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Essentials of
The Living World
First Edition
GEORGE B. JOHNSON
2
Evolution and Ecology
PowerPoint® Lectures prepared by Johnny El-Rady
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Introduction
Evolution and ecology are two key concepts
Evolution: Changes that occur in organisms’ traits
over time
Ecology: How organisms live in their environment
The great diversity of life on earth is the result
of evolution
And evolution can be said to be the
consequence of ecology over time
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2.1 Darwin’s Voyage on HMS Beagle
In 1831, Charles Darwin
took on the role of
naturalist of the ship
HMS Beagle
Fig. 2.1
The Beagle set sail on a
five-year navigational trip
around the world
Charles Darwin (1809-1882)
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2.1 Darwin’s Voyage on HMS Beagle
Fig. 2.3
Most of the time was spent around South America
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2.1 Darwin’s Voyage on HMS Beagle
Darwin studied a wide variety of plants and
animals across the globe
Particularly on the Galapagos Islands
In 1859, he published his book On the
Origin of Species
In it he proposed that evolution occurs
through natural selection
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2.2 Darwin’s Evidence
At first, Darwin was fully convinced that
species were immutable
However, his observations eventually
convinced him that evolution took place
Fossils of extinct species resembled living species
in the same area
Galapagos finches differed slightly in appearance
but resembled those on the S. American mainland
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In all Darwin observed 14 different finch species
They differed mainly in beaks and feeding habitats
Fig. 2.5 Four Galapagos finches and what they eat
He believed it was “descent with modification” from
a common ancestor Or evolution
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2.3 The Theory of Natural Selection
Darwin was influenced
by Thomas Malthus’s
Essay on the Principle
of Population (1798)
Populations increase
geometrically, while
food supply increases
only arithmetically
Fig. 2.6
Thus, food supply will limit population growth
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2.3 The Theory of Natural Selection
Darwin was also familiar with variation in
domesticated animals
Breeders use artificial selection to produce
animals/plants with particular traits
Darwin proposed that such trait selection
could also occur in nature
A process he termed natural selection
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Fig. 2.7
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2.3 The Theory of Natural Selection
Darwin drafted a preliminary transcript in 1842
However, he shelved it for 16 years
Probably because of its controversial nature
Alfred Russel Wallace (1823-1913)
independently developed a similar theory
Correspondence between the two spurred
Darwin to publish his theory in 1859
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2.3 The Theory of Natural Selection
Darwin’s Origin of Species
was disturbing to many
It suggested that
humans and apes have
a common ancestor
Darwin presented this
argument directly in a later
book, The Descent of Man
Fig. 2.8 Darwin greets
his monkey ancestor
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2.4 The Beaks of Darwin’s Finches
Darwin collected 31 finch species from the
Galapagos Islands in 1835
Ornithologist John Gould determined that
these finches were a closely-related group
They differed only in their bills
Darwin observed a correlation between the
beaks and the food source of the birds
He concluded that the beaks had been
shaped by evolution
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2.4 The Beaks of Darwin’s Finches
Fig. 2.10 A diversity of finches on a single island
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2.4 The Beaks of Darwin’s Finches
In 1938, David Lack set out to test Darwin’s
hypothesis
Lack’s five-month observation seemed to
contradict Darwin’s proposal
Lack found many different species of finch
feeding together on the same seeds
So was Darwin wrong or is there something
else going on?
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2.4 The Beaks of Darwin’s Finches
In 1973, Peter and Rosemary Grant embarked
on a study of the medium ground finch
Geospiza fortis feeds preferentially on small
tender seeds abundantly available in wet years
It resorts to larger, harder seeds in dry years
The Grants found out that the beak depth
changed predictably year after year
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2.4 The Beaks of Darwin’s Finches
Fig. 2.11
Large-beaked finches
increase in number
Small-beaked finches
are more common
The Grants’ research supported Darwin’s hypothesis
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2.5 How Natural Selection
Produces Diversity
Darwin believed that the Galapagos finches
all evolved from a single common ancestor
The ancestor came from the South American
mainland
New arrivals occupied different niches and
were subject to different environmental
pressures
This resulted in a cluster of species
A phenomenon termed adaptive radiation
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2.5 How Natural Selection
Produces Diversity
The 14 finch species that Darwin studied
now occupy four types of niches
1.
2.
3.
4.
Ground finches
Tree finches
Warbler finches
Vegetarian finch
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Fig. 2.12 An evolutionary tree of Darwin’s finches
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2.6 What is Ecology?
The term was coined by Ernst Haeckel (1866)
Gr. logos, study of
Ecology
Gr. oikos, house
Thus, ecology is the study of how
organisms interact with their environment
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2.6 What is Ecology?
There are five levels of ecological organization
1. Populations
Individuals of the same species living together
2. Communities
Populations of different species living together
3. Ecosystems
Combination of communities and associated
non-living factors
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2.6 What is Ecology?
There are five levels of ecological organization
4. Biomes
Major terrestrial assemblages of organisms
that occur over wide geographical areas
5. The Biosphere
All biomes together with marine and
freshwater assemblages
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2.7 A Closer Look at Ecosystems
Ecosystems: the fundamental units of ecology
All organisms in an ecosystem require energy
Almost all energy comes from the sun
Sun
Plants
Herbivores
Carnivores
Food chain
Energy is lost at each step of the food chain
This limits the number of steps
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2.7 A Closer Look at Ecosystems
Raw materials are not used up when
organisms die
They are recycled back into the ecosystem
for use by other organisms
Rainfall and temperature are the two most
important factors limiting species distribution
These physical conditions with their sets of
similar plants and animals are called biomes
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Fig. 2.14 Biomes of North America
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2.8 How Species Evolve to Occupy
Different Niches Within an Ecosystem
The niche of a species may be defined as its
biological role in the community
It is not synonymous with habitat
Habitat  place
Niche  pattern of living
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2.8 How Species Evolve to Occupy
Different Niches Within an Ecosystem
The principle of competitive exclusion
No two species with the same niche can coexist
Persistent competition is rare in nature
So species try to find ways to reduce competition
In resource partitioning, species avoid
competition by
1. Living in different portions of the habitat
2. Using different resources
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Fig. 2.15 Resource partitioning among sympatric lizard species
Anolis lizards
Same pattern has evolved independently
on different Caribbean Islands
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2.8 How Species Evolve to Occupy
Different Niches Within an Ecosystem
Character displacements
The changes that evolve in two species to
reduce niche overlap
This is clearly seen among Darwin’s
finches
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Fig. 2.16 Character displacement in Darwin’s finches (genus Geospiza)
Bills of similar sizes
when living apart
Bills of different sizes
when living together
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2.9 Patterns of Population Growth
Innate capacity for increase
The rate at which a population grows in the
absence of limits
Also termed the biotic potential
Realized rate of population increase (r)
# of individuals added minus the # lost
Thus:
r = (birth + immigration) – (death + emigration)
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Exponential Growth
To determine the population growth rate,
r must be adjusted for population size
population growth rate = rN
# of individuals in
the population
Realized rate of
population growth
A population exhibits exponential growth at
first
However, the growth rate slows down as
resources become depleted
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Fig. 2.17 Exponential growth in a population of bacteria
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Carrying Capacity (K)
Is the number of individuals that can be
supported in a particular area indefinitely
Population growth is limited by shortages of
some important factor
Space, water, nutrients
Growth of a population is approximated by the
following logistic growth equation
population growth rate = rN
K–N
K
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This relationship is graphically represented
as a sigmoid growth curve
Growth fluctuates
around K
Fig. 2.18
Exponential
growth at first
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Life History Strategies
The particular set of adaptations that adjusts
an organism’s growth rate to its environment
r-selected life history
K-selected life history
Rapid growth
Slow growth
Short lifespan
Long lifespan
Transient environments
Stable environments
Large no. of offspring
Small no. of offspring
No parental care
Parental care
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Life History Strategies
Fig. 2.20 K-selected life history
Fig. 2.19 r-selected life history
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2.10 Human Populations
Throughout most of our history, human
populations have been regulated by
Food availability
Disease
Predators
Two thousand years ago, the human
population was ~ 130 million
It took one thousand years for it to double
And another 650 years for it to double again
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2.10 Human Populations
Starting in the 1700s, technological changes
gave humans more control over their
environment
These changes allowed humans to expand
the carrying capacity of their habitats
Currently, the human population is growing at
a rate of ~ 1.3% annually
Doubling time at this rate is only 54 years!
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Fig. 2.21 History
of human
population size
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Population Pyramids
Human population growth is not uniform
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Population Pyramids
Some countries, like Mexico, are currently
growing rapidly
Fig. 2.22
Birth rate much
higher than
death rate
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Population Pyramids
A population’s age structure and sex ratio can
be used to assess its demographic trends
Fig. 2.23
Triangular pyramid
=> Rapid future
growth
More or less
rectangular pyramid
=> Stable population
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Consumption in the Developed World
The vast majority of the world’s population is
in developing countries
However, the vast majority of resource
consumption is in the developed world
This disparity can be quantified by calculating
the ecological footprint
The amount of productive land required to
support a person throughout his or her life
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Fig. 2.24 Ecological footprint of individuals in different countries
Resource use by humans
is now 1/3 greater than the
amount that nature can
sustainably replace
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