Speciation
Reading: Freeman, Chapters 25-26
Many of the figures, and most of my examples, are from Douglas Futuma’s
Evolutionary Biology, an excellent reference if you want to know more about
speciation
Species Concepts
• Biological
– “Species are groups of actually or potentially
interbreeding individuals that can mate and produce
fertile offspring”-idea was promoted by Ernst Mayr, an
evolutionist who worked on birds.
• In practice, this applies to most species, but in many cases, it is
simply impossible to test whether two species have the
potential to interbreed.
• Phylogenetic
– A species is a lineage, separate from other such lineages,
perpetuated ancestor to descendant, over time.
• Morphological
– Morphological criteria are used to define species.
Clearly, each school of thought has its strengths and its
drawbacks.
The biological species concept is great conceptually, but
virtually impossible to put into process in most situations.
Fossil species, species which do not reproduce in the lab
or zoo (the vast majority), and asexual species are sticky
issues.
The morphological species concept is what most
taxonomists actually use in practice, because it is expedient,
but it has many disadvantages.
It is subjective. Cryptic species, which look identical to
humans but are in fact reproductively isolated, are
problematic.
The phylogenetic species concept is becoming increasingly
used-it is most useful when the scientist has a very clear idea
what type of “lineages” they are looking at.
Speciation
• The theory of evolution must explain the origin of
new organisms. From the beginning, the origin of
species has been a focal point of evolutionary
theory.
– Ironically, Darwin was wrong about the origin of species.
He assumed that, given enough time, natural selection
would inevitably produce them. This is not actually the
case. It takes reproductive isolation.
– Macroevolution is the origin of new taxonomic groups.
– Speciation is the origin of new species. With extinction,
it is one of two keystone processes of macroevolution.
Cladogenesis
• Cladogenesis, the origin of lineages, is the budding
of a new species from a parent species that continues
to exist.
• Cladogenesis promotes biological diversity by
increasing the number of species.
• Although it culminates over thousands or millions of
years, cladogenesis is a real event. New species
originate by cladogeneis, which is ultimately
responsible for the origin of every major group of
animals.
Why Do We Have Separate Species At All?
• Sympatric species live in the same place. Without some
mechanism preventing allele and gene exchange among
sympatric species, distinct species would be impossible, we
would probably see a continuum from one form of life to
another.
• Barriers to allele flow are called isolation mechanisms.
• Isolation mechanisms allow sympatric species to exist.
• Without isolation mechanisms, closely related species
would hybridize: allele flow and recombination would
eventually transform them into a single,
polymorphic species.
The Evolution of New Species Results
From (and also causes) Barriers to Allele
Flow.
• At the time when a genetic or behavioral
mechanism evolves that keeps populations of a
species from interbreeding, two new species are
formed.
• As we shall see, this is frequently the result of a
geographic barrier, although it may be the result
of a chromosomal change or habitat preference.
There are Two Basic Categories:
Prezygotic Isolation Mechanisms
Postzygotic Isolation Mechanisms
• Presygotic isolation mechanisms prevent
mating, so that gametes of sympatric
species never form hybrid zygotes.
• Postzygotic isolating mechanisms act after a
mating has occurred, to prevent fertilization
or to prevent potential hybrids from passing
on their genes.
Prezygotic Isolation Mechanisms
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Habitat Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Gametes Die
Habitat Isolation
• Habitat isolation occurs because sympatric
species meet due to differences in their
habitat preference.
• Examples of habitat isolation:
• Sympatric species of spadefoot toads
(Scaphiopus) seldom meet because they
prefer different soil types.
• Many species of closely-related parasites,
such as bird lice, never meet because they
live and mate on different hosts.
Temporal Isolation
• Temporal Isolation: Occurs because species
mate at different times.
• Examples:
• Different species of plants frequently have
differing flowering seasons.
• Closely related species of fireflies
frequently mate at different times of night.
Behavioral Isolation
• Behavioral, or Ethological isolation
mechanisms include differences in courtship
behavior, differences in chemical signals or
vocalizations, and differences in color or
morphology that allow individuals to
recognize their own species.
• They are a very common mechanism
keeping closely-related sympatric animals
from interbreeding
Examples:
• Female fireflies respond
only to the light pattern
emitted by their own
species. Sympatric species
of fireflies emit different
light patterns.
• Experiment: Gulls
normally mate only with
their own species: artificial
hybridization can be induced
by modifying the contrast in
color between the eye and
the face. Thus, the isolation
mechanism is behavioral.
Mechanical Isolation
• Mechanical isolation occurs because the
sexual organs of closely-related sympatric
species are incompatible: they do not fit
together.
• This is thought to be an important isolation
mechanism in arthropods, particularly
insects and millipedes.
Sperm of various mammals
Gametic Mortality
Gametes are frequently very
specialized cells, which can
only perform well in the
reproductive tract of the
opposite sex of the same
species.
In many angiosperms, for
instance, pollen transferred to
the stigma of another species
will not germinate, or if they
do, will not form a pollen
tube.
Postzygotic Isolation
Mechanisms
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Zygote dies after fertilization
Hybrid Inviability
Hybrid Sterility
Low Hybrid Fitness
Hybrid Inviability
• Some species that do not ordinarily interbreed
occasionally do so. Frequently, the progeny of
these interspecific matings die at some point
during their development.
• Example: Hybrids between the frogs Rana pipiens
and Rana sylvatica do not survive more than a day
or so.
Hybrid Sterility
• Hybrid Sterility occurs when the hybrid of an
interspecific mating is unable to reproduce.
• Examples: Mules are the hybrid of a horse and a
donkey, they do not form normal sperm.
• Hybrids between Drosophila melanogaster and
D. pseudoobscura have atrophied testes and are
sterile,
Low Hybrid Fitness
• If interspecific hybrids do survive, they often
have very low fitness, this effectively keeps
them from spreading genes from one of their
parent species to the other parent species.
• Example: Dog-Wolf hybrids are perfectly
viable, but they are considered to be unsuitable
pets in most areas.
• Wild wolf populations do not accept hybrids,
they are killed on sight.
There Are Two Basic Types of
Speciation: Allopatric and
Sympatric
• Allopatric Speciation: Involves a
geographic barrier.
• Sympatric Speciation: Does not involve a
geographic barrier.
Allopatric Speciation
• Allopatric speciation involves a geographic barrier
that physically isolates populations of a species
and blocks gene flow.
• Once isolated, allopatric populations (living in
different places) accumulate genetic differences
due to natural selection, genetic drift, and new
mutations.
If the Geographic Barrier is
Removed, the Two Species May:
• 1) meld together by allele flow and recombination to
once again form a single species.
• 2) remain reproductively isolated.
• The fate of the new, incipient species, depends upon
whether isolation mechanisms have evolved during the
period of isolation.
• These isolation mechanisms may be premating or
postmating.
• Premating isolation mechanisms may evolve in
incipient species that have postmating isolation, to
reduce the probability of incorrect matings and the
subsequent loss of fitness.
Example of geographic isolation and possible allopatric
speciation: Two closely-related species of antelope squirrels
live on opposite sides of the grand canyon. On the South rim
is Ammospermophilus harrisi, on the North rim is
Ammospermophlus leucurus. Birds, and other species that can
cross the canyon, have not diverged into different species on
opposite sides.
Example: the Drongo
• The Drongo is a
black bird with a
crest of feathers, it is
highly variable in
behavior and
appearance
throughout its range.
• Each semi-isolated
population has its
own appearance.
• A “double” invasion probably occurred in Tasmania over
the course of the past few thousand years.
• The species Acanthiza pusilla is widespread on the
Australian continent. Tasmania has a slightly differentiated
population of this bird.
• Another, reproductively isolated species, A. ewingi, that is
even more differentiated from Australian Drongos in
appearance and morphology also inhabits Tasmania.
• During the last ice age, when sea level was lower, Tasmania
was part of island, this is probably when the ancestors of A.
ewingi invaded the island. Eventually they evolved
reproductive isolation from their Australian counterparts.
• When A. pusilla re-invaded the island more recently, the
two species were able to co-exist because they are
reproductively isolated.
Adaptive Radiation
on Islands
·Island chains frequently
produce many new species.
·Islands chains provide
barriers that facilitate
invasion and re-invasion by
different species.
·This is the probable
mechanism for the
proliferation of Darwin’s
finches on the Galapagos.
·The Hawaiian islands once
supported thousands of
unique Drosophila flies that
probably evolved by a similar
mechanism
How Long Does Allopatric Speciation
Take?
• Nobody knows for sure, and it depends upon the
group.
– McCune and Lovejoy, based on a study of
reproductive isolation in 40 pairs of allopatric fishes,
estimated that, for fishes, it takes between .8 and 2.4
million years for reproductive isolation to evolve.
• Hurt and Hedrick conducted interesting studies
of incipient allopatric speciation in the Sonoran
topminnows
– Poeciliopsis sonorensis and Poeciliopsis occidentalis
occupy different river drainages in Arizona
http://www.nativefish.org
• Based on molecular evidence, the two
species/subspecies have been isolated for
between one and two million years.
• McCune and Lovejoy found that males of
each species prefer to mate with females of
their own species, but given a choice, they
will hybridize.
• There was evidence of reduced hybrid fitness,
especially when hybrids were crossed with one of
the original species.
– Brood sizes were smaller, and there was an unusual,
male-biased sex ratio for these crosses.
• Are they separate species?
– Clearly, the answer is subjective. In this case, if the
populations were to mix together, the process of
reinforcement would probably complete the job….so
probably yes.
– Reinforcement-natural selection on females of these
incipient species pairs to avoid mating with males of
the wrong species, thus avoiding the cost of producing
unfit, hybrid offspring.
Sympatric Speciation
• Sympatric speciation results from intrinsic
factors, such as chromosomal changes and
nonrandom mating.
• Sympatric populations become genetically
isolated even though their ranges overlap.
Mechanisms of Sympatric Speciation
• Polyploidy: allopolyploidy and autopolyploidy
• Nonrandom mating: I.e., host shift
Polyploidy and Interspecific Hybridization
• Polyploidy: Disorders of meiosis cause the accidental
formation of gametes that are 2N rather than N. Union of
two of these gametes produces a zygote that is 4N. The
chromosome number has doubled, instantaneously
producing a potential new species! This produces an
autopolyploid.
• Interspecific Hybridization: Gametes of two species meet
and form a hybrid set, usually the hybrid set is sterile, but
sometimes it is not. This produces an allopolyploid, which
is usually infertile (the chromosomes can not pair during
meiosis), but may reproduce asexually. If the chromosomes
double by a disorder of meiosis, it produces a potentially
sexual species
• These mechanisms are probably common in plants, but
rare or absent in animals.
• Plants are frequently capable of self-fertilization, and
some can survive with double the normal number of
chromosomes, and most can propogate asexually.
• Many common agricultural plants are the products of
one, or both of the mechanisms above.
• Examples: Wheat is doubly autopolyploid, the product
of an interspecific hybridization, then a doubling of
chromosomes, then another interspecific hybridization,
then another doubling of chromosomes, to produce a
fertile hexaploid.
• Yellow bananas are allopolyploid, the product of two
interspecific hybridization events.
Host Shift
• Nonrandom mating due to host shift: Many species of
parasites mate on or nearby the host. A shift to a new
species of host therefore reproductively isolates the
parasites exploiting the different species of hosts.
• Example, the apple maggot: The best studied example of
this occurs in the apple maggot. Apple trees are not native
to the US, they were introduced here in the nineteenth
century. Following their introduction, the hawthorne
maggot began to feed on apples. The flies cue in to the
smell of their original host, so apple and hawthorne maggots
are now reproductively isolated: and considered to be
different species.
Parapatry
• Speciation is not always clear-cut, there are many examples
of SOME populations being reproductively isolated, while
OTHERS are able to interbreed. This is sometimes called
Parapatry.
• Example: A well known example involves California
garter snakes, Thamnophilus sp..
• Each population of land snakes is able to interbreed with the
populations closest to it, but not with more distant
populations. In some cases, however, a distant population
has come back around to encounter a distantly related
population. In these cases, they do not interbreed. A similar
relationship exists for water populations. Some water
snakes can even interbreed with the local land snakes
These figs are from Futuma’s Evolutionary Biology
Phylogeny, Taxonomy, and Systematics
• Phylogeny: The phylogeny of a group is a “family
tree” describing how species are related. The
branching pattern of different groups of organisms is
caused by repeated cladogenesis.
• Systematics is the study of phylogeny.
• Taxonomy: Is the process of describing and naming
organisms. Our modern process of taxonomy is
based on phylogeny, so an understanding of
phylogenetic relationships-names reflect relatedness.
The Taxonomic Heiriarchy
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Every species has a place in the taxonomic heirarchy:
Human
Mud-Dauber
Species
Homo sapiens Trypoxylon politum
Genus
Homo
Trypoxylon
Family
Hominidae
Sphecidae
Order
Primates
Hymenoptera
Class
Mammalia
Insecta
Phylum
Chordata
Arthropoda
Kingdom
Anamalia
Anamalia
Domain
Eukarya
Eukarya
Inferring Phylogeny
• The branching pattern of a phylogenetic tree
reflects its ideal place in our taxonomic hierarchy.
• Classification schemes are hypotheses of past
history based on the available evidence.
• Like all hypotheses, they make predictions that
can be tested by future study.
The phylogeny of a group of organisms
can be inferred from the following lines of
evidence
• Shared characteristics passed down from an
ancestor, called homologies.
– These are essential in inferring a phylogeny.
• Morphology and DNA sequences are very useful places to look
for homologies
• Biogeography
• The Fossil Record
Homology
• A character state is homologous in two species when it is
inherited by both from a common ancestor.
• The most widely accepted school of systematics today is
called cladistics.
• Cladistics infers the pattern of phylogeny based on
homologies. Groups are constructed based on shared
characteristics inherited from a common ancestor, that no
other group has.
• Trees are constructed by creating a nested series of such
groups.
Types of Characters Used in Cladistics
• Apomorphy-an evolutionary novelty for a group.
• Plesiomorphy-an evolutionarily primitive state
• Synapomorphy-a novel (derived) trait that a group has
inherited because the common ancestor of that group
had a novel characteristic and passed it on.
• Synplesiomorphy-an evolutionarily primitive trait that a
group has inherited because the common ancestor of
that group had inherited the primitive condition,
unchanged, from an earlier group.
• Note that these terms are relative-a synplesiomorphy for
one group may be a synapomorphy from the larger
group it came from.
Example of a Homology which is a
Synapomorphy
Homoplasy
• If a character has evolved more than once, if
possessed by two species but not present in the
common ancestor, it is called a homoplasy.
• One form of homoplasy is called convergent
evolution, it is quite common because
different species are often subject to similar
selective pressures.
• Homoplasy, when mistaken for homology, can
obscure the pattern of evolutionary history.
Example of a Homoplasy-not a
homology
Extinction
99.9999% of all species that have
ever lived are extinct
• Extinction has always been a major force in macroevolution.
• Species do not last forever…the mean expected lifespan of a
marine bivalve is about 14 million years, and the mean expected
lifespan of a terrestrial mammal might is about a tenth that.
• Climate change, natural disasters, and other phenomena have
always caused extinction.
• As some species originate, they inevitably drive other species
extinct.
• Extinctions, in turn, pave the way for speciation.
• An adaptive radiation is a wave of speciation that occurs as a new
habitat is colonized by a lineage, or in the wake of the extinciton
of another lineage.
• An adaptive radiation of mammals followed the extinction of the
dinosaurs.
Background Extinction
• At all times in history, groups of organisms have a
“background extinction rate”…species go extinct
because of normal ecological or evolutionary
processes, and as they disappear, other species take
their places.
– For instance, on oceanic islands, the arrival of a predator,
such as a monitor lizard or a snake, might precipitate the
extinction of ground-nesting birds. Any such birds that are
endemic to the island (that is, they live nowhere else) are
gone for good.
– But oceanic islands come and go, as geological forces shape
the Earth’s crust, and such extinctions are considered to be
“normal”.
Mass Extinctions
• The history of life on Earth has been punctuated by a series
of mass-extinctions.
– Extinction rates are much higher than background rates for a short
period of time.
– Some are better understood than others, but they have profoundly
influenced the evolution of life on Earth.
– Over the last 500 million years, there have been several major
mass extinctions (not counting the current one):
– here are some big ones
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1) The Late Devonian
2) Mid-Ordovician
3) Permian-Triassic
4) Late Triassic
5) Cretaceous-Tertiary.
The Human Mass-Extinction
• Since the development of agriculture, 10,000 years ago,
humans have modified an increasing proportion of the
Earth’s resources for our own purposes.
• Humans impact has caused extinction rates to be 10 to
1000 times greater than any time in the last 100,000 years.
• For example-one estimate for the recent background
extinction rate for birds is one species extinction per 400
years.
– If only this natural rate of loss affected the number of bird species,
no more than a couple of extinctions should have occurred in the
past 800 years.
– Scientists estimate that the actual loss during this time period lies
somewhere between 200 and 2,000.
• We have set in motion a mass extinction, one of the largest,
that will not culminate until thousands of years from now.
• Humans have extensively modified
the biosphere
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The human population passed 6
billion in the year 2000, and is growing
at a rate of almost 2% per year.
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Each human uses so much energy and so
many resources that our activities influence
virtually every aspect of the biosphere.
• In temperate areas, nearly all the land area that is
suitable for agriculture is plowed or fenced.
•
Worldwide, more than 35% of all land area is
used for farms or permanent pastures. Much of the
rest is grazed or logged on a regular basis.
From 35-45% of Global Net
Primary Productivity now goes
to serve human needs.
• In aquatic ecosystems as well, an
increasing amount of productivity is
harvested by humans. Nearly every
major fishery in the Northern
Hemisphere has showed strain from
overharvesting, and many have
collapsed.
Major sources of
anthropogenic extinction
• Habitat destruction and habitat
fragmentation
• Habitat Change and Disruption of
Ecosystem Processes
• Introduction of Exotics
• Overexploitation
Habitat destruction and
habitat fragmentation
• Most of the grasslands and forests of the Northern
Hemisphere were destroyed by the end of the
nineteenth century, the grasslands of the southern
hemisphere are now vanishing, and tropical forests
are disappearing at a rate of about 2% per year.
This type of destruction has become the norm for
most biological communities, as the human
population expands our economic needs require
resources from more and more land. The remaining
habitat is often broken into many small fragments,
which are separated by large areas of land under
cultivation or other human uses, effectively reducing
a single "continent" into many "islands".
Fragmented Habitats Support
Smaller Populations
• Essentially, every habitat fragment becomes
a biological "island" (analogous to continental
shelf islands, rather than the oceanic kind).
– As in the Mac Arthur Wilson model, the smaller
the island, the smaller the population of any given
species it can support.
– Small populations are at much greater risk of
extinction due to random events, such as
weather, disasters, and natural fluctuations in
their population and sex ratio.
This part of Canada used to be a continuous swath of natural
communities.
Here is a fragment seen from the air
• Additionally, smaller populations support
less genetic variation, which could lead
to the fixation of harmful alleles and the
ultimate extinction of the population (for
very small fragments), or simply inhibit
their ability to evolve in response to
changing conditions
Fragmented Habitats Frequently
Lack Critical Ecosystem
Processes
• Edge effects fundamentally alter habitat.
For certain species, this can be critical
to their ability to survive. For instance,
places where human habitation borders
nature preserves frequently have weedy
plants, fire is controlled, domestic cats
and dogs escape and prey on native
wildlife, and human noise and activity
disturb the behavior of certain animals.
• The edge habitats have different effects on
different species.
– Some large mammals, such as coyotes and
raccoons, reach much higher densities in edge
habitats because they are able to take advantage of
human resources (garbage), and return to the
safety of the preserve.
• Taking this a step farther,raccoons in the US, and red
foxes in England, have even penetrated urban areas to
become part of the city, reaching high densities.
– Other mammal species cannot tolerate edge
environments, wolves and mountain lions do not
like humans and cannot live on the edge (in cases
where they try, very bad things might happen.
This is a natural area in Massachusetts.
• Example: The brown headed cowbird is
a native to the United States. It is a
brood parasite, evicting the eggs of
other species to replace them with its
own. Cowbirds prefer edge habitats.
Now that forests are fragmented, there
are few safe areas from cowbirds, and
forest interior species such as bluebirds
are suffering a major loss of fitness in
some areas.
Habitat Change and
Disruption of Ecosystem
Processes
–Surviving areas of natural habitat often
change because humans have
fundamentally altered natural ecosystem
processes.
Examples
– Ladys' Slipper Orchids. There are probably
about 25, 000 species of orchids worldwide, and
they are being lost faster than they can be
classified.
– Orchids are typically tightly coevolved in
mutualistic relationships with other species, and
the loss of any of these relationships can lead to
extinction.
• Ladys' slippers are a very diverse group that
occupy a wide variety of habitats in the
Northern Hemisphere. They are in decline
even in protected areas, such as Indiana
Dunes.
• Human activities have altered their
ecosystems. Ladys' slippers need a
mutualistic fungus to germinate and
grow for the first few years. Airborne
nitrogen compounds (mostly from
automobiles) effectively "fertilize" vast
areas of ground and may put the
mutualistic fungi at an ecological
disadvantage.
• Also, the widespread application of
pesticides, the human tendency to
groom and "clean up" areas of open
sand and fallen wood, and the
introduction of the honeybee to North
America have caused the Andreneaid
bees that would normally pollinate these
plants to disappear from many areas.
– Pacific Salmon are very important
ecologically and economically off
the West Coast of North America.
– Salmon species have experienced
dramatic declines over the past
few decades due to a variety of
factors, many of which result from
human habitat modification.
– Hydroelectric dams have resulted
in increased juvenile mortality and
made many habitats inaccessible
to migrating salmon.
– Additionally, human logging and
agriculture has silted and modified
many of their upstream habitats,
causing a drop in recruitment.
Introduction of Exotics
• Human activities are creating the
worldwide equivalent of the "Great
American Faunal Interchange". This is
an uncontrolled experiment in
community ecology, with the potential
result of a massive loss of gamma
diversity worldwide caused by the loss
of endemic species.
Zebra Mussels
• In 1998, the zebra mussel was
discovered in Lake St. Claire near
Detroit. It was introduced to the
Great Lakes from the Caspian
Sea, probably in the ballast water
from a cargo ship, sometime
around 1985. This mode of
dispersal is very common, in 1982
the comb jelly (a ctenophore) was
introduced to the Black Sea in a
similar manner. Comb jellies
increased in number until they
amounted to an estimated 90% of
animal biomass in the Caspian.
• They have since spread throughout the Great Lakes
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region and throughout the Mississippi and Ohio River valleys.
It forms dense clusters of individuals, and can clog the water
intakes of electrical power stations, water stations, and other
industrial facilities.
Zebra mussels are incredibly effective filter feeders. Zebra
mussels actually make the water much clearer, but alter
native communities of organisms in the process. In the
Hudson River, phytoplankton biomass decreased 85% after
zebra mussels invaded, zooplankton decreased 70% as a
result.
The zebra mussel is a very effective competitor. Extinction of
native bivalves will almost certainly result from this
introduction.
You may have noticed that Chicago water tastes weird during
the summer, that is because the water is now clean enough to
allow the growth of cyanobacteria deep enough in the lake to
be pulled into the water intakes. The residue of cyanobacteria
toxins has an off taste.
Honey Bees
• Honey bees are native to Europe and Asia.
Apis mellifera, is a European species that is
widely cultivated for honey, beeswax, and as
a pollinator. European immigrants probably
introduced the honeybee to North America in
the nineteenth century (Native Americans
called it "white man's fly".) It is a very
effective competitor, and displaces native bee
species.
• Recently, honeybees themselves have taken
a hit, when the varolla mite was introduced in
the 1980's. The overuse of insecticides, and
widespread destruction of habitat, have
decimated North American bee populations,
both native and non-native.
– The Snake that Ate Guam.
Boiga irregularis, the brown tree
snake, is an arboreal snake native
to New Guinea, Australia, and the
Solomon Islands. It is a small,
nocturnal, rear-fanged snake.
– Boiga irregularis was introduced to
Guam in the late 1940s, probably
by hitching a ride in the wheel well
of a plane. Since that time, it has
literally eaten most of the endemic
birds of Guam to extinction. Since
there are no other native snakes
in Guam (other than a blind,
burrowing species), the bird fauna
there evolved no natural defenses.
• Thus, is an incredibly effective
predator of birds and their nests.
– In Australia, competition and predation
keep it in check, but the simpler ecosystem
of Guam has allowed it to increase in
numbers to up to 20 individuals per square
acre of jungle (among the highest ever
recorded for a snake).
– It also causes other problems in Guam,
including numerous power outages
resulting from large numbers of snakes
resting on power lines.
Garlic Mustard, Purple
Loostrife, Multiflora
Rose
– These are three more cases of an
introduced species being too good
at what they do. All three plants
were introduced intentionally in the
nineteenth century. Each of the
three has become so common that
it is likely to displace other species.
For example, in some East Coast
marshes, purple loosestrife
amounts to 90% of the vegetation,
displacing native sedges and other
plants.
Overexpolitation
• Stellar's Sea Cow-this huge
sirenian mammal lived in
the reached a length of 26
feet and could way seven
thousand pounds or more.
It existed on a diet of kelp,
and could not dive or swim
quickly.
• It was delicious, and was
hunted to extinction by
sailors within 30 years of its
discovery
What Makes A Species
Vulnerable to extinction?
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Endemism
Rarity
Small Population Size
Ecological Specialization
Beauty/Usefulness to humans/Competitor
with Humans
• Endemism- Species that are restricted
to a particular, small area, are more
vulnerable to extinction
• Rarity-Rarity is not the same thing as
endeminsm, endemics can be very common in
the restricted area where they do occur.
"Naturally rare" species have low population
densities, but may be widely distributed and have
respectable population sizes. We do not
completely understand the ecological factors that
make some species "naturally rare", but when a
common species gradually becomes rare, it is
often a prelude to extinction. "Naturally rare"
species can be a challenge to conservation,
because they are difficult to monitor and it is very
difficult to ensure that sufficient habitat is set
aside for them.
• Small Population Size-Small
population sizes render a species very
vulnerable to extinction, through
reduced genetic variation via genetic
drift, the potential for inbreeding
depression, demographic stochasticity
caused by random ecological disasters
and, for sexual species, the small
chance that every individual in the
population might be born the same sex.
• Ecological Specialization-Ecological
specialists are more prone to extinction
because there are only a few ways they can 'fit
themselves into" an ecosystem. They must
have certain interspecific relationships in order
to feed, obtain mates, have places to live, or
maintain competitive superiority. The loss of
other species in the community, or habitat
change due to human activity, can change
these factors, and render a formerly successful
species vulnerable to extinction.
• Useful to Humans or A Competitor of
Humans-Humans have a way of killing all the
pretty things, harvesting all the useful things,
and hunting to extinction everything that could
be perceived as a competitor. For instance,
fishermen in San Francisco are prone to
despising the California Sea Otter, despite its
important place in the ecosystem of the
California Coast, because of its status as a
competitor. They are protected now, however,
they were nearly hunted to extinction for their
pelts. Species that cross the paths of humans
sometimes suffer for it.
• Economic Considerations
•
Any conservation plan that does not take human
economics into account is prone to failure. It is very difficult
to set aside a habitat and protect it from all human activity.
The closest we have ever come are on military bases and
nuclear test sites (the conservation effect was unintentional
at first), and some private organizations (Nature
Conservancy) buy natural land and simply fence it off.
Even these exceptional preserves have neighbors, and are
occasionally eyed by developers and government
reclassification.
•
The vast majority of preserves must balance the needs
of human ecotourists, indigenous peoples, neighbors, and
government budget considerations against conservation
goals.
The Future of Our Own Species
– It is one of the strange ironies of our existence that,
though the actions of our species modify the
biosphere to an extent unprecedented in the history of
the earth, as individuals, we do not necessarily feel
any collective responsibility for our actions.
– The future of our own species will depend, to a very
large extent, upon decisions we make as individuals,
regarding our priorities. It is quite possible for our
species to survive for many thousands of years more,
but this is likely only if this generation takes
additional steps to ensure that the planet will remain
habitable to our own species.