Download - 24 Speciation
24 Speciation Sowhat is a species? Biological species concept
defined by Ernst Mayr
population whose members can interbreed & produce viable,
fertile offspring reproductively compatible Distinct species: songs
& behaviors are different enough to prevent interbreeding
Meadowlarks look so similar but are considered different species.
Cryptic speciestwo or more species that are morphologically
indistinguishable but do not interbreed. Meadowlarks Similar body
& colorations, but are distinct biological species because
their songs & other behaviors are different enough to prevent
interbreeding Biological species concept: Species are groups of
actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups.Ernst Mayr Eastern
Meadowlark Western Meadowlark Reproductive isolation is key to
speciation
Obstacle to mating or to fertilization if mating occurs geographic
isolation ecological isolation temporal isolation Speciation
requires the interruption of gene flow within a species whose
members formerly exchanged genes. The evolution of reproductive
isolation is important for understanding the origin of species.
behavioral isolation mechanical isolation gametic isolation
Allopatric Speciation
Ammospermophilus spp Geographic isolation Species occur in
different areas physical barrier allopatric speciation other
country Harriss antelope squirrel inhabits the canyons south rim
(Above). Just a few miles away on the north rim (L) lives the
closely related white-tailed antelope squirrel Barriers can form
when continents drift, sea levels rise and fall, glaciers advance
and retreat, or climates change. The populations evolve through
genetic drift and adaptation to different environments in the two
areas. Allopatric Speciation among Darwins Finches
Finch species in the Galpagos Islands evolved from a single South
American species that colonized the islands. The islands are far
apart, and have different environmental conditions. Sympatric
Speciation Ecological isolation
Species occur in same area, but occupy different habitats so rarely
encounter each other 2 species of garter snake, Thamnophis, occur
in same area, but one lives in water & other is terrestrial
Sympatric speciationspeciation without physical isolation. Gene
flow has been reduced between flies that feed on different food
varieties, even though they live in the same area Temporal
isolation Species that breed during different times of day,
different seasons, or different years cannot mix gametes sympatric
speciation same country Eastern spotted skunk (L) & western
spotted skunk (R) overlap in range but eastern mates in late winter
& western mates in late summer Behavioral isolation Unique
behavioral patterns & rituals isolate species identifies
members of species attract mates of same species courtship rituals,
mating calls The most comedic species of the Galapagos Islands is
the Blue Footed Booby, what a ridiculous outfit and expression!
Their name is in fact taken from the Spanish 'bobo' which means
clown. The Blue Footed Boobies above display part of their humorous
courtship ritual whereby they raise their feet one at a time and
then swivel their heads away from the prospective mate looking to
the sky. Other interesting Booby features are the highly evolved
airbag systems in their skulls which allow them to dive bomb into
the sea for fish from great height, and the egg and hatchling
nesting boundaries they make which are rings of Boobie poop. They
aren't the only Booby on the island there are also Masked and Red
Footed Boobies about. Blue footed boobies mate only after a
courtship display unique to their species Recognizing your own
species
courtship songs of sympatric species of lacewings Mechanical
isolation Morphological differences can preventsuccessful mating
Mechanical isolation For many insects, male &female sex organs
ofclosely related species donot fit together, preventingsperm
transfer The selection is intense because it directly affects
offspring production -- it is affecting sex itself Gametic
isolation sympatric speciation?
Sperm of one species may not be able to fertilize eggs of another
species mechanisms biochemical barrier so sperm cannot penetrate
egg receptor recognition: lock & key between egg & sperm
chemical incompatibility sperm cannot survive in female
reproductive tract Sea urchins release sperm & eggs into
surrounding waters where they fuse & form zygotes. Gametes of
different species red & purple are unable to fuse.
Dobzhansky-Muller model example: Speciation by Centric Fusion
In bats of the genus Rhogeessa, chromosomal rearrangements in
different lineages have led to speciation. Hybrids between some
types will not be able to produce normal gametes in meiosis. As
species diverge genetically, reproductive isolation increases.
Development of reproductive isolation may take millions of years,
or may develop in a few generations (e.g., the Rhogeessa bats).
Reduced hybrid fertility- example of postzygotic isolating
mechanism
Even if hybrids are vigorousthey may be sterile chromosomes of
parents may differ in number or structure & meiosis in hybrids
may fail to produce normal gametes Mules are vigorous, but sterile
Whats wrong with having 63 chromosomes? Odd number!Cannot pair up
in meiosis. Postzygotic isolating mechanismsreduce fitness of
hybrid offspring. Postzygotic mechanisms result in selection
against hybridization, which leads to reinforcement of prezygotic
mechanisms. Horses have 64 chromosomes (32 pairs) Donkeys have 62
chromosomes (31 pairs) Mules have 63 chromosomes! Hybrid breakdown
Hybrids may be fertile & viable in first generation, but when
they mate offspring are feeble or sterile In strains of cultivated
rice, hybrids are vigorous but plants in next generation are small
& sterile. On path to separate species Fig Formation of a
hybrid zone and possible outcomes for hybrids over time Isolated
population diverges Possible outcomes: Hybrid zone Reinforcement OR
Fusion Gene flow Hybrid OR Hybrid zones provide opportunities to
study factors that cause reproductive isolation A hybrid zone is a
region in which members of different species mate and produce
hybrids When closely related species meet in a hybrid zone, there
are three possible outcomes:---Strengthening of reproductive
barriers---Weakening of reproductive barriers---Continued formation
of hybrid individuals Barrier to gene flow Population (five
individuals are shown) Stability