Prezygotic isolation occurs before the formation of a zygote can take place. In most cases mating does not even occur. Forms of prezygotic isolation include spatial, behavioral, mechanical and temporal isolation. Postzygotic isolation occurs after members of two different species have mated and produced a zygote.

Postzygotic mechanisms include hybrid inviability, hybrid sterility and hybrid “breakdown.”

Postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

It looks like there are five major types of prezygotic barriers to reproduction: spatial isolation, temporal isolation, mechanical isolation, gametic isolation and behavioral isolation.

The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile.

Behavioral isolation is an important evolutionary mechanism that helps members of the same species identify each other as proper mates. If a continent splits, or a mountain range rises, one population can become two, and eventually the two populations may not be able to interbreed.

The ovaries are the main reproductive organs of a woman. The two ovaries, which are about the size and shape of almonds, produce female hormones (oestrogens and progesterone) and eggs (ova).

Defining reproductive status For the rest of the sample, reproductive status was inferred by age or the presence of a menstrual cycle in the previous year, with women > 18 years of age but < 42 classified as premenopausal (n=169) and those between 42 and 51.5 considered perimenopausal (n=74).

Reproduction means to reproduce. It is a biological process by which an organism reproduces an offspring who is biologically similar to the organism. Reproduction enables and ensures the continuity of species, generation after generation. It is the main feature of life on earth.

Many organisms – including microbes, plants, and some reptiles – do reproduce asexually. But the vast majority of living things reproduce sexually. Sexual reproduction allows for greater genetic innovation over time than clonal, improving the chances that an organism can solve the problems at hand.

Disadvantages of external fertilization include a reliance on water and the large amount of wasted sperm and eggs that never reach a corresponding gamete, even when the organisms releasing the eggs and sperm are in close proximity. Additionally, the eggs released can have no shells, as these would stop sperm.

Among vertebrates, external fertilization is most common in amphibians and fish. Invertebrates utilizing external fertilization are mostly benthic, sessile, or both, including animals such as coral, sea anemones, and tube-dwelling polychaetes.

Also, it is easier to find mates as the gametes just need to be released and can drift away with wind or water. This results in greater genetic variation. The disadvantages however, are that there is reduced chance of survival from predators and harsh environments.

External Fertilization occurs when an organism fertilised outside the body. The process of external fertilization usually requires a water body. Here both the eggs and sperms are released into the water. Majority of external fertilization occurs during spawning.

External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of species survival in a hostile environment.

Complete answer: Internal fertilization is the process when the syngamy (union of male and female gamete) occurs inside the female body after insemination using copulation. In contrast, External fertilization is the syngamy outside the female body, that is in the outer environment especially in water bodies.

Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization.