Darwin – Descent with Modification

Ch 22

• The Origin of Species– Focused biologists’ attention on the great

diversity of organisms

Figure 22.1

• Darwin made two major points in his book– He presented evidence that the many species

of organisms presently inhabiting the Earth are descendants of ancestral species

– He proposed a mechanism for the evolutionary process, natural selection

Concept 22.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species

• In order to understand why Darwin’s ideas were revolutionary– We need to examine his views in the context of

other Western ideas about Earth and its life

• The historical context of Darwin’s life and ideas

Figure 22.2

Linnaeus (classification)Hutton (gradual geologic change)

Lamarck (species can change)

Malthus (population limits)Cuvier (fossils, extinction)

Lyell (modern geology)

Darwin (evolution, nutural selection)

Mendel (inheritance)

Wallace (evolution, natural selection)

1750

American Revolution French Revolution U.S. Civil War

1800 1850 19001795 Hutton proposes his theory of gradualism.

1798 Malthus publishes “Essay on the Principle of Population.”

1809 Lamarck publishes his theory of evolution.1830 Lyell publishes Principles of Geology.

1831–1836 Darwin travels around the world on HMS Beagle.

Darwin begins his notebooks on the origin of species.1837Darwin writes his essay on the origin of species.1844

Wallace sends his theory to Darwin.1858

The Origin of Species is published.1859Mendel publishes inheritance papers.1865

The Scale of Nature and Classification of Species

• The Greek philosopher Aristotle– Viewed species as fixed and unchanging

• The Old Testament of the Bible– Holds that species were individually designed by

God and therefore perfect

• Carolus Linnaeus– Interpreted organismal adaptations as evidence that

the Creator had designed each species for a specific purpose

– Was a founder of taxonomy, classifying life’s diversity “for the greater glory of God”

Fossils, Cuvier, and Catastrophism• The study of fossils

– Helped to lay the groundwork for Darwin’s ideas• Fossils are remains or traces of organisms from

the past– Usually found in sedimentary rock, which

appears in layers or strata

Figure 22.3

• Paleontology, the study of fossils– Was largely developed by French scientist

Georges Cuvier

• Cuvier opposed the idea of gradual evolutionary change– And instead advocated catastrophism,

speculating that each boundary between strata represents a catastrophe

Theories of Gradualism

• Gradualism– Is the idea that profound change can take

place through the cumulative effect of slow but continuous processes

• Geologists Hutton and Lyell– Perceived that changes in Earth’s surface can

result from slow continuous actions still operating today

– Exerted a strong influence on Darwin’s thinking– Lyell – uniformitarianism – same geologic

processes are operating today as in the past and at same rate

Lamarck’s Theory of Evolution

• Lamarck hypothesized that species evolve– Through use and disuse and the inheritance

of acquired traits– But the mechanisms he proposed are

unsupported by evidence

Figure 22.4

Concept 22.2: In The Origin of Species, Darwin proposed that species change through natural selection

• As the 19th century dawned– It was generally believed that species had

remained unchanged since their creation, but a major change would challenge this thinking

• Darwin’s interest in the geographic distribution of species

– Was kindled by the Beagle’s stop at the Galápagos Islands near the equator west of South America

Figure 22.5

EnglandEUROPE

NORTHAMERICA

GalápagosIslands

Darwin in 1840,after his return

SOUTHAMERICA

Cape ofGood Hope

Cape Horn

Tierra del Fuego

AFRICA HMS Beagle in port

AUSTRALIA

TasmaniaNewZealand

PACIFICOCEAN

An

des

ATLANTICOCEAN

Darwin’s Focus on Adaptation

• As Darwin reassessed all that he had observed during the voyage of the Beagle– He began to perceive adaptation to the

environment and the origin of new species as closely related processes

• From studies made years after Darwin’s voyage– Biologists have concluded that this is indeed

what happened to the Galápagos finches

Figure 22.6a–c

(a) Cactus eater. The long,sharp beak of the cactusground finch (Geospizascandens) helps it tearand eat cactus flowersand pulp.

(c) Seed eater. The large groundfinch (Geospiza magnirostris)has a large beak adapted forcracking seeds that fall fromplants to the ground.

(b) Insect eater. The green warbler finch (Certhidea olivacea) uses itsnarrow, pointed beak to grasp insects.

Summary of Natural Selection

• Natural selection is differential success in reproduction– That results from the interaction between

individuals that vary in heritable traits and their environment

• If an environment changes over time– Natural selection may result in adaptation to

these new conditions

• Natural selection can produce an increase over time– In the adaptation of organisms to their

environment

Figure 22.11

(a) A flower mantidin Malaysia

(b) A stick mantidin Africa

• In 1844, Darwin wrote a long essay on the origin of species and natural selection– But he was reluctant to introduce his theory

publicly, anticipating the uproar it would cause

• In June 1858 Darwin received a manuscript from Alfred Russell Wallace– Who had developed a theory of natural

selection similar to Darwin’s

• Darwin quickly finished The Origin of Species– And published it the next year

Descent with Modification

• The phrase descent with modification– Summarized Darwin’s perception of the unity

of life– States that all organisms are related through

descent from an ancestor that lived in the remote past

• In the Darwinian view, the history of life is like a tree– With multiple branchings from a common

trunk to the tips of the youngest twigs that represent the diversity of living organisms

Figure 22.7

Hyracoidea(Hyraxes)

Sirenia(Manatees

and relatives)

Yea

rs a

goM

illio

ns

of y

ea

rs a

go Dei

no

ther

ium

Ma

mm

ut

Ste

go

don

Ma

mm

uth

us

Pla

tyb

elo

don

Bar

yth

eriu

m

Mo

eri

ther

ium

Elephasmaximus

(Asia)

Loxodontaafricana(Africa)

Loxodontacyclotis(Africa)

Natural Selection and Adaptation

• Evolutionary biologist Ernst Mayr– Has dissected the logic of Darwin’s theory into

three inferences based on five observations

• Observation #1: For any species, population sizes would increase exponentially– If all individuals that are born reproduced

successfully

Figure 22.8

• Observation #2: Nonetheless, populations tend to be stable in size– Except for seasonal fluctuations

• Observation #3: Resources are limited

• Inference #1: Production of more individuals than the environment can support– Leads to a struggle for existence among

individuals of a population, with only a fraction of their offspring surviving

• Observation #4: Members of a population vary extensively in their characteristics– No two individuals are exactly alike

Figure 22.9

• Observation #5: Much of this variation is heritable

• Inference #2: Survival depends in part on inherited traits– Individuals whose inherited traits give them a

high probability of surviving and reproducing are likely to leave more offspring than other individuals

• Inference #3: This unequal ability of individuals to survive and reproduce– Will lead to a gradual change in a population,

with favorable characteristics accumulating over generations

Differential Predation in Guppy Populations

• Researchers have observed natural selection– Leading to adaptive evolution in guppy

populations Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools and measured the average age and size of guppies at maturity over an 11-year period (30 to 60 generations).

EXPERIMENT

Pools with killifish,but not guppies priorto transplant

Experimentaltransplant ofguppiesPredator: Killifish; preys

mainly on small guppies

Guppies:Larger atsexual maturitythan those in“pike-cichlid pools”

Predator: Pike-cichlid; preys mainly on large guppies

Guppies: Smaller at sexual maturity thanthose in “killifish pools”

Figure 22.12

RESULTS After 11 years, the average size and age at maturity of guppies in the transplanted populations increased compared to those of guppies in control populations.

161.5185.6

67.5

Wei

ght

of g

upp

ies

at m

atur

ity (

mg)

Age

of

gupp

ies

at m

atur

ity (

days

)

92.3

48.5

Control Population: Guppiesfrom pools with pike-cichlids as predators

Experimental Population:Guppies transplanted topools with killifish aspredators

76.1

Males Females

85.7

58.2

Males Females

CONCLUSION Reznick and Endler concluded that the change in predator resulted in different variations in the population (larger size and faster maturation) being favored. Over a relatively short time, this altered selection pressure resulted in an observable evolutionary change in the experimental population.

The Evolution of Drug-Resistance

• In humans, the use of drugs– Selects for pathogens that through chance

mutations are resistant to the drugs’ effects

• Natural selection is a cause of adaptive evolution

• Drugs do not create resistant pathogens, they select for resistant individuals already in the population

The Fossil Record

• The succession of forms observed in the fossil record– Is consistent with other inferences about the

major branches of descent in the tree of life

Anatomical Homologies• Homologous structures between

organisms– Are anatomical resemblances that represent

variations on a structural theme that was present in a common ancestor

Figure 22.14Human Cat Whale Bat

• Comparative embryology– Reveals additional anatomical homologies

not visible in adult organisms

Figure 22.15

Pharyngealpouches

Post-analtail

Chick embryo Human embryo

• Vestigial organs– Are some of the most intriguing homologous

structures– Are remnants of structures that served

important functions in the organism’s ancestors

• Analogous structures– Similar solution to similar problems; but

doesn’t show relatedness

Molecular Homologies

• Biologists also observe homologies among organisms at the molecular level– Such as genes that are shared among

organisms inherited from a common ancestor

• Anatomical resemblances among species– Are generally reflected in their molecules,

their genes, and their gene products

Figure 22.16

Species

Human

Rhesus monkey

Mouse

Chicken

Frog

Lamprey14%

54%

69%

87%

95%

100%

Percent of Amino Acids That AreIdentical to the Amino Acids in aHuman Hemoglobin Polypeptide

Homologies and the Tree of Life

• The Darwinian concept of an evolutionary tree of life– Can explain the homologies that researchers

have observed

Biogeography

• Darwin’s observations of the geographic distribution of species, biogeography– Formed an important part of his theory of evolution

• Some similar mammals that have adapted to similar environments– Have evolved independently from different ancestors

Sugarglider

AUSTRALIA

NORTHAMERICA

Flyingsquirrel

Figure 22.17

Summarizing

• Evolution is a change in species over time

• Heritable variations exist within a population

• These variations can result in a differential reproductive success

• Over generations, this can result in changes in the genetic composition of the population