The Origin of Species

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Chapter 24 The Origin of Species
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Macroevolution is the origin of new taxonomic
groups, as opposed to microevolution, which is
genetic variation between generations.   A.  What
is a species?             1.  Biological species
concept                         - A species is a
population or group of populations whose members
have the potential to interbreed with one another
and produce viable offspring, but who cannot
produce viable offspring with other
species.   Figure 24.3 (p. 473) The biological
species concept is based on interfertility rather
than physical similarity.
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--gt Speciation is the process by which a new
species originates and involves the creation of a
population of organisms that are novel enough to
be classified in their own group.  There are two
processes by which this can occur              
- Anagenesis is the accumulation of heritable
traits in a population, that transforms that
population into a new species             -
Cladogenesis is branching evolution, in which a
new species arises as a branch of from the
evolutionary tree.  The original species still
exists.  This process is the source of biological
diversity.
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For a new species to form, there needs to be
isolation of some members of a species as a
separate population. Forms of isolation, that
interfere with breeding include
both..             2.  Prezygotic and postzygotic
barriers (Fig. 24.4, p. 474-5)                    
     ? Prezygotic barriers prevent mating or egg
fertilization if members of different species try
to mate. Examples                         a. 
Habitat isolation - Two species that live in the
same area, but occupy different habitats rarely
encounter each other.                         b. 
Behavioral isolation - Signals that attract mates
are often unique to a species  (e.g. different
species of  fireflies flash different patterns).
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                        c.  Temporal
isolation                                     -
Two species breed at different times of the day
or during different seasons.                     
       d.  Mechanical isolation                   
                  - Closely related species
attempt to mate, but are anatomically
incompatable.  (Example  flowering plants with
pollination barriers some plants are specific
with respect to the insect pollinator, often
occurs with butterflies/moths)                    
       e.  Gametic isolation                      
               - Gametes must recognize each
other.  (Example  fertilization of fish eggs,
chemical signals between sperm and egg allows
sperm to recognize the correct egg)
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- Postzygotic barriers prevent a hybrid zygote
from developing into a fertile adult.
Examples                         a.  Reduced
hybrid viability - Abort development of hybrid at
some embryonic stage.                          
b.  Reduced hybrid fertility - Meiosis doesnt
produce fertile gametes in vigorous hybrids.
                        c.  Hybrid
breakdown - First-generation hybrids are fertile,
but they cannot produce fertile offspring in the
next generation (e.g. different species of
cotton).  
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Figure 24.4 Reproductive Barriers
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There is a problem with the idea of biological
species concept --gt How do you get organisms to
breed to see whether viable offspring are
produced?  There are               3. 
Alternative concepts of species                   
      a.  Ecological species concept - Species
are defined by their use of environmental
resources their ecological niche (e.g. species
that are defined by their food source such as
butterflies with certain flowers)    
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                        b.  Morphological species
concept - Takes into consideration factors such
as body shape, size, etc.                  c. 
Paleontological species concept - Species in the
fossil record are characterized according to a
unique set of structural features.                
          d.  Phylogenetic species concept -
Recognizes species are sets of organisms with
unique genetic histories. This idea is based
often on molecular analyses such as DNA sequences.
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B.  Modes of speciation ? Figure 24.5 (p.
476)              1.  Allopatric
speciation                         - Allopatric
speciation describes speciation that takes place
in populations with geographically separate
ranges.  Gene flow is interrupted and new species
evolve. 2.  Sympatric speciation                  
       - Sympatric speciation describes
speciation that takes place in geographically
overlapping populations.  Chromosomal changes and
nonrandom mating reduce gene flow. Remember
Species arise when individuals in a population
become isolated one from the other.
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Figure 24.5 Two main modes of speciation
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 Examples of Allopatric speciation            
Figure 24.6 (p. 477) Allopatric speciation of
squirrels in the Grand Canyon. Animals like
birds do not show speciation like those animals
that are barred from breeding by the canyon.  
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Another place where adaptive radiation is
apparent is on island chains (e.g. Fig. 24.12).
This example is illustrative of what happened on
the Hawaiian islands. Would this example be
allopatric or sympatric speciation? Remember ?
Once geographic isolation has occurred, there
still must be changes that reproductively isolate
populations of individuals. If the populations
evolve so that they are now new species, they
cannot interbreed to produce fertile, viable
offspring. In other words, they need to be
reproductively isolated!
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Figure 24.12 Adaptive radiation
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2.  Sympatric speciation                        
- Sympatric speciation describes speciation that
takes place in geographically overlapping
populations.  This can occur by chromosomal
changes and nonrandom mating. Both can reduce
gene flow between organisms and cause populations
to evolve to new species. Example              
           - Reproductive barriers can arise by
polyploidy (greater than 2 sets of chromosomes). 
This mechanism is most common in
plants.                           Figure 24.8 (p.
478) Sympatric speciation by autopolyploidy in
plants.
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Figure 24.8 Sympatric speciation by
autopolyploidy in plants
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Figure 24.9 One mechanism for allopolyploid
speciation in plants
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                        - Animals diverge mostly
due to reproductive isolation.  Reproductive
isolation is a result of genetic factors that
cause offspring to rely upon resources not used
by previous generations.  (Example  switch to a
new food source) An extremely good example of
sympatric speciation in animals occurred in Lake
Victoria which has 200 closely related species of
Cichlids (fish) which probably all arose from one
ancestor with the driving force for speciation
being Competition for a limited resource (food)
within the lake, and adaptation to new food
sources. This gave rise to different species
that are kept from breeding with each other by
distinctive coloration patterns (Fig. 24.10)
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Figure 24.10 Does sexual selection in cichlids
result in reproductive isolation?
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• C.  From speciation to macroevolution
• How then do we get from the mechanism of
  speciation to evolution on a grand scale, i.e.
  macroevolution?
• There are two models to describe the tempo of
  speciation
• The Gradualism model suggests that change is
  gradual with the accumulation of unique
  morphological adaptation.
• The Punctuated Equilibrium model suggests that
  rapid change occurs, with a new species
  erupting from the ancestral lineage and then
  staying the same thereafter.
• Fig. 24.13

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However it does occur, we need to remember that ?
Speciation occurs when divergence leads to
reproductive barriers between the new and the
ancestral population. And this probably takes
vast amounts of time to occur. ? But how do
evolutional novelties emerge? For example, how
did something as complex as the eye first evolve?
We need to remember that ? Most evolutionary
novelties are modified versions of older
structures. And an extremely good example is the
eye as shown in Figure 24.14 (p. 483) A range
of eye complexity among mollusks.
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The lesson from the eye example is that  ?
Existing structures can be modified for brand new
functions.  These are called Exaptations
structures that evolve for one purpose but become
useful for another function.   Finally, we
should             Remember that evolution is
not goal oriented.  Differential reproduction is
only a reaction of individuals to their
environment.               Figure 24.24 (p. 481)
The branched evolution of horses. This figure
can give the illusion of goal-oriented evolution
of the horse, but it is only an illusion.
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