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Chapter 5
This lecture will help you understand:
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Natural selection
Evolution and biodiversity
Species and mass extinction
Levels of ecological organization
Population characteristics that predict growth
Population ecology
Conservation challenges
Central Case: Striking Gold in a Costa Rican Cloud Forest
• The golden toad of Monteverde, discovered in 1964, had disappeared 25 years later.
• Researchers determined that warming and drying of the forest was most likely responsible for
its extinction.
• As the global climate changes, more such events can be expected.
Evolution and natural selection
• Evolution = genetic change across generations
• Natural selection = process by which traits that enhance survival are passed on
to future generations more than those that do not
• This alters the genetic makeup of populations over time.
Natural selection shapes diversity
• Charles Darwin and Alfred Russell Wallace each proposed natural selection as
a mechanism for evolution and a way to explain the variety of living things.
• A trait that promotes success in natural selection is called an adaptive trait or
an adaptation.
• A trait that reduces success is maladaptive.
• A trait that is adaptive in one location or season may prove maladaptive in
another.
Natural selection and genetic variation
• For a trait to be heritable, genes in an organism’s DNA must code for the trait.
• Mutations are accidental changes in DNA.
• Mutations that are not lethal provide the genetic variation on which natural
selections act.
• In these ways, variable genes and variable environments interact in adapting to
environmental conditions.
Biodiversity
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Biodiversity, or biological diversity, is the sum of an area’s organisms,
considering the diversity of species, their genes, their populations, and their
communities.
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A species is a particular type of organism; a population or group of populations
whose members share certain characteristics and can freely breed with one
another and produce fertile offspring.
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A population is a group of individuals of a particular species that live in the
same area.
Biodiversity
Costa Rica’s Monteverde cloud forest is home to many species and possesses great
biodiversity.
Speciation
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Speciation: The process by which new species come into being
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It is an evolutionary process that has given Earth its current species richness—
more than 1.5 million described species and likely many million more not yet
described by science.
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Allopatric speciation is considered the dominant mode of speciation, and
sympatric speciation also occurs.
Allopatric speciation
1. Single interbreeding population
2. Population divided by a barrier; subpopulations isolated
Allopatric speciation
3. The two populations evolve independently, diverge in their traits.
4. Populations reunited when barrier removed, but are now different enough that
they don’t interbreed.
Allopatric speciation
Many geological and climatic events can serve as barriers separating populations
and causing speciation.
Phylogenetic trees
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Life’s diversification results from countless speciation events over vast spans
of time.
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Evolutionary history of divergence is shown with diagrams called
phylogenetic trees.
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Similar to family genealogies, these show relationships among organisms.
Phylogenetic trees
These trees are constructed by analyzing patterns of similarity among present-day
organisms.
This tree shows all of life’s major groups.
Phylogenetic trees
Within the group Animals in the previous slide, one can infer a tree of the major
animal groups.
Phylogenetic trees
And within the group Vertebrates in the previous slide, one can infer relationships
of the major vertebrate groups, and so on…
Extinction
• Evolution has not always progressed in a straight-forward manner, or from
simple to complex.
• The Burgess Shale fauna were complex and bizarre marine animals from 530
million years ago that vanished completely.
Extinction
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Extinction is the disappearance of an entire species from the face of the Earth.
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Average time for a species on Earth is ~1–10 million years.
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Species currently on Earth = the number formed by speciation minus the
number removed by extinction
Extinction
• Some species are more vulnerable to extinction than others:
• Species in small populations
• Species adapted to a narrowly specialized resource or way of life
• Monteverde’s golden toad was apparently such a specialist, and lived in small
numbers in a small area. It was endemic to the forest, occurring nowhere else.
Extinction
• Until 10,000 years ago, North America teemed with camels, mammoths, giant
sloths, lions, horses, saber-toothed cats, and other large mammals.
• Many scientists think their extinction was brought on by hunting after human
arrival.
Mass extinctions
Earth has seen five mass extinction events: 50%+ of species were wiped out.
Life’s hierarchy of levels
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Organismal
Population
Community
Ecosystem
Biosphere
Ecology
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Communities are made up of multiple interacting species that live in the same
area.
Ecosystems encompass communities and the nonliving material with which
their members interact.
Population ecology investigates how individuals within a species interact with
one another.
Community ecology studies interactions among species.
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Ecosystem ecology reveals patterns by studying living and nonliving
components of systems in conjunction.
Habitat and niche
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Habitat = the specific environment where an organism lives (including living
and nonliving elements: rocks, soil, plants, etc.)
Habitat selection = the process by which organisms choose habitats among the
options encountered
Niche = an organism’s functional role in a community (feeding, flow of energy
and matter, interactions with other organisms, etc.)
Specialists = organisms with narrow breadth and thus very specific
requirements
Population ecology
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Population = a group of individuals of a species that live in a particular area
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Several attributes help predict population dynamics (changes in population):
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Population size
Population density
Population distribution
Sex ratio
Age structure
Birth and death rates
Population size
Number of individuals present at a given time
The passenger pigeon was once North America’s most numerous bird, but is
now extinct.
Population density
Number of individuals per unit area
In the 19th century, the flocks of passenger pigeons showed high population
density.
Population distribution
Spatial arrangement of individuals
Age structure
• Or age distribution = relative numbers of individuals of each age or age class
in a population
• Age structure diagrams, or age pyramids, show this information.
Age structure
Sex ratio
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Ratio of males to females in a population
Even ratios (near 50/50) are most common.
Fewer females causes slower population growth.
Note human sex ratio biased toward females at oldest ages.
Population growth
Populations grow, shrink, or remain stable,
depending on rates of birth, death, immigration,
and emigration.
(crude birth rate + immigration rate) –
(crude death rate + emigration rate)
= growth rate
Survivorship curves
• Type I: survival rates are high when organisms are young and decrease sharply
when organisms are old.
• Type II: survival rates are equivalent regardless of an organism’s age.
• Type III: most mortality takes place at young ages, and survival rates are
greater at older ages.
Exponential growth
• Unregulated populations increase by exponential growth:
• Growth by a fixed
percentage, rather
than a fixed amount.
• Similar to growth
of money in a
savings account
Exponential growth in a growth curve
• Population growth curves show change in population size over time.
• Scots pine shows exponential growth
Limits on growth
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Limiting factors restrain exponential population growth, slowing the growth
rate down.
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Population growth levels off at a carrying capacity—the maximum population
size of a given species an environment can sustain.
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Initial exponential growth, slowing, and stabilizing at carrying capacity is
shown by a logistic growth curve.
Logistic growth curve
Population growth: Logistic growth
Logistic growth (shown here in yeast from the lab) is only one type of growth
curve, however.
Population growth: Oscillations
Some populations fluctuate continually above and below carrying capacity, as with
this mite.
Population growth: Dampening oscillations
In some populations, oscillations dampen, as population size settles toward
carrying capacity, as with this beetle.
Population growth: Crashes
Some populations that rise too fast and deplete resources may then crash, as with
reindeer on St. Paul Island.
Density dependence
• Often, survival or reproduction lessens as populations become more dense.
• Density-dependent factors (disease, predation, etc.) account for the logistic
growth curve.
• Other factors (e.g., catastrophic weather events) occur regardless of density, and
are density-independent factors.
Biotic potential and reproductive strategies
• Species differ in strategies for producing young.
• Species producing lots of young (insects, fish, frogs, plants) have high biotic
potential.
• Others, such as mammals and birds, produce few young.
• However, those with few young give them more care, resulting in better
survival.
r and K-selected species
r-selected species
• Many offspring
• Fast growing
• No parental care
K-selected species
• Few offspring
• Slow growing
• Parental care
Conservation of Biodiversity
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Ecotourism = tourism focused on visiting natural areas.
It provides economic incentive to local communities for conservation of nature
Conclusion
• The golden toad and other Monteverde organisms have helped illuminate the
fundamentals of evolution and population ecology.
• Natural selection, speciation, and extinction help determine Earth’s biodiversity.
• Understanding ecological processes at the population level is crucial to
protecting biodiversity.