Descent with Modification:

A Darwinian View of Life

Chapter 22

Overview: Darwin Introduces a Revolutionary Theory

 A new era of biology began on November 24, 1859 – the day Charles Darwin published

On the Origin of Species by Means of Natural Selection

 This manuscript:  Focused biologists’ attention on the great diversity of organisms

Darwins Two Major Points from

Origin of Species

1.

2.

Presented evidence that many species of organisms presently inhabiting the Earth are

descendents

of

ancestral species

(

common descent

) Proposed a mechanism for the evolutionary process  (

natural selection

)

a population’s

allele frequency

can

change

over generations if individuals that possess certain heritable traits leave

more offspring

than others   Results in

evolutionary adaptation

inherited characteristics that – accumulation of

enhance

organisms’ ability to

survive and reproduce

in specific environments

evolution

– change over time in genetic composition of a population and could eventually lead to

new species

Darwin’s Ideas Challenged Traditional Views

  The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species In order to understand why Darwin’s ideas were revolutionary, it is important to examine his view in context of other Western ideas about Earth and its life  There was MUCH resistance to the idea of evolution

On the Origin of Species

 Shook the deepest roots of Western culture  Challenged a worldview that had been prevalent for centuries

The Historical Context of Darwin’s Life and Ideas – Figure 22.1

1.

2.

3.

The Scale of Nature and Classification of Species

 

Aristotle

(Greek Philosopher) viewed species as

fixed and unchanging

Scale of Nature – all living things are arranged on a scale of

increasing complexity

 Old Testament of the

Bible

holds that species were individually designed by God and therefore are perfect  interpreted organismal adaptations as evidence that each species was designed to serve a specific purpose  

Carolus Linnaeus

(Swedish Botanist) developed two-part (

binomial

) system of naming organisms – still used today

taxonomy

– branch of biology concerned with naming and classifying organisms

Fossils, Cuvier, and Catastrophism

  Study of fossils (paleontology) helped lay groundwork for Darwin’s ideas  

Fossils

past - remains or traces of organisms from the Usually found in sedimentary rock, which appears in layers/strata Fossils in each layer provide a glimpse of some of the organisms that populated Earth at the time that layer formed

Formation of Sedimentary Rock – Figure 22.3

Fossils, Cuvier, and Catastrophism



Georges Cuvier

- noted that sedimentary strata contain unique groups of organisms  Deeper strata contain species that are often very different from those present today  Rather than follow evolution, Cuvier believed in

Catastrophism

- the idea that each strata boundary corresponds to some catastrophic event, such as flood

Theories of Gradualism

  

James Hutton

introduced the theory of gradualism 

Gradualism -

holds that geological changes are the cumulative product of slow, continuous processes

Charles Lyell

introduced the theory of uniformitarianism 

Uniformitarianism -

incorporates gradualism, and refers to the idea that geologic processes have not changed throughout earth’s history  So, there have been very few catastrophes and the earth must be older than 4-6000 years old Exerted a strong influence on Darwin’s thinking— applied the principle of gradualism to biological evolution

Lamarck’s Theory of Evolution

  

Jean Baptiste Lamarck

placed fossils in an evolutionary context and was the first person to present a functional mechanism describing evolution He used two common ideas of his time: 

Use and disuse

- organs that are used become stronger and more functional; those that are not used deteriorate 

Inheritance of acquired characteristics

- after acquiring a new trait, an organism is able to pass it on to offspring Lamarck’s theory was based on observations; they were unsupported by evidence

Lamarck v/s Darwin

Darwin’s Proposals

  In

The Origin of Species

, Darwin proposed that

species change through natural selection

 19th century—generally believed species remained unchanged since their creation, but a major change would challenge this thinking Darwin’s Research :   As a boy and into adulthood, Charles Darwin had a consuming interest in nature Soon after Darwin received his B.A. degree, he was accepted on board the HMS Beagle which was about to embark on a voyage around the world

The Voyage of the Beagle

   During his travels on the Beagle, Darwin:   Observed and collected many specimens of South American plants and animals Observed various adaptations of plants and animals that inhabited many diverse environments Geologic observations - found fossils of organisms high in the Andes Mountains; inferred that rocks containing fossils must have been raised there by a long series of earthquakes Galapagos Islands   Learned that most species there occur nowhere else in the world, but many resemble species in South America Species on the islands are related but, like finches, were adapted to different lifestyles

Voyage of the Beagle

Galapagos Finches

Darwin’s Focus on Adaptation

1.

2.

3.

4.

5.

Darwin reassessed all observations made during the voyage He began to perceive adaptation to the environment and the origin of new species as closely related processes 1.

2.

Species adapt to succeed in changing/different environments Confirmed years later—Galapagos finches’ beaks and behaviors are adapted to the specific foods available on their home islands In 1844, Darwin wrote a long essay on the origin of species and natural selection but reluctant to publish, anticipating the uproar it would cause In 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 next year

  

The Origin of Species

Evolution explains life’s unity and diversity Natural selection is a cause of adaptive evolution 1.

Descent with Modification Think of descent in terms of a branching tree 2.

3.

4.

5.

6.

Over time, organisms enter/experience new environments which lead to changes in the organisms These changes cause divergence, the branches of the tree Closely related species are closest together in the tree The trunk represents a common ancestor The Linnaean classification scheme fits the concept of an evolutionary tree; genus and species may differ, but still belong in the same family

Descent with Modification – Figure 22.7

Natural Selection and Adaptation

 1.

2.

3.

4.

5.

Darwin’s Observations: All species have the potential to

overproduce

offspring. If they continued to do so, they would use up all of the world’s resources Populations are

stable

, except for seasonal fluctuations, or unless changed by man altering the environment Environmental

resources are limiting

Individuals within a population have

varying characteristics

Much of the variation is

heritable

(genetic)

1.

2.

3.

Summary of Darwin’s Ideas

Natural selection

is differential success in reproduction (unequal ability of individuals to survive and reproduce).

Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up populations.

The product of natural selection is the

adaptation

of populations of organisms to their environment.

Natural Selection & Adaptation

Mayr’s Inferences Regarding Natural Selection

 Conditions lead to a struggle for existence among individuals of a population  Individuals with traits that allow them to outcompete others will survive and reproduce  Favorable traits are maintained, leading to gradual changes in the population

Thomas Malthus

 His ideas had a profound influence on Darwin’s thinking:  “man will overpopulate the planet unless there is disease, war, or famine” - eventually things will start dying  Populations in nature DO NOT GROW EXPONENTIALLY – there are always limits to population growth!

Artificial Selection

 Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits  Results in plants and animals bred as livestock/pets that often bear little resemblance to their wild ancestors (ex. wild mustard)

Summary of Natural Selection

1.

2.

3.

4.

5.

Overpopulation

- more organisms are born than can survive

Variation

within a population - there will be many variation for different traits among individuals

Competition

within the population - individuals will compete for survival: food, mates, shelter, etc.

Survival of the fittest

- those with traits best suited to the environment will be more likely to survive

Reproduction

- individuals that survive will pass their traits on to the next generation

Natural selection is differential success in reproduction – That results from the interaction between individuals that vary in heritable traits and their environment

Key Points to Remember

1.

2.

3.

4.

5.

Population - group of interbreeding individuals belonging to a particular species and sharing a common geographic area A population is the smallest unit that can evolve Natural selection occurs because of interaction between organisms and their environments Natural selection works to increase or decrease the occurrence of heritable traits Individuals are selected, but populations evolve

Evolution

1.

Theory

- an accepted hypothesis that has been tested over and over again without yet being disproved

2.

Definition

- Evolution is the change in the overall genetic makeup (allele frequency) of a population over time

3.

Three Basic Components

a. Individuals cannot evolve. Populations evolve.

b. Natural selection is the mechanism of evolution.

c. Evolution occurs by chance (NOT GOAL ORIENTED).

Darwin’s Theory Explains a Wide Range of Observations

  Natural Selection in Action: 1.

The evolution of insecticide resistance in insect populations 2.

The evolution of drug resistant HIV  Note: an environment does not create resistance, it selects for resistant individuals that were already present in the population.

And, natural selection depends on time and place. It favors those characteristics in a genetically variable environment that increase fitness in the current, local environment.

Evolution of Insecticide Resistance

1. By spraying crops with poisons to kill insects, humans have unwittingly favored the reproductive success of insects with inherent resistance to poisons.

2. Resistant individuals survive and reproduce, passing the gene for resistance to offspring.

3. Additional applications of the same insecticide will be less effective, and the frequency of resistant insects in the population will grow.

The Evolution of Drug Resistant HIV

 Researchers developed numerous drugs to combat HIV, but using these medications selects for viruses resistant to the drugs    A few drug-resistant viruses may be present by chance at the beginning of treatment The drug-resistant pathogens are more likely to survive treatment and pass on the genes that enable them to resist the drug to their offspring As a result, the frequency of drug resistance in the viral population rapidly increases

Antiviral Resistance

Darwin’s Theory Explains a Wide Range of Observations

  1.

The example of the insect resistance/HIV highlight 2 important points about natural selection: It is an editing mechanism, not a creative force; it only act on existing variation in population; cannot create favorable traits 2.

It favors traits that increase fitness in current, local environment; what is adaptive in one situation may not be adaptive in another In the cases described, natural selection brought about change rapidly enough that it could be

observed directly

Evidence of Evolution

 Evidence that the diversity of life is a product of evolution pervades every research field of biology.



Comparative Anatomy

 Anatomical Homologies  Embryological Homologies  Molecular Homologies  

Biogeography

 Continental Drift  Geographic Distribution of Species

Fossil Record Evidence

 Succession of Fossil Forms

Comparative Anatomy

 The study of different structures contributes to scientists’ understanding of the evolution of anatomical structures and of evolutionary relationships.

   Homologous structures Analogous structures Vestigial Structures

Homologies

 

Homologous structures

are similar structures occurring in different species that are believed to be derived from a common ancestor.

Analogous structures

are similar structures occurring in different species that are believed to be the result of convergent evolution (similar environmental pressures).

Homology

 Species with common ancestry should display underlying similarities, even in features that no longer match in function.



Homology

refers to similarities in characteristics resulting from common ancestry  3 types: anatomical, embryological & molecular

Anatomical Homologies

   Descent with modification is indeed evident in anatomical similarities between species grouped in the same taxonomic category.

The forelimbs of human, cats, whales, and bats share the same skeletal elements, even though the appendages have very different functions  These forelimbs are

homologous structures

variation on the ancestral tetrapod forelimb that represent The most likely explanation is that the basic similarity of these forelimbs is the consequence of the descent of mammals from a common ancestor – rather than all being uniquely engineered and unrelated.

Homologous Structures – Figure 22.14

Anatomical Homologies

 Many homologies that are not obvious in adult organisms become evident when we look at embryonic development:  All vertebrate embryos have tails posterior to the anus, as well as structures called pharyngeal (throat) pouches  Develop into gills in fishes and parts of ears/throat in humans

Embryological Homologies

Closely related organisms go through similar stages in their embryonic development. For example, all vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats.

Early Embryonic Development

Vestigial Homologies



Vestigial organs

- structures that have marginal, if any, importance to living organism, but had important functions in organism’s ancestors  Comparative anatomy confirms evolution is a remodeling process, an alteration of existing structures (not perfect = back/knee problems)

Vestigial Structures

The skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors. We would not expect to see these structures if snakes had an origin separate from other vertebrate animals.

Molecular Homologies

   All life forms use the same genetic machinery of DNA/RNA and the genetic code is essentially universal  Due to this, all species may have descended from a common ancestor Humans and bacteria share genes inherited from a distant ancestor Homologies and the Tree of Life  Molecular homologies basically follow the tree of life more closely related organisms have more similar homologies.

Molecular Homologies – Table 22.1

The data show the same pattern of evolutionary relationships that researchers find when they compare other proteins or assess relationships based on non molecular methods, such as skeletal anatomy.

Homologies mirror the taxonomic hierarchy of the tree of life – the Darwinian view of life predicts that different kinds of homologies in a group of organisms will all tend to show the same branching pattern through their evolutionary history.

Biogeography

 Darwin’s observations of the geographic distribution of species (biogeography) formed an important part of his theory of evolution.

  Species tend to be more closely related to other species from the same area than to other species with the same way of life but living in different areas.

Austraila is the home of marsupials (develop in pouch) – distinct from eutherians (live elsewhere and develop in uterus)  Some similar mammals that have adapted to similar environments have evolved independently from different ancestors (ex. Squirrels)

Biogeography

 Darwin’s experience in the Galapagos were pivotal to his thinking:  Islands generally have species found no where else in the world, yet it was observed by Darwin that most island species are closely related to species from the nearest mainland or neighboring island.

 This explains why 2 islands with similar environments in different parts of the world are populated not by closely related species, but rather by species that resemble those of the nearest mainland.

Biogeography

 Some similar mammals that have adapted to similar environments   Have evolved independently from different ancestors This is convergent evolution Sugar glider AUSTRALIA NORTH AMERICA Flying squirrel

Fossil Record

   We can see the development of present day species from ancestral species 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  Prokaryote fossils precede all eukaryotic fossils Darwinian view predicts evolutionary transitions leave signs in fossil record  Paleontologists have discovered fossils of many transitional forms

Endurance of the Darwinian View

 The Darwinian view of life endures in biology because it is supported by independent types of evidence:   Anatomical and molecular homologies that match patterns in space (biogeography) and time (the fossil record).

Natural selection can also explain how similar adaptations can evolve independently among distantly related species, such as sugar gliders and flying squirrels.

Summary of Darwin’s Theory

http://bcs.whfreeman.com/thelifewire/content/chp23/2302003.html

1.

2.

3.

4.

5.

6.

7.

Individual organisms in nature

differ from one another

some of this

variation is inherited

and Organisms in nature

produce more offspring than can survive

– and many that survive do not reproduce Members of each species must

compete for resources

Individuals

best suited to their environment survive and reproduce most successfully

– they pass their traits onto their offspring

Species change over time

– this is

caused by natural selection

– new species arise and other species disappear Species alive today have

descended with modifications from species that lived in the past

All organisms on Earth are united into a single tree of life by common descent