Evolution in the twenty-first century

A little over thirty years ago, during the eighties, some of the fundamental concepts of evolution seemed to be quite clear. Let us look at a few:

• The DNA of every living being is an encrypted message that perfectly describes how to build the corresponding phenotype (the adult individual). All the information is in the genes. (Mechanistic reductionism).
• Most DNA is unnecessary (junk DNA) and has been accumulated due to errors and repetitions in the transcription of the genomes of living beings.
• The best metaphor to represent the organization of a genome is a set of beads on a string.
• Genes are the repositories of inheritance, and each gene specifies a biological function.
• Evolutionary processes take place through random mutations that act on a single gene, which are subject to natural selection, which results in small incremental increases in the adaptation of living beings to the environment.

The end of the selfish gene

The German biologist August Weismann (1834-1914) was one of the most influential biologists of the late nineteenth century. His most important contribution was the theory of germinal plasma, also called in his honor Weismannism. According to this theory, there are two classes of cells in all multi-cellular living organisms (see Figure 1):

Figure 1

• Somatic cells, represented in the figure by an S, that make up most of the body and do not play any role in inheritance.
• Germ cells, represented in the figure by a G: the gametes, ovules and sperm, which pass the genetic information to the next generation.

Epigenesis and epigenetics

Fertilization of the egg by the sperm

In the previous post we saw that at the beginning of the 19th century the theory of epigenesis seemed to have won the game. However, after 1850, and for just over a century, a cascade of new discoveries tipped again the balance towards the theory of preformation. Let us see what they were:

1. The existence of a nucleus within animal and plant cells.
2. The confirmation that the nuclei of the male and female gametes are fused during fertilization. This put an end to spermism and ovism and made it clear that the new being begins in the zygote.
3. The confirmation that certain structures (the chromosomes) appear during cell reproduction inside the nucleus of the cell, which seem to play a very important role. It was also found that in the chromosomal endowment of the zygote, half of the chromosomes come from the father and the other half from the mother.
4. A new science (Genetics) was originated in the experiments of Thomas Hunt Morgan, who showed that the chromosomes are linked to Mendelian inheritance.
5. The works of Oswald Avery, who showed that DNA, a nucleic acid that appears in chromosomes, is the basis of Mendelian inheritance.
6. The discovery of the structure of DNA (a double helix), made in the early 1950’s by Francis Crick, James Watson and Rosalind Franklin.
7. The deciphering of the genetic code, which took place during the 1950s and the 1960s.

Preformation and epigenesis

The history of the evolution of scientific theories about embryonic life (the origin of living beings) is very curious. I will try to summarize it in a couple of posts.

Throughout the history of science, the problem of the embryonic development of multicellular organisms has been one of the most arcane mysteries that man has tried to solve. Faced with this problem, two conflicting solutions have essentially been proposed:

•         Preformation: During embryonic development there is just a growth of the pre-formed embryo, whose constitution and composition are completely ready-made from its origin. Everything is done from the beginning, whether in the egg, the sperm or the zygote.

•         Epigenesis: Embryonic development is a process. Not everything is decided from the beginning; everything happens because something - a vital force, or an external or internal action - forces the embryo to develop in one way, rather than another.