Chapter 22: Descent with Modification

Theory of Evolution• Change through time

– Descent with modification– Genetic changes of a population through time

• Explains Biodiversity– Variety of life forms, with adaptations to specific

environments– Form=Function

• Explains relationship between living organisms– How organisms are related by descent from

common ancestors

Theory of Evolution• Pattern of Evolution

– Observations from natural world– Collected from variety of fields

• Biology, geology, physics, chemistry

• Process of Evolution– Mechanisms causing change

• Unifying theory of biology– Explains and connects observations from natural

world

Types of Evolution• Microevolution= change in the gene pool of a

population over many generations• 4 Methods of Microevolution

– Mutations• May be deleterious or neutral

– Natural Selection• Process in which individuals with favorable inherited traits

are more likely to survive and reproduce

– Genetic Drift= chance events cause genetic changes from one population to the next

– Gene Flow= individuals or gametes move to a different population

Natural Selection and Mutation

Classification of Species• Before 1700s, major belief was organisms did not

change through time but were “perfect” and “permanent”– Greek philosopher Aristotle= arranged species on a

scala naturae• Simple to complex• Viewed species as fixed, non-changing

• 1700s: Carolus Linnaeus= founder of taxonomy– Branch of biology concerned with classifying

organisms– Developed the binomial format for naming species

• Homo sapiens

Classification of Species

• In the mid-1700s, studies of fossils found evidence of life-forms different from those currently living

• Early 1800s, French naturalist Jean Baptiste Lamarck explained the differences in fossil records by proposing organisms evolved through time

Fossil Record• Primarily based on imprints or remains

left in sedimentary rock– Relative ages of fossils can be

determined based on their position in rock layers

– Radiometric dating allows for more precise aging of fossils

– Whole organisms can also be preserved in mediums preventing decomposition

• Insects in resin• Wooly mammoth frozen in ice

• Documents change of organisms through time

• Documents large number of extinct species

Classification of Species

• Lamarck proposed:– Organisms could change its traits by using or not

using certain body parts– These changes could be passed

to offspring • Not supported by genetics

Charles Darwin

• Born in 1809• Life-long naturalist• After college, joined the crew of the

HMS Beagle to explore and map parts of the South American coastline– Collected specimens of South

American plants and animals

HMS Beagle in port

TheGalápagosIslands

NORTHAMERICA

ATLANTICOCEAN

Chile

SOUTHAMERICA

AFRICA

EUROPEGreatBritain

Equator

PACIFICOCEAN

Malay Archipelago

AUSTRALIA

Tasmania

NewZealand

Brazil

Argentina

Cape Horn

An

des

Mtn

s.

Cape ofGood Hope

PACIFICOCEAN

Charles Darwin: Voyage of HMS Beagle

Figure 22.5c

TheGalápagosIslands

PACIFICOCEANPinta

MarchenaGenovesa

EquatorSantiago

DaphneIslands

Fernandina

Isabela SantaCruz

SantaFe San

Cristobal

EspañolaKilometers

0 20 40 Florenza

Pinzón

Charles Darwin

• During voyage, made 2 important observations:– Animals and plants had characteristics specific to

the environment they inhabited– Organisms on islands were similar but different

from organisms on mainland• Darwin perceived adaptation to the

environment and the origin of new species as closely related processes

Figure 22.6

(a) Cactus-eater (b) Insect-eater

(c) Seed-eater

Charles Darwin

Charles Darwin

• 1844: wrote an essay on natural selection as the mechanism of descent with modification– Did not introduce his theory publicly

• 1858: Received manuscript from Alfred Russell Wallace– Developed a theory of natural selection similar to

Darwin’s• Darwin quickly finished The Origin of Species

and published it the next year

Charles Darwin

• 1859: The Origin of Species was published• Provided evidence of “descent with

modification”– Current species came from a succession of ancestors– As descendants spread into new habitats,

modifications (adaptations) accumulated as a result of new environmental factors

– Resulted in new species and increased diversity

• Proposed mechanism for evolution– Natural selection

Charles Darwin

• In the Darwinian view, the history of life is like a tree with branches representing life’s diversity

• Darwin’s theory meshed well with the hierarchy of Linnaeus

Figure 22.8Hyracoidea(Hyraxes)

Sirenia(Manateesand relatives)

†Deinotherium

†Mammut

†Platybelodon

†Stegodon

†Mammuthus

Elephas maximus(Asia)

Loxodonta africana(Africa)

Loxodonta cyclotis(Africa)

†Moeritherium

†Barytherium

60

Millions of years ago

34 24 5.5 2104 0

Years ago

Artificial Selection

• Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits– Artificial selection

Figure 22.9

Brusselssprouts

Kale

Selection for leaves

Selection for axillary (side) buds

Selection for apical (tip) bud

Cabbage

Broccoli

KohlrabiWild mustard

Selection for stems

Selection for flowers and stems

Darwin drew two inferences from two observations

• Observation #1: Members of a population often vary in their inherited traits

• Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals

Darwin drew two inferences from two observations

• Observation #2: All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce

• Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations

Natural Selection• Unequal reproduction is the

essential part of natural selection

• Individuals with characteristics allowing them to be better at…– getting food– escaping predators– tolerating environment– attracting mates – …will survive longer and

potentially produce more offspring with the adaptive characteristic

Natural Selection• Explains the match between organisms and

their environment…

3 Key Points to Evolution by Natural Selection

• Individuals do not evolve, populations evolve– Evolution occurs as adaptations accumulate in populations

over several generations• Natural selection only works on heritable traits with

variation in the population– Acquired traits are not passed to offspring

• Evolution is not working towards a specific goal or “perfect” organisms– Natural selection results from an organism interacting with

a specific environment– Characteristics favorable in one environment may not be

favorable in another

Natural Selection: Summary

• Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals

• Natural selection increases the adaptation of organisms to their environment over time

• If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species

Evidence of Natural Selection: Response to Introduced Plant Species

• Soapberry bugs use their mouthparts to feed on seeds within fruits

• Southern Florida: feed on balloon vine with large fruit– Longer beaks

• Central Florida: feed on goldenrain tree with small fruit– Shorter beaks

• Correlation between fruit size and beak size also observed in Louisiana, Oklahoma, and Australia

Soapberry bug with beak inserted in balloon vine fruit

Figure 22.13b

On native species, southern Florida

Museum-specimen average

On introduced species, central FloridaN

um

ber

of

ind

ivid

ual

s

10

8

6

4

2

0

10

8

6

4

2

0

Beak

Beak length (mm)

6 7 8 10 11

RESULTS

9

• In all locations, beak size has evolved in populations that feed on introduced plants– Fruits smaller or larger than native fruits

• Evolution by natural selection• In Florida, this evolution in beak size

occurred in less than 35 years

Evidence of Natural Selection: Response to Introduced Plant Species

Evidence of Natural Selection: Antibiotic resistant bacteria

• Staphylococcus aureus is commonly found on people– S. aureus became resistant to penicillin in 1945,

two years after it was first widely used• One strain, methicillin-resistant S. aureus

(MRSA) is a dangerous pathogen– S. aureus became resistant to methicillin in 1961,

two years after it was first widely used

Evidence of Natural Selection: Antibiotic resistant bacteria

• Methicillin= inhibits a protein used by bacteria in their cell walls– MRSA bacteria use a different protein in their cell

walls• When exposed to methicillin, MRSA strains are

more likely to survive and reproduce than nonresistant S. aureus strains– MRSA strains are now resistant to many antibiotics

Evidence of Natural Selection: Antibiotic resistant bacteria

Antibiotic Applied

Most bacteria killed,High resistance strains remain

Reproduction

Final population composed of high resistance strains

Evidence of Natural Selection: Antibiotic resistant bacteria

• Natural selection does not create new traits– Works on existing traits already present in the

population– Mutations are raw material for natural selection to

act on• Local environment determines which traits

will be selected for or selected against in any specific population

Evidence of Natural Selection: Comparative Anatomy

• Homology= similar characteristics resulting from a common ancestor

• Homologous structures= similar structure in features even if function is different

• Example: Vertebrate Forelimbs

Figure 22.15

Humerus

Radius

Ulna

Carpals

MetacarpalsPhalanges

Human Cat Whale Bat

Evidence of Natural Selection: Comparative Anatomy

• Comparative embryology reveals anatomical homologies not visible in adult organisms

Evidence of Natural Selection: Comparative Anatomy

• Vestigial structures= remnants of features with served important functions in the organism’s ancestors

Evidence of Natural Selection: Molecular Biology

• Homologies also occur at the molecular level – Genes shared among organisms inherited from a

common ancestor

Evidence of Natural Selection: Molecular Biology

• Evolutionary history contained in an organism’s DNA

• Closely related species will have more similarities in DNA than distantly related species

• Darwin proposed all life forms are related– Supported by molecular biology– All organisms use same genetic language- DNA and RNA– Same genetic code for nearly all organisms– Humans and bacteria share homologous genes

Evidence of Natural Selection: Evolutionary Trees

• Evolutionary trees= hypotheses about the relationships among different groups– Form nested patterns in evolutionary trees

• Made using different types of data– Anatomical similarities– DNA sequence data

Figure 22.17

Branch pointLungfishes

Amphibians

Mammals

Lizardsand snakes

Crocodiles

Ostriches

Hawks andother birds

Feathers

Amnion

Digit-bearinglimbs

Homologouscharacteristic

Tetrapo

ds

Am

nio

tes

Bird

s

1

2

3

4

5

6

Evidence of Natural Selection: Evolutionary Trees

• Convergent evolution= evolution of similar, or analogous, features in distantly related groups– Arise when groups

independently adapt to similar environments in similar ways

• Convergent evolution does not provide information about ancestry

Evidence of Natural Selection: Biogeography

• Biogeography= geographic distribution of species

• Island animals resemble species on closest mainland more than they resemble species on islands closer and farther away

• Explanation: Animals on islands migrated from mainland, natural selection changed animals over time into separate species– Galapagos Finches– Marsupials in Australia

Evidence of Natural Selection: Biogeography

• Alfred Russel Wallace (1823–1913) – Father of biogeography

• Best known as the co-discoverer of the principles of natural selection with Darwin

• Main contribution was the study of species distributions across large spatial scales

Evidence of Natural Selection: Biogeography

• Wallace (1860) observed that animals can vary considerably over very short distances– Could not be explained until continental drift was

proposed• Continental drift from movement of Earth’s plates

explains the distributions of some species• Type of macroevolution

Types of Evolution• Macroevolution= change in groups above the

level of species, often results in new taxonomic groups

• Methods of Macroevolution– Adaptive radiation= periods of change when many

new species originate from a common ancestor– Mass extinctions= large number of species go

extinct within a relatively short amount of time• 5 mass extinctions have occurred through geological time

with 50% or more species lost

– Plate tectonics and Continental drift= movement of Earth’s plates

Mass Extinction Event Mass Extinction Event

Evidence of Natural Selection: Biogeography

Evidence of Natural Selection: Biogeography

• Endemic species= species that are not found anywhere else in the world

• Islands have many endemic species– Closely related to species on the nearest mainland

or island• Darwin explained that species on islands gave

rise to new species as they adapted to new environments

Evolution is a Theory

• In science, theory accounts for many observations and data – Attempts to explain and connect a huge variety of

phenomena• Darwin’s theory of evolution by natural selection

– integrates diverse areas of biological study – stimulates many new research questions

• Ongoing research adds to our understanding of evolution