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Evolution notes
Chapters 16 and 17
Variation
• Bell curve is commonly seen in
populations. Each extreme trait is not as
common as the average trait
Causes of variations
– Mutation (genetically flawed copies of genes)
– Recombination (crossing over during meiosis)
– Random fusion of gametes (game of chance)
Gene Pool
• The total genetic information available in a
population
Hardy Weinberg Genetic
Equilibrium (assumption 1)
• The Population will NOT EVOLVE IF
– No net mutation occur: allele frequency does
not change overall because of mutations
• Allele frequency is a measure of how often a
certain allele exists in a population
• If mutations occur; allele frequency could change
and evolution occurs
Hardy Weinberg Genetic
Equilibrium (assumption 2)
– Individuals DO NOT leave or enter a
population
• Immigration = one or more individuals moving into
a population
• Emigration = one or more individuals moving out of
a population
• If individuals leave or enter, evolution can
occur
Hardy Weinberg Genetic
Equilibrium (assumption 3)
– The population is large
• If the population is small, only a small number of
organisms may have a certain trait and that trait
could easily disappear. Also the addition of a new
trait has a higher probability of becoming dominant
in a small population
• Genetic drift (allele frequency changes in a
population) is much more noticeable in small
populations
Hardy Weinberg Genetic
Equilibrium (assumption 4)
– Individuals mate randomly
• Many species do NOT mate randomly and
therefore certain traits will be favorable or become
more predominant than others. This causes
evolution
Hardy Weinberg Genetic
Equilibrium (assumption 5)
– Selection does not occur
• Natural selection (survival of the fittest) is the
most significant factor that disrupts genetic
equilibrium
• Sexual Selection: birds often choose mates
based on their color (the more colorful the male,
the more attractive). The fiddler crab chooses their
male based on the size of their claw. Anoles
prefer bright
colors on
the male’s
dewlap
Stabilizing Selection
• Organisms with the average trait have the
highest fitness. This shows a normal bell
curve
Directional Selection
• Organisms with traits that are towards one
extreme have the highest fitness.
– Giraffes developed longer necks
– Anteaters developed longer tongues
Disruptive Selection
• Individuals with either extreme have a
greater fitness than the average trait
– Finches of Galapagos island had small beaks
that were good for eating small seeds and big
beaks for large seeds,
but medium sized
beaks were too big
to get at the small
seeds and not
strong enough to
break apart the big seeds
Morphology
• Internal and external structural
appearance of an organism
– Scientists used to use this to classify species
– Now they use the biological species concept
(a population of organisms that can
successfully interbreed) in addition to
morphology to classify a species
• The many species alive today diverged from a
smaller number of earlier species
Geographic Isolation
• One species becomes separated from
some of its members by a barrier like a
river, mountain, or a canyon. After many
generations they may become so different
that they can no longer interbreed.
• These fish were
separated by
the isthmus of
panama
Reproductive isolation
• Even though they could produce offspring,
they do not breed in nature due to:
– Different mating seasons (wood and leopard
frogs)
– Different calls (various populations of frogs)
– Size variations (large and small breeds of
dogs)
Speciation
• The formation of a
new species
• Punctuated
equilibrium:
species remain
unchanged for a
long time and then
suddenly change
very rapidly (this is
opposite of
“gradualism”
Hominids
• Humans and their
immediate ancestors
Primates
• Anthropoids
– Marmosets, monkeys, apes, and humans
• Prosimians
– Earlier forms of primates like lemurs, lorises
and tarsiers
Anthropoids
•
•
•
•
•
•
Well developed collarbone
Rotating shoulder joints
Partially rotating elbow
Opposable thumbs
Similar dental formula
Larger cranial capacity
Humans
• Bipedalism: the ability to walk on two legs
• Cup shaped pelvis (to support upright
walking)
• S-shaped curve in spine (as opposed to a
C-shaped curve in chimpanzees)
• Shorter aligned toes
• Larger brain, adapted for production of
complex speech
Australopithecus afarensis
• “southern ape” of Africa. Lucy
was found in 1974. they lived 3 3.9 million years ago
• more recent Australopithecines
showed increasingly larger cranium
and other variations.
Not all are thought
to be our ancestors;
some are believed to
have given rise to
other apes
Homo habilis
• Tool maker
• Earliest known hominid in our same genus
• 1.6-2.5 million years old
Homo erectus
• Upright human
• Found in Java, China, Europe and Africa
• Probably the first hominid to travel out of
Africa
• Lived 1.8 - .5 million years ago
• They were hunters, used fire and many
lived in caves
Homo sapiens
• Neanderthal (lived 230,000 –
30,000 years ago)
• Had a slightly larger cranium
than us
– They were slightly shorter but
heavy built
– Used stone tools, lived in caves
– Wore clothing made from animal
skin
– Were likely killed off by or out
competed by modern Homo
sapiens