Speciation

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Speciation

• Reading Freeman, Chapters 25-26
• Many of the figures, and most of my examples, are
  from Douglas Futumas Evolutionary Biology, an
  excellent reference if you want to know more
  about speciation

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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.

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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.
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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.

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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.

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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.

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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.

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There are Two Basic CategoriesPrezygotic
Isolation MechanismsPostzygotic 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.

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Prezygotic Isolation Mechanisms

• Habitat Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametes Die

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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.

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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.

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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

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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.

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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.

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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.
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Postzygotic Isolation Mechanisms

• Zygote dies after fertilization
• Hybrid Inviability
• Hybrid Sterility
• Low Hybrid Fitness

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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.

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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,

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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.

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There Are Two Basic Types of Speciation
Allopatric and Sympatric

• Allopatric Speciation Involves a geographic
  barrier.
• Sympatric Speciation Does not involve a
  geographic barrier.

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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.

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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.

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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.
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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.

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• 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.

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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 Darwins finches on the
  Galapagos.
• The Hawaiian islands once supported thousands of
  unique Drosophila flies that probably evolved by
  a similar mechanism

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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

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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.

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• 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.

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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.

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Mechanisms of Sympatric Speciation

• Polyploidy allopolyploidy and autopolyploidy
• Nonrandom mating I.e., host shift

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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.

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• 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

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• 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.

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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.
  

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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

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These figs are from Futumas Evolutionary Biology
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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.

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The Taxonomic Heiriarchy

• 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

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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.

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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

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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.

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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.

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Example of a Homology which is a Synapomorphy
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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.

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Example of a Homoplasy-not a homology
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Extinction
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99.9999 of all species that have ever lived are
extinct

• Extinction has always been a major force in
  macroevolution.
• Species do not last foreverthe 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.

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Background Extinction

• At all times in history, groups of organisms have
  a background extinction ratespecies 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 Earths crust, and such
  extinctions are considered to be normal.

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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
• 1) The Late Devonian
• 2) Mid-Ordovician
• 3) Permian-Triassic
• 4) Late Triassic
• 5) Cretaceous-Tertiary.

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The Human Mass-Extinction

• Since the development of agriculture, 10,000
  years ago, humans have modified an increasing
  proportion of the Earths 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.

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• Humans have extensively modified the biosphere
• The human population passed 6 billion in the
  year 2000, and is growing at a rate of almost 2
  per year.
• 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.

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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.

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Major sources of anthropogenic extinction

• Habitat destruction and habitat fragmentation
• Habitat Change and Disruption of Ecosystem
  Processes
• Introduction of Exotics
• Overexploitation

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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".

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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.

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This part of Canada used to be a continuous swath
of natural communities.
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Here is a fragment seen from the air
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• 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

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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.

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• 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.

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This is a natural area in Massachusetts.
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• 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.

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Habitat Change and Disruption of Ecosystem
Processes

• Surviving areas of natural habitat often change
  because humans have fundamentally altered natural
  ecosystem processes.

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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.

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• 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.

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• 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.

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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.

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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.

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• They have since spread throughout the Great Lakes
  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.

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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.

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• 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.

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• 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.

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• 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.

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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.

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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

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What Makes A Species Vulnerable to extinction?

• Endemism
• Rarity
• Small Population Size
• Ecological Specialization
• Beauty/Usefulness to humans/Competitor with Humans

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• Endemism- Species that are restricted to a
  particular, small area, are more vulnerable to
  extinction

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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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.