2.2. Evolutionism
2.2.1. Lamarckism
In 1809, Jean–Baptiste Monet, Chevalier of Lamarck, published his work “Zoological philosophy” in which he exposed his theory about the evolution of living creatures.
Like Cuvier, Lamarck studied fossils. He observed there were fossils that had an intermediate appearance between older ones and present-day organisms. This made him think that one had developed from another.
Lamarck stated that “species evolve in a gradual and continuous way along their existence”. He included all living beings in his theory from the simplest to the more complex, even human being.
According to Lamarck, the evolution is based on:
- Tendency of organisms to complexity and perfection (an internal impulse given by God)
- Appearance of changes in the living beings (adaptations)
- “The function creates the organ” (the need provoked the apparition of new organs
or the development of others)
- “The organ that is not used atrophies” (This is the law of use and disuse:
when some organ is not necessary, becomes non-functional and finally disappears)
- Inheritance of acquired characteristics (the characteristics developed during the life
of the individual are conserved and transmitted to its offspring)
Nowadays, the Lamarck’s theory is not accepted, because:
- There are no proofs of the existence of this divine impulse to the perfection and complexity.
In fact, many times, changes in contrary direction are observed.
- The current knowledge about genetics denies the possibility that the traits acquired
along the life by the progenitor can be inherited by descendants.
Only the characteristics that are in genes can be transmitted to the offspring
However, the real value of Lamarckism is that it is the first theory opposite to the immutability of living beings (fixism) and proposes a scientific explanation to the changes they undergo along the time. Although it was surprising and creates a great controversy, the Lamarck’s ideas put the basis to run the scientific though of the epoch towards the problem of evolution.
The primitive giraffes
came from antelopes
with short necks
which lived in the savannah and ate the low leaves
of acacias.
During droughts, the food diminished and giraffes needed to stretch their necks to reach the higher leaves.
Due to the use and necessity, the organ was lengthening more and more.
The acquired characteristics (long necks and long legs) were transmitted to the offspring generation after generation.
2.2.2. Darwinism
In 1831, Charles Darwin joined to a scientific expedition around the world on board the Beagle.
During the following five years he visited South America,
Australia and Africa, studying the natural life and collecting specimens.
His observations let him to develop a new theory about the evolution of living beings.
He received a great influence from the works of:
- Charles Lyell (Principles of Geology, 1830-1833).
Lyell proposed a series of small, gradual and continuous changes to explain the Earth's history
- Thomas Robert Malthus (An essay on the principle of population, 1798).
Malthus showed the disproportion between the increment of the human population and the available food resources. Those people who have some advantage over the others will become victorious and will survive.
In the Galapagos Islands he found numerous proofs that were the key to elaborate his theory of the evolution. Studying the different species of finches he noticed that they had small variations in the shape of their beaks. The shape of the beak was related to the type of food they take. He deduced that the different environment of the different islands had provoked the selection of the birds with the most appropriate characteristic in each one. This meant that all the finch species were related and came from a common ancestor.
Darwin chose the term “natural selection” because the analogy of the activity that developed the cattle and plant breeders who selected the more suitable individuals as progenitors, in order to obtain the desired characteristics in the descendants (artificial selection).
In spite of the fact to be absolutely convinced that his theory was right, Darwin did not dare to publish it.
In 1858, the naturalist Alfred Russell Wallace sent him one of his works (About the tendency of varieties to apart from the original type) in which he exposed basically the same theory of the evolution.
This fact made that Darwin decided to make public his theory. Both scientists shared and exposed their discoveries together in the Linnaean Society of London.
Short later “The origin of the species” was published (1859). In this book Darwin sat the basis of his theory of the evolution by means of natural selection.
The foundations of the theory of Darwin-Wallace are:
- High reproductive capacity of living beings.
Living beings use to have many descendants, but only a part of them will survive and become adult. The cause that a species does not increment its number infinitely is that food resources are limited and only the fittest survive.
- Variability of the offspring.
The descendants of a couple with sexual reproduction are not identical among them. There are always slight differences that make them different.
- The natural selection
Among the members of a species a fight for survival is stablish (“struggle for existence”), especially if the resources are scarce. Only the best adapted get survive and reproduce (“survival of the fittest”) and transmit their characteristics to the offspring. The environment selects the individuals.
- Species change in a continuous and gradual form.
If the environmental conditions stay stable, the favourable variations will increase in each generation while the non-favourable ones will disappear.
In this theory the unit of evolution is the individual.
This theory provoked great polemic and it was not whole accepted due to Darwin could not explain the origin of the variability in the offspring.
Within a population of giraffes, individuals are different from each other (variability of the offspring) There are long-necked individuals and short-necked individuals.
During droughts, the long-necked giraffes would be able to reach the higher leaves. They will have more probabilities to survive and reproduce, while the short-necked giraffes will die (natural selection).
Generation after generation and in a gradual and continuous way, in the giraffes’ population there will be more individual with long neck and less individuals with shot neck each time.
Video: The making of a theory (HHMI)
2.2.3. Neo-darwinism or synthetic theory
During the first decades of the 20th century, important discoveries were made in the fields of genetics, biochemistry and ecology. The Mendel’s laws were rediscovered and it was proved that the hereditary factors (genes) were located in chromosomes. The explanation of the diversity within populations was found out: mutation (changes in genes) and recombination of genetic traits due to sexual reproduction.
All this made change the way in which evolution was understood. In 1937, Theodosius Dobzhansky published his book “Genetics and the origin of the species” in which he reinterpreted the Darwin’s ideas taking in account the new knowledge. His interpretation of the evolution is known as “synthetic theory”, because it fuses Darwinism and molecular genetics.
According to this theory, evolution occurs due to:
- Genetic variability:
Within a population there are a high number of different genotypes. This variety is produced at random by recombination and mutation.
Adaptation is the possession of any characteristic that improve the ability of an organism for use the environmental resources. The adaptation will increase its possibilities to survive and reproduce.
The best adapted genotypes will have higher biological efficacy, that is to say higher ability to produce offspring. It will be translated into a higher number of its descendants in the next generation and therefore into a higher propagation of its genes within the population in successive generations.
- Natural selection:
The worse genetic combinations of genes or disadvantageous mutations will be eliminated, while the fittest ones will be more abundant each time, due to the individuals which have them will survive more time and will reproduce more frequently. In this way the proportion of their genes will increase progressively in the population, making it evolve.
Therefore, natural selection provokes the variation in genes’ proportions within the populations and due to this fact, the population change. The unit of evolution is in this case, the population.
Changes are produced slowly and continuously and their effects are evident only long-term.
Among the ancestors of giraffes, animals of short necks and legs, mutations originated at random produced animals with longer necks and legs.
When the environmental conditions change (e.g. drought) the food becomes scarce. The mutation is then an advantage to the individual that have it. This individual will have more possibilities to survive and reproduce, leaving its genes to the next generation.
Natural selection favours the survival of individuals with adaptive advantages (long neck and legs).
In this way, with the pass of time,
the mutation will increase
its frequency in the population.
Each time there will be more giraffes of long neck and legs and fewer individuals with short neck and legs.
2.2.4. New evolutionary theories
a) Neutralism (Motoo Kimura, 1968)
Most part of mutations that occurs is neutral. They do not give advantages or disadvantages to the individual that have them. Natural selection does not act on them, neither favour nor disfavour them, therefore they accumulate in the population, provoking a gradual change of it with the pass of time. According to this theory is not the natural section, but the randomness, the responsible for the evolution.
b) Punctuated equilibrium (Stephen Jay Gould and Niels Eldredge, 1972)
According to this theory, species pass through two phases a long their existence:
- Stasis period:
This is a phase of stability which includes the most part of the existence of the species. During this time evolutionary changes do not occur or they are not important.
- Speciation period:
In short geological periods of time a large accumulation of evolutionary changes occur. These changes provoke the apparition of new species from the old one. These new species compete to each other and only some of them survive.
Eldredge and Gould maintain that the absence of transitional forms in the fossil register is not due to it is incomplete, but to the fact that some times the evolution goes away faster, in “leaps”. They do not deny the gradual character of the evolution, but defend that its rhythm is not uniform.
In this
case, the unit of evolution is the species.
c) The selfish gen (Richard Dawkins, 1976)
The unit of evolution is the gen. The bodies of the living beings act as machines that are used by genes to transmit themselves from a generation to another. The goal of genes is assure their own existence and perpetuate themselves.
According to this theory natural selection acts on determinate genes. Only survive those genes that get maintain in the living beings.
d) Endosymbiotic theory (Lynn Margulis, 1970)
According to this theory the symbiotic relationship between organisms is a key mechanism for the evolution. The origin of eukaryotic cells by endosymbiosis is widely accepted nowadays. This same mechanism could be the origin of new species by addition of genomes.
READING ACTIVITIES
After reading the text, copy and answer the following questions into your notebook:
2.1. Which is the difference between fixism and evolutionism?
2.2. How did Lamarck explain the evolution of living beings?
2.3. Which is the difference between “natural selection” and “artificial selection”?
2.4. What point of his theory can Darwin not explain? What was the consequence?
2.5. Why neo-darwinism is also known as “synthetic theory”?
2.6. What does it mean that adaptation is a consequence of natural selection?
2.7. Explain the sentence: “Natural selection acts on the phenotipes”
2.8. Which individual is the best adapted from the evolutionary point of view which survives longer or which leaves more descendants?
2.9. Give a reason to explain why this sentence is wrong: “Insects are getting used
to pesticides and passed some time they are not affected by them”.
2.10. Which evolutionary theory best explains the existence of “living fossils”,
those species that have barely changed since their inception?
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Eva Mª
López Rodríguez
Departamento
Biología y Geología
IES " J. S. Elcano"
Sanlúcar de Barrameda