The Origin of Species

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Chapter 24

• The Origin of Species

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Speciation

• The origin of a new species is the focal point of
  evolutionary theory. The appearance of a new
  species is the main point of diversity.
• Microevolution describes the adaptations that
  arise within a gene pool.
• Macroevolution occurs when evolutionary change
  occurs above the species level. These are big
  changes.

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2 Patterns of Evolutionary Change

• 1. Anagenesis
• 2. Cladogenesis

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1. Anagenesis

• Anagenesis is also called phyletic evolution and
  it is the accumulation of changes that happens
  gradually over time and transforms a given
  species into a species with different
  characteristics.
• An example would be how the color of a bird
  changes over time.

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2. Cladogenesis

• Cladogenesis is also called branching evolution
  and is the splitting of the gene pool into two or
  more separate pools. These pools give rise to
  new species. Cladogenesis promotes biodiversity
  by increasing the number of species.

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Ernst Mayr and the Biological Species Concept

• This concept defines a species as a population or
  group of populations whose members have the
  ability to interbreed in nature and produce
  variable, fertile offspring, and are unable to
  produce viable, fertile offspring with other
  populations.

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Reproductive Isolation

• Reproductive isolation is a mechanism by which
  many species are isolated from one another due to
  the existence of many different types of
  biological barriers.
• 1. Prezygotic barriers impede mating or hinder
  fertilization.
• 2. Postzygotic barriers prevent viable, fertile
  adults from forming.

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Prezygotic Barriers to Mating

• Impede mating/hinder fertilization
• 1. Habitat isolation
• 2. Temporal isolation.
• 3. Behavioral isolation.
• 4. Mechanical isolation.
• 5. Gametic isolation.

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1. Habitat Isolation

• Occurs as 2 species live in the same area, but
  encounter each other rarely, if ever.
• Example 2 species of garter snakes. One lives in
  the water, the other lives on land.

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2. Temporal Isolation

• This occurs when there are differences in
  breeding times, seasons, or years which prevents
  gamete mixing.
• Example Eastern and Western spotted skunk.
  Their geographic ranges overlap, but the Eastern
  skunk mates in the later winter, and the Western
  skunk in the late summer.

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3. Behavioral Isolation

• Courtship rituals which occur between closely
  related, but different individuals produce
  effective reproductive barriers.
• Example Blue footed boobies of the Galapagos.
  The courtship ritual for these organisms males do
  a dance where he shows off his blue feet to a
  female in high-step form.

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4. Morphological Isolation

• Occurs when differences in appearance and/or
  anatomy cause different species to be unable to
  mate.
• Example Plants of different color attract
  different pollinators preventing cross
  pollination. Also, differences in flower shape
  and design prevents cross pollination.

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5. Gametic Isolation

• This happens when the sperm of one species is
  unable to fertilize eggs of another species.
• Reasons for why this does not occur The sperm
  may not survive the reproductive tract of a
  female of a different species the sperm may not
  be able to penetrate the egg once it gets there.
• Example Sea urchins are a variety of closely
  related aquatic animals. They reproduce in a
  similar way, but they are distinct enough that
  their gametes do not fuse to form zygotes.

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Post Zygotic Barriers to Mating

• Prevent production of viable/fertile adults.
• 1. Reduced hybrid viability.
• 2. Reduced hybrid fertility.
• 3. Hybrid breakdown.

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1. Reduced Hybrid Viability

• Occurs when genes of different parent species
  interact and impair normal growth and development
  of the organism.
• Example A specific subspecies of salamander
  live in areas where they occasionally meet and
  breed. Often times the offspring do not develop
  fully and those that do are not very fit.

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2. Reduced Hybrid Fertility

• Sometimes 2 species can mate and reproduce a
  viable species that is sterile. The sterility is
  often a result of the two parents having a
  different number or structure of chromosomes.
  Thus, meiosis fails to produce normal gametes.
• Example A donkey and a horse mate and a mule is
  formed.

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• Donkey Horse Mule

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3. Hybrid Breakdown

• Occurs when 2 parents meet and produce offspring.
  When these offspring mate with each other or the
  parents, the resulting offspring is very weak and
  sterile.
• Example different strains of rice that each
  carry a number of recessive alleles. When the
  offspring mate, the recessives accumulate in the
  F1. Thus, the resulting F2 offspring are very
  weak and sterile.

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Limitations of the BSC

• These barriers to mating are not easily seen
  because we cant observe the matings of
  fossilized remains.
• We also cant evaluate the reproductive isolation
  of prokaryotes and other organisms.
• Additionally, there are a lot of animals we dont
  know much about making it difficult to apply this
  concept.

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Other Definitions for Species

• 1. Morphological species concept.
• 2. Paleontological species concept.
• 3. Ecological species concept.
• 4. Phylogenetic species concept.

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2 Main Ways of Speciation

• 1. Allopatric (other country) speciation.
• 2. Sympatric (same country) speciation.

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1. Allopatric Speciation

• Occurs when gene flow in a population is
  interrupted by a geographic barrier.
• Example Lakes may rise and fall separating
  groups of individuals. Rivers may split land that
  was formerly as one.
• Once the barrier has been set up and populations
  begin to diverge, mutations and natural selection
  take over and allele frequencies change as
  genetic drift alters the gene pool.

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1. Allopatric Speciation

• Allopatric speciation is likely to occur in these
  small populations as they are more affected by
  genetic drift than larger populations.
• To confirm allopatric speciation, scientists
  bring together 2 species in a laboratory and see
  if they can successfully breed and produce
  viable, fertile offspring.

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1. Allopatric Speciation

• The Galapagos ground finch Geospiza dificilis
• Females respond to songs from males from the same
  island and ignore songs from males of the same
  species from different islands (allopatric
  populations) This demonstrates that prezygotic
  barriers have developed in these allopatric
  populations and that they are on their way to
  becoming separate species.

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2. Sympatric Speciation

• This occurs in geographically overlapping
  populations. Even though direct contact remains
  between members of the same species, mechanisms
  such as chromosomal changes and nonrandom mating
  alter gene flow.

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2. Sympatric Speciation

• A general example
• Polyploidy occurs when accidents happen during
  cell division, and cells end up with extra sets
  of chromosomes.
• If a diploid cell becomes a tetraploid (4n) as a
  result of an error, and the organism is able to
  self-fertilize, it can become reproductively
  isolated in just one generation.

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2. Sympatric Speciation

• Example
• The North American apple maggot-fly usually
  colonizes Hawthorn trees--and eats haws.

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2. Sympatric Speciation

• Some flies have switched from haws to apples and
  lay their eggs on the apples while the fruit is
  still on the tree.
• They often compete with each other for territory
  on apple trees, rather than with other flies on
  the hawthorn trees.
• These flies are becoming more and more entwined
  with the cycle of the apple trees.

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2. Sympatric Speciation

• Apples ripen and fall about a month earlier than
  the haws.
• One month is a lifetime for these flies, so the
  switch from haws to apples by these flies is
  pushing the two gene pools apart.
• One fly is reliant upon the life-cycle of the haw
  tree, the other is reliant on the life-cycle of
  the apple tree and they are shifting their
  breeding cycles to account for their new habitat.

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Adaptive Radiation

• This occurs when a few organisms make their way
  into a new environment and give rise to diversely
  adapted populations of new organisms that have
  descended from a common ancestor.
• This is often seen following mass extinctions
  when numerous niches open up.

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The Tempo of Speciation

• Niles Eldridge and Stephen Jay Gould coined the
  term punctuated equilibrium to describe the tempo
  of speciation.
• Punctuated equilibrium is marked by periods of
  apparent stasis in the evolution of an organism
  and then is followed by points where speciation
  occurs rapidly.

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The Tempo of Speciation

• Sometimes we see fossils in the strata that never
  change and then disappear.
• This doesnt mean that the organism didnt change
  or evolve. It just means that the events that
  produced the new species may have occurred too
  rapidly to have been preserved in the fossil
  record.

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The Tempo of Speciation

• For example
• Say a species survived for 5 million years, and
  many of its major morphological changes occurred
  in the first 50,000 years (1). Often times this
  is too quick to be preserved in the fossil
  record. Then, it would seemingly appear
  suddenly, linger with no little/no change, and
  then become extinct.

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The Tempo of Speciation

• Stasis can also be explained this way. Often
  times, changes go undetected by the fossil
  record. Consider changes in biochemistry--they
  cant be detected by paleontologists.

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Macroevolution

• Macroevolution results as species diverge and
  speciate again and again resulting in differences
  that accumulate and become more pronounced.
  Speciation is the beginning of macroevolutionary
  change.
• These cumulative changes occur as a result of
  thousands of small speciation events.
• Thus, if you accept microevolution, you get
  macroevolution for free.

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The Arguments

• Many arguments against evolution fail to
  recognize the fact that many complex structures
  evolve in small increments from simple ancestral
  structures that perform the same basic function.
• Its hard to imagine millions of years when you
  cant comprehend what a million actually is.

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The Arguments

• For example, consider the human eye. It is a
  very complex structure that works to form an
  image and transmit the information to the brain
  for processing.
• How could such a complex structure evolve in
  gradual increments?
• How could a partial eye be of any use to our
  ancestors?

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The Arguments

• The flaw in the argument is the assumption that
  partial eyes have no use. Simple light sensors
  are useful, even though they cant focus an
  image.
• Many different eyes evolved on different
  organisms as they diverged from a common ancestor
  with light-sensing photoreceptors.

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Remember,

• Evolution is not goal oriented, it is the result
  of the interactions of organisms with their
  current environments. The most fit organisms
  survive. As the environments change over time,
  so to do the organisms.
• These small changes (microevolution) accumulate
  and slowly give rise to big changes
  (macroevolution).

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Remember Also,

• Speciation is a process, NOT an event.

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