Learning Objectives

• Charles Darwin and his contribution to the theory of evolution

• Define natural selection and how it relates to evolution

• The relationship between natural selection and evolution

• What is adaptive radiation?

• What is the difference between homologous and analogous?

• How do fossils provide a historical record of evolution?

• How is evolution observed at the molecular level?

• Hardy-Weinberg equilibrium

• What are the agents of evolution?

• Three types of selection: stabilizing, disruptive, and directional

Learning Objectives

• Human impact on natural selection Industrial melanism

• Guppies as an example of natural selection

• What is the biological species concept?

• What are the two categories of barriers to reproduction?

• What are the six isolating mechanisms that fall into the category of pre-zygotic barriers to reproduction?

• Post-zygotic barriers to reproduction

Charles Darwin, Father of Evolution

• Charles Darwin

(1809 -1882)

• English naturalist who first suggested an explanation for why evolution occurred

• Published in 1859, On the Origin of Species by Means of Natural Selection

Figure 14.1 The theory of

evolution by natural selection

was proposed by Charles Darwin

This brings us back to the Theory of

Evolution from Chapter 1

Natural Selection

• Charles Darwin’s work on evolution challenged

established worldviews

Darwin proposed a mechanism for evolutionary

change called natural selection

Darwin’s hypothesis for evolutionary change, after

much testing, eventually became accepted as theory

Darwin’s Voyage on HMS Beagle

• Darwin voyaged from 1831 - 1836 on the HMS Beagle, a ship

mapping the world’s coastlines

• Darwin observed firsthand different plants and animals in

various locales

• These observations played an important role in the

development of his thoughts about the nature of life on earth

Figure 14.3 The five-year voyage of HMS Beagle

Darwin’s Evidence

Darwin made several observations that helped lead him to believe that species evolve rather than remain fixed:

1) fossils of extinct organisms resembled those of living organisms

2) geographical patterns suggested that organismal lineages change gradually as individuals move into new habitats

3) islands have diverse animals and plants that are related to yet different from their mainland sources

Figure 14.5 Four Galápagos finches and what they eat

Darwin observed that, although all the finches

shared a common ancestor, their beak sizes had

evolved to suit their food. Darwin termed this

“descent with modification.”

Malthus’ Influence

• Thomas Malthus’ Essay on the Principle of

Population (1798) provided Darwin with a

key insight

while human populations tend to increase

geometrically, the capacity for humans to

feed this population only grows

arithmetically

Figure 14.6 Geometric and arithmetic progressions

Malthus contended that the human population

growth curve was geometric, while the human

food production growth curve was only

arithmetic

Darwin’s Malthusian Influence

• Darwin expanded Malthus’ view to include every organism

all organisms have the capacity to over-reproduce

only a limited number of these offspring survive and produce the next generation

• Darwin associated survivors with having certain physical, behavioral, or other attributes that help them to live in their environment

by surviving, they can pass their favorable characteristics on to

their own offspring

Figure 14.7 Sea turtle hatchlings

Many organisms, such as sea turtles, produce more offspring than will actually

survive

The Theory of Natural Selection

Darwin envisioned the frequency of favorable characteristics increasing in a population through a process called natural selection

favorable characteristics are specific to an environment; they

may be favored in one but not in another

organisms whose characteristics are best suited to their particular environment survive more often and leave more offspring

Survival and Fitness

• Darwin’s selection concept is often

referred to as the “survival of the fittest”

Darwinian fitness does not refer always to the

biggest or the strongest

fitness, in evolutionary theory, refers to

organisms who, due to their characteristics,

survive more often and leave more offspring

On the Origin of Species

• Darwin drafted his ideas in 1842 but hesitated to publish them for 16 years

• Another researcher, Alfred Russel Wallace, sent an essay to Darwin outlining a theory of evolution by natural selection

Wallace had independently arrived at the same mechanism for evolution that Darwin had

Darwin arranged for a joint presentation of their ideas in London

• Darwin finally published On the Origin of Species in 1859

The Beaks of Darwin’s Finches

• Darwin’s finches are a closely related group of distinct species all the birds are similar to each other except for the shape of their

beaks

genetic differences account for the physical differences in the beaks – How do these differences arise?

• birds with larger beaks make more of a protein called BMP4

Figure 14.9 A diversity of finches

on a single island

The Grants and the Finches

• Darwin supposed that the birds evolved from a single ancestor to become individual species who specialized in particular foods

• Peter and Rosemary Grant studied the medium ground finch on the island of Daphne Major in the Galápagos • Measured beak shape

over many years, recorded feeding preferences • Finches preferred to feed on

small, tender seeds • Finches switched to larger,

harder-to-crack seeds when the small seeds became hard to find

• Beak depth increased when only large, tough seeds were available

Figure 14.11 Evidence that natural selection alters beak

size in Geospiza fortis

Finches: Evolution in Action

• The Grants’ work with the

medium ground finch is an

example of evolution in action

average beak depth increased

after a drought

• only large-beaked birds were

able to crush the bigger seeds

and survive to make the next

generation

when wet periods returned, smaller

beaks prevailed at handling the

more plentiful small seeds

Punctuated equilibrium vs. Phyletic gradualism?

Adaptive Radiation

• Darwin’s finches on the Galápagos are an example of adaptive radiation in adaptive radiation, a cluster of

species changes to occupy a series of different habitats within a region

each habitat offers different niches to occupy

• a niche represents how a species interacts both biologically and physically with its environment in order to survive

each species evolves to become adapted to that niche

The Evidence for Evolution

• There are many lines of evidence supporting Darwin’s theory of evolution

fossil record comprises the most direct evidence of macroevolution

fossils are the preserved remains, tracks, or traces of once-living organisms

• they are created when organisms become buried in sediment

• by dating the rocks in which the fossils occur, one can get an accurate idea of how old the fossils are

History of Evolution

• Fossils in rock represent a history of evolutionary change

fossils are treated as samples of data and are dated independently of what the samples are like

successive changes through time are observed in the data

Figure 14.14

Testing the

theory of

evolution with

fossil titanotheres

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Mil

lio

ns o

f years

ag

o

35

45

40

50

Anatomical Record

• The anatomical record also reflects evolutionary

history

for example, all vertebrate embryos share a basic set

of developmental instructions

Figure 14.15 Embryos show our early evolutionary history

Homologous vs. Analogous

• Homologous structures are derived from the same body part present in an ancestor for example, the same bones might be put to different

uses in related species

• Analagous structures are similar-looking structures in unrelated lineages these are the result of parallel evolutionary

adaptations to similar environments • this form of evolutionary change is referred to as convergent

evolution

Figure 14.16 Homology among

vertebrate limbs

Molecular Evolution

• Traces of our evolutionary past are also

evident at the molecular level

organisms that are more distantly related

should have accumulated a greater number of

evolutionary differences than two species that

are more closely related

the pattern of divergence can be seen at the

DNA and protein levels

Figure 14.17 Molecules reflect

evolutionary divergence

Molecular Clock

• Evolutionary changes accumulate at a

constant rate in some proteins

this constancy is referred to as a molecular

clock

different proteins can evolve at different rates

Figure 14.18 The molecular clock

of cytochrome c

Vertebrate Phylogeny

Evolution of Systems

• The irreducible-complexity

fallacy refers to claims that

the molecular machinery of

the cell is irreducibly complex

• Yet natural selection acts on

the systems and not the

parts so that, at every stage

of evolution, the system is

functioning

for example, the mammalian

blood clotting system has

evolved in stages from much

simpler systems

• Variation within populations puzzled many scientists why don’t dominant alleles drive recessive alleles out of

populations?

• G.H. Hardy and W. Weinberg, in 1908, studied allele frequencies in a gene pool in a large population in which there is random mating, and in the

absence of forces that change allele frequencies, the original genotype proportions remain constant from generation to generation

because the proportions do not change, the genotypes are said to be in Hardy-Weinberg equilibrium

if the allele frequencies are not changing, the population is not evolving

Genetic Change in Populations

• Hardy and Weinberg arrived at their

conclusion by analyzing the frequencies of

alleles in successive generations

frequency is the proportion of individuals with

a certain characteristic compared to an entire

population

knowing the frequency of the phenotype, one

can calculate the frequency of the genotypes

and alleles in the population

Allele Frequencies

• By convention, the frequency of the

dominant allele is designated by the letter

p and that of the recessive allele by the

letter q

• Because there are only two alleles, the

sum of p and q must always equal 1

p + q = 1

• In algebraic terms, the Hardy-Weinberg

equilibrium is written as an equation

p2 + 2pq + q2 = 1

Hardy-Weinberg Equilibrium

Figure 14.22

Hardy-

Weinberg

equilibrium

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

360 cats

360/1,000 = 0.36

480 cats

480/1,000 = 0.48

160 cats

160/1,000 = 0.16

720 B 480 B + 480 b 320 b

720 B + 480 B = 1,200 B 1,200/2,000 = 0.6 B

480 b + 320 b = 800 b 800/2,000 = 0.4 b

Phenotypes

Genotypes

Frequency of alleles in the

population (total of 2,000)

Number of alleles in the

population (2 per cat)

Frequency of genotype in

the population (number in

a population of 1,000 cats)

Bb bb BB

b

B

q2 = 0.16

pq = 0.24

p2 = 0.36

p = 0.6

q = 0.4

p = 0.6

q = 0.4

b

B Eggs

Bb

bb

pq = 0.24

BB

Bb

Sperm

• The Hardy-Weinberg equilibrium only works if the following five assumptions are met

1. The size of the population is very large or effectively infinite.

2. Individuals mate with one another at random.

3. There is no mutation.

4. There is no immigration or emigration.

5. All alleles are replaced equally from generation to generation (natural selection is not occurring).

Hardy-Weinberg Assumptions

• Most human populations are large and

randomly mating with respect to most

traits and thus are similar to an ideal

population envisioned by Hardy and

Weinberg

for example, the frequency of heterozygote

carriers for recessive genetic disorders can be

estimated using the Hardy-Weinberg

equilibrium

Hardy-Weinberg and Humans

Agents of Evolution

• Five factors can alter the proportions of homozygotes and heterozygotes enough to produce significant deviations from Hardy-Weinberg predictions

1. mutation

2. migration

3. genetic drift

4. nonrandom mating

5. selection

Table 14.1 Agents of Evolution

Agents of Evolution: Mutation

• Mutation is a change in a nucleotide sequence in DNA

mutation rates are generally too low to significantly alter Hardy-Weinberg proportions

mutations must affect the DNA of the germ cells or the mutation will not be passed on to offspring

however, no matter how rare, mutation is the ultimate source of genetic variation in a population

Agents of Evolution: Migration

• Migration is the movement of individuals

between populations

the movement of individuals can be a

powerful force upsetting the genetic stability

of natural populations

• the magnitude of the effects of migration is based

on two factors

– the proportion of migrants in the population

– the difference in allele frequencies between the migrants

and the original population

Agents of Evolution: Genetic Drift

• Genetic drift describes random changes in allele frequencies

in small populations, the frequencies of particular alleles may be changed drastically by chance alone

in extreme cases, individual alleles of a given gene may be

• all represented in few individuals

• accidentally lost if individuals fail to reproduce or die

Animation: Simulation of

Genetic Drift

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Agents of Evolution: Founder Effect

• A series of small populations that are isolated from one another may come to differ strongly as the result of genetic drift

founder effect occurs when one of a few individuals migrate and become the founders of a new, isolated population at some distance from their place of origin

• the alleles that they carry will become a significant fraction of the new population’s genetic endowment

• the founder effect is important in the evolution of organisms on oceanic islands

Agents of Evolution: Nonrandom

mating

• Nonrandom mating occurs when

individuals with certain genotypes mate

with one another either more or less

commonly than would be expected by

chance

sexual selection is choosing a mate often

based on physical characteristics

nonrandom mating alters genotype

frequencies but not allele frequencies

Agents of Evolution: Selection

• Selection, according to Darwin, occurs if

some individuals, due to their inherited

characteristics, leave behind more

progeny than others • in artificial selection, a breeder selects for the

desired characteristics

• in natural selection, conditions in nature

determine which kinds of individuals in a

population are the most fit

• there are three types of natural selection

Figure 14.24 Three kinds of natural

selection

Stabilizing Selection

• Stabilizing selection is a form of

selection in which both extremes from an

array of phenotypes are eliminated

the result is an increase in the frequency of

the already common intermediate phenotype

for example, human birth weight is under

stabilizing selection

Figure 14.25 (a) Examples of

selection

Disruptive Selection

• Disruptive selection is a form of selection in which the two extremes in an array of phenotypes become more common in the population

selection acts to eliminate the intermediate phenotypes

for example, beak size in African black-bellied seedcracker finches is under disruptive selection because the available seeds are only large or small

Figure 14.25 (b) Examples of

selection

Directional Selection

Directional selection is a form of selection

that occurs when selection acts to eliminate

one extreme from an array of phenotypes In Drosophila, flies that fly toward light can be

selected against and those that avoid light selectively

bred, producing a population of flies with a greater

tendency to avoid light

Fossil records show that European black bears

increased in size during glacial periods, but

decreased in size between ice ages

Figure 14.25 (c) Examples of

selection

Sickle-Cell Disease

Sickle-cell disease is a hereditary disease

affecting hemoglobin molecules in the blood

The disorder results from a single nucleotide change

in the gene encoding beta-hemoglobin

• this causes the sixth amino acid in the chain to change from

glutamic acid (very polar) to valine (nonpolar)

• as a result, the hemoglobin molecules clump together and

deform the red blood cell into “sickle-shape”

Figure 14.27 Why the sickle-cell

mutation causes hemoglobin to clump

Sickle Cell: Non-Mendelian?

• Persons homozygous for the sickle-cell genetic

mutation frequently have a reduced lifespan

red blood cells that are sickled do not flow smoothly

through capillaries

oxygen transport is affected

• Heterozygous individuals make enough

functional hemoglobin to keep their red blood

cells healthy

Sickle-Cell Disease: Frequency

• The frequency of the sickle-cell allele is about 0.12 in Central Africa – How would we show this in the Hardy-Weinberg equation? one in 100 people is homozygous for the defective

allele and develops the fatal disorder

sickle-cell anemia strikes roughly two African Americans out of every thousand

• If natural selection drives evolution, why has natural selection not acted against the defective allele in Africa and eliminated it from the population there?

Heterozygote Advantage

The defective allele has not been eliminated

from Central Africa because people who are

heterozygous are much less susceptible to

malaria

the payoff in survival of heterozygotes makes

up for the price in death of homozygotes

• this is called heterozygote advantage

• stabilizing selection occurs because malarial

resistance counterbalances lethal anemia – what

would a graph of this selection look like?

Figure 14.28 How

stabilizing selection

maintains sickle-cell

disease

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Sickle-cell

allele in Africa

Falciparum

malaria in Africa

10–20%

1–5%

5–10%

Malaria

Peppered Moths and Industrial

Melanism

• The frequency of dark individuals relative to light individuals of the peppered moth (Biston betularia) has increased in industrialized areas darker forms may be less susceptible to predation

industrial melanism describes the evolutionary process in which darker individuals become more common than light individuals in a population as a result of natural selection

this phenomenon has been observed in other species of moths in industrialized areas in Eurasia and North America

Figure 14.29 Tutt’s hypothesis explaining

industrial melanism

• Due to a reduction in air

pollution, a reversal of

industrial melanism seems

to be occurring in England

and America

Industrial Melanism: Reversible?

Figure 14.30 Selection against

melanism

Natural Selection: Guppies

Guppies are colorful fish that are popular for

aquariums

on the island of Trinidad, guppies are found in two

very different stream environments

• in pools above waterfalls, the guppies are found along with

the killifish, a seldom predator of guppies

• in pools below waterfalls, the guppies are found in pools

along with the pike cichlid, a voracious predator of guppies

• guppies can move between pools by swimming upstream

during floods

Selection on Color in Guppies

• Guppy populations above and below

waterfalls exhibit many differences

guppies in high-predation pools are not as

colorful as guppies in low-predation pools

guppies in high-predation pools tend to

reproduce at an earlier age and attain

relatively smaller adult body sizes

these differences suggest the function of

natural selection

Figure 14.31 The evolution of

protective coloration in guppies

Guppies: Experiments in Selection

• John Endler conducted experiments on guppies to determine whether predation risk was really the driving selective force in this system

in a controlled laboratory setting, he created artificial pool environments in which he placed guppies in one of three conditions:

• with no predator present

• with killifish present (low predation risk)

• with cichlid present (high predation risk)

after 10 guppy generations, he found that the guppies from no- or low-predation risk pools were both larger and more colorful than the guppies from the high predation risk pool

he later found the same results in field experiments

Figure 14.32

Evolutionary

change in

spot number

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8

9

0 8 4

High

predation

Low

predation

No

predation 14

13

Sp

ots

pe

r fi

sh

12

11

10

12

Months

(a)

(b)

b: Courtesy H. Rodd

Speciation

• Speciation is the macroevolutionary

process of forming new species from pre-

existing species

it involves successive change

• first, local populations become increasingly

specialized

• then, if they become different enough, natural

selection may act to keep them that way

The Biological Species Concept

• Ernst Mayr coined the biological species concept, which defines species as

“groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”

• Populations whose members do not mate with each other and cannot produce fertile offspring are said to be reproductively isolated and, thus, members of different species

Barriers to Reproduction

• Barriers called reproductive isolating

mechanisms cause reproductive isolation

by preventing genetic exchange between

species

prezygotic isolating mechanisms

• prevent the formation of zygotes

postzygotic isolating mechanisms

• prevent the proper functioning of zygotes once

they have formed

Table 14.2 Isolating Mechanisms

Hint: This would be a

good table to review

for the exam!

Isolating Mechanisms

• There are six broad categories of prezygotic isolating mechanisms

geographical isolation

ecological isolation

temporal isolation

behavioral isolation

mechanical isolation

prevention of gamete fusion

• Geographical isolation occurs in cases when

species exist in different areas and are not able

to interbreed Kaibab squirrel found only in a 20 x 40 mile habitat on North Rim

of Grand Canyon

Abert’s squirrel is found on South Rim

• Widely distributed, several subspecies

Geographical

Kaibab squirrel

Abert’s squirrel

Figure 14.33 Lions and tigers

are ecologically isolated

(a)

(b)

(c)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

a-b: © Corbis RF; c: © Porterfield/Chickering/Photo Researchers

Ecological isolation results

from two species who occur

in the same area but utilize

different portions of the

environment and are unlikely

to hybridize

Lions & tigers once occupied

same geographical area but

different niches

• Lions: savannah

• Tigers: jungle

Ecological

Temporal isolation results from two species having different

reproductive periods, or breeding seasons, that preclude hybridization

• American toad and Fowler’s toad closely related but do not breed American toad breeds in early summer, Fowler’s toad later in the season

• Western spotted skunk breeds in the fall, while Eastern spotted skunk

breeds in late winter

Temporal

American toad Fowler’s toad

Eastern spotted

skunk

Western spotted

skunk

• Behavioral isolation refers to the often elaborate courtship and

mating rituals of some groups of animals, which tend to keep these

species distinct in nature even if they inhabit the same places

Behavioral

Mechanical isolation results from structural differences

that prevent mating between related species of animals

and plants

• Example: Snails whose shells spiral in opposite

directions cannot mate

Mechanical

Prevention of gamete fusion blocks the union of gametes

even following successful mating

Gametic

• Sea urchins spew

gametes into ocean,

sperm & eggs form

zygotes, develop

into larvae

• Giant Red Urchin

and Purple Urchin

occupy same

habitat along

western U.S.

• Gametes are

chemically/

genetically

incompatible

• If hybrid mating does occur and zygotes are

produced, many postzygotic factors may prevent

those zygotes from developing into normal

individuals

in hybrids, the genetic complements of two species

may be so different that they cannot function together

normally in embryonic development

even if hybrids survive the embryo stage, they may

not develop normally

finally, many hybrids are sterile

Post-Zygotic Isolation

Figure 14.34 Postzygotic isolation

in leopard frogs

• Similar in appearance,

overlapping ranges

• Fertilized eggs fail to

develop properly

Does Humanity Have a Hybrid Origin?

• One prominent geneticist proposes

that humans arose through a cross

between a male pig and female

chimpanzee

• Explains a number of anomalous

human characteristics:

Low fertility and non-uniformity of human sperm

Dermal features such as naked skin, subcutaneous fat, thermoregulatory

sweating, etc.

Facial features such as protruding cartilaginous mucous nose, eyebrows,

earlobes, philtrum

Many skeletal features, including short digits, thick tooth enamel, helical chewing

Behavioral traits such as nocturnal activity, swimming, tears

• Repeated backcrossing would account for the negligible differences

between human and chimp genomes

http://www.macroevolution.net/human-origins.html#.U4y9GPldWSp

Evolution in Action: The Grants

and Darwin’s Finches • https://www.youtube.com/watch?v=1XbY5

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