Солнечная система и ее тайны

Планеты Созвездия НЛО

The Darwinian Paradigm and Modern Evolution

The Darwinian Paradigm: a process of random variation in the structure of inherited biomolecules, with superimposed natural selection to achieve fitness. This is our modern view of evolution.

This is a general process that applies to a population of reproducing units. The mechanism of Darwinian evolution for life on Earth is based on reproduction by copying DNA molecules. Natural selection, as defined in the Darwinian paradigm, is the result of unequal reproductive success of the units in the population.

In other words, evolution occurs because the copying of DNA is not always perfect. Any change in a living form's DNA is called a mutation - most are lethal, but some improve the ability to survive and reproduce.

The three basic building blocks of the process are: replication, selection, and mutation, which then loops back to replication.

This picture, and the original Darwinian paradigm, is simple. The chemical structures of Earth's biomolecular systems have arisen through such Darwinian processes, with few exceptions that we will discuss later. Comparing the genomes of different living organisms, from microbes to plants and animals, results in a tree-like network of relationships - the universal phylogenetic tree of life (see figure below).

The phylogenetic tree of life shows how closely different organisms are related to one another. It is based on reading ribosomal RNA sequences - a small strand of RNA that carries genetic information and resides in the ribosome, because until recently it was impossible to read the full genome of an organism. With many full genomes now available, the basic structure of the tree has not changed, namely that there are three domains of life: bacteria, archaea, and eukarya. We - Homo Sapiens - are eukaryotes.

The commonality of the genomes among all life seems to point to a root of the tree, or a "last universal common ancestor" (LUCA). However, it is not possible to get any concrete evidence for its nature; it is very likely that LUCA was not a single organism, but an entire group.

Careful examination of organisms and their genes also reveals evidence of a form of evolution that extends the original simple Darwinian paradigm. We tend to think of genes only transferring from parent to offspring through reproduction. This process is called vertical gene transfer. However, many bacterial properties seem to be acquired by the lateral transfer of entire genes or even set of genes between closely related species (as in eukaryotes). Rarely this can even occur between very different bacteria, widely separated on the phylogenetic tree. The process is called horizontal gene transfer. We have to assume that it could have played an important role in the very first steps of life on Earth.

Солнечная система и ее тайны