Speciation

Assignment
Biosystematics
Topic: Speciation
Submitted to: Dr. Iftikhar Ahmad
Submitted by: Mujahid Hussain
Roll Number: 12
Class: M.Phil
12/20/2016Roll Number (12) Mujahid Hussain (M.Phil Botany)
1

Speciation (INTRODUCTION)
• Process of development of new species.
• Commonly results from the development of barriers to gene flow.
• Different types of isolating mechanisms are responsible for the
development of barriers.
2

Speciation (ISOLATING MECHANISMS)
• Isolating mechanisms operate
1. before sexual fusion (Prezygotic mechanism)
2. after sexual fusion (Post-zygotic mechanism)
3

ISOLATING MECHANISMS
Prezygotic mechanisms further classified into
1. Pre-pollination mechanisms
2. Post-pollination mechanisms
4

• Pre-pollination mechanisms include
1. Geographical isolation
2. Ecological isolation
3. Seasonal isolation
4. Temporal isolation
5. Ethological isolation
6. Mechanical isolation
5

• Post-pollination mechanisms include
1. Gemetophytic isolation
2. Gametic isoation
6

• Postzygtic mechanisms include
1. Seed incompatibility
2. Hybrid inviability
3. F1 hybrid sterility
4. F2 hybrid inviability
7

Pre-pollination mechamisms (Geographical isolation)
• Two species are separated geographically by a gap larger than
their pollen and seed dispersal.
• Example
• Platanu sorientais (Mediterranean region)
• Platanu occidentalis (North America)
• Both are well established species in their regions and readily
interbreed when brought into same area (vicarious species)
8

Pre-pollination mechamisms ( Ecological isolation)
• Two species occupy the same general area but occupy different
habitats
• Example
• Silene alba grows on light soils in open places
• Silene dioicaon grows on heavy soils in shade
9

Pre-pollination mechamisms (Seasonal isolation isolation)
• Two species occur in the same region but flower at different
seasons
• Example
• Sambucus racemose and Sambucus nigra flower nearly 7 weeks
apart
10

Pre-pollination mechanisms (Temporal isolation)
• Two species flower during the same period but during different
times of the day
• Example
• Agrostis tenuis flower in afternoon
• Agrosits stolonifera flowers in the morning
11

Pre-pollination mechanisms (Ethological isolation)
• Two species are interfertile but have different pollinators
• Example
• Humming birds are attracted to red flowers and hawk-moths to
white ones
12

Pre-pollination mechanisms (Mechanical isolation)
• Pollination between two related species is prevented by structural
differences between flowers
• Example
• Ophrys insectifera and ophrys apifera
13

Post-pollination mechanisms (Gametophytic isolation)
• Commonest isolating mechanism
• Cross-pollination occurs but the pollen tube fails to germinate or
if germinated it can’t reach and penetrate the embyo sac
14

Post-pollination mechanisms (Gametic isolation)
• Reported in several crop plants
• The pollen tube releases the male gametes into embryo sac, but
gametic or endospermic fusion does not occur
15

Post-pollination mechanisms (Gametophytic isolation)
• This is the commonest isolating mechanism
• Where in cross-pollination occurs but
• the pollen tube fails to germinate or if
• germinated, it can’t reach and penetrate the embryo sac.
16

Postzygotic mechanisms (After sexual fusion)
1. Seed incompatibility
2. Hybrid inviability
4. F2 hybrid inviability and sterility
17

Postzygotic mechanisms (Seed incompatibility)
• The zygote or even immature embryo is formed but fails to
develop and as such a mature seed is not formed.
• The phenomenonis commonly encountered in cross
• between Primula elatior andP. veris.
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Postzygotic mechanisms (Hybrid inviability)
• Mature seed is formed and manages to germinate but the F1
hybrid dies before the flowering stage is reached.
• The phenomenon is commonly encountered in crosses between
Papaver dubium and P. rhoeas
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Postzygotic mechanisms (F1 hybrid sterility: )
• F1 hybrids are fully viable and reach flowering stage but flowers
may abort or abortion may occur as late as F2 embryo formation,
• with the result that the F1 hybrid fails to produce viable seeds
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Postzygotic mechanisms (F2 hybrid inviability or sterility)
• F2 hybrid dies much before reaching the flowering stage or fails to
produce seeds.
21

abrupt speciation or gradual speciation
New species may develop through the mechanism of
1. abrupt speciation
2. gradual speciation
22

abrupt speciation
• The phenomenon of abrupt speciation is commonly involves
sympatric populations of two different species and as such is also
• known as sympatric speciation.
23

abrupt speciation
• It is commonly the result of a sudden change in chromosome
number or structure, producing instantly an almost irreversible
barrier between populations and thus effectively
• isolating them.
24

gradual speciation
• This is a more common phenomenon in nature. It may involve
phyletic evolution
• one species might evolve into something different from its
ancestor over a period of time
• (phyletic speciation). Alternatively, a population belonging to a
single species
• Might differentiate into two evolutionary lines
• through divergent evolution (additive speciation)
25

Phyletic Speciation
• It is the sequential production of species within a single
evolutionary lineage.
• Species A might,over a period of time, change through species B
and C into species D without ever splitting.
• The new species produced in this manner are variously called
successional species, palaeospecies, and allochronic species.
• The species which have become extinct in the process are termed
taxonomic extinctions.
26

Additive Speciation
• Additive speciation is the commonest mode of speciation, which
adds to the diversity of living organisms.
• Mayr (1963) suggested the occurrence of reductive speciation,
where by two previously independent species fuse into a third,
new species, themselves becoming extinct.
• Hybridization likewise produces new species but this always leads
to an addition in the number of species.
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Additive Speciation
• It is impossible to imagine that two evolutionary species can
actually fuse to produce a third species and themselves become
extinct.
• This may happen in a particular region, but not over the entire
range of these species
28

Additive Speciation
• The various modes of additive speciation are
1. Allopatric speciation
2. Allopatric introgressive speciation
3. Allo-parapatric speciation
4. Parapatric speciation
5. Stasipatric speciation
6. Sympatric speciation
29

Additive Speciation
• Additive speciation is the commonest mode of speciation, which
adds to the diversity of living organisms.
• Mayr (1963) suggested the occurrence of reductive speciation
• two previously independent species fuse into a third, new species,
themselves becoming extinct.
• Hybridization likewise produces new species but this always leads
to an addition in the number of species
30

Additive Speciation (Allopatric speciation)
• Lineage independence and consequent speciation result from
geographical separation of lineages
• may also result from the development of new
• species along the boundaries of a large central population.
31

Additive Speciation (Allopatric introgressive speciation)
• Although origin of species through hybridization is commonly
results from sympatric species
• examples of speciation involving two allopatric species, which had
contacts in the past,are also reported.
32

Additive Speciation (Allo-parapatric speciation)
• Such speciation occurs when two populations of an ancestral
species are separated, differentiate to a degree that is not
sufficient for lineage independence, and then develop lineage
independence during a period of parapatry (limited sympatry)
33

Additive Speciation (Allo-parapatric speciation)
• It differs from allopatric speciation in the sense that speciation is
completed after a period of sympatry and the process of attaining
lineage independence is potentially reversible because
• It is possible that two partly differentiated populations could form
a single evolutionary lineage
34

Additive Speciation (Parapatric speciation)
• This occurs when two populations of an ancestral species
differentiate despite the fact that no complete disjunction has
occurred.
• The daughter species may share a small fraction of their
respective ranges and interbreed within this narrow contact zone
and yet still differentiate
35

Additive Speciation (Stasipatric speciation)
• This is similar to parapatric speciation except that it results from
spontaneous chromosomal modifications.
• The resultant chromosome arrangement must be fully viable in the
homozygous state but of reduced viability in the heterozygous
state
36

Additive Speciation (Sympatric speciation)
• The process of ecological sympatric speciation is a slow one of
gradual speciation.
• The ecological differences in the habitats result in adaptive
• radiations in populations which gradually evolve into new species.
37

Speciation

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