4. Formation of new species

Unit 3: The origin and evolution of life 1 2 3 4 5 6

4. The formation of new species

We can define “species” from different points of view:

From a biological standpoint a species is “a group of natural populations which members can reproduce among them and produce fertile offspring”. It does not matter how similar individuals be. If mating does not produce descendants or these descendants are sterile the individuals do not belong to the same species.

From an evolutionary perspective a species is “a homogeneous group of organisms which live together and undergo changes over time”. As consequence of these changes, species modify and become new species.

Speciation is the process through which new species arise from other pre-exist species.

The ancestral species splits into two or more new species.

This process has given a huge diversity of organisms (biodiversity), millions of species, during the last 3,800 m.y.

A population of a determinate species can originate other or others populations by the apparition of a barrier which impedes that their members cross among them.

After the reproductive isolation, more genetic differences will be accumulating and the species will be more and more different with the pass of time.

Depending on the type of barrier, there are two models of speciation: allopatric and sympatric.

a) Allopatric speciation.

It is produce by geographical isolation.

As a result of the existence of geographical barriers, populations keep separated during a long period of time.

This provokes the arisen of evolutionary novelties in one or both populations due to differences between the natural environment of both geographical zones and to the fact that they cannot exchange genetic material between them.

If the separation continues during enough time, mechanisms of reproductive isolation will appear and both populations will become different species.

b) Sympatric speciation.

It is due to biological isolation.

As a result of the appearance of reproductive barriers that does not allow the reproduction between populations of the same species which share the same territory. They have the same effect that geographical barriers.

Isolation reproductive mechanisms can be classified into two groups:

- Prezigotic mechanisms: They impede the fertilisation and can be:

· Ecological isolation. Due to the existence of different habitats within the same ecosystem (differences in temperature, humidity, light)

· Seasonal isolation. Sexual maturity occurs in different moments of the year.

· Ethological isolation. Due to different behaviour during courtship or mating.

Signals are not recognised and the potential partner flees or attacks.

· Anatomical isolation. The structure or size of reproductive organs make impossible the copulation.

· Gametic isolation. Gametes are incompatible.

· Genetic isolation. Changes in the number of chromosomes produce sterility or unviability of hybrids.

- Postzigotic mechanisms: They act after fertilisation. They impede that the new individual produce descendence although it has mated. These mechanisms can provoke:

· Unviability of the hybrid embryo (e.g. embryos of goat and sheep died)

· Sterility of the hybrid adult (e.g. mule, hybrid between donkey and horse, is viable but sterile)