The document discusses speciation, which is the process by which new species develop from existing species. There are several factors that can lead to speciation, including geographical isolation, reproductive isolation, genetic drift, and variations due to natural selection. The main types of speciation discussed are allopatric (geographical isolation), sympatric (evolution of new niches), parapatric (non-random mating reduces gene flow), and peripatric (isolation of small populations). Examples are given of each type, including Darwin's finches through allopatric speciation and apple maggot flies through sympatric speciation. Reproductive isolating mechanisms that can occur both before and after mating are also outlined.

1. Speciation
M. Hemalatha
Ph.D Scholar
Centre for Plant Breeding and Genetics
Tamil Nadu Agricultural University

2. Species
• A species is a population of organisms consisting of
similar individuals which can breed together and
produce fertile offspring.

3. What is speciation?
The process by which new species develop from the
existing species is known as speciation.

4. Factors leading to speciation
• Geographical isolation
• Reproductive isolation
• No flow of genes between separated groups of
population
• Genetic drift
• Variations due to natural selection.

5. Types of Speciation
• Allopatric Speciation
• Sympatric Speciation
• Parapatric Speciation
• Peripatric Speciation

6. Allopatric Speciation
• It occurs when members of a population become
geographically isolated from one another.
• This may be a result of geographical changes.
• Alternatively, species members may emigrate, resulting
in population separation by dispersal; this is commonly
known as vicariance.

7. Example of allopatric speciation
• The divergent populations of finches inhabiting the
Galapagos Islands and are known as ‘Darwin’s finches’.
• Galapagos Islands hosted a population of finches - relatively
similar in morphology with slight differences in features such
as body size, color and beak length or shape.
• Different food sources were available for the birds on each of
the different islands, and the differences in beak shape were
an adaption toward acquiring the particular food source.

9. Sympatric Speciation
• Sympatric speciation is the evolutionary process whereby
species are formed from a single ancestral species while
inhabiting the same geographic area.
• In contrast to allopatric speciation, the distribution ranges of
species which evolve through sympatry may be identical or
they may only overlap.
• Rather than geographic distance prompting a reduction
of gene flow between populations, sympatry occurs when
members of one population make use of a new niche.

11. Example of Sympatric Speciation
• Apple maggot flies mated only on hawthorn trees, but after
apple trees were introduced into North America, they began to
mate on the apple trees as well.
• Thus, sympatric speciation occurred and new species were
formed when the species of apple maggot flies feeded only on
their respective trees even if they were not geographically
isolated and also if their adult populations looked identical.

13. Parapatric Speciation
• It is an extremely rare case of speciation that occurs when a population is
continuously distributed within a geographic area without any specific barriers
to gene flow.
• The population does not mate randomly within the population, but rather
individuals mate more commonly with their closest geographic neighbors,
resulting in uneven gene flow.
• Non-random mating may increase the rate of dimorphism within populations, in
which varied morphological forms of the same species are displayed.
• The result of parapatric speciation is one or more distinct sub-populations
(known as ‘sister species’), which have small, continuous overlaps in
their biogeographic range and are genotypically dimorphic.

14. Example of Parapatric Speciation
Anthoxanthum odoratum
 Some of these plants live near mines
where the soil has become contaminated
with heavy metals.
 The plants around the mines have
experienced natural selection for
genotypes that are tolerant of heavy
metals.
 Meanwhile, neighboring plants that don’t
live in polluted soil have not undergone
selection for this trait.

15.  The two types of plants are close enough that
tolerant and non – tolerant individuals could
potentially fertilize each. However, the two
types of plants have evolved different
flowering times.
 This change could be the first step in
cutting off gene flow entirely between the
two groups.
 Although continuously distributed,
different flowering times have begun to
reduce gene flow between metal - tolerant
plants and metal - intolerant plants.

16. Peripatric Speciation
• Peripatric speciation is a form of allopatric speciation
that occurs when populations that have become isolated
have very few individuals.
• Through this process, the population goes through
a genetic bottleneck.
• Within the small sub-population, organisms which are
able to survive within the new environment may carry
genes that were rare within the main population but that
cause a slight variation to behavior or morphology.

17. • Through repeated matings, the frequency of these, once
rare, genes increases within the small population. This
is known as the ‘founder effect’.
• Over time, the characteristic that was determined by the
gene becomes fixed within the population, leading to an
isolated species that is evolutionarily distinct from the
main population.

21. Artificial Speciation
• Artificial speciation is the form of speciation that can be
achieved by the input of human influence.
• By separating populations, and thereby preventing breeding,
or by intentionally breeding individuals with desired
morphological or genotypic traits, humans can create new,
distinct species.
• This is also known as ‘artificial selection’; most modern
domesticated animals and plants have undergone artificial
selection.

22. Evolution by artificial selection

23. Hybridization
 Often the hybrid offspring are sterile, but occasionally
they are fertile and are reproductively isolated from their
“parent” species.
 In the latter case, a new species is formed. The sunflower
species was produced by the hybridization of two other
sunflower species.

24. Hybridization in Sunflowers

25. Reproductive Isolating Mechanisms
Pre - zygotic mechanism
Pre-pollination
mechanisms
1. Geographical
isolation
2. Ecological isolation
3. Temporal isolation
4. Behavioural isolation
5. Mechanical isolation
Post-pollination
mechanisms
1. Gametophytic
isolation
2. Gametic isoation
Post - zygotic
mechanism
1. Seed
incompatibility
2. Hybrid inviability
3. F1 hybrid sterility
4. F2 hybrid
inviability
5. F2 hybrid sterility

26. Geographical isolation
• Species occur in different areas.

27. Ecological isolation
• Species occur in same region, but occupy different
habitats so rarely encounter each other.
• They are reproductively isolated.

28. Temporal isolation
• Species that breed during different times of day,
different seasons, or different years cannot mix
gametes.

29. Behavioural isolation
• Unique behavioral patterns & rituals isolate species

30. Mechanical isolation
• Morphological differences can prevent successful
mating.

31. 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

32. 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

33. 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 and P. veris.

34. 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

35. 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

36. F2 hybrid inviability or sterility
• F2 hybrid dies much before reaching the flowering stage or
fails to produce seeds.

37. Rate of speciation
• Does speciation happen gradually or rapidly??
– Gradualism
– Punctuated Equilibrium

38. Gradualism
• Gradual divergence
over long spans of time
• Big changes occur as
the accumulation of
many small ones

39. Punctuated Equilibrium
• Rate of speciation is not
constant
– rapid bursts of change
– long periods of little or no
change
– species undergo rapid
change when they 1st bud
from parent population