REPRODUCTIVE ISOLATION -

According to the biological species concept, members of one species are reproductively isolated from members of all other species.

Accordingly, reproductive isolation, that prevents the exchange of genes between populations, is the first step leading to the formation of new species.

ISOLATING MECHANISM -

Pre-zygotic isolating mechanisms -

1. Ecological isolation: Different habitat requirements separate groups, even though the inhabitants may exits in the same general location, e.g., Head and body lice are morphologically very similar, yet they live in different habitats on a single human body.
2. Head lice live and lay eggs in the hair on the head of a human, whereas body lice live and lay their eggs in clothing. Both suck blood for nutrition.
3. Geographical isolation: Geographic barriers such as emerging mountains; islands, rivers, lakes, oceans, moving glaciers, and other geographic barriers keep groups isolated e.g., different tortoises are found on different Galapagos Islands; surrounding oceans keep tortoise populations isolated.
4. Temporal Isolation: Two species living in different habitat cannot meet, e.g. fresh water fish cannot breed with sea fish.
5. Behavioural Isolation: Different animals have their own kind of courtship behaviour which is recognised by the fair sex of the same species only.
6. Seasonal isolation: Differences in breeding seasons prevent gene flow, even when populations are found in the same area e.g. Such isolation is very common in plants and frequently occurs in insects and certain other invertebrates.
7. Mechanical isolation: Differences in matting behavior prevent reproduction e.g.,many animals have evolved complicated, courtship activities before breeding.
8. Gamete isolation: Sperm and egg are incompatible. Gamete isolation is a common isolating mechanism in many plant and animal species.

Post-zygotic isolating mechanism -

1. Hybrid inviability: Zygotes or embryo fail to reach reproductive maturity e.g., hybrid embryos formed between two species of fruit flies fail to develop.
2. Hybrid sterility: Fertilization is successful between two species, but hybrid progeny are sterile e.g., a mule is a sterile hybrid produced from a mating between a female horse and a male donkey. Hinny is sterile hybrid between female ass and male horse (stallion).
3. Hybrid breakdown: Sometimes hybrids are fertile and give rise to offspring which do not continue to survive in F2 generation.