Download The Origin of Species

The Origin of Species
What is a species?
Modes of speciation
From speciation to macroevolution
Ernst Mayr in 1942
• An evolutionary biologist-introduced a definition
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of a biological species
Defines a species as a population whose
members have the potential to interbreed with
one another to produce viable, fertile offspring
In other words, genetic exchange is possible and
therefore reproductive isolation
Prezygotic Barriers to mating
• Impede mating between species or hinder
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fertilization of ova if different species attempt to
mate
1. habitat isolation- two species that live in
different habitats within the same area
encounter each other rarely
Ex. Two species of garter snakes in the genus
Thamnophis occur in same areas, but one lives
in water and other is terrestrial
Prince Albert National Park wolf have
become separated from other wolves
and therefore have become sick
because of inbreeding.
2. Behavioral isolation
• Special signals that attract mates, as well
as behavior unique to a species are
probably the most important reproductive
barriers
• Ex. Male fireflies of various species signal
to females of their kind by blinking their
lights in a particular rhythm
• Usually elaborate courtship behaviors
12 different species of fiddler crabs on the same
beach in Panama could be distinguished by the
display of waving their large cheliped, elevating
the body, and moving around in their burrow
Behavioral (courtship) isolation
3. Temporal isolation
• Two species that breed during different times of
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day, different seasons, or different years cannot
mix their gametes
Ex. The geographical range of two species of
skunk, the western and eastern species overlap
but since one mates in the summer and one in
the winter they never encounter one another
3 species of orchids genus Dendrobium live in
the same rain forest but flower on different days
4. Mechanical isolation
• Closely related species may attempt to
mate but fail to consummate the act
• Ex. Flower anatomy is often adapted to
certain pollinators for pollen transfer
• Ex. If insects of closely related species
attempt to mate the reproductive organs
may not fit together and therefore no
sperm transfer
Pollination of Scotch broom by a bumble
bee; nectar is unavailable to lighter
honeybees that can’t trip release mechanism
5. Gametic isolation
• Even if the gametes meet, they rarely fuse
to form a zygote
• Ex. If internal fertilization the sperm may
not survive the environment of the female
reproductive tract of another species
• Gamete recognition may depend on
specific molecules on the outside of the
cells
POSTZYGOTIC BARRIERS
• 1. reduced hybrid viability- when
prezygotic barriers are crossed and hybrid
zygotes are formed, genetic
incompatibility between the two species
may abort development of the hybrid
• Ex. Species of frog of the genus Rana,
occasionally hybridize but the hybrids
generally do not complete development
2. Reduced hybrid fertility
• Even if two species mate and produce
hybrid offspring, reproductive isolation is
intact if the hybrids are sterile
• Ex. A mule is a cross between a horse
and a donkey but remain different species
when they mate because the mule is
sterile
3. Hybrid breakdown
• In some cases when species cross-mate,
the offspring are viable and fertile, but
when these hybrids mate with one
another, offspring of the next generation
are feeble or sterile
• Ex. Cotton species can produce fertile
offspring but breakdown occurs in the
next generation
However…
• The division of species based on
reproductive barriers does not work in all
cases
• Ex. Fossils are divided into species based
on morphology not on reproductive
barriers because not enough is known
about it
• Also for asexual organisms comparative
morphology and biochemical means- ex.
2 Modes of speciation
• 1. allopatric speciation- when a
population forms a new species while
geographically isolated from its parent
population
• Ex. A mountain range may emerge, a
land bridge, such as the Isthmus of
Panama may separate species, islands
separation
Adaptive radiation
• An example of allopatric speciation- the
evolution of many diversely adapted
species from a common ancestor
• Ex. In the Hawaiian archipelago- a
species inhabits an island and is blown to
the next island and becomes adapted to
that island and eventually cannot
interbreed with the original parent species
Adaptive radiation-Darwin’s finches
also called divergence
Convergent Evolution
• when species evolve to look alike
due to similar environmental
conditions but are not genetically
related!
2. Sympatric speciation
• A new species can originate in the
geographic midst of the parent species
• In plants, when an error occurs in meiosis
and the egg or sperm have more
chromosomes than normal but can fuse
with a normal gamete and the result is
called polyploidy
Sympatric speciation in animals
• Can result from a subset of a species
becoming reproductively isolated because
of a switch to a habitat, food source or
other resource –ex. A wasp that pollinates
figs-each fig species is pollinated by a
particular species of wasp which mates
and lays its eggs in the figs
• Or in a rigid female preference of a particular
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male- ex. In Lake Victoria in East Africa there
are several species of cichlid fish that have
evolved
The females prefer a certain color male but in an
aquarium when the lighting changed and they
looked the same the females did not
discriminate between the colors of males
2 Models of evolution
• 1. Punctuated equilibrium model-
species diverge in spurts of relatively rapid
change. In other words, species undergo
most of their morphological modification
as they first bud from a parent species
and then change little
• Backed by fossil record
Punctuated equilibrium
Stephan J. Gould’s model-abrupt
environmental changes
2. Gradualism model
• Species descended from a common
ancestor gradually diverge more and more
in morphology as they acquired unique
adaptations
• This is what Darwin believed
Gradualism
“evo-devo” genes
• Genes that control development play a
major role in evolution
• Many macroevolutionary changes may
have been associated with mutations in
genes that regulate development
• Such changes can affect the time of
development events or the spatial
organization of body parts
An evolutionary trend does NOT
mean…
• That evolution is goal-oriented
• Trends may occur because a species with
certain characteristics endure longer and
speciate more often than those with other
characteristics