Transcript Postzygotic isolating mechanism

Opener
Chapter 22: Origin
of Species
Concept of Species
• Sympatric species: These co-occur and differ
phenotypically in their behavior and utilize different
parts of habitat.
• Subspecies: geographically distant populations of
same species that are distinct from one another
• Intermediate populations may be present between
geographically separated populations
Fig. 22.1
The Biological Species Concept
• Biological species are defined as groups of populations that
either have the potential to or can interbreed with each other
to produce fertile offspring
• Populations which do not mate with each other or who
cannot produce fertile offspring are reproductively isolated
(members of different species).
• Reproductively isolated mechanisms prevent genetic
exchange between species
• Two types – Prezygotic isolating mechanism and
Postzygotic isolating mechanism
• Prezygotic isolating mechanism
• Prevents mating and formation of zygotes between two species
• Ecological isolation-Species occur in same area but utilize different
portions of environment. For example: Tigers and Lions in India
• Geographic isolation- Species occur in different area which are often
separated by physical barrier
• Behavioral isolation-Species differ in mating rituals. Use different
signals via sensory modes for communication, sound production,
pheromones etc
• Temporal isolation- Species reproduce in different seasons or
different times of the day. Example: Five species of genus Rana
• Mechanical isolation- Structural differences prevent mating.
Example: Structure of male and female copulatory organs may be
incompatible
• Prevention of gamete fusion-Gametes of one species function poorly
with gametes of another species or within the reproductive tract of
another species.
Fig. 22.3
• Postzygotic isolating mechanism
• Prevent proper functioning of zygotes after they form
or prevent development of zygote into reproducing
adults
• Hybrid in viability or infertility – Hybrid adults do
not survive in nature or are sterile or have reduced
fertility
• Genetic complements are different that they cannot
function in embryonic development
• Example: Hybridization between sheep and goats
produces embryos that die in early developmental
stages.
Fig. 22.5
Postzygotic
isolation in
leopard frogs –
mating is rare,
fertilized eggs
with many
problems (in
laboratory
conditions)
Fig. 22.6
Reinforcement
encourages
prezygotic
isolating
mechanisms till
they are
completely
effective
Fig. 22.7
How genetic drift and natural selection can influence
reproductive isolation?
Genetic drift and natural selection can increase reproductive
isolation
Adaptation to different environments can lead to reproductive
isolation through the accumulation of differences Ex: Anolis lizards
with their dewlaps
Fig. 22.8
How geography influences speciation?
Allopatric speciation – populations are geographically isolated
Colonization of one or few individuals in distant places, barriers to
movement can split population, extinction of intermediate
populations can leave population isolated.
Evolve into separate species because there is no gene flow
• Sympatric speciation – Speciation occurs without
geographic separation
• Produced due to polyploidy and disruptive selection
• Polyploidy – Autopolyploidy and allopolyploidy
• Autopolyploidy – errors in meiosis creates a new
species resulting in tetraploidy
• Allopolyploidy – result of hybridization of two
species
• Disruptive selection – create population containing
individuals exhibiting different phenotypes
Fig. 22.10
• Species Clusters: Evidence of Rapid Evolution
Adaptive radiation
Species evolving
from a common
ancestor inhabits a
new environment
with many
resources and few
competing species
Species evolving
from a common
ancestor inhabits a
new place or old
place where abrupt
extinction of many
species
Increased accessibility to new
environments (key innovation)
Fig. 22.11
Adaptation
occurs as
species respond
to different
environments
on different
islands
Adaptation
occurs as
sympatric
species get
driven by
need to
minimize
competition
for available
resources
Fig. 22.12
Character displacement involves evolution of divergent
adaptations that minimizes competition between two species for
available resources
Fig. 22.16
Fig. 22.17.a
The relationship
between speciation and
evolutionary changes
are on a continuum
between gradualism and
punctuated equilibrium
Gradualism are
the slow and
steady
accumulation of
changes over
long periods of
time
Fig. 22.17.b
Punctuated
equilibrium are
rapid and
discontinuous
separated by long
periods of stasis
Fig. 22.18
Mass extinctions have
happened five times
due to asteroids hitting
earth and global
climate change
Human activities are
creating a sixth mass
extinction
Adaptive radiation
occurs in remaining
species
Species diversity
rebounds after mass
extinctions
• The future of evolution
• Human influences on environment can lead to
evolutionary adaptation and formation of new species
• Decreased population size due to habitat
fragmentation and pollution will increase genetic drift
and small populations may evolve or become extinct
• Human gene pool is becoming homogenized as people
from different ethnic origins reproduce
Fig. 22.19
Tab. 22.1
Tab. 22.1.contd.
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