↣ Theories of evolution

 Several theories have been proposed to explain evolution.

4.1 Lamarckisim

The French naturalist Jean-Baptiste de Monet, Chevalier de Lamarck (1744- 1829) was the first person to propose a logical theory of evolution. He had to confront supporters of fixism, such as Cuvier.

 As Cuvier did, Lamarck observed that some fossils appeared to be an intermediate stage between primitive organisms and present-day animals. This fact allowed him to conclude that some animals preceded other animals. 

Lamarck's theory, which is known as the theory of inheritance of acquired traits, explains that animals develop the structures and organs they need in order to adapt to the environment they live in ('the necessity creates the organ'). These new characteristics are transmitted to offspring, so species change over time.

º Although this theory is not accepted today, it still has some good ideas: 

↦ It acknowledges an evolutionary process and proposes a mechanism in order to explain it. 

↦ It recognises the relationship between anatomical structures and their purpose. 

↦ It explains that organisms adapt to the environment they live in. 

º However, Lamarck made some mistakes: 

↦ He claimed that acquired characteristics are transmitted from parents to offspring. 

↦ He claimed that animals have an internal need to improve themselves and become more complex.

4.2 Darwinism

This theory was developed by Charles Darwin and Alfred Russel Wallace, Both Darwin and Wallace were British naturalists during the second half of the 19th centun Their theory forms the base of our current understanding of biological evolution.

Darwinism consists of five basic principles: 

1. The individuals that make up a population of living things have a wide variety of anatomical, physiological and behavioural differences.

2. There are more individuals in each species than the number that reach reproductivo age. The number of individuals in each species remains more or less constant, so it can be deduced that many die. 

3. There is competition between the individuals of a species for space, food and procreation. This is known as the struggle for existence. It explains why many individuals are born but do not survive. 

4. Individuals whose characteristics help their survival in a certain environment are favoured over those who are badly adapted. This is referred to as natural selection. 

5. When the surviving individuals reproduce, the more favourable characteristics are transmitted to their offspring.

 As a result, survival of the fittest takes place: individuals that have favourable characteristics survive and these progressively become more common in the population; the ones that do not have favourable characteristics are affected and the characteristics gradually disappear in the population. 

After many generations, and if many important changes have occurred, new species may appear.

4.3. Neo-Darwinism 

Darwin was unable to explain why there was some initial variation between individuals of the same species. However, as scientific knowledge advanced, especially that of genetics, Darwin's theory was revised, improved and completed.

The discovery of Mendel's laws improved our understanding of the inheritance of characteristics and their mechanisms of transmission. The reasons behind the variation within a species were also discovered: 

↦ Sexual reproduction creates new combinations of genes. 

↦ Genetic recombination during meiosis.

↦ Mutations that lead to rapid changes in genes.

Neo-Darwinism considers that natural selection does not affect isolated individuals. Instead, it affects the population the individual belongs to. The interaction of these individuals varies and the population is then gradually modified. When many changes are accumulated, the population evolves and a new species is formed.

4.4 Neutralism

The Japanese biologist Motoo Kimura created this theory. He explained that most mutations create genes that are neither beneficial nor harmful for the individuals that have them, so natural selection does not affect them. If those individuals continue to produce offspring and become isolated from the rest of the population they belong to, a new species may originate. ACcording to this theory, the rhythm of evolution is more constant than previously thought.

4.5 Theory of punctuated equlibrium

The US palaeontologists Niles Eldredge and Stephen Jay Gould proposed a different theory. According to this theory, the evolutionary process does not always take place in a slow gradual way. Instead, in many cases, new species are formed rapidly. 

This theory is based on paleontological studies that have observed the sudden appearance of groups of fossils that seem to be unrelated to previous organisms.

Eldredge and Gould explained that some macro mutations can affect specific genes responsible for regulating other genes. They also explained that in order for a new species to appear so suddenly, a favourable environment is needed. This could happen after a change in climate, the extinction of another species or any geological disaster. 

This theory can explain the appearance of trilobites as well as the main groups of living things we have today. 

4.6. Endosymbiosis theory 

The US biologist Lynn Margulis explained that symbiotic relationships between organisms have been an essential mechanism in evolution. 

Her endosymbiosis theory proposes that eukaryotic cells originated as a result of two different types of bacteria joining together. 

Primitive eukaryotic cells acquired a capacity to absorb and digest some cells using the process of phagocytosis, like the ones that evolved into mitochondria and chloroplasts. 

4.7. Evolutionary developmental biology 

This theory is based on the discovery of a group of genes known as Hox genes (or gene toolkit'). They act as switches for other gene groups and organise the different areas of the body during embryonic development. This means that mutations in small groups of these genes can be responsible for big physical changes that can lead to a new species. 

For example, there is a group of genes that controls the number, length and separation of parts of the skeleton for fish's fins and tetrapods' limbs. Mutations in some of their genes can duplicate or reduce the number of thought that the different types of limbs vertebrates have were formed this way. ements, as well as their length. It is thought that the different types of limbs vertebrates have were formed this way.