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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
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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
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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
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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
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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