NOTES: CH 17 – Evolution of Populations
Vocabulary:
Fitness
Genetic Drift
Adaptive radiation
Divergent evolution
Punctuated Equilibrium
Gradualism
Convergent evolution
Gene flow
17.1 – Genes & Variation
● Darwin developed his theory of natural selection without knowing how heredity worked…or how variations arise
● VARIATIONS are the
● All of the discoveries in genetics fit perfectly into evolutionary theory!
Genotype & Phenotype
● GENOTYPE: the particular
an organism carries
● an organism’s genotype, together with environmental conditions, produces its PHENOTYPE
● PHENOTYPE: all physical, physiological, and behavioral characteristics of an organism (i.e.
,
)
Natural Selection
● NATURAL SELECTION acts directly on……PHENOTYPES!
● How does that work?...some individuals have phenotypes that are better suited to their environment…they
survive & produce more offspring (higher fitness!)
● organisms with higher fitness pass more copies of their genes to the next generation!
Do INDIVIDUALS evolve?
 NO!
 Individuals are born with a certain set of genes (and therefore phenotypes)
 If one or more of their phenotypes (i.e. tooth shape, flower color, etc.) are poorly adapted, they may be unable to
 An individual CANNOT evolve a new phenotype in response to its environment
So, EVOLUTION acts on…
 POPULATIONS!
 POPULATION =
 In a population, there exists a RANGE of phenotypes
 NATURAL SELECTION acts on this range of phenotypes  the most “fit” are selected for
.
17.2: Evolution as Genetic Change in Populations
Mechanisms of Evolution (HOW evolution happens)
1) Natural Selection (from Darwin)
3) Migration (a.k.a. Gene Flow)
2) Mutations
4) Genetic Drift
DEFINITIONS:
 Species : group of organisms that
 Population: group of individuals of the same species that
(& therefore interact, interbreed, & share the same GENE POOL)
 GENE POOL:
population.
of all members of a particular
 Allele frequency =
number of times other alleles occur (usually expressed as a %)
in a gene pool compared with the
1) Mechanism for Evolution: NATURAL SELECTION
 All organisms struggle for survival by competing for resources especially in an overpopulated environment thus…
-low levels of fitness =
-high levels of fitness =
 Natural Selection:
-Imagine that green beetles are easier for birds to spot (and hence, eat).
 Brown beetles are a little more likely to survive to produce offspring
 Pass their genes for brown coloration on to their offspring
 Next generation:
.
What is Fitness?
 FITNESS:
generation (relative to other genotypes)
-If brown beetles consistently leave more offspring than green beetles…
-The brown beetles have a
in the next
relative to the green beetles.
Fitness is a relative thing
 A genotype’s fitness depends on the
in which the organism lives.
 The fittest genotype during an ice age, for example, is probably not the fittest genotype once the ice age is over.
 The fittest individual is not necessarily the
 A genotype’s fitness includes its
(pass its
to the next generation)
There cannot be
without
in the first place!
Sources of Genetic Variation: MUTATION
 MUTATION:
that affects genetic information (random—not predictable)
-a mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown
coloration
-that would make the genes for brown beetles more frequent in the population
 Single mutation can have a large effect
 in many cases, evolutionary change is based on the
of many mutations
 can be beneficial, neutral, or harmful
 mutations do not “try” to supply what the organism “needs.”
 The individuals which happen to have the mutations giving them the
environment will be the ones that survive
 hence the “good” mutations will be “passed down” to the next generation.
to the
 Not all mutations matter to
-All cells in our body contain DNA
-Mutations in non-reproductive cells won’t be passed onto offspring
Cause of mutations:


Sources of Genetic Variation – MEIOSIS!
Gene Shuffling (How chromosomes line up in meiosis)
This shuffling is important for evolution because it can introduce new combinations of genes every generation.
3) Mech. for Evolution: MIGRATION (a.k.a. GENE FLOW)
 Some individuals from a population of brown beetles might have joined a population of green beetles.
-would make the genes for brown beetles more frequent in the green beetle population.
4) Mechanism for Evolution: GENETIC DRIFT
 In a population, an allele can become more or less common
-remember genetics and probability!
 Definition:
(gene)
 most effective with
 SO…Gene pools can change without
… an allele can become common
in a population by chance alone.
GENETIC DRIFT: Example
 Imagine that a population of green and brown beetles
 Several green beetles were killed when someone stepped on them and therefore, they had no offspring.
 The next generation would have a few more brown beetles than the previous generation—but just by chance.
 These chance changes from generation to generation are known as
Genetic Drift Example: FOUNDER EFFECT
 A small group of individuals move to new habitat (the “
” group)
 Their alleles and allele frequencies may be different that that of the original population
 So the new population that they found will have different
than the original
group…BY CHANCE!!
Genetic Drift Example: BOTTLENECK EFFECT
● a population experiences an event (storm, sickness, over hunted by humans) that causes it to decrease in # to
just a few individuals
● the allele frequencies in the
may be different than the original population
● Example:
 Of all of the mechanisms covered, the “strongest” influence is that of NATURAL SELECTION…
Natural Selection
 Natural selection on single gene traits can lead to changes in the
-Ex: brown vs. green beetles
 Natural Selection on polygenic traits—affects distribution of phenotypes in 3 ways
1) Directional Selection
2) Stabilizing Selection
3) Disruptive Selection
● Imagine the range of phenotypes in a population are graphed into a distribution
curve:
1) Directional Selection
 If organisms at one end of the curve have
organisms in the middle or at the other end of the curve
-Finch beaks in the Galapagos
-Result: specific beak size increased
than
Examples:
Peppered Moth example:
● 100 years after the first dark moth was discovered in 1848,
● the light variety continued to dominate in unpolluted areas outside of London
2) Stabilizing Selection
 Individuals near
have higher fitness that individuals at either end of the
curve
-Human baby birth weight
 Babies born v. underweight less likely to survive
 Larger babies have a hard time being born (think size of birth canal)
3) Disruptive Selection
 Individuals at
are more fit than those in the center
-Intermediate type is selected against
 Ex: Bird beak size
-if medium seed size becomes less common, birds that can eat the smallest and largest seeds will survive
Evolution (Change over time) vs. Genetic Equilibrium
● if a population is NOT evolving, allele frequencies in the gene pool do not change, and the population is in
GENETIC EQUILIBRIUM.
● The Hardy-Weinberg principle states that allele frequencies remain constant unless 1 or more factors cause
those frequencies to change…
● for Hardy-Weinberg equilibrium to occur, the following conditions must be met:
1) Large population
2)
3)
(no immigration or emigration)
4)
(no mating preference for particular phenotype)
5) No natural selection (all genotypes have an = chance of surviving & reproducing)
HOWEVER, in nature:
1)
& may mate with one another
2) there are always mutations (chance with every DNA replication)
3) gene flow often occurs between populations
4)
5) natural selection is always occurring **Therefore, in nature there will always be changes in populations
(“microevolution”)
;
Hardy-Weinberg Equation:
● p = frequency of dominant allele (A)
● q = frequency of recessive allele (a)
●p+q=1
● frequency of possible diploid combinations (AA, Aa, aa):
p2
+
(AA)
2pq
+
(Aa)
q2
=
1
(aa)
Example Problem:
● If the frequency of a recessive allele is 35% in a population of 1500 people, how many people would you predict
would be carriers of this allele, but would not express the recessive phenotype?
Example Problem:
● In a population with 2 alleles for a particular locus, B and b, the allele frequency of B is 0.78. If the population
consists of 172 individuals, how many individuals are heterozygous? How many will show the recessive
phenotype?
Nonrandom Mating leads to Sexual Selection!
● SEXUAL SELECTION: the selection of mates
strength, coloration)
(e.g. size,
SEXUAL SELECTION: Females
-females tend to increase their fitness by increasing the quality of their offspring by
(and are therefore “choosier” or more selective when finding a mate)
SEXUAL SELECTION: Males
-males increase their fitness by maximizing the
**as a result, in vertebrate species, the male is typically the “showier” sex
-
17.3: The Process of Speciation
Central Idea: How does natural selection (and other mechanisms of evolution) lead to the formation of a
new species?
 Speciation:
 Remember:
-Species: a group of organisms that breed with one another and produce fertile offspring
-Population: members of the same species that
 If a genetic change
fitness, that allele will eventually be found in many members of the
population
 As new species evolve, populations become
from each other.
 When members of two populations cannot reproduce to produce fertile offspring =
 At this point, species have
 Question: How does reproductive isolation develop?
3 kinds of Isolating Mechanisms
1) Behavioral Isolation Two populations are physically able to interbreed but have different courtship rituals or
other types of behavior
2) Geographic Isolation: Geographic barriers (
being exchanged, including advantageous mutations and variations
) prevent genes from
3) Temporal Isolation: Species reproduce at
reproduce with each other
and therefore are unlikely to
17.4: Molecular Evolution
● the analysis of genomes enables us to study evolution at the molecular level
● MOLECULAR CLOCK: uses mutation rates in DNA to
estimate the
independently
● the more differences there are between the DNA sequences of the
2 species, the
since the 2 species
shared a common ancestor
Molecular Evolution & Cladograms
● we can use this information to generate a CLADOGRAM –
a diagram that
.