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BIOLOGY
Chapter 19 THE EVOLUTION OF POPULATIONS
19.1 Population Evolution
19.2 Population Genetics
19.3 Adaptive Evolution
FIGURE 19.1
Living things may be single-celled or complex, multicellular
organisms. They may be plants, animals, fungi, bacteria, or archaea.
This diversity results from evolution.
POPULATION EVOLUTION
• Mechanisms of inheritance were not
understood when Darwin & Wallace
developed their ideas of natural selection.
• Modern synthesis – relationship between
natural selection and genetics (1940s)
• Microevolution – change of a population
over time
• Macroevolution – processes that gave
rise to new species & higher taxonomic
groups with divergent characteristics
POPULATION GENETICS
• The study of how selective forces change a
population through changes in allele & genotypic
frequencies.
• Allele frequency/gene frequency – the rate at which
a specific allele appears within a population
• Change in frequency = evolution
• Gene pool – the sum of all alleles in a population
• Genetic drift – random change in gene pool with no
advantage
• Founder effect – the event that starts change in
allele frequencies
HARDY-WEINBERG PRINCIPLE OF EQUILIBRIUM
• Developed in the early 20th century by an
English mathematician, Hardy, & German
physician, Weinberg.
• A population’s allele & genotype frequencies
are stable unless some kind of evolutionary
force is acting upon the population.
• Assumptions: NO mutations, migrations,
emigration, or selective pressure for or against
genotype, plus an infinite population
• Gives us a mathematical model to estimate the
alleles or genotypes in a stable population and
compare to real population
HARDY-WEINBERG PRINCIPLE OF EQUILIBRIUM
• Different alleles = different variables p & q
• If only 2 alleles the p + q = 1
• More interested in frequencies of
genotypes, not alleles – the population’s
genetic structure
• By observing phenotypes, can only know
the genotype of the homozygous recessive
individuals, so calculations can provide an
estimate of remaining genotypes:
• p2 + 2pq + q2 = 1
FIGURE 19.2
When populations are in the
Hardy-Weinberg equilibrium, the
allelic frequency is stable from
generation to generation and the
distribution of alleles can be
determined from the HardyWeinberg equation.
If the allelic frequency measured
in the field differs from the
predicted value, scientists can
make inferences about what
evolutionary forces are at play.
19.2 POPULATION GENETICS
• Distribution of phenotypes among
individuals = population variation
• Influenced by several factors:
• Genetic Variance – diversity of alleles
and genotypes within a population,
inbreeding
FIGURE 19.3
The distribution of phenotypes in this litter of kittens illustrates population variation.
(credit: Pieter Lanser)
FIGURE 19.4
Genetic drift in a population can lead to
the elimination of an allele from a
population by chance.
Small populations are more susceptible
to forces of genetic drift.
In this example, rabbits with the brown
coat color allele (B) are dominant over
rabbits with the white coat color allele (b).
In the first generation, the two alleles
occur with equal frequency in the
population, resulting in p and q values of
.5. Only half of the individuals reproduce,
resulting in a second generation with p
and q values of .7 and .3, respectively.
Only two individuals in the second
generation reproduce, and by chance
these individuals are homozygous
dominant for brown coat color. As a
result, in the third generation the
recessive b allele is lost.
FIGURE 19.5
A chance event or catastrophe can reduce the genetic variability
within a population.
Magnifies genetic drift by drastically reducing population size – ex.
Tornado or hurricane
FIGURE 19.6
Gene flow can occur when an individual travels from one
geographic location to another. Introduces new genes into
a population.
19.2 POPULATION GENETICS
• Mutations – changes in organisms DNA,
drive diversity, accumulates in population
over time.
• Nonrandom Mating –
• Simple mate choice – females prefer
certain male characteristics
• Assortative mating – preference to mate
with phenotypically similar individuals
• Physical location can influence
nonrandom mating
ENVIRONMENTAL VARIANCE - FIGURE 19.7
• Some species – gender is determined by
environment – temperature-dependent sex
determination (TSD)
• Geographical variation – cline, variation
across an ecological gradient
The sex of the American alligator (Alligator
mississippiensis) is determined by the temperature at
which the eggs are incubated. Eggs incubated at 30°C
produce females, and eggs incubated at 33°C produce
males. (credit: Steve Hillebrand, USFWS)
ENVIRONMENTAL VARIANCE - FIGURE 19.7
• Some species – gender is determined by
environment – temperature-dependent sex
determination (TSD)
• Geographical variation – cline, variation
across an ecological gradient
The sex of the American alligator (Alligator
mississippiensis) is determined by the temperature at
which the eggs are incubated. Eggs incubated at 30°C
produce females, and eggs incubated at 33°C produce
males. (credit: Steve Hillebrand, USFWS)
19.3 ADAPTIVE EVOLUTION
• Selecting for beneficial alleles (increasing
frequency) and against deleterious alleles
(decreasing frequency)
• But works on organism as a whole – so
evolutionary fitness selects for individuals
with greater contributions to the gene pool
of the next generation.
• Relative fitness measures how an individual
compares to the others in the population
FIGURE 19.8
Figure 19.8 Different types of
natural selection can impact the
distribution of phenotypes within
a population.
In (a) stabilizing selection, an
average phenotype is favored.
In (b) directional selection, a
change in the environment shifts
the spectrum of phenotypes
observed.
In (c) diversifying selection, two
or more extreme phenotypes are
selected for, while the average
phenotype is selected against.
FREQUENCY-DEPENDENT SELECTION
FIGURE 19.9
-
Favors phenotypes that are
either common (positive) or
rare (negative)
-
Negative increases genetic
variance & positive decreases
it
-
Like a game of rock-paperscissors, orange beats blue,
blue beats yellow, & yellow
beats orange
A yellow-throated side-blotched
lizard is smaller than either the bluethroated or orange-throated males
and appears a bit like the females of
the species, allowing it to sneak
copulations. (credit:
“tinyfroglet”/Flickr)
FIGURE 19.10
Sexual dimorphism is when the male and the female of the same species look completely different.
One gender selects mates from the other based on certain traits.
(a) peacocks and peahens,
(b) Argiope appensa spiders (the female spider is the large one)
(c) wood ducks.
Handicap principle – large appendages or bright colors carry risks, survive risks = better fitness
Good genes hypothesis – bright colors or ornaments indicates better ability to fight off disease or
higher metabolisms, so they have better fitness
NO PERFECT ORGANISM
• Natural selection is the driving force in evolution and can generate
populations that are better adapted to survive and successfully reproduce
in their environments.
• CAN NOT produce the perfect organism – only select on existing
variation in a population
• Limited by existing genetic variance & new alleles that arise from
mutations and gene flow
• Limited because it works at the level of the individual not the alleles – this
could link some beneficial alleles to some detrimental ones – looks at the
net affect of the combinations on the individual as a whole
• Constrained by relationships between different polymorphisms.
• Not all evolution is adaptive – natural selection selects fittest individuals,
but gene flow and genetic drift do the opposite
• Evolution has no purpose – it is the sum of various forces influences the
genes of a population
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