Transcript Speciation, Reproductive Isolation, and Patterns of Evolution

Speciation,
Reproductive Isolation,
and
Patterns of Evolution
AP Biology
Evolution IV
Speciation
• Species: a group of individuals capable of
interbreeding
• Speciation: the formation of new species
(3 main types)
– Allopatric Speciation
– Sympatric Speciation
– Adaptive Radiation
Allopatric Speciation
http://www.nodvin.net/snhu/SCI219/demos/Chapter_4/
Allopatric Speciation
• Occurs when a population is divided by a
geographic barrier
– Barriers: mountains, rivers, regions excluding
vital resources (water, food) areas covered
with volcanic lava
• Interbreeding between populations not
possible (reproductive isolation)
• Gene frequencies can diverge due to
natural selection, mutation, and genetic
drift
Sympatric Speciation
apple maggot flies
apples
hawthorns
Sympatric Speciation
• The formation of new species without the
presence of a geographic barrier
• Occurs by one of 3 ways:
– Balanced Polymorphism
– Polyploidy
– Hybridization
Balanced Polymorphism
• Suppose a population of insects
possesses a polymorphism for color.
Each color provides a camouflage to a
different substrate (rock, tree stump, etc..).
• When not camouflaged, they are eaten
• Thus, only insects with the same color can
associate and mate
• Similarly colored insects are reproductively
isolated…so gene pools can diverge.
Polyploidy
• More than 2 sets of chromosomes found in
diploid (2n) cells.
• Often occurs in plants (occasionally
animals) where triploid (3n), tetraploid (4n)
and higher chromosome numbers exist.
• Caused by nondisjunction in meiosis
• Tetraploid individuals will continue to
produce diploid gametes – making them
reproductively isolated very quickly.
Hybridization
• Occurs when 2 different forms of a species
mate and produce offspring along a
geographic boundary called a hybrid zone.
• The genetic variation of the hybrids is
greater than that of either parent
• This permits hybrids to adapt to
environmental conditions beyond the range
of either parent.
• Hybrids can eventually diverge from parent
forms when faced with selective pressures
Adaptive Radiation
Adaptive Radiation
• Relatively rapid evolution of many species
from a single ancestor.
• Ancestral species colonizes an area where
diverse geographic or ecological
conditions are available for colonization.
– i.e. many available ecological niches for a
population to spread into.
– Examples: Darwin’s Finches, Australian
Marsupials
Patterns of Evolution
• Evolution can take place along the lines of
the following patterns:
– Divergent Evolution
– Convergent Evolution
– Parallel Evolution
– Coevolution
Divergent Evolution
• Describes two or more species that
originate from a common ancestor.
• This may happen as a result of allopatric
or sympatric speciation or by adaptive
radiation
Convergent Evolution
• Describes two unrelated species that
share similar traits.
• Similar traits arise because each species
has independently adapted to a similar
niche
• These traits are called: Analogous Traits
Convergent Examples
• Sharks, porpoises, and penguins have torpedoshaped bodies with peripheral fins. These traits
arise as a result of adaptations each species has
made to aquatic life…not due to a common
ancestor.
• The eyes of squids and vertebrates are
physically and functionally similar. However they
are not from a recent common ancestor, both
evolved independently to perform similar
functions.
Parallel Evolution
• Describes two related species or two
related lineages that have made similar
evolutionary changes after their
divergence from a common ancestor
• Example:
– Species from two groups of mammals, the
marsupial mammals and the placental
mammals, have independently evolved similar
adaptations when ancestors encountered
comparable environments
Coevolution
• Describes the evolution of one species in
response to new adaptations that appear
in another species
• An example: evolutionary arms race
between predators and prey…or
– Plants and plant eating insects
– Pollinators and flowering plants
– Pathogens and animal immune systems