Speciation – Definition, Causes, Types, Process

What is Speciation?

Speciation is the evolutionary process by which new species are formed from a pre-existing species. It is the process where a single ancestral population is gradually divided into two or more distinct species. This process occurs when population of a species becomes isolated from each other and the gene flow between them is stopped. As a result of this isolation the populations start showing variations due to mutation natural selection and genetic drift.

With the passage of time these variations is accumulated and the populations become genetically different. When these differences become significant the individuals of the populations are no longer able to interbreed and produce fertile offsprings. This condition is referred to as reproductive isolation. Thus speciation is an important process of evolution which is responsible for the origin of new species and increase in biodiversity.

Definition of Speciation

Speciation is the evolutionary process through which new, genetically distinct species arise from an existing species, often due to reproductive isolation and genetic differentiation between populations. This can occur via mechanisms such as allopatric or sympatric speciation.

Factors Influencing Speciation

Some of the main factors influencing speciation are given below–

• Geographical isolation– It is the process where population of a species is separated by physical barriers like mountains rivers oceans etc. Due to this separation gene flow is stopped between populations. These isolated populations undergo independent evolution which finally leads to formation of new species.
• Natural selection– Natural selection is an important factor of speciation. When populations live in different environments they face different selective pressures. The organisms having favourable characters survive and reproduce. This continuous selection results in divergence of populations and speciation.
• Genetic drift– Genetic drift is the random change in gene frequency which mostly occurs in small populations. By chance some genes may increase or decrease in number. Over a long period this random process may cause genetic differences and speciation.
• Mutation– Mutation is the sudden change in genetic material. It introduces new variations in a population. These variations when accumulated over generations contribute to genetic divergence and helps in speciation.
• Reproductive isolation– Reproductive isolation is the condition where individuals of two populations cannot interbreed and produce fertile offsprings. It may be prezygotic or postzygotic. Once reproductive isolation is developed speciation becomes permanent.
• Polyploidy -Polyploidy is mainly seen in plants. It is the condition where chromosome number is doubled. This sudden chromosomal change results in immediate reproductive isolation and formation of new species.
• Sexual selection– Sexual selection is based on mating preferences. When individuals prefer certain traits during mating it leads to separation of populations. This difference in mating behaviour helps in reproductive isolation and speciation.
• Hybridization– Hybridization may also influence speciation. Sometimes hybrids formed between two species are fertile and adapted to new conditions. These hybrids may develop into a new species.

Speciation Types

1. Allopatric Speciation

It is the most common type of speciation. It occurs when a population is divided into two or more geographically isolated populations by a physical barrier such as mountain river or ocean. Due to this separation the gene flow is reduced to zero and populations diverge by natural selection mutation and genetic drift until reproductive isolation is formed. A common example is Darwin’s finches of Galapagos islands.

2. Peripatric Speciation

It is a special type of allopatric speciation. In this type a small population breaks away from a large parent population and occupies a new isolated area. As the population size is very small genetic drift and founder effect plays a major role in fixing random alleles and reducing genetic variation. This accelerates divergence from ancestral population. It is commonly seen in island colonization such as Hawaiian Drosophila.

3. Parapatric Speciation

This type of speciation occurs when populations are adjacent to each other without a complete physical barrier. The populations diverge across a continuous habitat due to environmental gradient or abrupt ecological change. Some gene flow occurs at the hybrid zone but strong divergent selection leads to reproductive isolation. An example includes grasses developing heavy metal tolerance near mining areas.

4. Sympatric Speciation

It occurs when new species are formed from a single ancestral population living in the same geographic area. There is no physical barrier present. Reproductive isolation develops due to behavioural changes ecological niche differentiation or sexual selection. A well known example is apple maggot fly which shifted from hawthorn to apple plants.

5. Polyploidy (Instant Speciation)

It is a rapid type of sympatric speciation mostly found in plants. It occurs due to the formation of more than two complete sets of chromosomes during cell division. The polyploid individuals become reproductively isolated from parent population as hybrid offspring becomes sterile due to mismatched chromosome numbers.

6. Ecological Speciation

It is the speciation driven by adaptation to different ecological conditions. Divergent natural selection leads to reproductive isolation. This type of speciation can occur in allopatric sympatric or parapatric conditions provided ecological differences are present.

7. Founder Effect Speciation

It occurs when a new population is established by a very small number of individuals. Strong genetic drift leads to rapid evolutionary divergence from original population. This concept is closely related to peripatric speciation.

8. Mutation Order Speciation

In this type different populations face similar selection pressure but diverge because they acquire different beneficial mutations by chance. Reproductive isolation arises even though environments are similar.

9. Hybrid Speciation

It occurs when two different species interbreed and produce a hybrid population that becomes reproductively isolated from both parent species. This type is rare in animals but common in plants and often associated with polyploidy.

10. Artificial Speciation

It is the formation of new species by human interference. It is achieved through selective breeding laboratory experiments or controlled environmental conditions. Artificial reproductive isolation has been demonstrated in laboratory populations of Drosophila.

1. Allopatric Speciation

• Allopatric speciation is the process of speciation that occurs when members of a population are geographically isolated from each other. Due to this isolation the individuals are unable to mate and genetic exchange between the populations is prevented or interfered.
• The geographical isolation may occur in different ways such as formation of a new river branch erosion forming a new valley or migration of a group of organisms to a distant place where return is not possible. An example includes seeds floating to an island across ocean.
• The extent of geographical separation required for speciation depends upon the biology of the organism and its dispersal capacity. Organisms with high dispersal ability require stronger barriers to stop gene flow.
• In flying insects even if populations occupy neighboring valleys individuals may move between populations and gene flow may continue. In such cases allopatric speciation is less likely to occur.
• In organisms with limited dispersal ability such as rodents the formation of a lake or valley can effectively prevent movement. Continued gene flow becomes unlikely and chances of speciation increases.
• Allopatric speciation is broadly classified into two types based on the mode of geographical separation.
  • Dispersal – It occurs when a few individuals of a species migrate to a new geographical area and become isolated from the parent population.
  • Vicariance – It occurs when a natural physical barrier suddenly separates a population into two or more isolated groups.

How does allopatric speciation occurs?

Allopatric speciation occurs through the following steps–

1. Geographical isolation– It is the first step of allopatric speciation. A population is separated into two or more groups by geographical barriers such as mountains rivers oceans glaciers or formation of land bridges. Due to this separation gene flow between populations is completely stopped.
2. Independent evolution of isolated populations– After isolation the populations remain separated for long period of time. As migration is absent each population evolves independently in their own habitat. The gene pools become separated and no interbreeding occurs.
3. Genetic variation and divergence– Genetic differences are gradually developed between the isolated populations. These differences are produced due to natural selection genetic drift and mutation. Different environmental conditions causes selection of different adaptive characters.
4. Development of reproductive isolation– As genetic divergence increases reproductive isolating mechanisms are formed. These may be prezygotic barriers such as difference in mating behaviour or postzygotic barriers such as sterile hybrids. Even if the populations come in contact again they fail to interbreed.
5. Formation of new species– With the completion of reproductive isolation the isolated populations are converted into separate species. This completes the process of allopatric speciation.

Examples of allopatric speciation

Some of the important examples are listed below–

1. Darwin’s finches– Darwin’s finches of Galapagos islands is a classic example. An ancestral species reached different islands and became geographically isolated. Due to different food habits and environmental conditions different beak shapes were developed. Over time many distinct species were formed.
2. Snapping shrimp (Alpheus species)– The formation of Isthmus of Panama separated marine populations into Atlantic and Pacific sides. Due to long geographical isolation different species of snapping shrimp evolved on each side.
3. Grand Canyon squirrels– The formation of Grand Canyon separated squirrel populations on the North Rim and South Rim. The inability to cross the canyon stopped gene flow and resulted in formation of distinct species.
4. House mice of Madeira island– On Madeira island house mice populations were isolated in different valleys. Due to genetic drift and chromosomal changes different populations evolved with different karyotypes leading to reproductive isolation.

2. Peripatric Speciation

• Peripatric speciation occurs when individuals present at the periphery or border of a large population split away from the main population. These isolated individuals in course of time give rise to a new species.
• It is sometimes difficult to differentiate peripatric speciation from allopatric speciation as both involve geographical isolation. However in peripatric speciation the isolated population is very small and located at the edge of the parent population.
• When the separated population enters a different biological niche such as feeding on different food or surviving under different environmental conditions peripatric speciation takes place.
• The populations that split away are generally small in size. Due to this the proportion of certain characters in the new population may differ from the original population.
• For example if a bird population is mostly blue in colour with few red birds and a small group separates in which red birds are more in number. In the new population red colour becomes common.
• The descendants of this isolated group are likely to show the same character and differ from the main population. This change in gene frequency is referred to as genetic drift.
• Over a period of time many changes accumulate and along with genetic drift they result in the formation of a new species.

How does peripatric speciation occurs?

Peripatric speciation occurs through the following steps–

1. Isolation of a small peripheral population– It is the initial step of peripatric speciation. A small group of individuals gets separated from a large central population and colonizes a new area. This area is usually at the margin of the original population such as islands mountain tops or isolated habitats. Due to this separation gene flow is completely reduced.
2. Founder effect– The isolated population consists of very few individuals. Because of this it carries only limited genetic variation of the original population. This condition is referred to as founder effect. The gene pool of the new population is different from the parent population from the beginning.
3. Strong genetic drift– As the population size is very small genetic drift becomes very strong. Random changes in allele frequencies occurs very rapidly. Some alleles may get fixed while others may be lost completely. This causes rapid genetic divergence.
4. Action of natural selection– The peripheral population faces new environmental conditions. Natural selection acts on the limited gene pool and favours traits suitable for the new habitat. Due to this adaptation takes place very fast compared to large populations.
5. Development of reproductive isolation– Due to combined effect of founder effect genetic drift mutation and natural selection reproductive isolation is developed. If the peripheral population comes in contact with ancestral population again they fail to interbreed.
6. Formation of new species– With complete reproductive isolation the small peripheral population becomes a new species. This completes the process of peripatric speciation.

Examples of peripatric speciation

Some important examples are given below–

1. Hawaiian Drosophila– Hawaiian fruit flies are the best example of peripatric speciation. Small populations colonized different islands. Due to founder effect and genetic drift rapid diversification occurred leading to formation of large number of species.
2. Hawaiian sword tailed crickets (Laupala)– mThese crickets evolved on different Hawaiian islands from small founder populations. Rapid changes in mating behaviour resulted in reproductive isolation and formation of new species.
3. Reef hermit crabs (Calcinus)– In reef hermit crabs small populations colonized remote reef areas. Founder effect and sexual selection caused rapid divergence resulting in peripatric speciation.
4. Mexican prairie dog (Cynomys mexicanus)– mThis species evolved from a small isolated population due to climatic changes. Isolation of peripheral population and genetic drift resulted in formation of a new species.
5. North American spruce (Picea)– mRed spruce and black spruce are believed to be formed due to peripatric speciation. Small populations were isolated in glacial refugia leading to genetic divergence.

3. Parapatric Speciation

• Parapatric speciation occurs when subpopulations of the same species are largely isolated from each other but still share a small overlapping region in their geographical range.
• This type of speciation may arise due to partial geographical barriers or due to uneven distribution of individuals within the population.
• The chances of parapatric speciation are very less as some amount of gene flow continues between the neighboring populations.
• It can occur among a series of neighboring subpopulations where each population can interbreed with the adjacent one. However the populations present at the extreme ends are unable to interbreed with each other. This condition is referred to as ring species.
• In such populations mating does not occur randomly. Individuals usually mate with their nearest geographical neighbors resulting in unequal gene flow.
• Non random mating increases the rate of dimorphism within the population where different morphological forms of the same species are seen.
• Parapatric speciation results in the formation of one or more distinct subpopulations which are referred to as sister species. These populations show small continuous overlaps in their biogeographical range and are genotypically dimorphic.

How does parapatric speciation occurs?

Parapatric speciation occurs through the following steps–

1. Continuous distribution of population– In parapatric speciation the population occupies a continuous geographical area. There is no physical barrier such as mountain or river. However individuals usually mate with nearby individuals and not with distant ones.
2. Environmental variation across habitat– mThe habitat shows gradual environmental changes such as difference in soil type temperature altitude or pollution. Due to this different parts of the population experience different environmental conditions.
3. Divergent natural selection– Because of environmental differences natural selection acts differently on neighbouring populations. Each group becomes adapted to its local condition. This divergent selection is strong enough to reduce effectiveness of gene flow.
4. Restricted gene flow– Although the populations are adjacent gene flow is limited due to distance and selection against maladapted individuals. Interbreeding occurs only in narrow contact zone.
5. Formation of hybrid zone– At the boundary of two populations hybrids are produced. These hybrids usually show low fitness as they possess intermediate characters not suitable for either environment.
6. Reinforcement– As hybrids are less fit natural selection favours individuals that avoid interbreeding. This leads to development of prezygotic isolation such as difference in flowering time or mating behaviour.
7. Formation of new species– With strengthening of reproductive isolation neighbouring populations finally become distinct species. This completes parapatric speciation.

Examples of parapatric speciation

Some important examples are listed below–

1. Sweet vernal grass (Anthoxanthum odoratum)– This grass shows parapatric speciation in mine areas. Populations growing on metal contaminated soil evolved metal tolerance. Adjacent populations on normal soil remained non tolerant. Difference in flowering time prevents interbreeding.
2. Ensatina salamanders– Ensatina salamanders in California form a ring species. Neighbouring populations can interbreed but populations at the ends of the range do not interbreed and behave as different species.
3. Ring species of gulls (Larus)– mDifferent populations of Larus gulls are distributed around Arctic region. Adjacent populations interbreed but terminal populations are reproductively isolated showing parapatric speciation.
4. Caucasian rock lizards (Darevskia)– Different species of rock lizards are adapted to different climatic zones. Hybrid zones are present but strong selection maintains distinct species.

4. Sympatric Speciation

• Sympatric speciation is the evolutionary process in which new species are formed from a single ancestral species while living in the same geographical area.
• In this type of speciation the distribution range of the populations may be completely similar or may partially overlap unlike allopatric speciation.
• Instead of geographical isolation reducing gene flow sympatric speciation occurs when a group of individuals starts utilising a new ecological niche within the same area.
• This may occur when a herbivorous insect begins feeding on a new or novel plant source to which it was not ancestrally associated or when a new plant species is introduced into the same geographical region.
• As insects generally reproduce or lay eggs on the type of fruit or plant on which they were born individuals gradually become specialised in feeding and mating on specific hosts.
• Due to this specialisation gene flow between populations feeding on different plants is reduced which leads to reproductive isolation.
• Since new species arise from populations living in highly overlapping or even identical environments sympatric speciation is distinct from other types of speciation.
• Sympatric speciation is considered to be more common in bacteria than in multicellular organisms as bacteria can exchange genes among themselves as well as transfer genes to their offspring.

How does sympatric speciation occurs?

Sympatric speciation occurs through the following steps–

1. Same geographical area– In sympatric speciation the populations live in the same geographical region. There is no physical barrier such as mountain river or ocean separating them. All individuals are theoretically capable of interbreeding.
2. Ecological differentiation– Within the same habitat some individuals start using different resources or occupy different ecological niches. This may include difference in food source host plant or microhabitat. This ecological shift reduces interaction between groups.
3. Disruptive selection– Natural selection favours extreme phenotypes rather than intermediate ones. Individuals adapted to specific niche survive better. Over time two distinct groups are formed within the same population.
4. Assortative mating– Individuals begin to mate preferentially within their own group. This may be due to difference in mating behaviour colour flowering time or habitat preference. This assortative mating reduces gene flow.
5. Development of reproductive isolation– Due to continued ecological divergence and mating preference reproductive isolating mechanisms are formed. These may be prezygotic such as temporal isolation or behavioural isolation.
6. Instant isolation by polyploidy– In plants sympatric speciation may occur suddenly by polyploidy. Polyploid individuals cannot interbreed with diploid parents resulting in instant reproductive isolation.
7. Formation of new species– With complete reproductive isolation the groups living in same area become separate species. This completes sympatric speciation.

Examples of sympatric speciation

Some important examples are listed below–

1. Apple maggot fly (Rhagoletis pomonella)– This fly originally fed on hawthorn fruits. Some populations shifted to apple trees. As mating occurs on host plant apple feeding and hawthorn feeding flies do not interbreed even in same area.
2. Cichlid fishes– Cichlid fishes in African lakes show sympatric speciation. Differences in feeding habits colour and mating preference caused formation of many species within the same lake.
3. Howea palms– Two species of Howea palms evolved on Lord Howe Island. They grow in same area but on different soil types. Difference in flowering time prevents cross pollination.
4. Three spined sticklebacks– In some lakes two forms of sticklebacks evolved from same ancestor. One feeds near surface water while other feeds near bottom. Disruptive selection and assortative mating maintains reproductive isolation.
5. Polyploidy in plants– Many plant species show sympatric speciation due to polyploidy. Polyploid plants are immediately isolated from diploid parents and form new species.

5. Artificial Speciation

• Artificial speciation is the type of speciation that is achieved through the involvement of human intervention.
• In this process human beings create new and distinct species by separating populations and preventing reproduction between them.
• It may also be brought about by deliberately breeding individuals that possess desired morphological or genotypical characters.
• This process is also referred to as artificial selection. Artificial selection has been practiced in the development of most modern domesticated plants and animals.
• Although the evolution of present day crops and livestock has taken thousands of years the process of artificial selection can be easily observed in organisms having short life cycles.

How does artificial speciation occurs?

Artificial speciation occurs through the following steps–

1. Human intervention– Artificial speciation occurs due to direct or indirect involvement of humans. It does not occur naturally. Humans control mating environment selection pressure or breeding conditions.
2. Isolation of populations by humans– Populations are deliberately separated in laboratory farms or controlled environments. Due to this separation gene flow between populations is restricted.
3. Artificial selection– Humans select individuals with desired characters such as size colour behaviour or productivity. These selected traits are allowed to reproduce while others are eliminated.
4. Exposure to different conditions– Separated populations are kept under different food habits temperature light or habitat conditions. This causes divergence in behaviour physiology or morphology.
5. Selection against hybrids– In some experiments hybrids formed between two populations are prevented from reproducing. This strengthens reproductive isolation between populations.
6. Development of reproductive isolation– After many generations populations show mating preference within their own group. They refuse to mate with individuals from other group even when brought together.
7. Formation of new species– With complete reproductive isolation the populations behave as separate species. This completes artificial speciation.

Examples of artificial speciation

Some important examples are listed below–

1. Fruit flies (Drosophila)– In laboratory experiments Drosophila populations were separated and reared on different food sources such as starch and maltose. After several generations flies preferred to mate within same group showing reproductive isolation.
2. Maze experiment in Drosophila– Fruit flies were allowed to breed only after choosing specific habitats such as light or dark conditions. After many generations the groups refused to interbreed.
3. Houseflies (Musca domestica)– Houseflies were selected for different responses to gravity in laboratory. Over time reproductive isolation developed between selected populations.
4. Domesticated plants– Maize was developed from wild teosinte through artificial selection. Continuous selection produced a form very different from ancestral plant.
5. Animal hybrids– Mule produced by crossing horse and donkey is an example of artificial hybridization. Although sterile it shows human controlled breeding.

6. Polyploidy (Instant Speciation)

Polyploidy is a type of instant speciation in which a new species is formed due to increase in the number of chromosome sets. It occurs when an error during cell division causes chromosomes to fail to separate properly. This condition is referred to as non disjunction. As a result gametes or offspring are produced with extra sets of chromosomes such as tetraploid instead of diploid. These polyploid individuals become reproductively isolated immediately from the parent population. When a polyploid individual mates with a diploid parent the offspring formed has uneven chromosome number and is usually sterile. Due to this complete reproductive isolation is established in a single generation. Polyploidy is very common in plants and leads to formation of new species without gradual evolutionary changes.

How does polyploidy (instant speciation) occurs?

Polyploidy speciation occurs through the following steps–

1. Non-disjunction during cell division– It is the initial step of polyploidy. During meiosis or mitosis chromosomes fail to separate properly. Because of this error gametes or cells are formed with extra sets of chromosomes.
2. Formation of unreduced gametes– Due to non-disjunction some gametes retain the full diploid set of chromosomes instead of haploid set. These gametes are referred to as unreduced gametes.
3. Chromosome doubling– When unreduced gametes fuse or when chromosome duplication occurs without cell division the offspring formed contains more than two complete sets of chromosomes. The chromosome number may change from diploid (2n) to tetraploid (4n).
4. Immediate reproductive isolation– The polyploid individual becomes reproductively isolated instantly. When a polyploid individual mates with a normal diploid individual the offspring produced is usually triploid (3n). These offspring are sterile as chromosomes fail to pair properly during meiosis.
5. Types of polyploidy formation– Polyploidy may occur as autopolyploidy where chromosome duplication happens within same species. It may also occur as allopolyploidy where hybridization between two different species is followed by chromosome doubling.
6. Formation of new species– Because of complete reproductive isolation in a single generation polyploid individual behaves as a new species. This completes instant speciation.

Examples of polyploidy (instant speciation)

Some important examples are listed below–

1. Wheat (Triticum species)– Modern wheat is an example of allopolyploidy. It originated due to hybridization between different grass species followed by chromosome doubling. The new polyploid wheat species is fertile and reproductively isolated from parent species.
2. Cotton (Gossypium)– Cotton species are formed through allopolyploidy. Hybridization between two different species followed by chromosome duplication resulted in new polyploid species with improved characters.
3. Tobacco (Nicotiana)– Some species of tobacco originated through polyploidy. Chromosome doubling restored fertility in hybrids leading to formation of new species.
4. Ferns– Polyploidy is very common in ferns. Many fern species have originated due to autopolyploidy and allopolyploidy resulting in instant speciation.
5. Flowering plants– Polyploidy is widely seen in angiosperms. Many plant species have originated suddenly due to chromosome multiplication and are reproductively isolated from their parent populations.

7. Hybrid Speciation

Hybrid speciation is the process in which a new species is formed by hybridization between two different parental species. In this type of speciation individuals from two distinct species interbreed and produce hybrid offspring. In most cases hybrids are sterile but in hybrid speciation the hybrids become fertile and form a stable population. Fertility is commonly restored by chromosome doubling which is referred to as polyploidy. Due to difference in chromosome number the hybrid population becomes reproductively isolated from both parent species. In some cases hybrid speciation may also occur without chromosome doubling when the hybrid acquires characters that allow it to survive in a new habitat or ecological niche. As a result gene flow with parent species is prevented and a new species is formed.

How does hybrid speciation occurs?

Hybrid speciation occurs through the following steps–

1. Hybridization between two species– It is the first step of hybrid speciation. Two closely related but genetically distinct species interbreed and produce hybrid offspring. In most cases these hybrids are weak or sterile.
2. Combination of parental genomes– The hybrid individual contains genetic material from both parent species. This mixed genome produces new combinations of characters which are not present in either parent species.
3. Restoration of fertility (in some hybrids)– In plants fertility of hybrids may be restored by chromosome doubling. This process is referred to as allopolyploidy. Chromosome doubling allows proper pairing of chromosomes during meiosis.
4. Immediate reproductive isolation– The fertile hybrid becomes reproductively isolated from both parent species. When it mates with either parent the offspring formed have unbalanced chromosome numbers and are usually sterile.
5. Homoploid hybrid speciation– In some cases hybrid speciation occurs without chromosome doubling. The hybrid occupies a new ecological niche where parent species cannot survive. Natural selection favours the hybrid type and maintains isolation.
6. Formation of a stable hybrid population– With continuous reproduction among hybrids a stable population is formed. Over time it behaves as an independent species.
7. Establishment of new species– Because of reproductive isolation and ecological adaptation the hybrid population becomes a new species. This completes hybrid speciation.

Examples of hybrid speciation

Some important examples are listed below–

1. Sunflower (Helianthus species)– New sunflower species originated by hybridization between Helianthus annuus and Helianthus petiolaris. These hybrid species can grow in extreme habitats where parent species cannot survive.
2. Wheat (Triticum)– Hybridization followed by chromosome doubling resulted in formation of fertile wheat species. This is an example of allopolyploid hybrid speciation.
3. Honeysuckle maggot fly– Hybrids formed between two fly species shifted to a new host plant honeysuckle. This ecological shift prevented interbreeding with parent species.
4. Heliconius butterflies– Some species of Heliconius butterflies are formed due to hybridization. These hybrids show unique colour patterns and are reproductively isolated from parent species.
5. Mariana mallard– This bird species is believed to have originated through hybridization between two different duck species resulting in a distinct hybrid lineage.

8. Mutation Order Speciation

Mutation order speciation is a type of speciation in which new species are formed when populations adapt to similar environmental conditions but fix different advantageous mutations. In this process populations are usually isolated from each other and experience the same type of selection pressure. However the specific mutations that arise in each population occur by chance and differ in their order of appearance. Natural selection favours these beneficial mutations and they become fixed in different populations. Over time accumulation of different mutations leads to genetic incompatibility between populations. As a result hybrids formed show reduced fitness sterility or inviability. Even though the populations remain ecologically similar reproductive isolation develops and new species are formed.

How does mutation order speciation occurs?

Mutation order speciation occurs through the following steps–

1. Isolation of populations– In this type of speciation populations are separated from each other by geographical or reproductive isolation. Due to this separation gene flow between populations is very limited or absent.
2. Similar environmental conditions– The isolated populations live under almost similar environmental conditions. They face same type of selection pressure and ecological requirements.
3. Random origin of mutations– In each population different beneficial mutations arise by chance. Although the environment is same the order and type of mutation occurring in one population is different from the other.
4. Fixation of different mutations– Natural selection favours these advantageous mutations. Over time different alleles become fixed in different populations. This causes genetic divergence without ecological divergence.
5. Development of genetic incompatibility– As different mutations accumulate interactions between genes become incompatible. When individuals from two populations interbreed the hybrid shows reduced fitness.
6. Postzygotic isolation– Hybrids formed are weak sterile or inviable due to genetic incompatibilities. This leads to development of postzygotic reproductive isolation.
7. Formation of new species– With complete reproductive isolation populations become separate species. This completes mutation order speciation.

Examples of mutation order speciation

Some important examples are listed below–

1. Mediterranean land snails (Albinaria)– Different species of Albinaria are ecologically similar and occupy similar habitats. Their divergence is attributed to chance fixation of different mutations.
2. California salamanders (Batrachoseps)– These salamanders show speciation without major ecological differences. Mutation order speciation explains their genetic divergence.
3. Damselflies– Some damselfly species are genetically distinct but ecologically similar. Their speciation is consistent with mutation order process.
4. Crickets– Certain cricket species show divergence in reproductive traits while living in similar environments. This pattern supports mutation order speciation.

9. Founder Effect Speciation

Founder effect speciation is a type of speciation which occurs when a new population is formed by a very small number of individuals isolated from a large ancestral population. Due to this isolation the new population carries only a limited portion of the genetic variation of the parent population. This sudden change in gene pool is referred to as founder effect. Because the population size is very small genetic drift acts very strongly and causes rapid changes in allele frequencies. Along with genetic drift natural selection also acts in the new environment leading to quick evolutionary divergence. As a result reproductive isolation develops between the founder population and the ancestral population and a new species is formed.

How does founder effect speciation occurs?

Founder effect speciation occurs through the following steps–

1. Isolation of a small population– It begins when a very small number of individuals get separated from a large ancestral population. These individuals colonize a new area such as island mountain region or isolated habitat. Due to this separation gene flow with parent population is stopped.
2. Formation of a founder population– The newly established population is very small in size. It carries only a limited sample of genetic variation present in the original population. Many alleles are lost while few alleles may become common.
3. Strong genetic drift– Because of small population size genetic drift acts very strongly. Random changes in allele frequency occurs. Some alleles may get fixed purely by chance which causes rapid genetic divergence.
4. Genetic restructuring– Loss of genetic variation changes the interaction between genes. Certain gene combinations become important in the new population. This altered genetic background creates a different evolutionary pathway.
5. Action of natural selection– The founder population faces new environmental conditions. Natural selection acts on the limited gene pool. This leads to rapid adaptation and further divergence from ancestral population.
6. Development of reproductive isolation– Due to combined effect of drift genetic reorganization and selection reproductive isolating mechanisms develop. Even if the founder population comes in contact with parent population interbreeding does not occur.
7. Formation of new species– With complete reproductive isolation the founder population becomes a distinct species. This completes founder effect speciation.

Examples of founder effect speciation

Some important examples are listed below–

1. Hawaiian Drosophila– Hawaiian fruit flies are the classic example. Small populations colonized different islands. Founder effect and genetic drift caused rapid diversification resulting in formation of hundreds of species.
2. Hawaiian sword tailed crickets (Laupala)– These crickets evolved from small founder populations on different islands. Changes in mating behaviour resulted in reproductive isolation.
3. Cave planthoppers– Cave planthoppers of Hawaii show rapid radiation due to founder effect. Small isolated populations evolved into distinct species.
4. Mexican prairie dog (Cynomys mexicanus)– This species originated from a small isolated population during climatic changes. Founder effect and drift caused divergence from ancestral species.
5. Reef hermit crabs (Calcinus)– Small populations colonized remote reef areas. Founder effect and sexual selection resulted in formation of new species.

10. Ecological Speciation

Ecological speciation is the process by which new species are formed due to adaptation to different ecological conditions. It occurs when populations living in different environments experience different selection pressures related to food source climate habitat or predators. Due to divergent natural selection different alleles which are beneficial in a particular environment are selected in each population. These adaptive differences reduce gene flow between populations as individuals migrating to another environment show poor survival. This condition is referred to as immigrant inviability. Reproductive isolation develops as a direct result of ecological adaptation and mating preference linked with adaptive traits. Ecological speciation may occur in presence or absence of geographical isolation and results in formation of distinct species.

How does ecological speciation occurs?

Ecological speciation occurs through the following steps–

1. Population exposed to different environments– In ecological speciation populations live in different environments or ecological conditions. These environments may differ in food source climate soil type predators or habitat. Because of this populations face different selection pressures.
2. Divergent natural selection– Natural selection acts differently on populations living in different environments. Traits that are favourable in one environment may be unfavourable in another. Due to this different adaptive characters are selected in each population.
3. Adaptive divergence– With continuous selection populations gradually become different in morphology physiology or behaviour. These adaptive changes are directly related to survival and reproduction in their respective environment.
4. Reduction of gene flow– Individuals migrating to another environment show poor survival. This is referred to as immigrant inviability. Because of this gene flow between populations is reduced even without physical barriers.
5. Development of prezygotic isolation– As a byproduct of adaptation reproductive barriers are formed. These include habitat isolation temporal isolation or assortative mating where individuals prefer mates with similar adaptive traits.
6. Formation of low fitness hybrids– When hybridization occurs hybrids usually show intermediate characters. These characters are not suitable for either environment. Hence hybrids have low survival or reproductive success.
7. Establishment of reproductive isolation– Due to continued selection against hybrids and migrants reproductive isolation becomes stronger. Populations can no longer interbreed successfully.
8. Formation of new species– With complete reproductive isolation populations adapted to different ecological niches become separate species. This completes ecological speciation.

Examples of ecological speciation

Some important examples are listed below–

1. Three spined sticklebacks (Gasterosteus aculeatus)– Marine sticklebacks colonized freshwater habitats. Due to different selection pressures freshwater populations evolved reduced armour and body size. Assortative mating and habitat preference maintain reproductive isolation.
2. Apple maggot fly (Rhagoletis pomonella) – Flies shifted from hawthorn to apple as host plant. Mating occurs on host fruit and difference in fruit ripening time caused habitat and temporal isolation leading to ecological speciation.
3. Howea palms– Two species of Howea palms evolved on Lord Howe Island. Adaptation to different soil types resulted in difference in flowering time preventing interbreeding.
4. Sweet vernal grass (Anthoxanthum odoratum)– Grass populations growing on metal contaminated soil evolved metal tolerance. Adjacent populations on normal soil remained non tolerant. Early flowering in mine populations caused reproductive isolation.
5. Heliconius butterflies– Different species evolved distinct wing colour patterns due to ecological adaptation for mimicry. Hybrids show low fitness and reduced mating success.
6. Pea aphids– Different populations became specialized on different host plants. Aphids migrating to alternative host show poor survival which reduces gene flow.

Speciation Process: How Does Speciation Occur?

Speciation is the process by which new species are formed from a single ancestral population. It occurs through gradual changes that involve isolation genetic variation and development of reproductive barriers.

1. Isolation or Interruption of Gene Flow

The first step in speciation is the interruption of gene flow between the members of a population.

2. Geographical Isolation

In many cases speciation begins due to geographical separation. Physical barriers such as mountains rivers or oceans divide a population into isolated groups. This condition is referred to as allopatric speciation. Due to the barrier genetic exchange between populations is prevented and independent evolution takes place.

3. Ecological or Behavioural Isolation

Speciation may also occur without complete geographical separation. Populations may become isolated by occupying different habitats using different food sources or breeding at different times. This type of isolation is commonly associated with sympatric or parapatric speciation.

4. Genetic Divergence

After isolation the separated populations begin to show genetic differences due to various evolutionary forces.

5. Natural Selection

Each population adapts to its own environment. Differences in climate food or predators lead to fixation of different traits in each population. This process is referred to as ecological speciation.

6. Genetic Drift

In small populations random changes in gene frequency occur. Genetic drift may cause rapid divergence especially when a small group forms a new population such as in founder effect.

7. Sexual Selection

Differences in mating behaviour colour or mating preference may develop. Individuals may prefer mates from their own population which reduces interbreeding between populations.

8. Mutation

New mutations arise randomly in each population. These mutations add to genetic differences over time.

9. Development of Reproductive Isolating Mechanisms

As divergence continues populations develop barriers that prevent successful interbreeding.

10. Prezygotic Isolation

These barriers act before fertilization. They include behavioural isolation temporal isolation mechanical isolation and gametic isolation.

11. Postzygotic Isolation

These barriers act after fertilization. Hybrid offspring may be weak sterile or may not survive.

12. Reinforcement

When partially diverged populations come into contact hybridization may occur. If hybrids show low fitness natural selection favours individuals that avoid mating with the other population. This strengthens reproductive isolation.

13. Instantaneous Speciation

In some cases speciation may occur rapidly.

14. Polyploidy

It is commonly observed in plants. Errors during cell division produce individuals with extra chromosome sets. These individuals become immediately reproductively isolated from the parent population.

15. Hybridization

Hybridization between two species may sometimes result in a new reproductively isolated population which can survive in a specific environment.

Causes of Speciation

The different causes of speciation are listed below–

1. Geographical isolation
   It is the most common cause of speciation. It occurs when a population is physically separated by barriers such as mountains rivers oceans or by migration to distant areas. Due to this separation gene flow is stopped and populations evolve independently. This type of speciation is referred to as allopatric speciation.
2. Reproductive isolation without geographical barrier
   Speciation may occur even in absence of physical barriers. It is caused due to differences in habitat preference breeding season or mating behaviour. Because of this gene flow is reduced within the same geographical area and speciation is initiated. This is referred to as sympatric or parapatric speciation.
3. Natural selection
   Natural selection is one of the major driving forces of speciation. When populations live in different environmental conditions they face different selection pressures. These pressures lead to accumulation of different adaptive traits and over time reproductive isolation is developed between the populations.
4. Disruptive selection
   In this type of selection extreme phenotypes are favoured over intermediate forms. It usually occurs within the same population. Due to this the population is divided into two distinct groups which may finally result in formation of new species.
5. Sexual selection
   Speciation may occur due to differences in mating preferences. Variation in mating signals or choice of mates leads to reproductive isolation. Over time this isolation results in divergence of populations into different species.
6. Genetic drift
   Genetic drift refers to random changes in allele frequencies in a population. It is more effective in small populations. Due to chance events populations become genetically different which may lead to speciation even in absence of natural selection.
7. Founder effect
   It is a special case of genetic drift. When a small number of individuals establish a new population the gene pool is very limited. The new population shows different genetic composition from the parent population which may result in rapid speciation.
8. Polyploidy
   Polyploidy is a sudden increase in number of chromosomes due to error during cell division. It results in instant speciation as polyploid individuals are reproductively isolated from diploid parents. This type of speciation is common in plants.
9. Hybridization
   Sometimes hybridization between two different species produces a hybrid which is fertile and reproductively isolated from both parents. If the hybrid is better adapted to a new environment it may give rise to a new species.
10. Mutation
    Mutation causes sudden genetic changes. When different mutations accumulate in isolated populations genetic incompatibility develops. This mutation order speciation results in formation of new species.
11. Reinforcement
    When two diverging populations come in contact again and produce unfit hybrids natural selection favours individuals that avoid interbreeding. This strengthens reproductive isolation and completes the process of speciation.