2. 2
• All life on Earth is related.
• Humans, beetles, plants, and bacteria all share a common ancestor.
• Natural selection acts to promote traits and behaviors that increase
an organism’s chances of survival and reproduction.
• Mutations and other sources of variation among individuals, as well
as the evolutionary forces that act upon them, alter populations and
species.
Introduction
3. 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. (credit “wolf”: modification of work by Gary Kramer; credit “coral”:
modification of work by William Harrigan, NOAA; credit “river”: modification of work by Vojtěch Dostál; credit “fish"
modification of work by Christian Mehlführer; credit “mushroom”: modification of work by Cory Zanker; credit “tree”:
modification of work by Joseph Kranak; credit “bee”: modification of work by Cory Zanker)
4. 19.1: Population Evolution
Population genetics
• Study of allele frequencies in a population, (i.e. studying if they are
changing from generation to generation, or not)
• Evolution and Flu Vaccines
• ABO blood type system
• Evolution is a significant change in allele frequencies of a
population over generations
• Equilibrium is no significant change in allele frequencies of a
population over generations
• Gene pool is the sum of all the alleles in a population.
5. Hardy–Weinberg Principle of Equilibrium
• States that population in genetic equilibrium will
maintain consistent allele frequencies from one
generation to the next.
• Genetic equilibrium relies on maintenance of
these criteria:
1. No mutation takes place
2. No genes are transferred to or from other sources
(no immigration or emigration)
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
5
6. The HW Principle can be written as an equation:
p + q = 1
• For 2 alleles, p and q
p = frequency of allele B for black coat color
• Black cat is BB or Bb
q = frequency of allele b for white coat color
• White cat is bb
and random mating in that population leads to the next
generation, such that:
p2 + 2pq + q2 = 1
6
8. • If all 5 assumptions for Hardy-Weinberg
equilibrium are true, allele and genotype
frequencies do not change from one generation to
the next
• In reality, most populations will not meet all 5
assumptions
• To determine this, look for changes in frequency from
one generation to the next, and then suggest
hypotheses about what process or processes are most
likely at work to cause changes to the frequencies (i.e.
try to figure out what is causing evolution)
8
10. 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 Hardy-
Weinberg 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.
• https://www.youtube.com/watch?v=oG7
ob-MtO8c
11. 19.2: Population Genetics
• Population variation is the distribution of phenotypes (physical traits)
among individuals.
• Influenced by a number of factors, genetic structure and environment (Figure
19.3).
• Polymorphism is when individuals of a population display different
phenotypes, or express different alleles of a particular gene (i.e. fur color).
12. Figure 19.3
• The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)
13. Genetic Variance
• Natural selection and some other evolutionary forces can only act
upon heritable traits (an organism’s genetic code).
• Alleles are passed from parent to offspring.
• Acquired traits are not heritable.
• For example: If an athlete works out in the gym daily, building muscle strength, would
the athletes offspring grow up to be a body builder?
• Heritability is the fraction of phenotype variation that can be attributed to genetic
differences, or genetic variance among individuals in a population.
• Genetic variance is the diversity of alleles and genotypes within a population.
• When scientist breed species, such as zoo animals, they try to increase a
population’s genetic variance to preserve that particular phenotype.
• Inbreeding is mating of closely related individuals which can cause abnormalities and
susceptibility to disease.
15. Figure 19.4
• Genetic drift in a population can lead to
the elimination of an allele from a
population by chance. 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.
16. 16
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17. Figure 19.5
• A chance event or catastrophe can reduce the genetic variability within a population.
• Genetic drift can lead to the loss of alleles in isolated populations.
• Alleles that initially are uncommon are particularly vulnerable.
19. Bottleneck Effect
• Can occur when a population’s size is drastically
reduced (often due to human activity or natural
disaster)
• Survivors may constitute a random genetic
sample of the original population
• Results in loss of genetic variability (loss of
alleles) which cannot be regained simply by
increased breeding of the survivors
19
21. Figure 19.6
• Gene flow occurs when an individual travels from one geographic location to
another.
22. Other Evolutionary Forces
• Mutations are changes to an organism’s DNA and are an important
drive of diversity in populations.
• Nonrandom mating: if individuals nonrandomly mate with their
peers, the result can change the population.
• i.e. female peahens may prefer peacocks with bigger, brighter tails.
• Assortative mating: an individual’s preference to mate with partners
who are phenotypically similar to themselves.
• Environmental variance: have an influence on phenotypes.
• Sun exposure (skin color), temperature-dependent sex determination (TSD) in
some turtles and other reptiles (Figure 19.7).
23. 19.3: Adaptive Evolution
• There are various ways in which natural selection can shape
populations.
• Adaptive evolution: when natural selection only acts upon heritable traits:
selecting for beneficial alleles and therefore increasing their frequency in the
population and decreasing the harmful alleles frequency.
• Evolutionary (Darwinian) fitness: selection for individuals with greater
contributions to the gene pool of the next generation.
• Natural selection acts at the level of the individual.
24. Fitness and its measurement
• Fitness
• Individuals with the highest fitness leave more surviving offspring
in the next generation than individuals with less fitness
• Fitness has many components
1. Survival
2. Sexual selection – some individuals are more successful at attracting mates
3. Number of offspring per mating
24
28. 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.
31. Scenario: Only large and small seeds are available for food.
• Birds with intermediate-sized beaks are at a disadvantage with both
seed types – they are unable to open large seeds and too clumsy to
efficiently process small seeds
Disruptive, stabilizing, or directional?
31
32. • Occurs when the fitness of a phenotype depends on
its frequency within the population
• Positive frequency-dependent selection
• Favors the most common form
• Tends to eliminate variation because “oddballs” stand
out (and get eaten!)
• Negative frequency-dependent selection
• Rare phenotypes are favored by selection
• Rare forms may not be in “search image” of the predator,
so they are overlooked.
32
“Frequency-dependent selection”
33. Figure 19.9
• A yellow-throated side-blotched lizard is
smaller than either the blue-throated or
orange-throated males and appears a bit
like the females of the species, allowing it
to sneak copulations. (credit:
“tinyfroglet”/Flickr)
34. 34
Positive frequency-
dependent selection:
• Predator notices the
unusual phenotype more
• Common phenotype has
the survival advantage
dark brown
medium brown
light brown
PercentofColorType
TakenbyFishPredators
20 40 60 80 100
20
40
60
80
100
Color Type Frequency in Population
Color type of
water boatman
Negative frequency-
dependent selection
• Predator prefers to eat the most
common color of “water
boatman” (insect)
• Common phenotype has a
survival disadvantage
35. • Oscillating selection
• Selection favors one phenotype at one time
and another phenotype at another time
• The overall effect will be to maintain genetic
variation in the population
• Medium ground finch of Galápagos Islands
• Birds with big bills favored during drought (only big
seeds available)
• Birds with smaller bills favored in wet conditions
(smaller seeds in abundance)
35
36. Sexual Selection
• Sexual dimorphism: when males and females of certain species are
often quite different from one another in ways beyond the
reproductive organs.
• i.e. males are often larger, and display many elaborate colors and
adornments, like the peacock’s tail, while females tend to be smaller and
duller in decoration. (Figure 19.10).
37. Figure 19.10
• Sexual dimorphism is observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one),
and in (c) wood ducks. (credit “spiders”: modification of work by “Sanba38”/Wikimedia Commons; credit “duck”:
modification of work by Kevin Cole)
