SpeciesSpecies ““AA species species is a group of similar individual is a group of similar individual

organisms that can usually breed among organisms that can usually breed among themselves to produce themselves to produce fertile offspringfertile offspring.”.”

Ernst MayrErnst Mayr Geneticists and molecular biologists Geneticists and molecular biologists

define a species as a group of organisms define a species as a group of organisms that share the that share the same gene poolsame gene pool..

SpeciationSpeciation The formation of a new species. The formation of a new species. Two types:Two types:Allopatric: similar species living in a Allopatric: similar species living in a

separate geographical area.separate geographical area.Sympatric: species of same genus Sympatric: species of same genus

living together in same area.living together in same area.

AllopatricAllopatric

1.1. Population moves into new area with Population moves into new area with many ecological niches unfilled.many ecological niches unfilled.

2.2. Population explosion follows with Population explosion follows with increased variation.increased variation.

3.3. Migration into fringe environments.Migration into fringe environments.4.4. Geographical barrier arises so Geographical barrier arises so

geographically isolated.geographically isolated.5.5. Isolated population have different Isolated population have different

mutationsmutations

6.6. Different populations are exposed Different populations are exposed to different environmental selection to different environmental selection pressures.pressures.

7.7. Some subspecies develop genetic Some subspecies develop genetic and chromosomal differences that and chromosomal differences that mean they cannot breed with the mean they cannot breed with the parent population.parent population.

8.8. Now are genetically isolated so Now are genetically isolated so therefore a new species.therefore a new species.

A cline: when we move in a particular A cline: when we move in a particular direction through a range of species direction through a range of species and find a continuous increase or and find a continuous increase or decrease in some characteristics.decrease in some characteristics.

Isolating mechanisimsIsolating mechanisims1.1. Geographical barriers: mountains, rivers, Geographical barriers: mountains, rivers,

continental plates moving, oceans, continental plates moving, oceans, deserts.deserts.

2.2. Ecological barriers: populations may Ecological barriers: populations may have developed genetic differences to have developed genetic differences to cope with different ecological niches or cope with different ecological niches or habitats.habitats.

3.3. Reproductive barriers: under natural Reproductive barriers: under natural conditions, sympatric populations of conditions, sympatric populations of different species can be prevented from different species can be prevented from mating by pre or post zygotic isolating mating by pre or post zygotic isolating mechanisims.mechanisims.

PrezygoticPrezygotic

Habitat isolation (nocturanl/diurnal)Habitat isolation (nocturanl/diurnal)

Temporal isolation (differences in breeding Temporal isolation (differences in breeding times)times)

Biochemical incompatibilityBiochemical incompatibilitysperm and egg fail to unite - due to chemical sperm and egg fail to unite - due to chemical differences, eg, hormone signals.differences, eg, hormone signals.

Prezygotic 2Prezygotic 2

BehaviouralBehaviouralDifferences in courtship behaviourDifferences in courtship behaviourare a major isolating factor.are a major isolating factor.

Mechanical differencesMechanical differences Appearance and colouration patterns can Appearance and colouration patterns can differ enough between closely related differ enough between closely related species to make them “unattractive” to species to make them “unattractive” to each other.each other.The fit of the mating apparatus of many The fit of the mating apparatus of many insects is very specific.insects is very specific.

PostzygoticPostzygoticThese mechanisms stop gene flow between These mechanisms stop gene flow between species even if mating is successful.species even if mating is successful.

Hybrid SterilityHybrid SterilityThe resulting offspring from two different species The resulting offspring from two different species may be healthy, but is sterile, eg. horses and may be healthy, but is sterile, eg. horses and donkeys can breed to produce mules, which are donkeys can breed to produce mules, which are sterile.sterile.

Hybrid inviabilityHybrid inviabilityThe zygote fails to develop.The zygote fails to develop.

Hybrid breakdownHybrid breakdown

When Isolating Mechanisms When Isolating Mechanisms FailFail

Sometimes isolating mechanisms are Sometimes isolating mechanisms are weak. When this happens, species weak. When this happens, species hybrids can occur.hybrids can occur.

Eg:Eg:○ the native grey duck and the introduced the native grey duck and the introduced

mallard duck hybridise freely. mallard duck hybridise freely. ○ The endangered black stilt is interbreeding The endangered black stilt is interbreeding

with the pied stilt, producing a range of with the pied stilt, producing a range of intermediate hybrids.intermediate hybrids.

Sympatric Sympatric Remember non geographical Remember non geographical

isolationisolationTo illustrate the following example is To illustrate the following example is

imaginary:imaginary:there is a bird population on an there is a bird population on an island.island. E.gE.g

Sympatry involves natural selection Sympatry involves natural selection but allopatry does notbut allopatry does not

Other ways of speciationOther ways of speciation Polyploidy – instant speciation: the abrupt Polyploidy – instant speciation: the abrupt

formation of a new species with more than formation of a new species with more than two sets of chromosomes. E.g. wheat.two sets of chromosomes. E.g. wheat.

Chromosomal rearrangement: duplications Chromosomal rearrangement: duplications or delations may make individuals or small or delations may make individuals or small populations that are genetically populations that are genetically incompatible with the rest of their species.incompatible with the rest of their species.

Isolation by time.Isolation by time.