Download Speciation

9/7/2014
Changing Environments Accelerate
Evolution
• Evolution is always occurring, just slowly
– Bacteria and viruses change all the time, so we’re
constantly coming up with new antibiotics, antivirals,
and vaccines to treat them
Speciation
• New habitats open up opportunities for evolution
• Leads to adaptive radiations – where many
groups evolve from a single
ancestor
• i.e. after the fall of dinosaurs –
many mammal groups
evolved
Chapter 24
Pg. 488 - 506
Species
Allopatric Speciation
• A species is a group of individuals capable of
interbreeding and producing fertile offspring
– Also defined in terms of reproductive isolation
• Speciation is the formation of new species –
anything that fragments a population and
isolates small groups of individuals may cause it.
– It occurs by…
• Begins when a population is divided by a geographic barrier
(physical separation)
– Interbreeding is prevented
– Common barriers include mountain ranges or rivers, but can also
include regions without vital resources (no water, no food, volcanic
lava, paved road, etc.)
• Once isolated, gene frequencies in the two populations can
diverge due to natural selection (environments may be slightly
different), mutation, or genetic drift
– If gene pools diverge enough, interbreeding will not occur if the
barrier is removed
– A reproductive barrier is formed
– New species are formed
• Allopatric speciation
• Sympatric speciation
• Adaptive radiation
Basically:
Geographic barrier
isolation
Reproductive barriers
New species
Sympatric Speciation
Reproductive
Differential
evolution
• Formation of new species without a geographic
barrier; develop a genetic difference preventing
them from interbreeding
• May occur via…
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Balanced polymorphism
Polyploidy
Hybridization
Habitat isolation
Behavioral isolation
Temporal isolation
Reproductive isolation
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Balanced Polymorphism
• When among subpopulations, it may lead to
speciation
• Example: population of insects possess
polymorphism for color; each color provides
camouflage to different substrate – no camo
means the insect is eaten.
– Under these circumstances, only insects with the
same color can associate and mate.
– Thus, similarly colored insects are reproductively
isolated from other subpopulations, and their
gene pools diverge as an allopatric speciation.
Hybridization
• Occurs when two distinctly different forms of a species (or closely
related species that are normally reproductively isolated) mate
and produce progeny along a geographic boundary called a hybrid
zone
– May cause reduced hybrid viability (development may not occur), reduced
hybrid fertility (hybrids may be sterile), and/or hybrid breakdown (when
hybrids mate, offspring may be weak or sterile)
• Sometimes the genetic variation of the hybrids is greater than
either parent
– It also permits the population of hybrids to evolve adaptations to
environmental conditions in the hybrid zone beyond the range of either
parent
– Exposed to different selection pressures, hybrids eventually diverge from
both parent populations
Behavioral Isolation
• Example: sticklebacks, small saltwater fish, have elaborate mating
behavior. At breeding time, in response to increased sunlight, the
males change color and develop a red underbelly. He builds a nest
and courts the female with a dance that involves both partners. If
either partner fails in any step of the mating dance, no mating
occurs and no young are produced.
• Example: male fireflies signal to females of their kind by blinking
the lights on their tails in a particular pattern. Females respond
only to characteristics of their own species, flashing back to
attract males. If, for any reason, the female does not respond with
the correct pattern, no mating occurs. The two animals become
isolated from each other.
Polyploidy
• Possession of more than the normal 2 sets of
chromosomes found in diploid (2n) cells
• Often occurs in plants (only occasionally animals)
– 3n (triploid)
– 4n (tetraploid)
– Higher numbers are also found
• Occurs as a result of nondisjunction of all chromosomes
during meiosis – produce two viable diploid gametes and
two sterile gametes with no chromosomes
– Tetraploid zygote = diploid sperm + diploid egg
• Since normal meiosis in the tetraploid will continue to
produce diploid gametes, reproductive isolation with
other individuals in the population (and thus speciation)
occurs immediately in a single generation
Habitat Isolation
• Two organisms live in the same area but
rarely encounter each other
• Example: two species of one genus of
snake can be found in the same
geographic area, but one inhabits water
while the other is mainly terrestrial
Temporal Isolation
• Temporal refers to time
• Example: a flowering plant colonizes a
region with areas that are warm and
sunny and areas that are cool and shady.
Flowers in warmer regions become
sexually mature sooner than flowers in
cooler areas. This separates flowers in
the two different environments into two
separate populations.
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Reproductive Isolation
• Closely related species may be unable to mate
because of a variety of reasons
– Things that prevent mating are called prezygotic
barriers (mechanical isolation)
• Differences in structural of genitalia may prevent
insemination
• Difference in flower shape may prevent pollination
• Small male dog and large female dog may have difficulty
mating
Reproductive isolation:
• Reproductive isolation is the existence of biological
factors (barriers) that impede members of two species
from producing viable, fertile offspring
• Two types:
1) Prezygotic Isolation – prevent mating
2) Postzygotic Isolation- prevent a fertilized egg from
developing a fertile adult
– Things that prevent the production of fertile offspring
once mating has occurred are called postzygotic
barriers (gametic isolation)
• Particular zygote may not be viable
Prezygotic Isolation:
Postzygotic Isolation:
Prezyotic barriers include:
1. Temporal Isolation: species may breed at different times of day;
different seasons or different years (ex. wood frog mate in march,
leopard frog mate in april)
2. Behavior Isolation: use of signals or behaviors to attract mates
that are unique to the species (ex. visual like firefly, auditory like
crickets, pheromones)
3. Mechanical Isolation: species reproductive organs are
anatomically incompatible (ex. shells of two species of snails, one
has moving inward spiral, other has outward spiral)
4. Gamete/Genetic Isolation: gametes unable to fuse and form a
gamete (sperm can’t survive in female reproductive tract)
5. Habitat Isolation: two species that may live in same geographical
region but not in same habitat so they can’t mate aka competitive
exclusion principle, each with its one niche (ex. one species of
garter snakes live on land other liv in water(
Postzygotic barriers include:
1. Reduced hybrid viability – when zygote is
formed, genetic incompatibility caused
development to cease
2. Reduced hybrid fertility – viable offspring
unable to reproduce; horse (64 chromosomes)
and a donkey (62 chromosomes) make a mule
(63 chromosomes)
3. Hybrid breakdown – two species mate, produce
viable, fertile hybrids but when hybrids mate
their offspring are weak or sterile
Adaptive Radiation
Adaptive Radiation Examples
• Relatively rapid evolution of many species
from a single ancestor
• Occurs when ancestral species is introduced
to an area where diverse geographic or
ecological conditions are available for
colonization
• Variants of ancestral species diverge as
populations specialize for each set of
conditions
• The marsupials of Australia began with the colonization
and subsequent adaptive radiation of a single ancestral
species.
• The 14 species of Darwin’s finches on the Galapagos
Islands evolved from a single ancestral South American
mainland species.
• Adaptive radiations occurred after each of the 5 big mass
extinctions. With up to 90% of species going extinct, the
periods following extinctions provided numerous
ecological opportunities for species to colonize.
– Colonization was followed by competition, which, in turn,
promoted speciation.
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Parapatric Speciation:
• Occurs where populations share a common
border; the borders are open to gene flow.
• Hybrid zone – adjoining populations meet
and reproduce hybrid offspring
Rate of Speciation
• Fossil record lacks transitional
forms between two species
• Is the fossil record incomplete or
accurately reflects evolution as it
occurs
• Punctuated Equilibrium- long
periods of stasis, punctuated by
periods of rapid speciation, trigger
by changes in environment
• Gradualism- continuous evolution
over long periods, gradually
accumulate adaptations
Extinction
•Is the norm
•Most species go
extinct- but new
ones survive
•There have
been at least 5
major mass
extinctions
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