In this lesson we looked at the following objectives:
In the lesson we learnt that allopatric speciation is where a population is divided into two or more separate populations by some kind of geographic barrier such as a road or a new dam.
Sympatric speciation is where individuals in the same population become reproductively isolated from each other even though they occupy the same geographic range. Factors that could lead to them becoming reproductively isolated from each other are things like changes in courtship behaviour, changes in feeding behaviour, changes in colouration.
Remember, speciation is when populations diverge from each other genetically. This is due to the accumulation of different alleles in each population. Because the populations are reproductively isolated from each other there can be no gene flow between them. The accumulation of mutations in a population is an example of a phenomenon called genetic drift.
In the lesson we looked at several examples:
North American Black Caps which migrate from Africa to southern europe but have recently split into a migratory population that head to northern europe and one that continues its’ southern migratory route.
Sparrows in San Diego that have altered tail plumage in city environments so have become reproductively isolated from their wild cousins.
Canadian lake sticklebacks – this was actually an example of the reverse of speciation so a bit of a red herring in this case. The reason is inadverantly an impact of humans on the environment. The introduction of cray fish into the environment has lead to decreased isolation of two separately reproducing populations, possibly due to increased turbidity of the lake water meaning that sticklebacks can no longer recognise visual cues which define members of separate populations.
Lizards that inhabit savannah grassland – without human intervention grassland fires break out which stops woodland from growing in place of the savannah grass. Humans have suppressed the fires due to the danger they pose to human life and property. This has allowed trees to grow on the savannah which has fragmented the lizard’s habitat separating them into separate populations.
In discussing speciation it is important to note that mechanisms of reproductive isolation can occur before the formation of a zygote (fertilised egg) or after the formation of a zygote. Barriers that prevent the formation of a zygote could include things like geographic barriers, differences in courtship behaviour that prevent individuals from recognising each other as members of the same species or different receptor molecules on the ovum which prevent the sperm from binding with it.
Post zygotic mechanisms include having an uneven number of chromosomes in the zygote. This is the reason that mules are infertile – they cannot form gametes due to having an uneven number of chromosomes resulting from the fusion of horse and donkey gametes.
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