Speciation

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Speciation

• Speciation splitting of one species into 2
  different species.
• What is a species? Based on ability to reproduce.
• Biological species concept a species is a
  group of organisms that interbreed under natural
  conditions and that are reproductively isolated
  from each other.
• Reproductively isolated dont produce fertile
  hybrids.
• Natural conditions artificial breeding doesnt
  count. For example, artificial insemination,
  keeping 2 species locked up together.
• .

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Speciation
In contrast, the older morphological species
concept members of the same species look
similar to each other. Many examples of
organisms that look similar but cant produce
fertile offspring. Problems with biological
species concept Doesnt work with fossils
or extinct species. Doesnt work with
asexual species , such as most bacteria.
How to deal with what is natural.
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Reproductive Isolation

• How do populations become reproductively
  isolated?
• Result of gene mutations and altered allele
  frequencies due to selection and genetic drift.
• To maintain as a single species, there must be
  gene flow between populations matings between
  members of separated populations that allow
  mixing of alleles.
• In the absence of gene flow, mutations in
  different populations will be independent, and
  allele frequencies will change independently of
  each other.
• In most cases, migration is the key to gene flow.
  
• Once gene flow stops genetic divergence occurs.
  The two populations gradually become genetically
  different.
• Speciation sometimes occurs very quickly, other
  times more slowly. In most cases it is not an
  instantaneous event.

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Genetic Divergence
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Reproductive Isolation Mechanisms

• Pre-zygotic (before mating) vs. post-zygotic
  (after mating).
• Pre-zygotic different breeding seasons (either
  yearly or time of day), different signals sight,
  sound, smell, behavior, mechanical
  problemsgenitalia dont fit together. Also
  gametic isolation sperm dont detect the egg or
  cant fertilize it.
• Post-zygotic hybrid inviability hybrids dont
  survive to birth or adulthood hybrid sterility
  as with the mule, a sterile hybrid.
• Even having hybrids that are significantly less
  fit (able to survive and reproduce) than
  purebreds is an isolating mechanism, a way to
  prevent the two populations from fusing into one
  population.

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Allopatric Speciation

• The simplest and most common mechanism of
  speciation is allopatric speciation 2 groups of
  one species are isolated geographically, and
  diverge into separate species.

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More Allopatric Speciation

• Geographical barriers mountains, oceans, rivers.
  A few members of a species manage to cross by a
  rare chance event.
• This is the mechanism by which Darwins finches
  evolved into separate species in the Galapagos
  islands. Only very rarely can birds cross the
  ocean to get to other islands.
• Or, the barrier develops slowly as conditions
  change the gradual formation of the Grand Canyon
  split a population into 2 isolated groups, that
  have diverged into separate species, the Kaibab
  and Albert squirrels.

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Sympatric Speciation

• Geographical isolation is the easiest way for
  species to form, but there are other possible
  mechanisms. Sympatric speciation means
  speciation that occurs within the same
  geographical location.
• An example cichlid fish in Lake Barmobi Mbo in
  Cameroon, Africaan isolated volcanic lake. Nine
  species, all more closely related to each other
  (by DNA evidence) than to similar fish in other
  lakes. Lake has no distinct geographical zones,
  and the fish can easily swim anywhere in it.
  They feed in different locations, but all breed
  in the same location, close to the bottom. An
  example of sympatric speciation, but the
  mechanism is not clear.

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Sympatric Speciation by Polyploidy

• About half of all flowering plants are polyploid
  more than 2 copies of each gene.
• Polyploids are the result of failure of cell
  division (mitosis or meiosis) to separate the
  chromosomes into 2 cells.
• New polyploids are usually sterile, or their
  offspring are sterile extra chromosomes with no
  homologue to pair with in meiosis leads to
  unbalanced (aneuploid) gametes, which will
  produce dead offspring.
• Triploids (3 copies of each gene) are sterilethe
  source of seedless fruit.
• In animals, sexual reproduction is essential
  every generation, so most polyploids fail to
  reproduce need a similarly polyploid mate to
  produce fertile offspring.
• In plants, vegetative propagation is possible for
  many generations without sex, and in many species
  one plant contains both sexes. So, reproduction
  with a polyploid partner is easier in plants than
  in animals.

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Hybrid Zones

• When two populations of a species are separated
  by a geographical barrier, they diverge
  genetically. Sometimes the barrier is removed
  and the two groups come into contact with one
  another. The region of contact is a hybrid
  zone.
• Several possibilities exist
• If the two groups have only diverged a bit,
  fertile offspring will result, and the two groups
  will merge back into a single species.
  Geographical differences may exist within the
  species different subspecies or varieties, but
  all can interbreed freely.
• If the two groups have diverged to the point that
  no fertile offspring result from their matings,
  sexual selection will occur to deter further
  matings. New pre-zygotic reproductive isolation
  mechanisms, especially behavioral differences,
  arise to reinforce the division between the two
  species.
• This is called parapatric speciation 2 species
  forming while in contact with each other in a
  restricted region.

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Parapatric Example

• Shrimp in Panama. Up until 3 million years ago,
  North and South America were separated by the
  ocean. As dry land rose between the continents,
  shrimp who once freely interbred were separated
  by a barrier. When put together in the lab (or
  by going through the Panama Canal) now, they
  attack each other rather than mating.

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Patterns of Speciation

• What happens after 2 species separate from each
  other?
• In some cases, the species exists for millions of
  years, gradually changing in response to external
  conditions but always maintaining as a single
  distinct species.
• In other cases, many new species will form from a
  single species in a very short time this is
  adaptive radiation. This often happens on
  isolated islands, where a new species is blown in
  by a storm, and finds many different ecological
  niches to fill. Darwins finches are an example
  of this.
• They are thought to have originated with a small
  group of finches that blew over about 1 million
  years ago, to islands with no dangerous predators
  and very few other land birds.
• Also, sometimes a key innovation will arise,
  and new niches will suddenly be open to the
  possessors of the innovation. Or, a disaster
  will allow a sudden expansion of a few surviving
  species. Mammals grew rapidly in number of
  species following the extinction of the dinosaurs.

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Extinction

• Extinction can happen none left of the species.
• Various events can cause extinction being
  outcompeted for a critical resource, having the
  climate change too rapidly to adapt.
• Mass extinctions are caused by catastrophic
  events. The Earth has had several mass
  extinction events, where the vast majority of
  species die out over a short period of time.
  This is what is seen when one moves between
  various geological ages. Asteroids hitting the
  Earth are responsible for at least some of these,
  but probably not all.