6.2.12 Isolation and Speciation Flashcards
What is genetic isolation?
When two populations of the same species become reproductively isolated from each other, they can eventually become genetically isolated
If the two populations are no longer reproducing with each other, then they do not interchange genes with each other in the production of offspring
Changes that occur in the allele frequencies of each group are not shared, so they evolve independently of each other, which can lead to the formation of two populations that are no longer successfully able to interbreed
When the genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring, speciation has occurred
There are different mechanisms of speciation:
Allopatric and sympatric
Evolution causes speciation: the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
Genetic isolation between the new population and the pre-existing species population is necessary for speciation
There are two different situations when speciation can take place:
Two populations of a species are separated by a geographical barrier and become genetically isolated from each other
Two populations of species are living in the same area (experiencing similar environmental selection pressures) but still become genetically isolated from each other
What is allopatric speciation?
Allopatric speciation occurs as a result of geographical isolation
It is the most common type of speciation
Allopatric speciation occurs when populations of a species become separated from each other by geographical barriers
The barrier could be natural like a body of water, or a mountain range
It can also be man-made, like a motorway
This creates two populations of the same species who are reproductively separated from each other, and as a result, no genetic exchange can occur between them
If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species
The changes in the alleles/genes of each population will affect the phenotypes present in both populations
Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)
Example of allopatric speciation:
Imagine there is a population of trees that are all one species
A new mountain range forms that divides the population into two
The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them
The two populations experience different selection pressures
Over thousands of years, the divided populations form two distinct species that can no longer interbreed
What is sympatric speciation?
Sympatric speciation takes place with no geographical barrier
A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
Something has to happen that splits or separates the two populations:
This can occur via:
Ecological separation
Behavioural separation
Ecological separation: Populations are separated because they live in different environments within the same area
For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering, so a population growing in soil with a slightly different pH may flower at a different time from another population, leading to reproductive separation (and eventually genetic isolation) of the two populations
Behavioural separation: Populations are separated because they have different behaviours
For example differences in feeding, communication or social behaviours, such as courting behaviours to attract a mate
Example of sympatric speciation:
A species of fish lives in a lake
Some individuals within the population feed on the bottom while others remain higher up in the open water
The different feeding behaviours separates the population into different environments
Here, behavioural separation has led to ecological separation
The separated groups experience different selection pressures
Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding
Over time natural selection causes the populations to diverge and evolve different courtship displays
They can no longer interbreed; they are now considered separate species
What is the importance of mutations?
Both allopatric and sympatric speciation are reliant on mutations occurring within individuals in populations
Without mutations, there are no new alleles of genes for selection to act on
The changes in genetic material caused by mutations are important as these changes are what produce the differences in physiology, behaviour and morphology between populations over many generations, eventually leading to speciation
