Chapter 16: Evolution of Populations and Speciation

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Chapter 16 Evolution of Populations and
Speciation
16-1 Variation of Traits in a Population
16-2 Disruption of Genetic Equilibrium
16-3 Formation of Species
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16-1 Genetic Equilibrium
I. Variation of Traits in a Population

• Variation ALLOWS for natural selection to act
  upon a population.

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(1) Population Genetics (population is SMALLEST
unit capable of evolving)

• Study of evolution from a GENETIC perspective
  (i.e., gene pool changes).

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(2) Bell Curve (within a LARGE sample population)

• Measurable TRAITS display a BELL-CURVE pattern
  (i.e., MEDIAN
  form is FAVORED).

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(A) Causes of Genotypic Variation in a Population

• Genetics CAN increase VARIATION in a population
  in THREE ways

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(1) MUTATION ? from mutant copies of individual
GENES.
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(2) RECOMBINATION AND CROSSING-OVER during
meiosis add variation to sex cells.
NOTE On average, between 2-3 crossovers occur
on each pair of chromosomes during meiosis.
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(3) RANDOM fusion of GAMETES ensure offspring are
NOT clones of parents.
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II. Allele Frequencies and the Gene Pool

• Some alleles are MORE NUMEROUS than other
  alleles in gene pools.

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(1) Gene Pool

• TOTAL genes AVAILABLE in a population of a
  species (a dynamic NOT static pool of genes).
• NOTE SUM of ALL changes in a GENE POOL over a
  LONG period of TIME is known as EVOLUTION.

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(2) Allele Frequency (a calculation)

• Frequency of an ALLELE among ALL alleles in a
  POPULATION. (e.g.,
  PREDICT the likelihood of a certain PHENOTYPE
  being expressed).

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(A) Predicting Phenotype

• Certain PHENOTYPES are EXPRESSED more often than
  OTHER TYPES.

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(1) Phenotypic Frequency

• Frequency of a SPECIFIED phenotype within an
  entire POPULATION.

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Critical Thinking
(1) Do you think it is easier to analyze genotype
in organisms with complete dominance or in
organisms with incomplete dominance?
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III. Hardy-Weinberg Genetic Equilibrium (i.e., a
theoretical state)

• A model showing HOW allele frequencies in a
  population CAN remain CONSTANT. (i.e.,
  evolution does NOT occur).

(1) NO mutations occur (NO allelic frequency
change).
(2) Individuals neither ENTER nor LEAVE the
population.
(3) Population is LARGE (ideally, infinitely
large)
(4) Individuals mate RANDOMLY.
(5) Natural selection does NOT occur.
NOTE This model ALLOWS us to consider what
forces DISRUPT genetic equilibrium in a
population (i.e., allow a POPULATION to EVOLVE).
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Sample Problem Using the Hardy-Weinberg
equation of
p2 2pq q2 1 (p dominant
allele) (q recessive allele)
Calculate the expected number of heterozygotes
(Aa) and homozygous dominant (AA) individuals in
a population of 100 tigers, 10 of which are
albino white (homozygous recessive, aa).
q2 (10/100) 0. 1
Therefore q 0.32
p 1-q or 0.68
Therefore, the frequency of homozygous dominant
individuals is p2 or 0.46, or approximately 46
And the frequency of heterozygous individuals is
2pq or 0.44, or approximately 44
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16-2 Disruption of Genetic Equilibrium

• A VIOLATION of Hardy-Weinberg equilibrium can
  result in EVOLUTION (i.e., FIVE evolutionary
  forces).

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I. Mutation (introduces VARIATION, disrupts
equilibrium)

• Mutations produce NEW alleles INTO a population.

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II. Migration (disrupts equilibrium)

• REMOVES or ADDS new members to a gene pool
  (i.e., new alleles/genes).

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(1) Immigration (WIDENS gene pool)

• MOVEMENT of NEW genes INTO a population.

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(2) Emigration (SHRINKS gene pool)

• Movement of genes OUT of a population.

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(3) Gene Flow (FLUIDITY of the waves in the gene
pool)

• NET movement of genes BETWEEN populations of
  SAME species.

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III. Genetic Drift (MORE likely experienced in
SMALL populations)

• CHANGE in allelic frequencies as a result of
  RANDOM events (chance).

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IV. Nonrandom Mating (includes Assortative Mating)

• Results from GEOGRAPHIC PROXIMITY and a SUITABLE
  MATE (negative drawback, inbreeding).

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(1) Assortative Mating (e.g., NON-random mating)

• Selection of a MATE with SIMILAR physical traits
  (to ITSELF).

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V. Natural Selection (selective forces of NATURE
for OR against)

• An ONGOING process that DISRUPTS genetic
  equilibrium.
• (NOTE 4 types of natural selection include)

(A) Stabilizing Selection
(B) Directional Selection
(C) Disruptive Selection
(D) Sexual Selection
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(A) STABILIZING Selection (selects AGAINST the
extremes)

• Individuals with AVERAGE form of a trait have
  the HIGHEST fitness (i.e., Selected FOR by
  nature)

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(B) DIRECTIONAL Selection (selection is SHIFTED
one side)

• Individuals with a MORE EXTREME form of a trait
  (one extreme OR the other, NOT BOTH) have
  GREATEST fitness.

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(C) DISRUPTIVE Selection (mean is selected
AGAINST)

• Individuals with EITHER extreme variation have
  GREATEST fitness.

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Critical Thinking
(2) Human newborns with either a very high or
very low birth weight are more likely to die in
infancy. What type of selection does this seem
to be?
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(D) Sexual Selection (survival ALONE does NOT
further evolution)

• GENES of REPRODUCERS, rather than those of
  merely SURVIVORS, are FAVORED through natural
  selection.

NOTE The development of trait that may SEEM
harmful (coloration patterns) can enhance the
reproductive fitness if it encourages MATING.
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16-3 Formation of Species
I. The Concept of Species

• Existing species are MODIFIED versions of
  ANCESTRAL species.

NOTE As species EVOLVE in the world, it has
been a challenge to exactly DEFINE a species.
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(1) Speciation (an evolutionary processforming a
NEW species)

• Members become ISOLATED (over time) AND evolve
  into a NEW species (Allopatric and Sympatric
  Speciation).

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Critical Thinking
(3) What effect might a very short generation
time, such as that of bacteria, have on
speciation?
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(A) Morphological Concept of Species

• DEFINED on evidence of STRUCTURE and APPEARANCE
  (i.e., its morphology).

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(1) Morphology

• Structure AND appearance of an organismusing as
  SPECIES identifier has 2 LIMITATIONS

• Phenotypic variation AMONG members of a single
  population, and
• (2) Interbreeding of different species
  producing FERTILE offspring.

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(B) The Biological Species Concept (Ernst Mayr)

• A population of INTERBREEDING organisms
  producing fertile offspring BUT cannot breed with
  OTHER groups.

NOTE Limitations include our LACK of ability to
classify FOSSILS as species (unable to determine
EXTINCT reproductive compatibilities) as well as
with ASEXUAL organisms.
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II. Isolating Mechanisms

• Speciation begins with ISOLATIONTWO TYPES of
  isolation DRIVE speciation.

(A) Geographic Isolation (Allopatric Speciation)
(B) Reproductive Isolation (Sympatric Speciation)
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(A) Geographic Isolation

• PHYSICAL separation due to geographical barriers
  (can lead to DIVERGENT evolution)

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(B) Reproductive Isolation (TWO types FOLLOWS
geographical isolation)

• Genetic BARRIERS between members DUE to
  mutations OR incompatible recombinations (TAKES
  TIME).

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(1) Prezygotic Isolation (FIRST type of
reproductive isolation)

• Reproductive isolation that occurs BEFORE
  fertilization of members of TWO populations.
• (Ex Incompatible MATING CALLS
  in wood frogs)

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(2) Postzygotic Isolation (SECOND type of
reproductive isolation)

• Reproductive isolation that occurs AFTER
  fertilization. (Ex
  Offspring are born STERILE, never develop, OR die
  early).

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Critical Thinking
(4) Where populations of two related species of
frogs overlap geographically, their mating calls
are different. Where the species dont overlap,
their calls are identical. What type of
isolating mechanism is in operation?
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III. Rates of Speciation (2 Theories)

• MILLIONS of years NEEDED for speciation,
  HOWEVER, NEW species have formed within a few
  THOUSAND years.

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Critical Thinking
(5) Highways may provide an effective geographic
isolation mechanism for some slow-moving animals.
Why are such artificial barriers not likely to
result in complete speciation?
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(1) Punctuated Equilibrium (theory of speciation)

• SHORT QUICK bursts of RAPID genetic change
  followed by LONGER periods of LITTLE to NO change.

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(2) Gradualism (Darwins belief of speciation)

• Occurs over a LONGER period of a million years
  or so, resulting in SMALLER, GRADUAL genetic
  changes at a STEADY pace.

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Extra Slides AND Answers for Critical Thinking
Questions
(1) Incomplete dominance is easier because
heterozygotes have a different phenotype from
homozygous dominant individuals.
(2)This is an example of stabilizing selection
because the intermediate phenotype is favored.
(3) A very short generation time speeds up the
rate of evolution and therefore can increase the
rate of speciation.
(4) This is an example of a prezygotic isolating
mechanismit prevents mating between the two
species.
(5) Such artificial barriers are unlikely to be
in place long enough for speciation to occur.
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Revisiting Reproduction and Inheritance

• The distribution of the genes in successive
  generations is determined by the characteristics
  of the genes and by the interaction between genes
  and environment.

Assessing Prior Knowledge

• In tiger populations, a recessive allele causes
  an absence of fur pigmentation, producing an
  albino or white tiger with pink eyes. What
  genotype would an orange, heterozygous tiger have?

• Would you suggest the white phenotype in tigers
  to be selected FOR or selected AGAINST by natural
  selection?

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