ORIGIN OF REPRODUTIVE ISOLATION

 What is reproductive isolation??

It is a prevention of interbredding between the population of two different or closely related species.

Can lead to the formation of new species (speciation).

Reproductive isolation functions in two mechanism---

1)Prezyotic 

2)Postzygotic

PREZYGOTIC MECHANISM 

It works before the formation of zygote.i.e before fertilization has occurred. 

In this the nature tries to prevent fertilization.

It includes:-

a)Ecological barrier

It is also known as geographical isolation.Here the species lives in two different habitat.

Due to their differences in habitat mating is not possible. 

Eg.Suppose species '1'lives in freshwater and species '2'lives in terrestrial. So fertilization is not possible between them.

b)Temporal isolation 

In this isolation,the breeding seasons of species are different. 

Due to the differences in breeding season,they don't mate. 

Eg.Suppose species '1' has breeding season from April till November and another species '2' has  breeding season from December to March. So mating is not possible between the two.

Most common is the Red-legged frog (November-late April) and Yellow legged frog(late April -June).

c)Behavioral isolation

In this,their is a difference in sexual behavior. 

Eg.During rainy season the female frog makes a very peculiar sound to attract the male member of his own species. 

This sound is only identified by the opposite member of his own species. 

d)Mechanical isolation

In this the sex organs of two different species is incompatible due to which mating is not possible. 

Evolution has led to the development of genital organs with increasingly complex and divergent characteristics,which can cause mechanical isolation between species. 

e)Gametic isolation 

In those species were external fertilization occurs,in them this type of mechanism comes into play.

When sperms and ova of different species are released into any medium they are been fertilized by the members of their same species.

Reason-The chemicals present of these gametes can identify the gamete of their own species and gets fertilized.

Eg-Giant Red Urchin and Purple Urchin they sh share same habitat but they don't mate as their gametes don't recognize one another, maintaining species integrity. 

POSTZYGOTIC MECHANISM 

In this when nature cannot prevent the members of two different species to reproduce, hence in this situation the following mechanism operates.

a)Hybrid inviability

Sperms and eggs of two different species may combine but the zygote don't survive. 

Embryo dies after few cleavage or some time before death.

Eg.Panthers pardus and Panthera tigris 

They mate,zygote forms but embryo miscarried or is stillborn.

b)Hybrid sterility

Here when two members of different species mate,the offspring that are born is sterile.

Eg-Panthera tigris(tiger) and Panthera leo(lion)

When they cross the offspring that is formed is Liger and it is sterile.

c)Hybrid breakdown

Two related species can hybridize and their F1 offspring are fertile.

But successive generation follows lowers viability thus cannot become a established population. 

Eg.Rice cultivars

F1 Hybrid is fertile and viable but F2 generation is stunted and sterile.

Two important conclusions can be drawn from above:-

HYBRIDIZATION 

Generally hybridization is carriedout by our scientists to get desired high level breeds of species (both in plant and animals).But hybridization is not itself supported by nature.It is against natural selection and rejected by nature.

SPECIATION 

Reproductive isolation leads to origin of new species. If two species are separated by any geographical barrier, then both the species will be individually adapted to their changes in habitat and ultimately leads to formation of new species. 

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https://youtube.com/channel/UC8ViaVUP4SaSFRUA1lZSUhw