Chapter 22—Descent with

Modification: A Darwinian View of

Life

When in doubt…don’t follow the

crowd

I. Historical Context for

Evolutionary Theory

• Charles Darwin’s view of life contrasted with conventional paradigms – Biblical time scale

• Earth was only few thousand years old

– Scale of Nature • Aristotle—species are perfect and do not evolve

– Old Testament • Creation account that species were individually

designed and non-evolving

– Natural theology • adaptations of organisms are evidence of Creator’s

design and every creature has a purpose

• These paradigms were 2,000 years old

Scientist Topic/Process Explanation

Linnaeus

Hutton

Lyell

Lamarck

Malthus

Cuvier

Historical Context of Darwin’s Life

and Ideas

Scientist Topic/Process Explanation

Linnaeus Taxonomy

Hutton Gradualism

Lyell Uniformitarianism

Lamarck Evolution

Malthus Populations

Cuvier Paleontology/

Catastrophism

Paleontology

Voyage of the HMS Beagle (1831-1836)

II. The Darwinian Revolution

• Charles Darwin (1809-1882)

– British naturalist

– Proposed the idea of evolution by natural

selection

– Collected clear evidence to support his

ideas

– Wrote The Origin of Species

Darwin’s Finches (14 species on

Galapagos Islands) • Differences in beaks

– associated with eating different foods

• Finches with beak differences allowed them to… – successfully feed

– successfully compete

– successfully reproduce

– pass successful traits onto their offspring

• Darwin’s conclusions – when original South American finches reached islands, they

adapted to available food in different environments/islands • Adaptive radiation—

– Over many generations, the finches changed anatomically & behaviorally, separated into different species

Adaptive Radiation—one

species becomes many

Artificial Selection—breeding of

domesticated plants and animals

If we can make this happen, can’t natural selection happen in nature?

What did Darwin say?

• Variation – every population of organisms includes differences between

individuals

– much of the variation is heritable

• Over-population – organisms reproduce more than the environment can support

– some offspring survive & some offspring don’t survive

• Competition/Struggle for Existence – for food, mates, nesting spots, escaping predators

• Put together all three and you get natural selection – survival is not random, environment filters heritable variations

– fittest are the ones that survive to reproduce

– leads to gradual change in a population over many generations (evolution)

―Descent with Modification‖

≈ 99% of all species that

ever lived are now extinct

(lots of evolutionary

dead-ends)

Summary—Darwinian view of life

1. The diverse forms of life have arisen by

descent with modification (evolution)

from ancestral species

2. The mechanism of modification/evolution

has been natural selection working over

enormous tracts of time

Subtleties of Natural Selection

1. A population is the smallest unit that can

evolve (individuals cannot evolve!)

2. Natural selection can only amplify or diminish

heritable variations (traits gained during one’s

life are not passed on)

3. Specifics of natural selection are situational–

environments vary and so do adaptations (sometimes you win and sometimes you lose )

Evidence of Evolution

Topic Explanation

1.Insecticide Resistance

An insecticide does

not create resistant

individuals, it selects

for resistant insects

already present in

the population

“editing” process

(The same is true for

any selective

pressure in nature)

Why do bugs

become resistant?

2. Drug Resistance

The 3TC drug is a

cytosine mimic

that blocks

copying of RNA

into DNA by

reverse

transcriptase

How does HIV

become

resistant?

3. Anatomical Homologies

Homology—similar characteristics resulting from common

ancestry, serve different functions

Homologous Structures

Vestigial Organs—Examples?

All share a

common 4-

limbed

ancestor

Anatomical

“remodeling”

4. Embryological Homologies

Anatomical

similarities

between embryos

5. Molecular Homologies

Molecular similarities in the sequences of DNA & proteins

6. Biogeography Convergent evolution/

Analogous structures

evolving similar solutions to

similar “problems”

• similar functions/structures

• different origin

• no close evolutionary

relationship

Marsupial mammals vs.

Placental mammals

Dolphins vs. Fish

7. Fossil Record

Prokaryotes →

Eukaryotes

Vertebrates:

Fish → amphibians →

reptiles → mammals &

birds

Why would whales

have pelvis & leg

bones if they were

always sea creatures?

Why is evolution by natural

selection not ―just a theory‖?