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XBIO: Chapter 11 – The Evolution of Populations NOTES
11.1 Genetic Variation Within Populations
KEY CONCEPT: A population shares a common gene pool.
I. Genetic variation in a population increases the chance that some individuals will survive.
A. Population Genetics – the study of evolution from a genetic point of view
B.
A population with a lot of genetic variation has a wide range of phenotypes
C.
If the environment changes and there are greater variations in phenotypes, the chances that some
organisms will survive is higher
D. Genetic variation is stored in a population’s gene pool
1. Gene Pool – the combined alleles of all of the individuals in a population
2. Allele frequency – a measure of how common a certain allele is in the population
3. Phenotype frequency – how often a phenotype occurs out of all the phenotypes
II. Genetic variation comes from several sources.
A. Mutation – random change in the DNA of a gene, can form a new allele
1. Can be passed on to offspring if the change occurs in a reproductive
cell (gamete)
2. Increases the genetic variation in
gene pools
B.
Recombination – reshuffling of genes during
meiosis, when gametes are made, each
parent’s alleles are arranged in new ways
11.2 Natural Selection in Populations
KEY CONCEPT: Populations, not individuals, evolve.
I. Natural selection acts on distribution of traits.
A. Normal distribution – the type of distribution of phenotypes where the
frequency is highest near the mean value and decreases toward each
extreme
Example - Human height tends to follow a normal distribution
II. Natural selection can change the distribution of a trait in one of three ways.
A. Microevolution – the observable change in the allele frequencies of a
population over time (occurs on a small scale, within a single population)
B.
Natural selection can lead to microevolution
C.
Three ways in which the distribution of a trait may change
1. Directional Selection – individuals that display a more extreme form of a trait have greater
fitness than individuals with an average form of the trait
Example – The rise of drug resistant bacteria
2. Stabilizing Selection – individuals with the average form of a trait have the highest fitness
Example – Wasps eat smaller gall flies, woodpeckers eat larger gall flies
3. Disruptive Selection – individuals with either extreme variation of a trait have greater fitness
Example – Lazuli buntings (a type of bird) vary widely in the brightness of their feathers Bright blue male buntings are dominant and aggressive; they typically target young bluish
brown and leave the dull brown young alone
QUESTIONS FOR REVIEW
1. ?
11.3 Other Mechanisms of Evolution
KEY CONCEPT: Natural selection is not the only mechanism through which populations evolve.
I. Gene flow is the movement of alleles between populations.
A. Gene Flow – the process of genes moving from one population to another
1. Emigration – movement of individuals out of a population
2. Immigration – movement of individuals into a population
3. The lack of gene flow increases the chance that two populations will evolve into different
species
II. Genetic drift is a change in allele frequencies due to chance.
A. Genetic Drift – the phenomenon by which allele frequencies in a population change as a result of
random events, or chance
1. Effects small populations more
2. If an allele disappears completely and variation is reduced so much – extinction is likely
B.
Two processes commonly cause populations to become small enough for genetic drift to occur
1. Bottleneck Effect – an event occurs, the population is greatly
reduced, and only a few organisms survive, surviving
organisms do not represent the allele frequencies from the
original population

Example -
Northern elephant seals have reduced genetic variation probably
because of a population bottleneck humans inflicted on them in the
1890s. Hunting reduced their population size to as few as 20
individuals at the end of the 19th century. Their population has since
rebounded to over 30,000 — but their genes still carry the marks of
this bottleneck: they have much less genetic variation than a
population of southern elephant seals that was not so intensely
hunted.
2. Founder Effect – a few individuals that do not represent the overall allele frequencies of the
larger population leave to start a new population

Example –
Eastern Pennsylvania is home to beautiful farmlands and countryside, but it's also a gold mine of
information for geneticists, who have studied the region's Amish culture for decades. Because of
their closed population stemming from a small number of German immigrants -- about 200
individuals -- the Amish carry unusual concentrations of gene mutations that cause a number of
otherwise rare inherited disorders, including forms of dwarfism.
One form of dwarfism, Ellis-van Creveld syndrome, involves not only short stature but polydactyly
(extra fingers or toes), abnormalities of the nails and teeth, and, in about half of individuals, a
hole between the two upper chambers of the heart. The syndrome is common in the Amish
because of the "founder effect."
When a small part of a population moves to a new locale, or when the population is reduced to a
small size because of some environmental change, the genes of the "founders" of the new society
are disproportionately frequent in the resulting population.
If individuals in the group tend to marry within it, there's a greater likelihood that the recessive
genes of the founders will come together in the cells that produce offspring. Thus diseases
of recessive genes, which require two copies of the gene to cause the disease, will show up more
frequently than they would if the population married outside the group.
In the Amish, in fact, Ellis-van Creveld syndrome has been traced back to one couple, Samuel King
and his wife, who came to the area in 1744. The mutated gene that causes the syndrome was
passed along from the Kings and their offspring, and today it is many times more common in the
Amish population than in the American population at large.
III. Sexual selection occurs when certain traits increase mating success.
A. Sexual selection (nonrandom mating) - occurs when certain traits increase mating success, females
are choosy about their mates because their investment in the
offspring is higher (in most cases)
1. Competition between males (ex. head butting of
bighorn sheep), winner gets to mate with the female
2. Males display certain traits that attract the female
(ex. peacocks fanning out their tails)
IV. There are five factors that can lead to evolution. (11.4)
11.5 Speciation Through Isolation
KEY CONCEPT: New species can arise when populations are isolated.
I. The isolation of populations can lead to speciation.
A. When the gene flow between two populations
stops – the populations are said to be isolated
B.
As the gene pools change, the isolated
populations become more and more
genetically different
C.
Reproductive Isolation – occurs when
members of different populations can no
longer mate successfully
D. Speciation – the rise of two or more species
from one existing species
II. Populations can become isolated in several ways.
A. Behavioral Barriers – isolation caused by difference in courtship or mating behaviors
Example - Potential Mates meet but cannot figure out what to do about it. For example before birds mate they go through
intricate courtship rituals that involves singing, head bobbing, wing spreading, or prancing that a female has to genetically
recognize before she will mate.
B.
Geographic Barriers – isolation caused by a physical barrier that divides a population into two or more
groups
C.
Temporal Barriers – exists when timing prevents reproduction between populations
Example - Potential mates occupy overlapping ranges but reproduce at different times. Extreme example is seen in the
periodical cicadas of the eastern United States. These insects mature underground and only emerge to reproduce every
17 years.
1.
Geographic Isolation – the physical separation of members of a population LEADS TO…

2.
Allopatric Speciation – “different
homelands” - happens when species
arise as a result of geographic isolation
because there is no longer gene flow
between the two populations
Reproductive Isolation – results
from barrier to successful breeding between
population groups in the same area LEADS TO…

Sympatric Speciation – occurs when
two subpopulations become
reproductively isolated within the same
geographic area
11.6 Patterns in Evolution
KEY CONCEPT: Evolution occurs in patterns.
I. Evolution through natural selection is not random.
A. The environment controls the direction of natural selection
B.
When the environment changes, different traits may become advantageous
C.
Convergent Evolution – evolution toward similar characteristics in unrelated species (ex. any
organisms showing analogous structures)
D. Divergent Evolution – when closely related species evolve in different directions, are closely related
but have
different
appearances because of adapting to different environments
II. Species can shape each other over time.
A. Coevolution – the process in which two or more species evolve in response to
changes in each other
III. Species can become extinct.
A. Extinction – the elimination of a species from Earth
B.
Often occurs when a species as a whole is unable to adapt to a change in its environment
C.
May occur as result of a change in the environment, the introduction of a new predator, a decrease in
food supply
IV. Speciation often occurs in patterns
A. Punctuated Equilibrium - bursts of evolutionary activity are followed by long periods of stability
B.
Gradualism – the idea that speciation occurs at a regular, gradual rate
C.
Adaptive Radiation – the diversification of once
ancestral species into many descendant species