Chapter 18
Diversity and Variation
Natural Selection:
• Not all members of a population
necessarily have an equal chance of
surviving and reproducing.
• Some individuals are born with a
mutation that gives them a survival
advantage or adaptation
• The better adapted individuals are
more "fit“, tend to survive and
reproduce, passing on their
adaptations to the next generation
more so than the less "fit" members
of the population
How does natural selection shape a population?
• First, think of variation in terms of a bell
shaped curve:
– what does the y-axis represent?
• The number of individuals
– What does the x-axis represent?
• the value of a particular
characteristic– like size, color, etc.
– What does it mean when a variable
exhibits continuous variation?
• It has a wide range over which it
varies, like human height
Stabilizing Selection:
• Organisms with extreme characteristics die or fail to
reproduce, resulting in populations of individuals that
possess intermediate characteristics.
•Infants weighing significantly less or more than 7.5 pounds have higher rates
of infant mortality. Selection works against both extremes.
Lizards and Stabilizing Selection
• http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/CDA2
2_2f/CDA22_2f.htm
Directional Selection
• tends to favor
phenotypes at
one extreme of
the range of
variation.
The graph
shows the
decrease in size
of pink salmon
caught in two
rivers in British
Columbia since
1950, driven by
selective fishing
for the large
individuals.
Disruptive Selection
• favors individuals at
both extremes of
variation: selection is
against the middle of
the curve
http://wps.prenhall.com/wps/media/obj
ects/487/499541/CDA22_2/CDA22_2d/C
DA22_2d.htm
What’s taxonomy?
The theories and techniques of describing, grouping and naming
organisms
•Biologists do not think of species simply as a long alphabetical list.
•Since Linnaeus, the father of modern taxonomy, species have been
arranged in a taxonomic hierarchy:
•Species are grouped in genera.
•The gray wolf species Canis lupus and the golden
jackal Canis aureus , for example, are grouped in the
genus Canis .
•Genera are grouped into families;
•the genus containing dogs and wolves combines
with several other genera, such as the fox genus
Vulpes , to make up the family Canidae.
•Several families combine to make up an Order
• Carnivora, in this example.
•Orders make a Class (Mammalia).
• Classes make a Phylum (Chordata).
• Phyla make up one of the five Kingdoms (Animalia).
Why do we classify organisms?
•
To make sense of the diversity of organisms
•
Put them in order
• To understand the relationships organisms have with each
other.
• Morphological (form), physiological (structure), ecological
(environment), genetic and molecular characteristics can all
be used to classify organisms.
• Nucleic acid structure is probably the best indicator of
relatedness
Why else is classification
important?
Practical and scientific reason—
• identifying wild relatives of crops–
that provide breeders with useful
alleles
• Benefits to public health--disease
control
• Identifying unknown species that
might help understand
ecosystems
• Understanding the “unity of life”
What’s a Species?
The basic taxonomic group is the species
All individuals and populations of a particular
type of organism that can interbreed with one
another.
Species may look different, but as long as they
can produce FERTILE offspring they are still
members of the same species.
The Species Concept
Species are interbreeding natural populations which
are reproductively isolated from other such groups.
Species remain separate from one
another in three basic ways:
• potential mates do not meet;
•potential mates meet but do not breed
•potential mates meet and breed but do not produce
fertile or viable offspring.
Appearance Isn’t Everything
•Organisms may appear to be alike and be different species.
•For example, Western meadowlarks (Sturnella neglecta) and Eastern
meadowlarks (Sturnella magna) look almost identical to one another, yet do not
interbreed with each other
•Thus, they are separate species according to this definition.
Evidence Used to Classify
Organisms
•
•
•
•
morphology
anatomy /development
the fossil record
molecular data
Placing Organisms in Groups
Ex. All animals with hair are classified
as mammals
The Steller’s jay and the blue jay
Using Homologous Structures
Similarities of biological structures that results
from evolution form a common ancestor.
Forearm of frogs, lizards, birds, humans, cats, etc.
Homologous structures are are also those that develop from
similar embryological origins
Adult fish, salamanders, turtles, chickens,
rabbits, and humans bear virtually no
resemblance to one another.
Yet these animals are virtually
indistinguishable as embryos.
Can you pick out which one is the
human?
How about now?
Which one is the human embryo?
Why should animals that have markedly different adult forms and function
develop from such similar embryos?
Far back in vertebrate
history, they all had a
common ancestor, probably
some type of primitive fish,
that developed from a
similar type of embryo.
As the various types of vertebrates evolved, they each retained
this basic vertebrate embryo as part of their life cycle– even
though its parts gave rise to different adult organs.
Analogous Structures
Similarities in form or function that are not a result of evolution from a
common ancestor, but is evidence of convergent evolution (similar
environmental pressures).
Ex. wings of insect,
bird, bat and pterosaur
Analogous vs. Homologous
Structures
Chemical homologies
•The more similar the genetic code (DNA), the more closely related two
species are (the more recently they evolved from one another)
•So, the longer the period since two species have diverged from a
common ancestor, the greater the number of differences in genes and
proteins between the species.
Amino Acids as Evidence for Evolution
Packet
The Linnean classification system
The system uses
homologies to group
species into larger and
more general categories.
Kingdom, phylum, class,
order, family, genus,
species
Binomial nomenclature?
The scientific naming of
species where each species
receives a Latin or Latinized
name of two parts
• the first part indicates the
genus and the second is
the specific epithet.
•For example, Juglans regia
is the English walnut;
Juglans nigra, the black
walnut.
How do you write a scientific name?
•Capitalize genus
•underline/italicize both names
•lower case species name
Ex.
American Black Bear
Ursus americanus
Brown Bear
Ursus arctos
•Subspecies Syrian (Brown) Bear (Ursus arctos syriacus)
•Subspecies Grizzly Bear, (Ursus arctos horribilis)
•Subspecies Kodiak Bear, (Ursus arctos middendorffi)
Ways to Classify Organisms
Current classification systems
attempt to reflect the evolutionary
relationships of organisms
Orthodox• Originally, organisms classified using the Linnaean system were grouped
together based simply on physical similarities
•Ex. Dividing birds into orders based on their beaks and claws
• This method can be subjective
Cladistics
•Assumes each
group has an
ancestor that other
species do not
share
•Groups species
according to
ancestry and
homologous
characteristics.
Ex. All mammal species have mammary glands, but no other organism do.
•Therefore all mammals must be descended from a species that has no other
living descendants.
•Mammals form a clade, or branch on this diagram of the history of the
animals.
Phylogeny
•Comparing the anatomical structures
of many different organisms.
•homologous structures
•for example, the front legs of a
horse, wings of a bird, flippers of
a whale, and the arms of a
person are all homologous to
each other.
•When different organisms share a
large number of homologous
structures, it is considered strong
evidence that they are related to
each other.
Making a Cladogram
Step 1: determine which of the
characteristics each animal has.
Step 2: Draw a cladogram to illustrate the
ancestry of these animals. The diagram should
reflect shared characteristics as time proceeds.
SETS
#1
Backbone
TRAITS
Tuna
x
#2
Placenta
#3
Foramen magnum fwd.
Horse
Human
x
x
x
TOTALS of X’s
x
x
1
2
3
Notice how the
different animals are
all at the same time
level (across the top)
since they all live
today.