Speciation

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Chapter 17

• Speciation

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17.1 Species are reproductively isolated lineages
on the tree of life

• Speciation the divergence of biological
  lineages and the emergence of reproductive
  isolation between lineages
• It comes down to what a species is

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Speciation

• The splitting of one species into two or more
  species.
• OR
• The transformation of one species into a new
  species over time.

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Figure 24.1 Two patterns of speciation
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We can recognize many species by their appearance

• Groups of organisms that mate with one another
  are commonly called species
• Carolus Linnaeus developed a binomial
  nomenclature system that we still use today
• He classified certain species only by appearances
  alone

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• He used a morphological species concept which
  assumes that species are made of individuals that
  look alike (vise versa)
• Using morphology as a way of identifying species
  has limitations
• Do not always look alike
• Cryptic species (two or more species look almost
  identical but do not interbreed)
• Cannot rely on appearance alone
• Scientists now use genetic and behavioral data as
  well to group species

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Cryptic Species
http//ntdtv.org/en/news/science-technology/2011-1
1-24/dna-barcoding-uncovers-new-butterfly-species-
in-mexico.html
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Reproductive isolation is key

• Reproductive isolation a state in which two
  groups of organisms can no longer exchange genes
• Most important factor in long-term isolation
• Ernst Meyer proposed the biological species
  concept species are groups of actually or
  potentially interbreeding natural populations
  which are reproductively isolated from other such
  groups

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Figure 24.9 Ensatina eschscholtzii, a ring
species
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The lineage approach takes a long-term view

• Lineage species concept when one species splits
  into two or more daughter species, which
  thereafter evolve as distinct lineages
• Lineage is an ancestor-descendant series of
  population over time
• Either ends in extinction or another speciation
  event
• Can happen over thousands of generations or
  rather quickly

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Lineage species concept
http//earthlingnature.wordpress.com/2012/06/05/wh
ats-a-species-2-vertical-species-concepts/
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17.2 Speciation is a Natural Consequence of
Population Subdivision

• Not all evolutionary changes result in new
  species
• Speciation requires the interruption of gene flow
  within a species
• Is genetic change prevents reproduction how can
  such a change spread thorough a species in the
  first place?

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Incompatibilities between genes can produce
reproductive isolation

• Lets look at figure 17.3 which illustrates how
  this can happen

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Dobzhansky-Muller model of hybrid incompatibility
http//www.nature.com/scitable/content/dobzhansky-
muller-model-of-hybrid-incompatibility-7883
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Reproductive isolation develops with increasing
genetic divergence

• As species diverge genetically, they become more
  reproductively isolated
• Can happen over millions of years or develop in
  just a few generations

http//www.sci.sdsu.edu/class/bio100/Lectures/Lect
12/lect12.html
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17.3 Speciation may occur through geographic
isolation or in sympatry
http//www.tokresource.org/tok_classes/enviro/syll
abus_content/4.1_biodiversity/index.htm
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Physical barriers give rise to allopatric
speciation

• Allopatric speciation (geographic speciation)
  speciation that results when a population is
  divided by a physical barrier
• Dominant mode of speciation
• Examples water, mountains, dry land formed by
  continental drift or climate changes
• Usually large populations

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Allopatric speciation
http//evolution.berkeley.edu/evosite/evo101/VC1bA
llopatric.shtml
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• Can also happen when members of a pop. Cross a
  barrier and establish a new isolated pop.
• Example Finches of the Galapagos
• The environments and food sources are different
  on all the islands

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Allopatric speciation
http//science.kennesaw.edu/jdirnber/Bio2108/Lect
ure/LecEvolution/Evol4MacroEvol.html
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Sympatric speciation occurs without physical
barriers

• Speciation without physical isolation is called
  sympatric speciation
• Can happen with disruptive selection where
  certain organisms have distinct microhabitats
  where mating takes place
• Example flies that lay their eggs in different
  types of fruits (page 338 read)

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Sympatric speciation
http//evolution.berkeley.edu/evosite/evo101/VC1eS
ympatric.shtml
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• Most common is polyploidy or the duplication of
  sets of chromosomes
• Can happen either from chromosome duplication in
  a single species autopolyploidy
• or from the combining of the chromosomes of two
  different species allopolyploidy

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sympatric speciation due to polyploidy (more
common in plants than animals)
Allopolyploid allo other, so two
species-more common, can reproduce asexually, or
occasionally, sexually
Autopolyploid auto self, so one species-- in
this case, the tetraploid can self-fertilize or
mate with other tetraploids
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• Happens much more in plants than animals
• Reason being plants can self-fertilize

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Figure 24.6 Two modes of speciation
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17.4 Reproductive Isolation is Reinforced When
Diverging Species Come into Contact

• Reproductive isolation may be incomplete when two
  diverging species come back into contact
• This will result in hybridization
• If hybrids are less fit, selection will favor
  parents that do not produce hybrids
• Selection results in strengthening, or
  reinforcement of isolating mechanisms that
  prevent hybridization

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• Prezygotic isolating mechanisms- mechanisms that
  prevent hybridization from occurring are called
• Postzygotic isolating mechanisms- mechanisms that
  reduce the fitness of hybrid offspring
• Postzygotic mechanisms result in selection
  against hybridization, which leads to the
  reinforcement of the prezygotic mechanisms

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Figure 24.5 A summary of reproductive barriers
between closely related species
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Prezygotic mechanisms prevent hybridization

• Prezygotic isolating mechanisms, which come into
  play before fertilization, can prevent
  hybridization in several ways
• 4 Examples

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Mechanical isolation

• structural differences in genitalia or flowers
  prevent copulation or pollen transfer
• Examples
• male dragonflies must grasp females with special
  appendages
• floral anatomy is often adapted to a specific
  pollinator that transfers pollen only between
  members of the same species.

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Mechanical isolation
Can occur in even very closely-related species
(genital openings cannot be aligned)
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Temporal Isolation

• Two species breed at different times of the day
  or in different seasons
• Example
• Brown trout breed in the spring and rainbow trout
  in the fall

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Temporal isolation
Breed at different times
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Behavioral isolation

• Little or no sexual attraction exists
• Examples
• Fireflies and blinking signals
• Other species specific courtship behavior
• Male gypsy moths and female pheromones.

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Behavioral Isolation
Only other blue-footed boobies recognize and
respond to these mating behaviors
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Gametic Isolation

• Sperm cannot reach egg to fertilize it (Important
  in water species)
• Examples
• sperm may not be able to survive in the
  environment of the females reproductive tract
• gamete recognition may be based on specific
  molecules coating the egg which only adhere to
  complementary molecules on sperm cells of the
  same species.

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Gametic isolation
Sperm and eggs of a purple sea urchin and a red
sea urchin are releases, but do not fuse outside
their species
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Postzygotic isolating mechanisms result in
selection against hybridization

• Genetic differences between two diverging
  lineages may reduce the survival and reproductive
  rates of hybrid offspring
• Three ways

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Low hybrid zygote viability

• Hybrid zygotes may fail to mature either dying or
  developing phenotypic abnormalities
• This prevents them from becoming reproductively
  capable adults

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Reduced hybrid viability
Salamander subspecies occasionally hybridize, but
embryos are often miscarried, or live young are
frail
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Low hybrid adult viability

• Hybrid offspring may have lower survivorship than
  non-hybrid offspring
• Also called hybrid breakdown

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Low hybrid adult viability
Hybrids between these strains of rice
(technically still one species) are viable and
fertile, but an accumulation of too many
recessive alleles makes them
small and sterile
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Hybrid infertility

• Hybrids mature into infertile adults
• Example the offspring of horses and donkeys
  (mules) are sterile
• Healthy but produce no descendants

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Reduced hybrid fertility
A mule (hybrid of horse and donkey) is viable,
but not fertile
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• Hybrids that are less fit will leave less species
  that can produce viable offspring
• Individuals that avoid breeding with members of
  other species will have a selective advantage
• Any trait that contributes to avoidance of
  hybridization will be favored

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Hybrid zones may form if reproductive isolation
is incomplete

• If reproductive isolation is not complete, it
  will result in the formation of hybrid zones
  where those two groups overlap
• When they first start they are crosses between
  purebred individuals
• Over many generations this will result in many
  recombinants of the original two species

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Page 344 Lets read about hybrid zones