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BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
1
BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
Chapter 17 – Evolution of Populations
Chapter Mystery – Epidemic
In 1918, an epidemic began that would go on to kill more than 40 million
people. A doctor wrote: “Dead bodies are stacked about the morgue like cordwood.”
What was this terrible disease? It was a variety of the same influenza virus
that causes “the flu” you catch again and again. How did this strain of a common
virus become so deadly? And could that kind of deadly flu epidemic happen again?
The answers to those questions explain why we can’t make a permanent
vaccine against the flu, as we can against measles or smallpox. They also explain
why public health officials worry so much about something you may have heard
referred to as “bird flu.” Look for evolutionary processes that might help explain
how new strains of influenza virus appear all the time. Then, solve the mystery.
17.1
Genes and Variation
Guiding Question: How do genes make evolution possible?
1. Genetics Joins Evolutionary Theory
a. Variation is the raw material for natural selection
b. Main focus of genetic study is the genotype
• Alleles inherited from each parent
c. Evolution’s main focus is the phenotype
• Which traits make an organism better adapted?
• All the genes and alleles available in a population = gene pool
⇒ Population is a group of organisms of the same species
living in one area
d. The number of times an allele occurs in a gene pool compared to the
total = allele frequency
• Evolution is any change in the allele frequency in a population
e. Natural selection acts on phenotypes, not genotypes.
2. Sources of Genetic Variation
BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
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BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
a. Mutations – any change in a sequence of DNA
• You have about 300 mutations not inherited from your parents
(most are neutral, some may hurt, some may help)
b. Genetic recombination by sexual reproduction
• Crossing over
• Independent assortment
c. Lateral gene transfer
• Passing genes from one organism to another organism that is not
its offspring, swapped like trading cards
3. Single Gene Traits
a. Traits controlled by one gene with two alleles
b. Mendelian heredity – brown eyes is dominant over blue
c. Phenotype is two options (simple dom./rec.) or three different options
(incomplete/codom)
4. Polygenic Traits
a. Traits controlled by many genes
b. “The beyond Mendel” – polygenics
c. Phenotype can be a range of different possibilities, not just one or the
other
17.2
Evolution as Genetic Change in Populations
Guiding Question: What causes a population’s gene pool to change?
1. How Natural Selection Works
a. Fitness – the ability of a living thing to survive and reproduce
• In genetic terms – each time an organism reproduces, it passes
copies of its genes on to its offspring
• Fitness – success in passing genes to the next generation (i.e.
reproducing)
BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
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BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
b. Natural selection – single gene traits
• Change in allele frequencies = change in phenotype frequencies
• Population = 10 spotted rabbits & 30 solid rabbits
⇒ Spotted becomes more advantageous
⇒ Next generation 20 spotted/20 solid
⇒ Next generation 30 spotted/10 solid
c. Natural selection - polygenic traits
• Can affect the relative fitness of phenotypes
• Produces three types of selection – each a slight change in the
overall curve of phenotypes
• Directional
⇒ selects for individuals at one end of the curve
⇒ moves the whole curve left or right (still bell)
• Stabilizing
⇒ selects for individuals in the middle of the curve
⇒ the curve becomes more narrow and taller
• Disruptive
⇒ selects for individuals at either end of the curve
⇒ the curve becomes two hills separated by a valley
d. Genetic Drift
• Random change in the allele frequency of a population
⇒ By chance not natural selection
⇒ Genetic Bottlenecks – population reduction due to natural
disaster or disease – new population’s allele frequencies can
be much different than original
⇒ Founder Effect – a few individuals colonize a new habitat
e. Evolution vs. Genetic Equilibrium
BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
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BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
• Genetic
equilibrium
=
allele
frequencies
are
stable
and
unchanging
• Sexual reproduction does not alter allele frequencies in and of
itself
f. Hardy-Weinberg Principle
• Predicts
genotype
frequencies
in
a
population
in
genetic
equilibrium
⇒ p2 + 2pq + q2 = 1, and p + q = 1
⇒ p = allele frequency of dominant allele
⇒ q = allele frequency of recessive allele
⇒ probability of genotype AA = p2
⇒ probability of genotype aa = q2
⇒ probability of genotype Aa = 2pq
• Five conditions can disrupt Hardy-Weinberg equilibrium and cause
evolution to occur:
⇒ Nonrandom mating (sexual selection is occurring)
⇒ Small population size (genetic drift usally doesn’t affect
large populations)
⇒ Immigration or emigration (can disrupt allele frequencies)
⇒ Mutations (can introduce new alleles not previously
present)
⇒ Natural selection (differential fitness)
17.3
The Process of Speciation
Guiding Question: How do new species form?
1. Isolating Mechanisms
a. Species – a population whose members can interbreed and produce
fertile offspring
BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
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BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
b. Speciation – forming a new species; population that could interbreed
before, but can’t now
c. Reproductive isolation – members of two populations that can no longer
interbreed or produce fertile offspring
• Behavioral Isolation – when populations have different courtship
rituals or other behaviors involved in reproduction
⇒ e.g. Eastern meadowlark and Western meadowlark
• Geographical Isolation – when populations are separated by
geographic barriers, such as mountains or rivers.
⇒ e.g. Abert’s squirrel and Kaibab squirrel
• Temporal Isolation – when populations reproduce at different
times.
⇒ e.g. orchids that must be pollinated on day but not all
bloom together
2. Speciation in Darwin’s Finches (a well known example)
a. Founder effect
• Original species likely blown during storm or got lost
b. Geographical isolation
• Finches don’t usually fly over water
c. Changes in gene pool
• Beak size and shape is polygenic
d. Behavioral isolation
• Different beaks can affect sexual selection
e. Ecological competition and continued evolution
• Two species occupying same niche will both likely not make it
• Less competition = higher fitness
BHS Biology – Chapter 17 – Darwin’s Theory of Evolution
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BIG IDEA: EVOLUTION
Essential Question: How can populations evolve to form new species?
17.4
Molecular Evolution
Guiding Question: What can genes tell us about an organism’s evolutionary
history?
1. Timing Lineage Splits: Molecular Clocks
a. Molecular clocks – uses known mutation rates in DNA to estimate the
length of time that two species have been evolving
• Neutral mutations accumulate in DNA of different species at
about the same rate (assumption)
• Two
species
accumulate
evolving
different
independently
neutral
from
each
mutations
other
through
will
time
(assumption)
• More differences = more time passed since common ancestor
2. Gene Duplication
a. Organisms may carry multiple copies of the same gene, extra copies
undergo mutations
b. The mutated gene may have a new function that is different from the
original gene.
c. Multiple copies of a duplicated gene can turn into a gene family –
produce similar but different proteins (e.g. hemoglobin)
3. Developmental Genes and Body plans
a. Some genes, called Hox genes, control the forms of animals’ bodies
b. Small changes in Hox genes during embryological development can
produce major changes in the adult.
c. Some scientists think that changes in Hox genes may contribute to
major evolutionary changes (e.g. insects and crustaceans)