturalist on
the HMS Beagle.
- The Beagle sailed on Dec. 27, 1831 and eventually ended up on the Galapagos
Islands.
o On the voyage, Darwin read Charles Lyell’s book “Principles of
Geology”, which explained Jean Baptiste Lamarck’s theory of evolution.
Lyell agreed with Lamarck but said that this change occurred over time
and very slowly.
 This theory said that evolution occurs through use and disuse of
physical features by individual members of species.
 These acquired “traits” would then be passed on from
parent to offspring.
o The book also talked about the principles of uniformitarianism, which
holds that the geological structure of Earth resulted from the cycles of
observable processes and that these same processes operate continuously
through time.
- On the Galapagos, Darwin was struck by the fact that the plants and animals on
the island resembled the plants and animals he saw on the coast of South
America.
- He noted that these things must have migrated to the islands from South America
a long time ago
o He didn’t publish his findings right away because although he was sure
species evolved, he didn’t have a mechanism of how.
 In 1859, he finally published a book called “The Origin of Species
by Means of Natural Selection”.
 Natural Selection means a process through which
populations change in response to their environment.
 This means that individuals who have traits that better
allow them to survive in their environment are more likely
to survive and reproduce, thus passing those traits to their
offspring.
Darwin’s Theories
- Darwin’s ideas are summed up in two theories; Descent with Modification and
Modification by Natural Selection.
o Descent with Modification – the newer forms of species appearing in the
fossil record are actually the modified descendants of older species.
 He inferred that all species had descended from one or a few
original types of life.
 This theory accounts for the fact that similar organisms
arise in the same geographic locations.
o Modification by Natural Selection – states how evolution actually occurs.
 He got the idea from English economist Thomas Malthus.
 He stated that there are things in place to keep the
populations under control. Examples being disease, war,
and famine. These things keep the human population under
control. If they didn’t exist, eventually humans would
cover the earth and we’d run out of food sources.
 Darwin proposed that the environment might affect individual
organisms in a population in different ways because individuals of
a species are not identical.
 Some organisms may have traits that better enable them to
cope with their environment. These organisms with
favorable traits will likely leave more offspring in the
environment.
o If a trait both increases the reproductive success of
an organism and is inherited, then that trait will tend
to be passed on to many offspring. A population of
organisms adapt to their environment as their
proportion of genes for favorable traits increase.
 In an evolving population, a single
organism’ s genetic contribution to the next
generation is termed fitness. Thus, an
individual with high fitness is well adapted
to its environment and reproduces more
successfully than an individual with low
fitness.

An organism with a favorable trait is said to
have an adaptive advantage.
Evolution in Process – Evidence of Evolution
- If a species has changed over time, then the genes that determine their
characteristics should also have changed (usually due to mutations).
Similarities in Macromolecules
- The amino acid sequence of proteins is genetically determined by an organisms
DNA.
- If evolution has taken place, then species that descended from a common ancestor
in the distant past should have more amino acid sequence differences between
their proteins than do species that share a common ancestor.
o For example: humans and gorillas have few amino acid sequence
differences (because we “share” a recent common ancestor), but humans
and frogs have many amino acid sequence differences because the
common ancestor we share is a much more distant relative.
Looking at Nucleic Acids
- Nucleotide changes (such as substitution) cause changes in the amino acid
sequence of a protein.
- Scientists can directly estimate the number of nucleotide changes that have taken
place in a gene since 2 species diverged from a common ancestor. They do this
by comparing the exact nucleotide sequence of each species.
o They construct a phylogenetic tree, which shows how organisms are
related through evolution.
Anatomy and Development Suggest Common Ancestry
- Comparison of the anatomy (structure) of different types of organisms often
reveal basic similarities in body structures, even though their functions may be
very different.
o Vestigial structures: a structure which is reduced in size and function;
considered to be evidence of an organism’s evolutionary past.
 For example: humans have a tailbone, but no tail. Monkeys have a
tailbone with a tail.
o Homologous structures: are structures that share a common ancestry.
 For example: the forelimbs of all vertebrates are made from the
same basic bones, even though they don’t all share the same
function.
o Analogous structures: serve identical functions and look somewhat alike,
but they have different embryological development and may have very
different internal anatomy.
 Example: the wings of a humming bird and a humming moth.
- The evolutionary history of organisms can also be seen in the development of
embryos.
Patterns of Evolution
- Coevolution: the change of 2 or more species in close association with one
another.
o Example: predator/prey relationships.
- Convergent evolution: organisms that appear to be very similar are not closely
related at all.
o Example: a shark and porpoise.
- Divergent Evolution: 2 or more related populations or species become more and
more dissimilar.
o Can be because of adaptive radiation, which means that many related
organisms evolve from a single ancestral species, but change in response
to their environment.
o Can be because of artificial selection, which means that divergent
evolution is sped up artificially.
 Example: domestic dogs are all the same species but we breed
dogs for certain traits.
 Pit bulls, rotweilers, Dalmatians, etc.

- Gradualism: the model that evolution occurs gradually over long periods of time.
- American biologist Stephen Jay Gould and Niles Eldredge hypothesize that major
environmental changes in the past have caused evolution to occur in spurts.
o Punctuated equilibrium: the model of evolution that says evolution occurs
in spurts. It is characterized by rapid change in species (due to
environmental changes) and is separated by period of little or not change.