Introduction to
Evolutionary Ecology
(Part I: Week 4)
•Biological Evolution
•Natural Selection
•Biological Diversity
•Speciation
•Phylogenetic Trees
•Extinction
•Units of Ecological Organization
Biodiversity
• Biodiversity, or biological diversity, is a quantitative
description of an area’s organisms, considering the
diversity of species, their genes, their populations, and
their communities.
• 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.
Natural Selection
• Natural selection rests on three indisputable facts:
1. Organisms produce more offspring than can
survive.
2. Individuals vary in their characteristics.
3. Many characteristics are inherited by offspring
from parents.
THEREFORE,
• Some individuals will be better suited to their
environment; they will survive and reproduce more
successfully.
• These individuals will transmit more genes to future
generations.
• Future generations will thus contain more genes from
better-suited individuals.
• Thus, characteristics will evolve over time to resemble
those of the better-suited ancestors.
Fitness = likelihood that an individual will
reproduce
and/or
the number of offspring an individual
produces over its lifetime
Adaptive trait, or adaptation =
a trait that increases an individual’s
fitness
• Evidence of natural
selection is all
around us:
1. In nature …
Textbook example:
Diverse types of bills
in Hawai’ian
honeycreepers
Figure 4.23a
2. In our
domesticated
organisms.
Dog breeds, types of cattle,
improved crop plants—all
result from artificial
selection
Natural selection (and
genetic engineering)
conducted by human
breeders & geneticists.
Speciation
• The process by which new species come into being
• Speciation 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.
• Allopatric speciation is considered the dominant mode of
speciation
• Parapatric and (possibly) sympatric speciation also
occur.
Allopatric Speciation
1. Single interbreeding
population
2. Population divided
by a barrier;
subpopulations
isolated
Figure 5.2
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.
Map of Relative Terrestrial Biodiversity
248
Phylogenetic trees
• Life’s diversification results from countless speciation
events over vast spans of time.
• Evolutionary history of divergence is shown with
diagrams called phylogenetic trees.
• 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.
Figure 5.4
• Within the group Animals, one can infer a tree of the
major animal groups.
Figure 5.4
• Within the group Vertebrates, one can infer relationships
of the major vertebrate groups, and so on…
Figure 5.4
Humans and chimps share 96% of the same DNA! http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html
Extinction
• Extinction is the disappearance of an entire species from
the face of the Earth.
• Average time for a species on Earth is ~1–10 million
years.
• Species currently on Earth =
• the number formed by speciation
• minus the number removed by extinction.
Raphus cucullatus
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.
End of Cretaceous
End of Ordovician
End of Devonian
End of Permian
Now!
Quaternary
End of Triassic
Life’s Hierarchy of Levels
Figure 5.7
• Ecology deals mainly
with levels from the
organism up to the
biosphere.
Figure 5.7
Ecology
• The study of:
-
the distribution and abundance of organisms,
-
the interactions among them,
-
the interactions between organisms and
their abiotic environments
Habitat and Niche
• Habitat = the specific environment where an
organism lives (including living and nonliving
elements: rocks, soil, plants, etc.)
• Niche = an organism’s functional role in a
community (feeding, flow of energy and matter,
interactions with other organisms, etc.)
• Population = a group of individuals of a species
that live in a particular area
• Several attributes help predict population
dynamics (changes in population):
»•
»•
»•
»•
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Population size
Population density
Population distribution
Age structure
Sex ratio
Population Size
• Number of individuals present at a given time
Population size for the golden toad was
1,500+ in 1987, and zero a few years later.
Population Density
• Number of individuals per unit area
Population Density in Central and
South America (persons/km2)
1960
2000
Population Distribution
• Spatial arrangement of individuals
Clumped
Random
Uniform
Figure 5.8
Age structure
• Or age distribution =
relative numbers of
individuals of each age or
age class in a population
• Age structure diagrams,
or population pyramids,
show this information.
Figure 5.9
Age structure
Pyramid weighted
toward young:
population growing
Pyramid weighted
toward old: population
declining
Figure 5.9
Sex ratio
• 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.
Population growth rate =
(birth rate + immigration rate) –
(death rate + emigration rate)
Exponential Growth
• Population
growth
curves show
change in
population
size over time.
• Scots pine
shows
exponential
growth
Figure 5.10
Exponential Population Growth Projection
250000
0.40 % annual growth
200000
150000
1.21% annual growth
100000
Lethbridge
50000
0
2000
Regina
2005
2010
2015
2020
2025
2030
2035
2040
(2003-2004 rates extrapolated: data from StatsCan)
Limits on Growth
• Limiting factors restrain exponential population
growth, slowing the growth rate down.
• Population growth levels off at a carrying
capacity—the maximum population size of a
given species an environment can sustain.
• Initial exponential growth, slowing, and
stabilizing at carrying capacity is shown by a
logistic growth curve.
Logistic Growth Curve
Competitors
Figure 5.11
Oscillations
• Some populations fluctuate continually above and below
carrying capacity, as with this mite.
Figure 5.12b
Predator-Prey Population Patterns
(a form of oscillation)
Theoretically, the lynx population expands after the rabbit population expands,
until there are so many lynx that the rabbit population crashes again.
Dampening Oscillations
• In some populations, oscillations dampen, as population
size settles toward carrying capacity, as with this beetle.
Figure 5.12c
Crashes
• Some populations that rise too fast and deplete resources
may then crash, as with reindeer on St. Paul Island.
Figure 5.12d
Density Dependence
• Often, survival or reproduction lessens as
populations become more dense.
• Density-dependent factors (disease,
predation, etc.) account for the logistic
growth curve.
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-strategists
• r-strategists
•
Many offspring
•
Fast growing
•
No parental care
• K-strategists
•
Few offspring
•
Slow growing
•
Parental care
Terms come from:
r = intrinsic rate of
population increase.
(Populations can
potentially grow very
fast)
K = symbol for carrying
capacity. (Populations
tend to stabilize near K.)
Community Ecology
• Ecologists interested in how populations or species
interact with one another study community ecology.
• Community = a group of populations of different
species that live in the same place at the same time
•
e.g., Monteverde cloud forest community–golden
toads, quetzals, trees, ferns, soil microbes, etc.
Interactions between Trophic Levels
Figure 5.14b
Food Chains and Webs
• We can represent feeding interactions (and thus
energy transfer) in a community:
– Food chain: Simplified linear diagram of who eats
whom
– Food web: Complex network of who eats whom
Food Web for Temperate Deciduous Forest
Figure 5.14a
Keystone Species
• Species that have especially great impacts on other
community members and on the community’s identity
• If keystone species are removed, communities
change greatly.
A “keystone” holds an arch together.
Figure 5.15a
• When the keystone sea otter is removed, sea urchins
overgraze kelp and destroy the kelp forest community.
Figure 5.15b
Predation
• One species, the predator, hunts, kills, and consumes the
other, its prey.
Figure 5.16
Predation drives adaptations in prey
Cryptic coloration:
Camouflage to hide
from predators
Warning coloration:
Bright colors warn
that prey is toxic
Mimicry:
Fool predators
(here,
caterpillar
mimics snake)
Figure 5.18
Competition
• When multiple species seek the same limited resource
Interspecific competition is between two or
more species.
Intraspecific competition is within a species.
Interspecific Competition
• Different outcomes:
• Competitive exclusion =
one species excludes the other from a
resource.
• Species coexistence =
both species coexist at a ratio of
population sizes, or stable equilibrium.
Interspecific Competition
• Adjusting resource use, habitat use, or way of life
over evolutionary time leads to:
• Resource partitioning = species specialize in different
ways of exploiting a resource.
• Character displacement = physical characters evolve
to become different to better differentiate resource use.
Resource Partitioning
• Tree-climbing
bird species
exploit insect
resources in
different ways.
Figure 5.20
Parasitism
• One species, the
parasite, exploits
the other species, the
host, gaining
benefits and doing
harm.
Figure 5.21
Hemiparasite
Dendropthora sp.
&
Host
Miconia ligustrina
Mutualism
• Both species benefit one another.
• Hummingbird pollinates flower while gaining nectar for itself.
Figure 5.22
Succession
• A series of regular, predictable, quantifiable
changes through which communities go
• Primary succession: Pioneer species colonize a newly
exposed area (lava flows, glacial retreat, dried lake bed).
• Secondary succession: The community changes following
a disturbance (fire, hurricane, logging).
Terrestrial Succession
Figure 5.23
Aquatic Succession
1. Open pond
2. Plants begin to cover
surface; sediment
deposited
3. Pond filled by sediment;
vegetation grows over
site
Figure 5.24
Invasive species
• A species that spreads widely and rapidly becomes
dominant in a community, changing the community’s
normal functioning
• Many invasive species
are non-native, introduced
from other areas.
• Purple loosestrife invades
a wetland.
Figure 5.25
Zebra Mussels
Purple Loosestrife
Crested Wheat Grass
Challenges
• Earth’s biodiversity faces a mass extinction event caused
by human actions.
• Climate change may alter communities and cause species
extinctions.
• Invasive species pose a new threat to community stability.
• Conservation efforts need to consider local economies
and social conditions in order to succeed.
Solutions?
• There may still be time to avoid most species extinctions
threatened by human actions, if we take proactive steps to
counteract environmental degradation.
• We are learning more about climate change. Further
studies are important, to determine stresses in an altered
global environment. To some extent, we have committed
ourselves to warming in the long term and high latitude
environments will experience the greatest warming.
• Ecological restoration efforts can sometimes remove
invasive species and restore original communities
(debatable issue).