Slide 1
Reproductive isolation can be achieved either through prezygotic
mating isolation or postzygotic mating isolation as we will now discuss.
Slide 2
Postzygotic isolating mechanisms refer to situations where a
zygote forms but the embryo cannot survive and develop into a mature
individual or an individual is born and survives to adulthood but cannot
reproduce successfully by producing fertile offspring
Slide 3
Hybrid inviability refers to a situation where the genes from the two different species cannot function properly together and the embryo dies before birth.
This is seen in matings between sheep and goats. Sheep and goats are closely related but they cannot produce hybrids because of gene incompatibility which leads to loss of the embryo.
Slide 4
Hybrid Infertility refers to a situation where hybrid
offspring are born but they cannot reproduce.
An example of this is when a donkey is crossed with a horse to produce a mule. Male donkeys have a diploid chromosome number of 62 (so their sperm contain 31 chromosomes) and female horses have a diploid chromosome number of 64 - so their eggs have 32 chromosomes. When a sperm with 31 chromosomes fertilizes an egg with 32 chromosomes - the result is that the zyogte has 63 chromosomes - an uneven number. The unpaired chromosome causes problems during meiosis and the mule usually cannot produce normal sperm or eggs.
Slide 5
But there are always exceptions. In 2007 a mule in
Colorado named Kate gave birth to a foal (this has been documented a few other
times as well).
So far, nobody can adequately explain how this happened - although researchers are working hard to figure out how a female mule can successfully reproduce.