4. Reproductive Isolating Mechanisms
Speciation forms when gene flow is prevented and reproductive isolating mechanisms are built up in the different populations.
Over time, new phenotypes may develop through mutation. If the populations become reproductively isolated and there is no gene flow for a long time, the two populations will show divergent evolution. This could lead to speciation.
There are two types of reproductive isolating mechanisms (RIMs).
Prezygotic Isolating Mechanisms
Prezygotic isolating mechanisms are those (potentially due to mutation) that prevent successful fusing of gametes (fertilisation), preventing a zygote (fertilised egg) from being formed.
Read the different types of prezygotic isolating mechanisms below, and then answer the questions.
Geographical Isolation - preference for a different location, or separation by a geographic barrier.
Ecological (Habitat) Isolation - occupying or breeding in different habitats in the same area.
Structural (Morphological) Isolation - different structures, e.g. reproductive parts.
Temporal isolation - different timing of mating, active at different times.
Behavioural Isolation - differences in courtship or mating behaviours.
Gametic Isolation - differences in gametes, making them incompatible.
Postzygotic Isolating Mechanisms
Sometimes, sperm may still successfully fertilise eggs of different species (despite the prezygotic isolating mechanisms), forming a hybrid.
Postzygotic mechanisms are barriers that occur after fertilisation has taken place, so that hybrids are prevented from producing viable offspring themselves.
There are 3 postzygotic isolating mechanisms.
Hyrbid Inviability (premature death)
The zygote, embryo or offspring produced is inviable (unable to survive), and dies early in its development.
Hybrid Sterility (infertile)
The hybrid offspring is sterile (infertile and cannot breed). This is usually because the sterile cannot produce normal gametes during meiosis.
The hybrid can reproduce, but subsequent generations are infertile or have reduced ability to breed.
A single barrier might not completely isolate a gene pool, but more than one isolating mechanisms could isolate the populations and lead to speciation.
Reproductive isolation in islands. Less predators