Speciation

Species and Speciation
ObjectivesAt the end of this lecture, you should be able to understand the following
What is a species?
What maintains species?
Reproductive barriers
What makes new species? (Mechanisms of speciation)
Allopatric speciation
Parapatric speciatin
Sympatric speciation
Adaptive radiation
Cospeciation vs coevolution

Objectives
What is a species?
What maintains species?
Reproductive barriers
What makes new species? (Mechanisms of speciation)
Allopatric speciation
Parapatric speciatin
Sympatric speciation
Adaptive radiation
Cospeciation vs coevolution

What is a “species”?
Biological species concept:

Proposed by Ernst Mayr in 1942

A species is a set of individuals that can potentially interbreed in nature
Alternate definitions of species
There is no one all-encompassing definition of “species”.

Several other definitions have been proposed.

Each of these definitions explains the concept of a species to some extent.
Phylogenetic species concept
A species is the smallest set of organisms that share a common ancestor and can be differentiated from other such sets.

Morphological / Phenetic species concept
A species is a set of individuals with similar morphologies or phenotypes, that can be differentiated from other such groups.
(…contd.)

Recognition species concept
Individuals that recognize each other as potential mates belong to the same species.

Ecological species concept
Individuals that occupy the same ecological niche belong to the same species. This concept accommodates asexual and sexual species.
Objectives
What is a species?
What maintains species?
Reproductive barriers
What makes new species? (Mechanisms of speciation)
Allopatric speciation
Parapatric speciatin
Sympatric speciation
Adaptive radiation
Cospeciation vs coevolution

What maintains species?(What keeps species from mixing?)
Reproductive isolation - the existence of reproductive barriers.

Prezygotic barriers

Postzygotic bariers

Prezygotic barriers
Impedes mating between species

Habitat isolation
similar species that occupy different habitats within the same geographic area may rarely encounter each other, and therefore there are small chances of gametes mixing.

Temporal isolation
closely related species that breed at different times of the day or in different seasons cannot mix gametes even if ranges overlap.
(…contd.)

Behavioral isolation
courtship behaviors are unique to each species; mates recognize and respond to behaviors only of their species.

Mechanical isolation
morphology of different species prevent successful mating.

Gametic isolation
if members of different species attempt to mate, fertilization of ova is hindered.
Refer Campbell textbook for examples of prezygotic barriers.
Postzygotic barriers
These barriers prevent mixing of species if prezygotic barriers are overcome and fertilization between two species takes place.

Reduced hybrid viability
genes of different parent species may interact and impair the hybrids development.
(…contd.)

Reduced hybrid fertility
hybrid is sterile, and cannot pass on hybrid genes.

Hybrid breakdown
if hybrids are fertile, and they attempt to mate with other hybrids or individuals from parent species, their offspring remain sterile.
Refer Campbell textbook for examples of postzygotic barriers.
Do species ever interbreed?
Very closely related species may interbreed in a laboratory, or naturally if some environmental conditions change; this may indicate that the two speciated very recently in evolutionary time.

Sometimes when two species interbreed, and the hybrid offspring is viable and fertile, new species may arise.
Objectives
What is a species?
What maintains species?
Reproductive barriers
What makes new species? (Mechanisms of speciation)
Allopatric speciation
Parapatric speciatin
Sympatric speciation
Adaptive radiation
Cospeciation vs coevolution

What is Speciation
Mechanism by which two or more separate species are produced.

Allopatric speciation
Parapatric speciation
Sympatric speciation
Adaptive radiation
1. Allopatric speciation
Some members of a population are separated from others by a geographic barrier.

Gene flow between the two populations is greatly reduced (barrier is almost impervious to gene flow)

The two populations evolve separately in their respective habitats

If the two populations come together sympatrically, they can no longer interbreed.
Ancestral squirrel population divided by the formation of the Grand Canyon
Reduced gene flow between the two separated populations.
North rim – Kaibab squirrel (Sciurus aberti kaibabensis)
South rim – Abert’s squirrel (Sciurus aberti aberti)
An example: Kaibab squirrel (left) and Abert’s squirrel (right) on opposite rims of the Grand Canyon
2. Parapatric speciation
Population is not split, but is large and extends over a large range.

Some members of the population may never interact;
or change in environment across range causes differential selection pressures.

Gene flow between members of the population is restricted, and the two groups evolve separately.

They can no longer interbreed.
Limited interaction among individuals occupying a large range can reduce gene flow
Change in environment - Parapatric speciation may be happening in the grass species Anthoxanthum odoratum. Selection works on populations living near mines where the soils are contaminated with heavy metals, and selects
for metal tolerant species. These populations are diverging from the original metal intolerant population.
3. Sympatric speciation
No geographic barriers between individuals of a population.
Gene flow between individuals is not restricted.
Population may be divided by the following mechanisms
Some individuals may exploit a new niche
Disruptive selection may favor two different traits
Occurrence of polyploidy by failure of a cell to divide during meiosis
(…contd.)

Gene flow is reduced between individuals of the two sympatrically separated groups

Thus individuals of the two different groups evolve separately within the same geographic range.
New niches - Original hosts of apple maggot flies in America were the native hawthorns. When apples were introduced to the continent, some flies started laying their eggs in apples. Females mostly lay eggs in the type of fruit they grew up, and males mostly look for mates on fruits they grew up. The two groups speciated sympatrically.

Disruptive selection – cichlids in Africa’s Lake Victoria speciated as a result of disruptive selection, i.e. females prefer certain extreme color variants and this preference is heritable.
Under normal light females choose males of their own species based on color.

Under monochromatic light, females cannot tell males of the two species apart, and they interbred.
Two cichlid species Pundamilia pundamilia (top) and P. nyererei (bottom) –
Polyploidy - A species of goatsbeard plant Tragopogon miscellus is a tetraploid hybrid of T. dubius and T. pratensis.
T. mirus is an allopolyploid (a sterile hybrid that turns into a fertile polyploid) hybrid of T. dubius and T. porrifolius.
4. Adaptive radiation
Often occurs in island archipelagoes
Individuals of a population migrate to/ colonize new habitat
Gene flow restricted between isolated population and parent population
Resource environment and predator pressures different in new habitat
New niches available for occupation
Individuals in new habitat adapt accordingly, occupying new niches
Darwin’s finches – at least 13 different species of finches on the galapagos islands occupying different niches on different islands.
Hawaiian silverswords – diverged into several species occupying different
niches on different
islands.
Objectives
What are species?
What maintains species?
Reproductive barriers
What makes new species? (Mechanisms of speciation)
Allopatric speciation
Parapatric speciatin
Sympatric speciation
Adaptive radiation
Cospeciation vs coevolution

What is Cospeciation
Two lineages speciating in parallel
Most likely occurs between host and parasite pairs
Example
Lice on gophers switch from one gopher to another allowing gene flow between lice.
If part of the gopher population speciates by any mechanism, the lice associated with it will also speciate as their gene flow with original lice population is restricted.
Cospeciation is different from Coevolution
Coevolution:
Species do not “form” in parallel
Two species influence each others mutual evolution through close association
Examples are predator-prey and host-parasite associations, competitive species, mutualistic species.
The cheetah and gazelle coevolve as a result of predator prey relationship – the gazelle evolves to outrun the cheetah, while the cheetah evolves to run fast enough to catch the gazelle.
Other examples
The yucca moth lays eggs in the yucca flower, in the process pollinating it. The two have closely evolved in a mutualistic relationship. Another similar system is the figs and fig wasp system – each species of fig is pollinated by a unique species of wasp that can lay its eggs only in the flower of that particular fig species.

The bullhorn acacia (Acacia cornigera) and the stinging ant (Pseudomyrmex ferruginea) share a mutualistic relatonship and have coevolved to suit each other’s requirements. The ant defends the plant against herbivores, in return receiving food from the plant that appears to be produced for the sole purpose of feeding the ants.
Quiz yourself
According to Ernst Mayr, what classifies a group of organisms as belonging to the same species?
Prezygotic / postzygotic barriers prevent gametes of different species from mixing. Name these.
Prezygotic / postzygotic barriers keep hybrid zygotes from forming new species. Name these.
If such barriers exist, then how do new species arise? What is the mechanism called?
Match the definitions in column A with mechanisms in column B
Column B
Allopatric speciation

Parapatric speciation

Sympatric speciation

Adaptive radiation

Cospeciation

Coevolution

Population extends over a large range; individuals do not interact, or environment in small part of range changes; reduced gene flow.
Members of a population colonize new habitat and exploit new niches there.
Population is split by a geographic barrier, and gene flow is restricted between the separated populations.
Population is not split; some members exploit new niche, or there is disruptive selection, or occurrence of polyploidy.
Two species are closely associated with each other and mutually influence each others evolution.
Two lineages speciate in parallel.
Column A