Chapter 16 The Evolution of Populations and Species
1. Explain the difference between the morphological concept of species and the biological species
concept.
A. Morphological concept of species
1. For many years, scientists used the internal and external structure and appearance of
an organism—its morphology—as the chief criterion for classifying it as a species
2. The morphological concept of species has limitations
a. There can be phenotypic differences among individuals in a single population
b. Some organisms that appear different enough to belong to different species can
interbreed in the wild and produce fertile offspring
B. The Biological Species Concept
1. According to the biological species concept, as proposed by German-born, American
biologist Ernst Mayr (1904–), a species is a population of organisms that can
successfully interbreed but cannot breed with other groups
2. While this definition is useful for living animals, it does not provide a satisfactory
definition for species of extinct organisms, whose reproductive compatibility cannot
be tested nor is it useful for organisms that reproduce asexually
a. Thus, our modern definition of species includes components of both the
morphological and biological species concepts
1) A species is a single type of organism
2) Members of a species are morphologically similar and can interbreed to
produce fully fertile offspring
3) The many species alive today diverged from a smaller number of earlier species
2. Define geographic isolation, and explain how it can lead to speciation.
A. Speciation – the formation of species
B. Geographic isolation is the physical separation of members of a population
1. Populations may be physically separated when their original habitat becomes
divided
a. A deep canyon could develop, a river could change course, or a drying climate
in a valley could force surviving fragments of an original population into
separate mountain ranges
b. Once the subpopulations become isolated, gene flow between them stops
c. Natural selection and genetic drift cause the two subpopulations to diverge,
eventually making them incompatible for mating
C. Reproductive Isolation
1. Barriers prevent successful breeding between populations in the same area
a. Prezygotic – occurs before fertilization
i. Example: Different mating seasons or mating calls
b. Postzygotic – happens after fertilization
i. Examples: The new offspring is sterile or it dies before it can reproduce
3. Forces That Change Population Allele Frequencies
A. Population genetics – the study of populations from a genetic point of view
1) Within a population individuals may vary in observable traits
a. Some organisms will be on either extreme but most will be in the middle (bell curve)
j. Causes of Variation
1) Environmental pressures
2) Heredity
3) Mutations
4) Recombination
a. Independent assortment
b. Crossing over
5) Random fertilization
k. Natural selection alters the proportions of alleles within populations
2) Allele Frequency and the Gene Pool
a. Gene Pool – total amount of genetic information available in a population
b. Allele frequency – determined by dividing the number of a certain allele by the total
number of alleles in the population
c. Example: there are 2 types of eye color, brown and blue (B and b), in a set of 10 gametes
- What is the allele frequency of B?
3) Populations change in response to evolutionary forces
a. Allele Frequencies
i. G.H. Hardy (English mathematician) and Wilhelm Weinberg
(German physicist)
a) Independently demonstrated that dominant alleles do not automatically
replace recessive allele
b) Use of algebra and theories of probability
c) Showed frequency of alleles in a population and ratio of heterozygote
individuals to homozygote individuals do not change from generation to
generation
(1) Unless the population is acted on by processes that favor a certain allele
(2) For example dominant lethal alleles (the individual dies before it can be
passed on)
(3) Hardy-Weinberg Principle: frequencies of alleles in a population do not
change unless evolutionary forces act on the population
i. Principle holds true if population is:
a. Large enough to not mate with relatives
b. Not acted on by the five evolutionary forces:
i. Mutation
ii. Gene flow – movement of individuals in and out
population = migration
1. Immigration – individuals move into a population
2. Emigration – individuals move out of a population
iii. Nonrandom mating
1. Mate selection is due to geographic proximity
a. Think of interbreeding and the likelihood of
mutations
2. Assortive mating: choosing a mate with similar
characteristics, thus, similar genes
iv. Genetic drift - the phenomenon of changes in allele
frequency in a population because of random events
(chance)
1. Example: in a small population, if 1 individual
doesn’t mate, it disrupts the allele frequency
v. Natural selection
2
2
ii. p +2p+q = 1
a. p2: frequency of individuals that are homozygous for the allele
A
b. 2pq: frequency of individuals that are heterozygous for the
alleles A and a
c. q2: frequency of individuals that are homozygous for the allele a
d. Predicts genotype frequency
e. Phenotype frequency – the number of individuals with a
particular phenotype divided by the total number of individuals
in the population
4. Discuss the types of selection
A. Directional Selection
1) Individuals that display a more extreme form of a trait have greater fitness than
individuals with an average form of the trait
2) Alleles promoting the extreme phenotype become less common in the population
a. Characterizes the evolution of single gene traits
b. For example antibiotic resistance in disease-causing bacteria
B. Stabilizing Selection
1) Individuals with the average form of a trait have the highest fitness - The average
represents the optimum for most traits; extreme forms of most traits confer lower fitness
on the individuals that have them
2) Causes the frequency of the intermediate phenotypes to increase
a. The population has fewer individuals that have alleles promoting extreme types
b. Very common in nature
C. Disruptive selection
1) Individuals with either extreme variation of a trait have greater fitness
than individuals with the average form of the trait
D. Sexual Selection
1) Females tend to choose the males they mate with based on certain traits
Chapter 16 The Evolution of Populations and Speciation
 Biologists study many different traits in populations, such as size and color.
 Traits vary and can be mapped along a bell curve, which shows that most individuals have
average traits, while a few individuals have extreme traits.
 Variations in genotype arise by mutation, recombination, and the random fusion of gametes.
 The total genetic formation available in a population is called the gene pool.
 Allele frequencies in the gene pool do not change unless acted upon by certain forces.
 The Hardy-Weinberg genetic equilibrium, a theoretical model of a population in which no
evolution occurs, tends to maintain the population as it is.
 Evolution can take place if the genetic equilibrium of a population is disrupted.
 Immigration can bring new genes into a population, causing evolution.
 Nonrandom mating can alter the genotypes of a population, but it does not affect allele
frequencies.
 Genetic drift operates in small populations; the contribution or lack of contribution of the genes
of one or a few organisms can change the population’s gene pool significantly.
 Stabilizing selection encourages the formation of average traits.
 Directional selection encourages the formation of more-extreme traits, such as a very long
tongue in anteaters.
 Disruptive selection selects for extreme traits rather than average traits.
 In sexual selection, the development of traits that may seem harmful can actually enhance
reproductive fitness if they encourage mating.
 According to the biological species concept, a species is a population of organisms that can
successfully interbreed and cannot breed with other groups.
 Speciation means species formation, and it always begins with a population that has become
isolated.
 Geographic isolation results from the division of an original population.
 Reproductive isolation results from barriers to successful breeding. Prezygotic isolation occurs
before fertilization. Postzygotic isolation occurs after fertilization and results in wasted gametes.
 Some scientists think that enormous phenotypic changes in species occur in sharp (punctuated)
steps, rather than along a gradual curve, as Darwin proposed.
Vocabulary List
Allele frequency
Assortative mating
Bell curve
Biological species concept
Directional selection
Disruptive selection
Emigration
Equilibrium
Gene flow
Gene pool
Genetic drift
Geographic isolation
Hardy-Weinberg genetic
Immigration
Morphology
Phenotype frequency
Population genetics
Population genetics
Postzygotic isolation
Prezygotic isolation
Punctuated equilibrium
Reproductive isolation
Sexual selection
Speciation
Stabilizing selection