Chapter 17:
Processes of Evolution
Unit 6: Evolution
Case Study: Rise of the Super Rats
 Rats are one of the most notorious pests
of all time
 Cost us about ____________
each year
 Rodenticide warfarin: very effective
when first used
 Rats developed resistance – inherited
gene that made the chemical ineffective
 This is an example of:
__________________________________
Variation in Populations
 _______________
: one group of individuals of
the same species in a specified area
 Species share:

Morphological traits

Physiological traits

Behavior traits
 Populations show variation in traits:

Qualitative: __________________________

Quantitative: _________________________
The Gene Pool
 The genes of a population comprise the
_______ ________, a pool of genetic resources
 Alleles are the primary source of variation in
appearance
 Polymorphism: occurs when genes have three
or more alleles that persist in a population with
a frequency of at least 1%

Example: _______________________
 ________________

: two distinct traits
Example: male / female
Mutations
 Mutations are the source of new alleles
 We can predict average mutation rates

In humans: ________ mutations per person
per generation
 Many mutations give rise to structural,
functional, or behavioral alterations that
reduce an individual’s chance of surviving
and reproducing
Mutations


Mutations can be:

____________ : usually arise from drastic
changes in the phenotype

_____________: alter the base sequence of
DNA but have no effect on survival or
reproduction

______________: enhances the survival or
reproduction – occurs every so often
Natural selection will favor the transmission
of beneficial mutations on to the next
generations
Stability and Change in Allele Frequencies

Researchers typically track _____________
_________________ in populations (how often
a certain allele occurs)

The relative abundance of alleles of a given gene
among all individuals of a population

For a starting point, they use a reference called
genetic equilibrium, when a population is not
evolving with respect to a certain gene

Genetic equilibrium can ONLY be reached
when five conditions are met
Genetic Equilibrium
 Genetic equilibrium can occur only when:
Mutations do not occur
 The population is infinitely large
 The population stays isolated from all others of
the same species
 Mating is random
 All members of the population survive and
produce the same number of offspring

 In nature, _______________________________
Microevolution

If all 5 conditions are never met, change is
occurring within a population

Results in small-scale changes in the population’s
allele frequency; called __________________

Four processes of microevolution:

mutation

natural selection

genetic drift

gene flow
When is a population not evolving?


The Hardy-Weinberg Formula can be used to track
whether a population is in genetic equilibrium or not

A mathematical formula which tracks allele
frequency for a specific trait

Applied rules of probability to sexually reproducing
populations and found that the gene pool can only
be stable when __________________________
Researchers can use the formula to estimate the
frequency of carriers of alleles that cause
genetic traits and disorders
Hardy-Weinberg Genetic Equilibrium

Hardy-Weinberg formula:
p2 + 2 pq + q2 = 1
p+q=1
where p and q are the frequencies of alleles A and a

You can draw this on a Punnett square:
Natural Selection

Natural selection: the differential survival and
reproduction among individuals

Natural selection influences all levels of biological
organization

Selection can be:
Directional Selection

Directional selection occurs when allele
frequencies shift in a _____________________

Forms at one end of the range of phenotypic
variations become more common than the
intermediate forms

Examples:

Peppered Moth, pocket mice (predation)

Resistance to antibiotics
Directional Selection

Butterfly wing color:
medium-blue is between
two phenotypic extremes
(white and dark purple)

Orange arrows identify
which forms are being
selected against over time
Figure 17.5, page 270
Peppered Moth

Peppered moth: a classic example of the
directional selection process

Feed at night and rest on tree trunks during day

Light gray lichens grew on trees

In preindustrial England, light colored moths
were the most common

Due to air pollution starting in 1850s, the
dominant allele shifted to dark colored moths

Air pollution was _________________________
Peppered Moth

Figure
17.6,
pg 271
Since the advent of pollution controls starting in
1950s, allele frequency has begun to shift back to
light colored moths
Pocket Mice
Figure
17.7,
pg 271

Rock pocket mice in Arizona are another example of
directional selection

Light brown granite; dark basalt (lava rock)
Resistance to Antibiotics

Human activity can also influence directional selection

Antibiotics: toxins that kill bacteria by interfering in
physiological processes

Since 1940s, have been widely prescribed in the U.S.

Overuse of antibiotics puts tremendous selection
pressure on bacteria

Bacteria divide quickly and form huge populations with
great genetic variation

E. coli can divide every _________________

Likely that some will survive antibiotic treatment

Resistant strains are becoming the norm
Selection Against or in Favor of
Extreme Phenotypes
 Stabilizing Selection: works
against phenotypes at the fringes
of a range
 Disruptive Selection: favors
phenotypes at the fringes of a
range
Stabilizing Selection

Stabilizing selection: intermediate forms of a
trait in a population are favored

Does NOT _______________________________

Examples:

Birth weight for babies – mid-sized babies
selected for most often

Body mass size of sociable weaver birds

Body mass represents a trade-off
between risks of starvation and predation
Stabilizing Selection
Stabilizing eliminates
the ______________
Figure 17.8a, pg 272
Disruptive Selection

Disruptive selection: forms at both ends of a range
of variation are favored

Intermediate forms are ______________________

Example: selection for bill size in black-bellied
seedcracker finch in Africa

Females and males have either large or small bills
(no intermediate ones)

2 different types of plants: hard or soft seeds

Feeding performance maintains this
dimorphism of beak size
Disruptive Selection
Disruptive selection eliminates
_______________________
Figure 17.8b, pg 272
Modes of Natural Selection
Fig 17.4,
page 269