The Origin and

Diversification of Life

on Earth

BIOL 108 Intro to Bio Sci

Chapter 10 pt 1

Rob Swatski Assoc Prof Biology

HACC-York 1

Learning Goals

Life on earth most likely

originated from nonliving materials.

Species are the basic units of biodiversity.

Evolutionary trees help us conceptualize and categorize

biodiversity.

Macroevolution and the

diversity of life.

An overview of the diversity of life on earth.

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10.1 Complex organic molecules arise in non-living

environments.

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Phase 1: The Formation of Small Molecules Containing Carbon and Hydrogen

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The Urey-Miller Experiments

The first demonstration that complex organic

molecules could have arisen in earth’s early

environment

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Why is it important that Urey and Miller’s experiment produced amino acids?

1. Because they are the building blocks of DNA

2. Because they are the building blocks of RNA

3. Because they are the building blocks of protein

4. Because they are the building blocks of triglycerides

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Take-Home Message 10.1

Under conditions similar to those on early earth, small organic molecules form which have some

chemical properties of life.

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Life on earth most likely originated from

nonliving materials.

10.2 Cells and self-replicating systems evolved together to create the first

life.

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- RNA appears on the scene

- RNA can catalyze reactions necessary for replication

Phase 2: The Formation of Self-Replicating, Information-Containing Molecules.

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The “RNA World” Hypothesis

A self-replicating

system

A precursor to cellular

life?

RNA-based life and DNA-

based life 13

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Life Is Defined by Two Characteristics:

1) the ability to replicate

2) the ability to carry out some sort of metabolism

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Phase 3: The Development of a Membrane, Enabling Metabolism,

and Creating the First Cells

Membranes make numerous aspects of

metabolism possible.

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How Did the First Cells Appear?

Spontaneously? Mixtures of

phospholipids Microspheres

Compartmental-ization within

cells

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Take-Home Message 10.2

The earliest life on earth appeared about 3.5 billion years ago, not long after

earth was formed.

Self-replicating molecules—possibly RNA—may have formed in earth’s early environment and later acquired or developed

membranes

Membranes enabled these self-replicating molecules to

replicate and make metabolism possible, the

two conditions that define life.

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10.3 What is a species?

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Biological Species Concept

Species: different kinds of organisms

Species are natural populations of organisms that:

• interbreed with each other or could possibly interbreed

• cannot interbreed with organisms outside their own group (reproductive isolation)

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Two Key Features of the Biological Species Concept:

1) Actually interbreeding or could possibly interbreed

2) “Natural” populations

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The biological species concept is not easily applied to bacteria and fossils. What other characteristics could we

use to decide whether two different bacteria are two different species?

1. Observe asexual reproduction.

2. Measure the size of each bacteria.

3. Compare the sequences of specific genes in the bacteria.

4. All of the above.

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Barriers to Reproduction

1) Prezygotic barriers

2) Postzygotic barriers

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Prezygotic Barriers

Make it impossible for individuals to mate

with each other

OR

Make it impossible for the male’s

reproductive cell to fertilize the female’s

reproductive cell

Examples of Prezygotic

Barriers

Courtship systems

Physical differences

Physical or biochemical

factors involving gametes

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Postzygotic Barriers

Occur after fertilization

Generally prevent the

production of fertile offspring

Hybrids

Which answer below is considered to be a postzygotic barrier to reproduction?

1. Two species of frogs mate at different times in the spring.

2. Birds about to mate need to go through specific courtship rituals.

3. The male gamete (pollen) of one flower is not compatible with the female organs of another flower.

4. Fertilization occurs, but the embryo dies before it is born.

5. There is a mechanical isolation that prevents fertilization.

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Take-Home Message 10.3

Species are generally defined as populations of

individuals that interbreed with each other or could

possibly interbreed.

Species cannot interbreed with organisms outside

their own group.

This concept can be applied easily to most

plants and animals, but for many other organisms it

cannot be applied.

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10.4 How do we name species?

We need an organizational system!

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Carolus Linnaeus and Systema Naturae

A scientific name consists of two parts:

1) Genus

2) Specific epithet

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Hierarchical System

Inclusive categories at the top…

…leading to more and more exclusive categories

below.

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Take-Home Message 10.4

Each species on earth is given a unique name,

using a hierarchical system of classification.

Every species on earth falls into one of three

domains.

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10.5 Species are not always easily defined.

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Difficulties in Classifying Asexual Species

Doesn’t involve fertilization or even two individuals

Does not involve any interbreeding

Reproductive isolation is not meaningful

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Chihuahuas and Great Danes generally can’t mate.

Does that mean they are different species?

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Difficulties in Classifying Ring Species

Example: insect-eating songbirds called greenish warblers

Unable to live at the higher elevations of the Tibetan mountain range

Live in a ring around the mountain range

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Difficulties in Classifying Ring Species

Warblers interbreed at southern end of ring.

The population splits as the warblers move north along either side of mountain.

When the two “side” populations meet at northern end of ring, they can’t interbreed.

What happened?!

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Difficulties in Classifying Ring Species

Gradual variation in the warblers on each side of the mountain range has accumulated…

…the two populations that meet have become reproductively incompatible…

…no exact point at which one species stops and the other begins

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Difficulties in Classifying Hybridizing Species

Hybridization

• the interbreeding of closely related species

Have postzygotic barriers evolved?

Are hybrids fertile?

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Morphological Species Concept

Focus on aspects of organisms other than reproductive isolation as defining features

Characterizes species based on physical features such as body size and shape

Can be used effectively to classify asexual species

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Which answer below would require the morphological species concept to delineate between the two species?

1. Dog and cat

2. Salmonella and E. coli

3. Cow and goat

4. Donkey and horse

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Take-Home Message 10.5

The biological species concept is useful when describing most

plants and animals.

It falls short of representing a universal and definitive way of distinguishing many life forms.

Difficulties arise when trying to classify asexual species, fossil

species, speciation events that have occurred over long periods

of time, ring species, and hybridizing species.

In these cases, alternative approaches to defining species

can be used.

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10.6 How do new species arise?

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Speciation

One species splits into two distinct species.

Occurs in two distinct phases

Requires more than just evolutionary change in a population

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Allopatric Speciation

Speciation with geographic isolation

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Speciation without Geographic Isolation

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Polyploidy

Error during cell division in plants

Chromosomes are duplicated but a cell does not divide.

This doubling of the number of sets of chromosomes is called polyploidy.

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Polyploidy

The individual with four sets can no longer interbreed with any individuals having only two sets of chromosomes

Self-fertilization or mating with other individuals that have four sets can occur.

Instant reproductive isolation, considered a new species.

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Take-Home Message 10.6

Speciation is the process by which one species splits into two distinct

species that are reproductively isolated.

It can occur by polyploidy or by a combination of reproductive isolation

and genetic divergence together.

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10.7 The history of life can be imagined as a tree.

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Systematics and Phylogeny

Systematics names and arranges species in a manner that indicated: • the common ancestors they share

• the points at which they diverged from each other

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Systematics and Phylogeny

Phylogeny • evolutionary history, of organisms

Nodes • The common ancestor points at which species diverge

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Take-Home Message 10.7

The history of life can be visualized as a tree; tracing

from the branches back toward the trunk follows the pathway of descendant back

to ancestor.

The tree reveals the evolutionary history of every species and the

sequence of speciation events that gave rise to

them. 61

10.8 Evolutionary trees show ancestor-descendant relationships.

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Monophyletic Groups

a group in which all of the individuals are more closely related to each other than to any individuals outside of that group

determined by looking at the nodes of the trees

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Which animals are represented in a monophyletic group indicated by the red

arrow?

1. Fish, bird, human, rat, and mouse

2. Bird, human, rat and mouse

3. Human, rat, and mouse

4. Rat and mouse

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Constructing evolutionary trees requires comparing similarities

and differences between organisms.

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Below are genetic sequences for a gene found in four different species. Which species are the

most closely related?

Species A: AGT-CTA-CTT-ACT-ATC-CTA Species B: AGT-CTA-CTT-ACC-ATC-CTA Species C: AGT-AAA-CTT-ACC-ATC-CTA Species D: AGA-CTA-TTT-ACC-ATG-CTA

1. Species A and B

2. Species A and C

3. Species A and D

4. Species B and D

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Take-Home Message 10.8

Evolutionary trees constructed by biologists are

hypotheses about the ancestor-descendant

relationships among species.

The trees represent an attempt to tell us which groups are most closely

related to which other groups based on physical features,

usually DNA sequences. 73