albia dugger miami dade college chapter 17 processes of evolution sections 1-6

Albia Dugger • Miami Dade College

Chapter 17Processes of

EvolutionSections 1-6

17.1 Rise of the Super Rats

• When warfarin was used to control rats, natural selection favored individuals with a mutation in the VKORC1 gene which resulted in warfarin resistance

• When warfarin resistance increased in rat populations, people stopped using warfarin to kill rats

• The frequency of the warfarin-resistance allele in rat populations declined, probably because rats that carry the allele are not as healthy as ones that do not

Rats Infesting Rice Fields in the Philippines

Natural Selection for Resistant Rats

17.2 Individuals Don’t Evolve, Populations Do

• Mutations in individuals are the source of new alleles in a population’s gene pool

• A change in an allele’s frequency in a population is called microevolution

Variation In Populations

• All individuals of a species share certain morphological, physiological, and behavioral traits

• A population is a group of interbreeding individuals of the same species in a specified area

• Individuals of a population with different alleles of shared genes vary in the details of their shared traits

Morphs

• Many traits have two or more distinct forms (morphs)

• A trait with only two forms is dimorphic

• Traits with more than two distinct forms are polymorphic

• Traits that vary continuously among individuals of a population may be influenced by alleles of several genes

Phenotypic Variations

An Evolutionary View of Mutations

• Mutations are the source of new alleles that give rise to differences in details of shared traits

• Lethal mutations result in death

• Neutral mutations have no effect on survival or reproduction

• Beneficial mutations convey an advantage

Table 17-1 p272

The Gene Pool

• Gene pool• All genes found in one population

• Alleles• Different forms of the same gene• Determine genotype and phenotype• Dimorphism and polymorphism

Allele Frequencies and Microevolution

• Allele frequency refers to the relative abundance of a particular allele of a given gene in a population

• Changes in allele frequency of a population (or a species) is called microevolution

• Microevolutionary processes include mutation, natural selection, genetic drift, and gene flow

Take-Home Message:

What mechanisms drive evolution?

• Individuals of a natural population share morphological, physiological, and behavioral traits characteristic of the species

• Different alleles are the basis of differences in the details of a population’s shared traits

• All alleles of all individuals in a population make up the population’s gene pool

• Changes in allele frequency (microevolution) are always occurring in natural populations

17.3 Genetic Equilibrium

• Natural populations are always evolving

• Researchers trace evolution within a population by tracking deviations from a baseline of genetic equilibrium – a theoretical state in which a population is not evolving

• Natural populations are never in genetic equilibrium

Genetic Equilibrium

• Under certain ideal conditions, the frequency of an allele in a sexually reproducing population’s gene pool should remain stable from one generation to the next

• Five conditions required for a genetic equilibrium:• Mutations do not occur• Population is infinitely large• Population is isolated (no gene flow)• Mating is random• All individuals survive and reproduce equally

The Hardy-Weinberg Formula

• The Hardy-Weinberg formula can be used to determine if a population is in genetic equilibrium

p2(BB) + 2pq (Bb) + q2(bb) = 1.0

• The frequency of the dominant allele (B) plus the recessive allele (b) equals 1.0

p + q = 1.0

A Two Allele System

one type of gametes two types of gametes one type of gametes

Predicted Proportions in a Population

Applying the Rule

• A population consists of 1,000 plants: 490 homozygous (BB), 420 heterozygous (Bb), and 90 homozygous (bb)

• Each plant makes two gametes:• All gametes made by BB individuals have the B allele• All gametes made by bb individuals have the b allele• Bb individuals have half B gametes half b gametes

Applying the Rule

• Frequency of the B allele:

p (B) = (980 + 420) ÷ 2,000 = 1,400 ÷ 2,000 = 0.7

• Frequency of the b allele:

q (b) = (180 + 420) ÷ 2,000 = 600 ÷ 2,000 = 0.3

Applying the Rule

• Predicted proportion of individuals in the next generation:

BB (p2) = (0.7)2 = 0.49Bb (2pq) = 2 (0.7 × 0.3) = 0.42bb (q2) = (0.3)2 = 0.09

Real World Situations

• Researchers used genetic equilibrium to determine the carrier frequency of an allele that causes hereditary hemochromatosis (HH)

• The allele’s frequency (q) was found to be 0.14

• Frequency of the normal allele (p) = 1.0 – 0.14 = 0.86

• Carrier frequency (2pq) is calculated at 0.24

Take-Home Message: How do we know when a population is evolving?

• Researchers measure genetic change by comparing it with a theoretical baseline of genetic equilibrium

• Allele frequencies are always changing in natural populations because ideal conditions can never be met

ANIMATED FIGURE: How to find out if a population is evolving

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17.4 Patterns of Natural Selection

• Natural selection occurs in different patterns depending on the organisms involved and their environment

Natural Selection

• Natural selection results from the differential survival and reproduction among individuals of a population that vary in details of their shared traits

• Natural selection occurs in three recognizable patterns depending on the organisms and their environment:• Directional selection• Stabilizing selection• Disruptive selection

Three Modes of Natural Selection

disruptive selection

stabilizing selection

directional selection

population before selection

Take-Home Message: Does evolution occur in recognizable patterns?

• Natural selection, the most influential process of evolution, occurs in patterns that depend on the organisms and their environment

17.5 Directional Selection

• Changing environmental conditions can result in a directional shift in allele frequencies

• Directional selection• Changing environmental conditions can shift allele

frequencies in a consistent direction• Forms of traits at one end of a range of phenotypic

variation become more common

Stepped Art

Range of values for the trait

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ANIMATED FIGURE: Directional selection



Directional Selection in Peppered Moths

• In preindustrial England, most moths were light-colored, and a dominant allele that resulted in darker coloration was rare

• In post-industrial England, pollution from coal-burning factories changed the colors of the forests

• Predatory birds ate more light-colored moths in soot-darkened forests, and more dark-colored moths in clean forests

• Light color is adaptive in areas of low pollution; dark color is adaptive in areas of high pollution

Directional Selection in Peppered Moths

ANIMATED FIGURE: Change in moth population



Directional Selection in Rock-Pocket Mice

• In rock-pocket mice, two alleles of a single gene control coat color

• Night-flying owls are the selective pressure that directionally shifts the allele frequency

• Most of the mice in populations that inhabit dark rock have dark gray coats

• Most of the mice in populations that inhabit light brown rock have light brown coats

Directional Selection in Rock-Pocket Mice

Directional Selection in Antibiotic Resistant Bacteria

• A typical two-week course of antibiotics can exert selection pressure on over a thousand generations of bacteria

• Antibiotics are used preventively in humans, cattle, pigs, chickens, fish, and other animals raised on factory farms

• Bacteria with alleles that allow them to survive antibiotic treatment (antibiotic resistant strains) are now common in hospitals and schools

Take-Home Message: What is the effect of directional selection?

• Directional selection causes allele frequencies underlying a range of variation to shift in a consistent direction

17.6 Stabilizing and Disruptive Selection

• Stabilizing selection • Natural selection that favors an intermediate phenotype

and eliminates extreme forms

• Disruptive selection • Natural selection that favors extreme forms of a trait and

eliminates the intermediate forms

Stepped Art


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ANIMATED FIGURE: Stabilizing selection



Stabilizing Selection: Sociable Weavers

• Body weight in sociable weavers is a trade-off between risks of starvation and predation

• Food supplies are limited – leaner birds do not store enough fat to avoid starvation

• Fatter birds may be more attractive to predators, and not as agile when escaping

• Birds of intermediate weight have the selective advantage

Sociable Weavers

Body Weight of Sociable Weavers

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ANIMATED FIGURE: Disruptive selection



Disruptive Selection in African Finches

• The bill of a blackbellied seedcracker is either 12 millimeters wide, or wider than 15 millimeters

• These finches feed on seeds of two types of sedge – one with hard seeds, one with soft seeds

• Small-billed birds are better at opening soft seeds; large-billed birds are better at opening hard seeds

Bill Size in African Finches

ANIMATED FIGURE: Disruptive selection among African finches



Take-Home Message: Natural selection can favor intermediate or extreme forms of traits

• With stabilizing selection, an intermediate phenotype is favored, and extreme forms are selected against

• With disruptive selection, an intermediate form of a trait is selected against, and extreme phenotypes are favored

albia dugger miami dade college chapter 17 processes of evolution sections 1-6

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