Evolution and the Origin of Life: From Darwin to Modern Theories
The Origin of Variability
Darwin could not explain how such variability arises on which natural selection acts, and how such variability is maintained generation after generation. Many of the differences between individuals are due to genetic variations, and these are generated by two processes: mutation and sexual reproduction.
Mutations as a Source of Variability
Mutations are changes that occur randomly in genes. Those that affect the gametes are transmitted to offspring. Types:
- Harmful mutations: Confer a survival disadvantage; they tend to be eliminated by natural selection because individuals who carry them are less likely to survive.
- Favorable mutations: Provide some advantage, as they improve the ability to survive.
- Neutral mutations: Not beneficial or harmful; natural selection neither favors nor removes them until there is a change in the environment.
Mutations generate heritable variation on which natural selection acts.
Sexual Reproduction as a Source of Variability
Sexual reproduction creates variability due to genetic recombination, which occurs during meiosis and the random union of gametes during fertilization. The mixture of parental genes generates new gene combinations in individuals that make them unique. Some of these features allow the offspring to survive adverse conditions.
Evolution by Natural Selection
The theory of evolution by Darwin and Wallace proposed that natural selection is the mechanism by which species change over time. It can be summarized in the following points:
- There is a struggle between organisms for survival; environmental resources are limited. If more individuals are born than can survive, there is a fight for survival among them.
- Variability exists among individuals of a population. Within the group of individuals of the same species that form a population, there may be some differences among them.
- The environment selects the best-adapted organisms. Individuals who exhibit a change beneficial to a particular environment will have a greater chance of survival.
Natural selection acts on variations that occur in individuals. Individuals with favorable variations will survive longer, reproduce more, and transmit the changes to their offspring. Individuals with unfavorable variations will be less likely to survive. Slowly and steadily, the species change.
Neo-Darwinism or Synthetic Theory of Evolution
By the year 1930, new knowledge in the field of genetics led a group of scientists to formulate a new theory of evolution. This theory proposed mutations, genetic recombination, and natural selection as the main engines of evolutionary change. This theory is called neo-Darwinism or synthetic theory and unifies different areas of biology, including genetics, paleontology, biochemistry, and ecology. The main features of neo-Darwinism are:
- Rejects Lamarckism.
- Genetic variability is due to two processes: mutation and recombination.
- Natural selection acts on genetic variability.
- Natural selection leads to changes in the set of alleles in a population.
- The population evolves, not individuals.
- Evolution takes place gradually.
Comparison Between Gradualism and Punctuated Equilibrium
Gradualism: Species form a single evolutionary lineage from the ancestral species. The transformation is slow, gradual, and continuous as a result of small changes over long periods. The transformation to a new species does not occur in isolated individuals but in the entire population.
Punctuated equilibrium: Species do not follow a single evolutionary lineage from the ancestral species, but several. Speciation occurs in fits, and periods of stasis alternate with periods of speciation. The transformation to the new species is produced from a small population that is isolated.
The Main Hypotheses on the Origin of Life
There are several hypotheses that attempt to explain the origin of life, including:
- Hypothesis of panspermia: Life originated in space and traveled in spore form from one planetary system to another.
- Prebiotic synthesis hypothesis: Life comes from organic molecules on Earth that arose from inorganic matter.
Prebiotic Synthesis
In 1923, Oparin and Haldane proposed that at some moment, a number of organic molecules may have formed from gases present in the primitive atmosphere. This was based on the idea that some 4500 million years ago, the planet was surrounded by an atmosphere without oxygen (reducing), consisting of methane, ammonia, hydrogen, and water vapor. When the temperature dropped, water vapor condensed, forming clouds leading to the rain that formed the oceans. Energy from the sun and lightning caused the inorganic compounds from the atmosphere to react, causing organic compounds that precipitated on the surface and were swept away by the rain to the oceans.
In 1953, Miller conducted a scientific experiment to test the hypothesis of Oparin and Haldane. He designed an experiment with a few areas that reproduced the alleged circumstances of the early Earth. In the first area, he combined water, methane, ammonia, and hydrogen. He heated the water, and the steam dragged the gases to the second area, where they were submitted to electric shocks. When they cooled, they precipitated.
Biological Evolution: The Origin of Biodiversity
Biological evolution is the transformation of species over time. There are two kinds of theories about the origin of species:
- Fixism: Species have remained unchanged since their inception.
- Evolutionism: Species can change and generate other species. Different evolutionary theories include Lamarckism, Darwinism, the synthetic theory, and punctuated equilibrium theory.
Fixism
Fixism is a theory stating that species have remained unchanged since their inception. This theory had its foundations in the interpretation of Genesis and other holy books. Creationism is accepted, explaining the origin of species as creations of God that remained unchanged through time. According to the Bible, it is estimated that the Earth has an age of 6000 years. The naturalist Carl Linnaeus, to whom we owe the current binomial classification of species, was a strong supporter of fixism. The discovery of fossils was a setback for those defending fixism because they were remains of living beings from a bygone era that no longer existed. This led to catastrophism, described by the naturalist Georges Cuvier, who explained that in the past, there had been living things different from today, which had remained unchanged and became extinct due to some natural disaster.
Lamarckism
Lamarckism was the first theory of evolution, proposed by the naturalist Jean-Baptiste de Lamarck. He thought that species transform into others continuously over time. It was based on:
- Organisms show a tendency toward complexity, evolving from simple forms to complex forms.
- The repeated use of an organ produces its development. The function creates the organ, and its disuse leads to degeneration. Thus, the original characters are being replaced by a series of acquired characters.
- Acquired characteristics are heritable. The changes induced by the environment that an organism acquires can be transmitted to offspring. This theory is also known as the theory of acquired characteristics.