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BIOSC 041 Descent with Modification: A Darwinian View of Life Reference: Chapter 22

Outline

v  Paradigm shifts in science v  Darwin’s world v  Fundamental tenets of evolution v  Examples and evidence v  Natural selection

Paradigm shifts in science

v  A new era of biology began in 1859 when Charles Darwin published The Origin of Species

v  The Origin of Species focused biologists’ attention on the great diversity of organisms

v  The Origin of Species arose from, and gave rise to, one of the greatest paradigm shifts in science, one that is still going on today

Paradigm Shift

v  Accepted view on a particular topic is challenged

§  Evidence accumulates from various sources

v  Accepted view shifts to new understanding

§  Seemingly overnight- but not really…

v  Examples

§  Geocentrism vs. Heliocentrism

§  Round vs. Flat Earth

§  Microbiology

§  Evolution

In Darwin’s Day…

v  Age of the Earth §  5,000-6,000 years old §  Earth was static

§  Oceans, mountains, etc. were fixed & did not change except through divine action (e.g. Flood)

v  Earth strata §  Formed in place, within the earth

In Darwin’s Day…

v  Animals & Plants §  Created at the same time §  Humans at the top of a “great chain of being”

v  Fossils §  Believed to be the bones of demons/dragons OR §  Strange mineral deposits that “grew” in the earth

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Paradigm Challenge

v  Evidence was mounting that challenged traditional views of a young Earth inhabited by unchanging species §  New views on geology §  New views on religion and human societies §  New views on biology

§  Earlier paradigm shift – microorganisms §  Anatomical and developmental similarities between

very different groups of organisms

Scala Naturae and Classification of Species

v  Greek philosopher Aristotle viewed species as fixed and arranged them on a scala naturae (ladder of nature)

v  The Old Testament holds that all species were individually designed by God and therefore perfect

v  Carolus Linnaeus interpreted organismal adaptations as evidence that the Creator had designed each species for a specific purpose

Ideas About Change over Time - Catastrophism

v  The study of fossils helped to lay the groundwork for Darwin’s ideas

v  Paleontology, the study of fossils, was largely developed by French scientist Georges Cuvier

v  Cuvier advocated catastrophism, speculating that each boundary between strata represents a global, geologic catastrophe (e.g. volcano, flood, etc.)

Ideas About Change over Time - Uniformitarianism

v  Geologists James Hutton and Charles Lyell perceived that changes in Earth’s surface can result from slow continuous actions still operating today

v  Lyell’s principle of uniformitarianism states that the mechanisms of change are gradual and constant over time

v  This view strongly influenced Darwin’s thinking

Darwin’s Research – H.M.S. Beagle

v  Darwin served as naturalist during a 5-year around the world voyage on the Beagle, where he collected specimens of South American plants and animals

v  Observed that fossils resembled living species from the same region, and living species resembled other species from nearby regions

v  Experienced an earthquake in Chile and observed the uplift of rocks

Darwin’s Research – H.M.S. Beagle

v  Darwin was influenced by Lyell’s Principles of Geology and thought that the earth was more than 6000 years old

v  His interest in geographic distribution of species was kindled by a stop at the Galápagos Islands west of South America

v  He hypothesized that species from South America had colonized the Galápagos and speciated on the islands

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Figure 22.5

Darwin in 1840, after his return from the voyage

The Galápagos Islands

NORTH AMERICA

ATLANTIC OCEAN

PACIFIC OCEAN

PACIFIC OCEAN

Pinta

Marchena Genovesa

Equator

Chile Santiago

Daphne Islands

Fernandina

Isabela Santa Cruz Santa

Fe San Cristobal

Española Kilometers

0 20 40 Florenza

Pinzón

SOUTH AMERICA

AFRICA

EUROPE Great Britain

HMS Beagle in port

Equator PACIFIC OCEAN

Malay Archipelago

AUSTRALIA

Tasmania New Zealand

Brazil

Argentina

Cape Horn

And

es M

tns.

Cape of Good Hope

Figure 22.2 1809

1798

1812

1795

1830

1790 1809 1831-36

1844

1859 1870

Lamarck publishes hypothesis of evolution.

Malthus publishes “Essay on the Principle

of Population.”

Hutton proposes principle of gradualism.

Charles Darwin is born.

Darwin travels around the world on HMS

Beagle.

The Galápagos Islands

Darwin writes essay on descent with modification.

On the Origin of Species is published.

While studying species in the Malay Archipelago, Wallace (shown in 1848) sends Darwin his hypothesis of natural selection.

1858 Cuvier publishes extensive studies of vertebrate fossils.

Lyell publishes Principles of Geology.

Darwin’s Focus on Adaptation

v  In reassessing his observations, Darwin perceived adaptation to the environment and the origin of new species as closely related processes §  Coined the term “descent with

modification” to describe his hypothesis for perceived similarities and differences between species

v  Modern evolutionary biologists have confirmed Darwin’s hypothesis regarding Galapagos finches and iguanas (and other species as well)

On The Origin of Species

v  In 1844, Darwin wrote an essay on natural selection as the mechanism of descent with modification, but did not introduce his theory to a wide public audience

v  Natural selection is a process in which individuals with favorable inherited traits are more likely to survive and reproduce

v  In 1858, Darwin received a manuscript from Alfred Russell Wallace, who had developed a theory of natural selection similar to Darwin’s

v  Darwin quickly finished The Origin of Species and published it the next year

Natural Selection

v  Darwin based his theory of natural selection on two key observations:

1.  Overproduction 2.  Heritable variation among individuals

Observation 1: Overproduction

v  All species tend to produce excessive numbers §  Leads to a struggle for

existence §  Now known as intraspecific

competition

v  Individuals compete with one another for food and other resources

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v  Variation exists among individuals in a population §  Now known as phenotypic variation §  Phenotype = physical trait expressed

v Much of this variation is heritable

Observation 2: Individual variation Darwin’s Big Leap

v  Observation à Inference §  Favorable traits that are heritable become more common §  Unfavorable traits become more rare

v  Why? §  Those individuals with traits best suited for the local

environment are more successful §  Leave behind more offspring §  Pass along more of their genes

The result of natural selection is evolutionary adaptation

(a) A flower mantid in Malaysia

(b) A Trinidad tree mantid that mimics dead leaves

(c) A leaf mantid in Costa Rica

Sources of Genetic Variation

v  Mutations and sexual recombination §  Mutations are changes in the DNA of an organism §  Sexual recombination shuffles alleles during meiosis

v  Sources of genetic variation occur at the level of the individual

v  Natural selection may act at the level of the individual

BUT

v  Evolution acts at the level of the population

Natural Selection does not act on the individual

v  Lamarck: believed that characteristics acquired by the individual can be passed on to offspring

Natural Selection acts on populations (next lecture)

v  Relies on heritable, phenotypic variation within a population

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Descent from a common ancestor

v  Darwin’s ideas explained three broad observations: §  The unity of life §  The diversity of life §  The match between organisms and their environment

v  Darwin never used the word evolution in the first edition of The Origin of Species

v  The phrase descent with modification summarized Darwin’s perception of the unity of life

v  The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past

Evolution is supported by an overwhelming amount of scientific evidence

1.  Fossil Record 2.  Comparative Anatomy 3.  Comparative Embryology 4.  Comparative Genomics 5.  Biogeography 6.  Artificial Selection 7.  Evolution in Real Time

1. Evidence of Evolution – Fossil Record

v  The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time

v  Fossils can document important transitions §  For example, the transition from land to sea in the

ancestors of cetaceans

Figure 22.20

Other even-toed ungulates

Hippopotamuses

†Pakicetus

†Rodhocetus

†Dorudon

Living cetaceans

Common ancestor of cetaceans

Millions of years ago 70 Key 60 50 40 30 20 10 0 Pelvis Tibia

Femur Foot

2. Evidence of Evolution – Homology & Comparative Anatomy

v  Comparative anatomy

§  Comparison of body structure between species

§  Homology §  Similarity in structures due to common ancestry

3. Evidence of Evolution – Comparative Embryology

v  Comparative Embryology §  the comparison of common structures that appear

during the development of different organisms

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4. Evidence of Evolution – Genomic Similarities

v  Evolutionary relationships among species §  Leave markers in DNA and proteins §  Can be determined by comparing genes and proteins of

different organisms §  Another type of homology

5. Evidence of Evolution - Biogeography

v  Biogeography is the geographic distribution of species v  Earth’s continents were formerly united in a single large

continent called Pangaea, but have since separated by continental drift

v  An understanding of continent movement and modern distribution of species allows us to infer when and where different groups evolved

Island Biogeography

v  Endemic species are species that are not found anywhere else in the world

v  Islands often have endemic species closely related to species on the nearest mainland or island

v  Darwin explained that species on islands gave rise to new species as they adapted to new environments

6. Evidence of Evolution - Artificial Selection

v  Humans have modified other species by selecting and breeding individuals with desired traits from a common ancestor

7. Evidence of Evolution - Direct Observations

v  Two examples provide evidence of natural selection

§  Natural selection of introduced plant species

§  Evolution of drug-resistant bacteria

Natural Selection in Response to Introduced Plant Species

v  Soapberry bugs use a “beak” to feed on seeds in fruits v  In southern Florida soapberry bugs feed on native

balloon vine with larger fruit; they have longer beaks v  In central Florida they feed on non-native goldenrain

tree with smaller fruit; they have shorter beaks v  Correlation between fruit size and beak size has also

been observed in Louisiana, Oklahoma, and Australia

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Soapberry Bug Evolution

v  In all cases, beak size has evolved in populations that feed on introduced plants with fruits that are smaller or larger than the native fruits

v  These cases are examples of evolution by natural selection

v  In Florida this evolution in beak size occurred in less than 35 years

Figure 22.13b

On native species, southern Florida

Museum-specimen average

On introduced species, central Florida N

umbe

r of i

ndiv

idua

ls

10

8

6 4

2

0

10

8

6 4

2

0

Beak

Beak length (mm) 6 7 8 10 11

RESULTS

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The Evolution of Drug-Resistant Bacteria

v  The bacterium Staphylococcus aureus is commonly found on people

v  One strain, methicillin-resistant S. aureus (MRSA) is a dangerous pathogen

v  S. aureus became resistant to penicillin in 1945, two years after it was first widely used

v  S. aureus became resistant to methicillin in 1961, two years after it was first widely used

The Evolution of Drug-Resistant Bacteria

v  When exposed to methicillin, MRSA strains are more likely to survive and reproduce than nonresistant S. aureus strains

v  MRSA strains are now resistant to many antibiotics v  Antibiotic resistance is not a “learned” adaptation; bacteria

have short generation times and large population sizes, which increase the rate at which they evolve

v  Drug-resistant bacteria are the result of spontaneous mutation and natural selection

A Different Cause of Resemblance: Convergent Evolution

v  Convergent evolution is the evolution of similar, or analogous, features in distantly related groups

v  Analogous traits arise when groups independently adapt to similar environments in similar ways

v  Convergent evolution does not provide information about ancestry

Natural Selection: A Summary

v  Individuals with heritable traits that are adaptive to the current environment survive and reproduce at a higher rate than other individuals

v  Over time natural selection increases the adaptation of a population of organisms to their environment

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

v  Natural selection does not create new traits, but edits or selects for traits already present in the population

v  The local environment determines which traits will be selected for or selected against in any specific population

v  Evolution can be viewed as both a pattern and a process

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What Is Theoretical About Darwin’s View of Life?

v  In science, a theory accounts for many observations and data and attempts to explain and integrate a great variety of phenomena

v  Darwin’s theory of evolution by natural selection integrates diverse areas of biological study and stimulates many new research questions

v  Ongoing research adds to our understanding of evolution §  Evolutionary biology is an active field- we don’t know

everything yet!