Speciation

General mechanisms of the process

As long as there is a continuous exchange of genetic information between different populations, they maintain their unity and belong to the same species. Evolutionary separation is triggered by isolation mechanisms.

When an individual population or a group of them becomes isolated, gene flow and the exchange of genetic information completely cease. Under such conditions, specific changes begin to accumulate in the isolated group under the constant influence of evolutionary factors. Over time, these changes lead to individuals losing the fundamental ability to interbreed with representatives of other populations of their ancestral species. As a result, the population transforms into a genetically independent closed system, and the original species splits into two or more new ones.

Classification of speciation pathways

In modern biology, according to the mechanism of origin, three main pathways of new species formation are distinguished:

1. Divergent speciation. The classic variant in which a single ancestral species splits into several new branches. The original groups undergo character divergence.
2. Phyletic speciation. Proceeds without the divergence of ancestral groups. Over a succession of generations, species "A" smoothly and entirely transforms into a completely new species "B". At the same time, the total number of species in nature does not change. The dynamics of this process are studied exclusively using paleontological material. However, this pathway does not exclude the possibility that at certain stages, other groups may separate (diverge) from the single phyletic trunk.
3. Hybridogenic speciation. Also occurs without initial divergence. The mechanism consists of the direct fusion of the genomes of two different species with the formation of a hybrid species. The main problem of such hybrids is sterility, which is successfully overcome through the transformation of the organism into a polyploid (due to a multiple doubling of the chromosome set).

Examples of the hybridogenic pathway and modes of divergence

A classic example of successful hybridogenic speciation is the cultivated plum, whose diploid chromosome set is 48 ($2n = 48$). It is the evolutionary product of the natural hybridization of blackthorn ($2n = 32$) and cherry plum ($2n = 16$). The process can be described by the following scheme: the haploid set of blackthorn $n = 16$ fuses with the haploid set of cherry plum $n = 8$. A hybrid with a set of $2n = 24$ is formed. Subsequently, chromosome doubling (polyploidization) occurs, and a fertile species with a set of $2n = 48$ is formed.

For the divergent pathway, two main modes of speciation are distinguished:

• Allopatric (geographic) — associated with spatial barriers.
• Sympatric (ecological) — occurs within the same range. Its characteristic feature is a strong morphological similarity between the newly formed species and the ancestral one (the only exception here being interspecific hybrids).

Evidence at the level of chromosomal rearrangements

Traces of major evolutionary events are reliably recorded in the genetic material. Clear evidence of chromosomal rearrangements during speciation is the structure of human chromosome 2. It clearly traces the remnants of the fusion of two ancestral chromosomes:

• In the middle part of the chromosome, rudimentary telomeres are detected (remnants of telomeric sequences that are normally found only at the ends).
• Elements of a second centromere are discovered (two centromeric sequence regions are present instead of the standard one).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed analysis of allopatric and sympatric modes
• Molecular mechanisms of population isolation
• Genetic structure of vestigial telomeres
• Paleontological evidence of macroevolution

Other topics in the "Macroevolution" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.