Speciation Contents History Key Components of the Concept of Speciation Modes of Speciation Speciation-Gradual and additive mechanism Speciation Types@ Allopatric Pperipatric Parapatric and Sympatric with examples

1. Speciation
Speciation
Contents
History
Key Components of the Concept of Speciation
Modes of Speciation
Speciation-Gradual and additive mechanism
Speciation Types@ Allopatric Pperipatric Parapatric and Sympatric with examples

2. Dr Praveen Mohil
Asstt. Professor
Department of Botany
University of Rajasthan, Jaipur
We can define speciation as a process by which the novel genetically independent group of
organisms are formed through the process of evolution.
History
Speciation is the evolutionary process by which populations evolve to become distinct species.
“Speciation is the process of formation of new species from existing populations.” The biologist
Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to
anagenesis, phyletic evolution within lineages.
Charles Darwin was the first to describe the role of natural selection in speciation in his 1859
book On the Origin of Species.
Speciation, the process by which new species arise, is a fundamental concept in evolutionary
biology. It involves the divergence of populations from a common ancestor into distinct lineages
that can no longer interbreed or exchange genetic material freely. The concept of speciation
provides crucial insights into the mechanisms driving biodiversity and the origin of biological
diversity across the planet.
Key Components of the Concept of Speciation:
Genetic Divergence: Speciation typically begins with populations of a single species becoming
genetically isolated from each other. This isolation can occur due to various factors such as
geographic barriers, ecological differences, or reproductive barriers. Over time, genetic
differences accumulate between populations through processes like mutation, genetic drift, and
natural selection.
Population --------isolation (due to geographical/ecological barriers)--------2 or more species
Reproductive Isolation: Reproductive isolation mechanisms prevent gene flow between
populations and contribute to the formation of distinct species. These mechanisms can be
prezygotic, occurring before fertilization, or postzygotic, occurring after fertilization. Examples
include differences in mating behaviors, mating times, morphology, and genetic
incompatibilities.
Population --------isolation (due to free gene flow)--------2 or more species
Divergent Selection: As populations become isolated and adapt to different environments or
ecological niches, natural selection drives them to diverge genetically and phenotypically.
Divergent selection favors traits that enhance survival and reproduction in specific environments,
leading to the accumulation of genetic differences between populations.
Population --------isolation (due to adaptation )--------2 or more species
Adaptive Radiation: Speciation can be facilitated by adaptive radiation, a process where a
single ancestral species rapidly diversifies into multiple species, each adapted to exploit different
ecological niches. Adaptive radiation often occurs in response to the availability of new habitats
or resources, leading to the proliferation of species with diverse traits.
Single Population --------seperation (due to new habitats )--------2 or more population

3. Modes of Speciation
Modes of Speciation: Speciation can occur through several modes, including allopatric,
sympatric, parapatric, and peripatric speciation. These modes differ in the degree of geographic
isolation between populations and the mechanisms driving reproductive isolation. Allopatric
speciation, where populations are geographically separated, is the most widely studied mode, but
other modes, such as sympatric speciation within the same geographic area, also play significant
roles in generating biodiversity.
Speciation-Gradual and additive mechanism
Gradual and additive mechanisms of speciation refer to processes by which new species arise
through the gradual accumulation of small genetic and phenotypic changes over time. These
mechanisms contrast with more abrupt or punctuated modes of speciation, which involve rapid
bursts of evolutionary change.
Characteristics of Gradual and Additive Mechanisms:
Accumulation of Genetic Variation: In gradual speciation, genetic variation accumulates
within populations through mutation, recombination, and other evolutionary processes. These
genetic changes are typically small and may initially have subtle effects on the phenotype.
Phenotypic Divergence: Over time, the accumulation of genetic variation leads to phenotypic
divergence between populations. Small differences in traits such as morphology, behavior, or
physiology gradually accumulate, reflecting the genetic changes underlying speciation.
Selection Pressures: Natural selection plays a key role in driving the divergence of populations.
Environmental factors, such as differences in habitat or resource availability, exert selection
pressures that favor individuals with traits adapted to local conditions. Gradual changes in
selective pressures contribute to the divergence of populations and the formation of new species.
Gene Flow and Isolation: Despite the gradual accumulation of genetic differences, gene flow
between populations may still occur, maintaining genetic exchange between them. However, as
populations become more genetically and phenotypically distinct, barriers to gene flow may
arise, leading to reproductive isolation and the completion of speciation.
Temporal Scale: Gradual and additive mechanisms of speciation typically operate over longer
time scales, spanning hundreds to thousands of generations. The gradual accumulation of genetic
and phenotypic changes allows for the slow divergence of populations and the eventual
emergence of reproductive barriers.
Examples of Gradual and Additive Speciation Mechanisms:
Allopatric Speciation: In allopatric speciation, geographic isolation leads to the gradual
divergence of isolated populations. Over time, genetic and phenotypic differences accumulate
due to local adaptation and genetic drift, eventually resulting in reproductive isolation and the
formation of new species.

4. Divergent Natural Selection: Differential selection pressures in different environments can
drive the gradual divergence of populations. For example, in a heterogeneous habitat,
populations may adapt to different ecological niches, leading to the accumulation of genetic and
phenotypic differences.
Sexual Selection: Sexual selection, driven by mate choice and competition for mates, can lead to
the gradual evolution of reproductive isolation between populations. Preferences for specific
traits or behaviors can drive the divergence of populations and contribute to speciation over time.
Molecular Evolution: Molecular evolution, such as changes in DNA sequences or gene
expression patterns, can occur gradually over evolutionary time scales. These molecular changes
may underlie phenotypic differences between populations and contribute to the process of
speciation.
Comparative table outlining the differences between gradual and additive mechanisms of
speciation:
Aspect Gradual Mechanism of Speciation Additive Mechanism of Speciation
Definition It occurs through the gradual
accumulation of small genetic and
phenotypic changes over time.
It results from the additive effects of
genetic and phenotypic changes that
accumulate gradually.
Genetic
Variation
Genetic variation accumulates
within populations over time
through mutation, recombination,
and other evolutionary processes.
Genetic variation is added to populations
through mutation, recombination, and
other mechanisms, contributing to the
divergence of populations.
Phenotypic
Divergence
Gradual divergence in phenotypic
traits occurs as a result of
accumulating genetic differences.
Phenotypic differences between
populations gradually emerge as a
consequence of additive genetic effects.
Selection
Pressures
Natural selection acts on
populations, favoring individuals
with traits adapted to local
environmental conditions.
Differential selection pressures in
different environments contribute to the
divergence of populations.
Gene Flow
and
Isolation
Gene flow between populations may
occur, maintaining genetic
exchange. Reproductive isolation
may gradually arise as populations
become more genetically and
phenotypically distinct.
Gene flow between populations may
occur, but barriers to gene flow gradually
emerge as populations diverge, leading to
reproductive isolation.
Temporal
Scale
Operates over longer time scales,
spanning hundreds to thousands of
generations.
Also operates over longer time scales,
allowing for the gradual accumulation of
genetic and phenotypic changes.
Examples Allopatric speciation, where
geographic isolation leads to gradual
divergence. Divergent natural
selection, sexual selection, and
molecular evolution can also
contribute.
Examples include allopatric speciation
with gradual divergence due to geographic
isolation, as well as cases where divergent
natural selection, sexual selection, or
molecular evolution drive gradual
divergence.

5. There are four geographic modes of speciation in nature, based on the extent to which
speciating populations are isolated from one another: Allopatric Pperipatric Parapatric and
Sympatric.
Speciation Types
Allopatric Speciation- caused by geographical isolation.
Examples of allopatric speciation include: Galápagos Finches( byCharles Darwin), Hawaiian
Honeycreepers (birds), East African Rift Lakes Cichlids (cichlid fishes).
Parapatric Speciation -This is a type of allopatric speciation in which the species are not
formed by any physical barrier. Instead, they are beside each other. This occurs by an extreme
change in the habitat. Though the individuals in these areas can interbreed, they develop
different characteristics and lifestyles. Or populations of the ancestral species diverge into
separate species without complete geographic isolation. This can occur due to localized
environmental differences or barriers to gene flow within a continuous geographic range.
Examples:
1.Hawthorn (Crataegus)- Crataegus monogyna and Crataegus laevigata-In Europe, these two
species of hawthorn trees are distributed in overlapping ranges but show distinct ecological
preferences. C. monogyna prefers lower elevations and drier conditions, while C. laevigata
prefers higher elevations and wetter conditions. Over time, adaptation to these different habitats
has led to reproductive isolation and the development of distinct species.
2.Monkeyflower (Mimulus) - Mimulus guttatus and Mimulus cardinalis. Mimulus species are
known for their rapid speciation and hybridization. In California, M. guttatus and M. cardinalis
are two closely related species that occupy different niches along streams and rivers. M. guttatus
typically grows in wet, lowland habitats, while M. cardinalis inhabits drier, higher elevations.
Despite some overlap in their distributions, they maintain genetic and phenotypic differences due
to adaptation to their respective habitats.
3.White Spruce (Picea glauca)- Picea glauca and Picea mariana. In northern North America,
white spruce (Picea glauca) and black spruce (Picea mariana) are two coniferous tree species
that coexist across a wide range of habitats. While they often occur together in mixed forests,
they display differences in ecological preferences. White spruce is typically found in moist, well-
drained soils of upland areas, while black spruce thrives in wet, poorly drained soils of bogs and
swamps. This divergence in habitat preference has contributed to reproductive isolation and the
formation of distinct species.
4.Tropical Milkweed (Asclepias curassavica). Asclepias curassavica and Asclepias incarnata
These two species are found in overlapping ranges across North and South America. A.
curassavica is more common in drier habitats, such as roadsides and disturbed areas, while A.
incarnata is often found in wetter habitats like marshes and wet meadows. Despite growing in

6. adjacent habitats, they maintain genetic differences due to adaptation to their respective
ecological niches.
Peripatric Speciation- is a form of speciation where a new species arises from a small, isolated
peripheral population that becomes reproductively isolated from the main population. This
isolation can occur due to geographical barriers or ecological factors. Over time, genetic drift,
natural selection, and other evolutionary processes lead to divergence and the eventual formation
of a new species.
Examples
1.Hawaiian Silverswords (Argyroxiphium spp. and Wilkesia spp.): The Hawaiian silverswords
are a group of flowering plants endemic to the Hawaiian Islands. They provide an excellent
example of peripatric speciation. Each species of silversword is typically found on a single island
or a few neighboring islands. Their isolation on different islands, coupled with different
environmental conditions and pollinators, has led to the evolution of distinct species from a
common ancestor.
2. Galápagos Finches (Geospiza spp.): While not plants, Galápagos finches provide a classic
example of peripatric speciation. These finches are famous for their adaptive radiation on the
Galápagos Islands. Different species of finches have evolved on different islands, each adapted
to exploit different food sources and habitats. This divergence likely started from a small
population of finches that colonized the islands and then diversified through peripatric
speciation.
3. Hawthorn Trees (Crataegus spp.): Hawthorn trees are found throughout the Northern
Hemisphere and represent a diverse group of species. In North America, there are many
examples of peripatric speciation within the Crataegus genus. For instance, Crataegus intricata
is a species endemic to the Appalachian Mountains, which likely arose from a small isolated
population that became reproductively isolated from other hawthorn populations.
4. Madagascar Orchids (e.g., Angraecum spp.): Madagascar is known for its remarkable
biodiversity, including a vast array of orchid species. Many orchids on the island have evolved
through peripatric speciation due to the geographical isolation of different regions and
microhabitats. Angraecum sesquipedale, famously associated with Charles Darwin and its long
nectar spur, is an example of an orchid that has likely undergone peripatric speciation in
Madagascar.
Sympatric Speciation - is a process by which new species evolve from a single ancestral species
occupying the same geographic region or habitat. This type of speciation occurs without any
physical barrier to gene flow, unlike allopatric speciation where geographic isolation plays a
significant role.
Sympatric speciation can occur through various mechanisms such as polyploidy, disruptive
selection, sexual selection, or habitat differentiation.
Examples
Polyploidy: Polyploidy is a common mechanism of sympatric speciation in plants where
individuals have more than two sets of chromosomes. This can occur through autopolyploidy
(multiple sets of chromosomes from the same species) or allopolyploidy (multiple sets of

7. chromosomes from different species).Eg. Wheat (Triticum). Bread wheat (Triticum aestivum) is
an allopolyploid species that originated from hybridization between Triticum turgidum and
another wild grass species. This hybridization event occurred sympatrically, leading to the
formation of a new species with increased genetic diversity and adaptation to various ecological
niches.
Disruptive Selection: Disruptive selection occurs when extreme phenotypes are favored over
intermediate phenotypes, leading to the divergence of a population into two or more distinct
groups.Eg. Hawthorn Flies (Rhagoletis pomonella)
Hawthorn flies are a classic example of sympatric speciation. They originally laid their eggs on
the fruit of hawthorn trees (Crataegus) but later adapted to lay eggs on introduced apple trees
(Malus) in the same geographic region. Disruptive selection, likely driven by differences in fruit
ripening times and chemical cues, led to the divergence of hawthorn flies into two specialized
populations, each adapted to exploit a different host plant.
Habitat Differentiation: Sympatric speciation can also occur when a single population occupies
different ecological niches within the same habitat, leading to reproductive isolation and
speciation.eg Mimulus aurantiacus.
Mimulus aurantiacus, commonly known as sticky monkeyflower, exhibits sympatric speciation
in response to soil type differentiation. Populations of sticky monkeyflower growing in
serpentine soil, which is high in heavy metals, have evolved different flowering times and
specialized traits compared to populations growing in non-serpentine soil. This habitat
differentiation has led to reproductive isolation and genetic divergence between the two
populations, potentially resulting in the formation of distinct species.

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