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Biology Chapter 15 Notes
Nature Selection and the Evidence for Evolution
On Dec. 2, 1831 English Scientist Charles Robert Darwin (1809-1882), set sail for five years on HMS
Beagle around the world.
 Developed the theory of evolution by natural selection and adaptation after gathering many
samples on his voyage.
 Natural Selection is a mechanism for change in population-survival of the fittest
 Two types of Adaptations
 Behavioral
 Structural
 Mimicry-resemble another species that is harmful
 Camouflage-a species being able to blend into its environment
 Anatomy
o Homologous structures- Evolutionary structures that meet different
needs but develop from the same body parts.
 Darwin concluded that living organisms evolved through
gradual modification of earlier organism-descent from a
common ancestor.
 Example: Human arm (picking up stuff and writing),
whale fin (swimming), dog leg (walking), and bat wing
(flying)
o Analogous structures-The body parts of organisms don’t have a
common evolutionary origin but a similar function
 Ex. Insect and bird wings are similar in function but not
in structure
o Vestigial structures- A Vestigial means trace. These are organs that
are reduced in size or function that serve no purpose now.
Examples:
o Python snake has a vestigial leg bone which now serves no
locomotion now
o Human appendix is a vestigial organ because it no longer functions
in digestion although the Koala’s appendix is used to digest leaves
o The eyes in mole-rats and cave fish are not used for sight. They are
blind.
Embryology-The study of the development of embryos
 An embryo is the earliest stage of growth and development of both plant and animal organisms
o During certain stages vastly different organisms show similarities
 The similarities of vertebrate embryos show that similar genes are present in all vertebrates.
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 These similar genes indicate that these various organisms have a common ancestor that passed
on the similar genes
o As the embryos develop their own genes become active and cause the dissimilarities to
occur. These are mutations of the original genes.
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All these changes lead to homologous structures and vestigial organs.
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Biochemistry-The science that deals with biological and chemical processes in organisms
 It provides strong evidence for evolution
Physical and well as environmental factors aid in the process of evolution.
The ideas of both John Hutton and Charles Lyell helped Darwin realize that the Earth was continually
changing over time.
 Hutton –Plate tectonic theory
 Lyell stated that the Earth was much older than thousands of years old because of the gradual
changes by elements and natural forces of the rocks and mountains. Therefore, organisms had
to change too.
Thomas Malthus: Population Controls –An Economist
 Malthus greatly influenced Darwin because his theory stated that human babies are born at a
faster rate than adults dies. Also, he realized that at a certain point (carrying capacity) if
humans continued to do so that we would run out of space and food.
 Ways to prevent endless growth in human populations are famine, disease, and war.
• Darwin realized that this population growth also applied to other animals and plants
Jean Baptiste de Lamarck (1744-1829) –Evolutionary Theory is the foundation of Biological Science. His
theory involved that organisms deliberately adapt in response to the conditions of the environment;
therefore, they can survive.
 Later evolutionary theorists determined that genes determined variations. Also mutations can
occur do to environmental factors.
 Populations are groups of organism with the same species in a given area whose members can
breed with one another.
Mechanism of Evolution
Terms:
1. Gene pool- all members of a population can interbreed, they and their offspring share a
common alleles
2. Allelic frequency (relative frequency)-The percentage of any specific allele in the gene pool
 Evolutionary change involves a change in the relative frequencies of alleles in the gene
pool of a population
3. Stabilizing selection- is natural selection that favors average individuals in a given population
and reduces variation in a population
4. Directional selection-occurs when natural selection favors one of the extreme variations of a
trait and can lead to the rapid evolution of a population
5. Disruptive selection-Individuals with the extreme variation that results eventually in no
intermediate forms of the trait and leading to the evolution of two new species
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6. Speciation- The evolution of new species, occurring when members of similar populations no
longer interbreed to produce fertile offspring within their natural environment.
Physical barriers can prevent interbreeding
 One of the most common ways in which new species form is when populations are separated.
7. Reproductive isolation-occurs when formerly interbreeding organisms can no longer mate and
produce fertile offspring (Two types)
 Genetic material of the populations becomes so different that fertilization can’t
occur
 Behavioral- two different species won’t mate together so isolation occurs
8. Geographic isolation-occurs whenever a physical barrier divides a population-like the splitting of
Pangaea caused land animals to become separated
9. Polyploid-Any individual or species with a multiple of the normal set of chromosomes
 Speciation rates of polyploids usually take only one generation but other
mechanisms occur slowly
a.) Gradualism is the idea that species originate through a gradual change in
adaptations
b.) In 1972, Niles Eldredge and Stephen J. Could proposed punctuated
equilibrium-states that speciation occurs relatively quickly, in rapid bursts,
with long periods of genetic equilibrium in between
Patterns of Evolution
10. Convergent evolution-occurs when distantly related species evolve to look alike
 Analogous structures are similar structures in appearance and function (also see
anatomy section of this chapter notes)
11. Divergent evolution-occurs when closely related species have evolved to look differently
 Adaptive radiation- The result of an ancestral species evolving into an array of species to
fit a number of diverse habitats
 Ex. Honeycreepers birds have resulted in a wide array of beak shapes
Evidence from fossil records indicates that modern-day birds, mammals, and reptiles descended from
reptilian ancestors including dinosaurs.
Evolutionary Theory evolves with Genetic drift-the absences of natural selection.
 This kind of random change in the frequency of a gene is called genetic drift.
 Can be due to an organism with a particular allele populates more offspring
than other alleles.
 Can be due to a natural disaster-Mount St. Helens eruption may have drastically
shrink a gene pool by killing many individuals and restricting gene flow among
the survivors.
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Gradual and Rapid Evolutionary Change
1. Gradualism-The theory that evolutionary changes occurs slowly and gradually (mentioned
earlier)
2. Genetic equilibrium- A population having the same frequency of alleles over generations
 Organisms that do the change very much from the time they appeared to the time they
disappeared.
Rapid evolution after long periods of equilibrium can occur in several ways.
 Small population of a species becomes isolated from the main part of the population
 Small population migrating to a new environment-Galapagos finches
 Dramatic change on Earth
 Mass extinction-a large amount of species suddenly die
3. Punctuated equilibria-describes long stable periods interrupted by brief periods of change
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