Crimean state medical University named after S. I. Georgievsky Submitted by

Ajay Sharma
Submitted to Anna Zukhova
Georgievsky

ability to interbreed with one another under natural condition and

produce fertile offspring and reproductively isolated from other species.

In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction.

have identified about two million species and still do not

know how many species inhabit Earth. Scientists estimate as many as 30 million species may inhabit Earth.

So, the big question is how did so many species form?

occurs when members of similar populations no longer interbreed to

produce fertile offspring within their natural environment
It occurs when groups in a species become reproductively isolated and diverge
A single species can generate multiple species

HENCE IT LEAD TO A NEW IDEAS ABOUT EVOLUTION
Speciation is the origin of new species.
Speciation increases our planet’s biological diversity.
Therefore, speciation is an important part of the theory of evolution.
Many scientist have different ideas about it.

individuals of a species from mating and producing viable and

fertile offspring

A new species will form when reproductive isolation occurs

The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviours and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species

barriers
Geographically separated populations aren’t necessarily distinct species.
Example: Northern Spotted

Owl and Mexican Spotted Owl.

Geographical Isolation

mechanisms can be divide in to two different types.
Isolating

mechanisms that occur before fertilization are called Prezygotic isolating mechanisms.
Isolating mechanisms that occur after fertilization are called postzygotic isolating mechanisms.

egg can be fertilized.
A zygote is an egg that

has been fertilized by a sperm
Premating isolating mechanisms include:
1) Ecological isolation.
2) Temporal isolation.
3) Behavioral isolation.
4) Mechanical incompatibility.
5) Gametic isolation

they live in different habitats.
Ex. The mountain bluebird lives at

high elevation while the eastern bluebird prefers low elevation

breed at different times of year.
Ex: The Monterey Pine and

the Bishop pine have different pollination periods.
Example Red and black sea urchins live in the same location, but release their gametes at different times of the year.

reproductive structures are incompatible
Closely related species may attempt to mate

but fail because they are anatomically incompatible and transfer of sperm is not possible
In animals with internal fertilization, male and female sexual organs may not fit together
EX. snails of species whose shells have left handed spirals may be unable to successfully copulate with snails whose shells have right handed spirals

of female reproductive tract in another species
For fertilization to occur,

the sperm must reach the ovum. For this to happen they usually attract each other through chemical means, but the chemicals might vary from one specie to another. In this case gametes won’t recognise each other and fertilization won’t take place.

of courtship may be different to the extent that sexual

union is not achieved
Ex: Fireflies produce patterns of flashes that attract their mates. Over 2000 species of fireflies are isolated based on the patterns of flashes
Members of the two species will not mate with each other because they use different songs to attract mates. Eastern meadowlarks will not respond to western meadowlark songs, and vice versa.
Example: Eastern and Western Meadowlarks

from developing into viable, fertile adults
There are three likely cases

that will occur to ensure that the hybrid does not reproduce Postmating isolating mechanisms include:
1. Hybrid inviability
2. Hybrid infertility
3. Hybrid breakdown

different species and 'inviability' refers to an organism that does

not thrive. In hybrid inviability, mating occurs and a hybrid is produced, but Hybrid very weak and can’t live outside the uterus.

Ex. Several species of the frog live in the same habitats but hybrids do not complete development

If Sheep and goats mate then Hybrid zygotes Die before birth

 The creation of an unsuccessful hybrid is also a form of post- zygotic barrier.

numbers are different, meiosis cannot produce normal gametes
EX. Male

donkey x Female horse Mule Mules:  Healthy (viable)

the invention of different medium of transport.

(2)It can be seen

in animals but it is rare in human as human beings can travel to any place to reproduce.(3)WHO says no to gene edited babies.