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Evolution
Mr. Wright, 2011
Sections 15.1 & 15.2
Evolutionary Theory
• Evolution – change over time
▫ How modern organisms descended
from their ancestors.
• Theory – a well-supported testable
explanation for something
Evolutionary Theory
• The main person behind
evolutionary theory was
Charles Darwin.
• Darwin sailed around the
world, recording his
observations as he
travelled.
• These observations led to
him proposing the idea of
evolution.
Darwin’s Observations
• Darwin was most influence by what
he saw on the Galapagos Islands.
• Each island had a unique climate:
▫ Some were dry and barren
▫ Some were moist and diverse
▫ Others were inbetween
Galapagos Tortoises
• Darwin found tortoises on each island, but they
looked different from those on other islands.
• How are they different, and what might this
mean?
Galapagos Finches
• Darwin also studied finches on each island.
• How are they different? What might this mean?
Adaptations
• An adaptation is any inherited characteristic
that increases an organism’s chance of
survival.
• These are key for outcompeting other
organisms.
• Let’s look at a few examples and figure out
what purpose they serve.
Adaptation - Porcupine
Adaptation - Hare
Adaptation – Walking Stick
Adaptation - Giraffe
Adaptation - Whale
Section 15.3
Variation is heritable.
• The variations that evolve over time can be
passed on to offspring if an individual manages
to reproduce.
Artificial Selection
• Artificial Selection is when breeders select for
certain desirable traits over time.
The Struggle for Existence
• All organisms compete with each other to
survive.
• Evolution selects for “better” organisms that
have a greater chance at making it.
Fitness
• Fitness – the ability of an
organism to survive and
reproduce in its environment
Survival of the Fittest
• The organisms that are the most fit
are the ones who survive. The weak
will perish.
Relation to Natural Selection
• Nature selects for the most fit individuals over
time – only the fit live to reproduce and pass on
their traits!
Common Descent
Homologous Structures
• Structures with different forms that develop
from the same embryonic tissue.
• Typically, the more similar two structures are,
the closer they are related.
Embryology
• Embryos are very similar to one another from
species to species – suggests a common
ancestor?
Darwin’s Theory - Summary
• Individuals differ, and variation is heritable.
• Organisms produce more offspring than will
survive, and not all will reproduce.
• Organisms compete for limited resources.
• Survival of the fittest!
• All species share a common ancestor they
descended from.
Section 16.1
Genes and Variation
• In the 1930’s, biologists began to connect
Darwin and Mendel’s work to each other.
• Evolution is controlled by gene frequencies.
Variation and Gene Pools
• Genetic variation is studied in whole
populations.
▫ Population – group of individuals that can
interbreed.
• Gene pool – ALL the different alleles that occur
in a population
Variation and Gene Pools
• Relative Frequency – the number of times that
an allele occurs in a gene pool compared to the
total number of alleles
• Evolution is any change in the relative
frequencies of alleles over time.
Variation and Gene Pools
• Relative Frequency of
the Green Allele =
12/15 = 4/5
• Relative Frequency of
the Purple Allele =
2/15
• Relative Frequency of
the Red Allele = 1/15
Sources of Genetic Variation
• Mutations
▫ Changes to the genetic sequence of an organism
• “Gene Shuffling”
▫ Remember that meiosis produces gametes that are
all genetically unique
▫ Produces many different combinations of alleles
but DOES NOT change their relative frequencies
in a population.
Single-Gene Traits
• Variation for a trait can be caused by a single
gene – you either have the trait, or you don’t.
Polygenic Traits
• Many traits are controlled by several genes
together – polygenic traits.
• Depending on how the genes combine, you can
have several different phenotypes.
• Generally forms a bell-curve.
Section 16.2
Review Concepts
• Fitness – an organism’s success at passing on its
genes
• Adaptation – a genetically controlled trait that
increases an individual’s fitness
• Natural selection determines what individuals
survive and reproduces – an organism either
passes on ALL genes, or none at all.
Natural Selection on Single-Gene Traits
• A forest lizard can appear in 3 colors: brown, red
(mutant), or black (mutant)
• What do you think might happen to the red
allele over time?
• What do you think might happen to the black
allele over time?
Natural Selection on Single-Gene Traits
• Due to natural selection, over time…
▫ If an allele increases fitness, it’s relative frequency
will increase.
▫ If an allele decreases fitness, it’s relative frequency
will decrease.
▫ If an allele has no effect on fitness, it will not be
affected by natural selection.
Natural Selection on Polygenic Traits
• Remember that polygenic traits often form a bell
curve.
• The fitness of individuals close to each other on
the curve is similar, but can vary over great
distances.
• Can be affected in three ways:
Directional Selection
• When individuals at one sight of the curve are
more fit than the other side, directional selection
occurs.
Stabilizing Selection
• When individuals at the center of the curve have
the highest fitness stabilizing selection occurs.
Disruptive Selection
• When the extremes have more fitness than the
individuals in the middle, disruptive selection
occurs.
Genetic Drift
• Sometimes allele frequency changes simply due
to chance – this is called genetic drift.
• Most common in small populations.
Founder Effect
• Genetic drift can also occur when a small group
goes off and colonizes a new habitat.
• This is known as the founder effect.
Section 16.2
Overview
• In order to study how evolution takes place, it
can be useful to study a situation where NO
CHANGE takes place.
• When allele frequencies remain constant it is
called genetic equilibrium. No evolution occurs.
• This is called the Hardy-Weinberg Principle, and
is made up of the following five rules.
Random Mating
• All members of the population must have an
equal opportunity to produce offspring.
• All alleles have an equal chance of being passed
on.
Large Population
• Genetic drift has less of an affect on a large
population.
No Immigration
• Immigrating individuals can either bring genes
into or out of the population.
No Mutations
• Mutations can affect the allele frequencies in a
population.
No Natural Selection
• All genotypes must have an equal chance of
surviving and reproducing.
Section 16.3
Speciation
• Speciation – the formation of a new species
▫ Species – group of organisms that can breed and
produce fertile offspring
• Speciation is a product of evolution.
• As species evolve, populations become
reproductively isolated and can no longer
interbreed.
• 3 ways that this can happen.
Behavioral Isolation
• When two populations are capable of
interbreeding but do not due to different
reproductive behaviors.
• Example: Bird songs
Geographic Isolation
• When a species is split into two new species by
geological events it is geographic isolation
• Example: separation by mountains
Temporal Isolation
• When two species do not reproduce due to time
differences it is called temporal isolation.
• Example: rain forest orchids
Example of Speciation - Assignment
• You will create a diagram showing the process of
speciation in an organism of your choosing. The
diagram should be at least 6 steps long.
• There is a good example of what I am looking for
on pages 408-410 in your book.
• The book uses finches as their example – you
must use something different. Tigers, turtles,
dung beetles, aardvarks, whatever floats your
boat.