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CHAPTER 23
THE EVOLUTION OF
POPULATIONS
I. POPULATION GENETICS
A.


THE MODERN EVOLUTIONARY SYNTHESIS INTEGRATED
DARWINIAN SELECTION AND MENDELIAN INHERITANCE
THE DEVELOPMENT POPULATION GENETICS, WITH ITS
EMPHASES ON QUANTITATIVE INHERIANCE AND VARIATION,
BROUGHT DARWINIAN THEORY AND MENDELIAN PRINCIPLES
OF INHERITANCE TOGETHER.
THE MODERN SYNTHESIS FOCUSES POPULATIONS AS UNITS
OF EVOLUTION.
B. THE GENETIC STRUCTURE OF A POPULATION IS
DEFINED BY ITS ALLELE AND GENOTYPE FREQUENCIES
A
POPULATION, A LOCALIZED OF
ORGANISMS BELONGING TO THE
SAME SPECIES, IS UNITED BY ITS
POOL, THE AGGREGATE OF ALL
ALLELES IN THE POPULATION.
C. THE HARDY-WEINBERG THEOREM DESCRIBES A
NONEVOLVING POPULATION


ACCORDING TO THE HARDY-WEINBERG THEOREM, THE
FREQUENCIES OF ALLELES IN A POPULATION WILL
REMAIN CONSTANT IF SEXUAL REPRODUCTION IS THE
ONLY PROCESS THAT AFFECTS THE GENE POOL.
IF P AND Q REPRESENT THE RELATIVE FREQUENCIES OF
THE DOMINANT RECESSIVE ALLELES OF A TWO-ALLELE
LOCUS, RESPECTIVELY, THEN P^2 + 2PQ + Q^2 = 1, WHERE
P^2 AND Q^2 ARE THE FREQUENCIES OF THE
HOMOZYGOUS GENOTYPES, AND 2PQ IS THE FREQUENCY
OF THE HETEROZYGOUS GENOTYPE.
23.3 HARDY-WEINBERG THEOREM
II. CAUSES OF MICROEVOLUTION
(CHANGES IN ALLELE FREQUENCIES)
A.

MICROEVOLUTION IS A GENERATION–TO–GENERATION
CHANGE IN A POPULATION’S ALLELE OR GENOTYPE
FREQUENCIES
FOR HARDY-WEINBERG THEOREM TO APPLY, THE POPULATION
MUST BE :
1.
2.
3.
4.
5.

VERY LARGE
BE TOTALLY ISOLATED
HAVE NO NET MUTATIONS
SHOW RANDOM MATING
HAVE REPRODUCTIVE SUCCESS FOR ALL INDIVIDUALS
MICROEVOLUTION CAN OCCUR WHEN ONE OR MORE OF THE
CONDITIONS REQUIRED FOR HARDY-WEINBERG EQUILIBRIUM ARE
NOT MET.
B. THE FIVE CAUSES OF MICROEVOLUTION ARE
GENETIC DRIFT, GENE FLOW MUTATION, NONRANDOM
MATING, AND NATURAL SELECTION

NATURAL SELECTION: THE INCREASE IN ALLELE FREQUENCIES
DUE TO THE IMPACT OF THE ENVIRONMENT
MUTATIONS: RANDOM CHANGE IN DNA SEQUENCE (+/- OR 0)
 NON-RANDOM MATING: OCCURS WHEN INDIVIDUALS CHOOSE MATES
BASED UPON THEIR PARTICULAR TRAITS.

INBREEDING: INDIVIDUALS MATE WITH RELATIVES
SEXUAL SELECTION: FEMALES CHOOSE MALES BASED UPON THEIR ATTRACTIVE
APPEARANCE OR BEHAVIORAL PATTERNS
GENE FLOW: INTRODUCTION OR REMOVAL OF ALLELES FROM THE
POPULATION WHEN INDIVIDUALS LEAVE (EMIGRATION) OR ENTER
(IMMIGRATION)THE POPULATION
 GENETIC DRIFT: RANDOM INCREASE OR DECREASE OF ALLELES

FOUNDER EFFECT: WHEN ALLELE FREQ. IN A GROUP OF MIGRATING
INDIVIDUALS ARE, BY CHANCE, NOT THE SAME AS THAT OF THEIR POPULATION
OF ORIGIN.
BOTTLENECK: WHEN THE POPULATION UNDERGOES A DRAMATIC DECREASE
IN SIZE.
23.4 GENETIC DRIFT: RANDOM INCREASE OR DECREASE IN
ALLELES
23.5 BOTTLENECK EFFECT
III. GENETIC VARIATION, THE SUBSTRATE FOR NATURAL
SELECTION
A. GENETIC VARIATION OCCURS WITHIN AND BETWEEN
POPULATIONS
 GENETIC VARIATION INCLUDES INDIVIDUAL VARIATION IN
DISCRETE AND QUANTITATIVE CHARACTERS WITHIN A
POPULATION, AS WELL AS PHYSICAL VARIATION BETWEEN
POPULATIONS.
B. MUTATION AND SEXUAL RECOMBINATION GENERATE
GENETIC VARIATION



MOST MUTATIONS HAVE NO EFFECT OR ARE HARMFUL,
BUT ARE ADAPTIVE. SEXUAL RECOMBINATION PRODUCES
MOST OF THE VARIATION THAT MAKES ADAPTATION
POSSIBLE IN POPULATIONS OF REPRODUCING ORGANISMS.
MUTATIONS: RANDOM CHANGE IN DNA SEQUENCE
SEXUAL REPRODUCTION: GENETIC RECOMBINATION
CROSSING OVER
INDEPENDENT ASSORTMENT
RANDOM JOINING OF GAMETES
OUTBREEDING (MATING WITH UNRELATED PARTNERS) ALSO
INCREASES VARIATION
C.
DIPLOIDY AND BALANCED POLYMORPHISM PRESERVE
VARIATION



DIPLOIDY: PRESENCE OF TWO COPIES OF EACH
CHROMOSOME (2N)
DIPLOIDY MAINTAINS A RESERVOIR OF LATENT VARIATION
IN HETEROZYGOTES.
BALANCED POLYMORPHISM MAY MAINTAIN VARIATION AT
GENE LOCI AS A RESULT OF HETEROZYGOTE ADVANTAGE
OR FREQUENCY DEVELOPMENT SELECTION.
HETEROZYGOUS ADVANTAGE- SOMETIMES CARRYING A
TRAIT, BUT NOT EXPRESSING IT IS ADVANTAGEOUS
FREQUENCY DEPENDENT SELECTION- WHEN THE LEAST
COMMON PHENOTYPE HAVE A SELECTIVE ADVANTAGE.

SOME GENETIC VARIATION MAY BE UNAFFECTED BY
NATURAL SELECTION.
IV. NATURAL SELECTION AS THE MECHANISMS OF
ADAPTIVE EVOLUTION
A.




EVOLUTIONARY FITNESS IS THE RELATIVE CONTRIBUTION
AN INDIVIDUAL MAKES TO THE GENE POOL OF THE NEXT
GENERATION
DARWINIAN IS MEASURED ONLY BY REPRODUCTIVE SUCCESS.
ONE GENOTYPE HAS GREATER RELATIVE FITNESS THAN
ANOTHER IF IT LEAVES MORE DESCENDANTS.
SELECTION FAVORS CERTAIN GENOTYPES IN A POPULATION BY
ACTING ON THE PHENOTYPE OF INDIVIDUAL ORGANISMS.
THE WHOLE ORGANISM IS THE OBJECT OF SELECTION
B. THE EFFECT OF SELECTION ON A VARYING CHARACTERISTIC
CAN BE STABILIZING, DIRECTIONAL, OR DIVERSIFYING

NATURAL SELECTION CAN –
ACT AGAINST EXTREME PHENOTYPES (STABILIZING
SELECTION),
FAVOR RELATIVELY RARE INDIVIDUALS ON ONE END
OF THE PHENOTYPIC RANGE (DIRECTIONAL
SELECTION),
OR FAVOR INDIVIDUALS AT BOTH EXTREMES OF THE
RANGE OVER INTERMEDIATE PHENOTYPES
(DIVERSIFYING SELECTION).
23.11 MODES OF SELECTION
C.
SEXUAL SELECTION MAY LEAD TO PRONOUNCED
SECONDARY DIFFERENCES BETWEEN THE SEXES

SEXUAL SELECTION LEADS TO THE EVOLUTION OF
SECONDARY SEX CHARACTERISTICS, WHICH CAN GIVE
INDIVIDUALS AN ADVANTAGE IN MATING.
MALE COMPETITION: AWARD THE STRONGEST MALES
– EX. EVOLUTION OF HORNS, ANTLERS, MUSCLES
FEMALE CHOICE: TRAITS OR BEHAVIORS IN MALES THAT ARE
ATTRACTIVE TO FEMALES
– EX. COLORFUL BIRD PLUMAGE, MATING SONGS, MATING RITUALS
D. NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS
 THE REASONS ARE:
STRUCTURES RESULT FROM MODIFIED
ANCESTRAL ANATOMY,
ADAPTATIONS ARE OFTEN COMPROMISES
THE GENE POOL CAN BE AFFECTED BY
GENETIC DRIFT
AND NATURAL SELECTION CAN ACT ONLY ON
AVAILABLE VARIATION.