14 de out. de 2022

Lamarck's and Darwin's theories in text books

Evolution of Species and Natural Selection

The principle of evolution postulates that the species that inhabited and inhabit our planet were not created independently, but descended from each other, that is, they are linked by evolutionary ties. This transformation, called the evolution of species, was presented and explained satisfactorily by Charles Darwin, in his treatise On the Origin of Species, in 1859.

The history of the evolution of life is documented through the record of fossils found by archaeologists and evolutionists.

The pressure generated by the environment on living beings represents one of the main causes of evolution. Natural environments often present negative and limiting factors, in addition to difficult realities such as predation and competition. Hostile and unstable environments drive the evolutionary process, as they strongly select only the survival of the fittest.

As a result of environmental pressure and the existence of genetic mutations, life has evolved and specialized, creating this whole range of different biomes and ecosystems that make up the biosphere. The group of animals alone currently has more than 1 million described species. At the top of the evolutionary line are the most complex and elaborate animals, the vertebrates, representing only 5% of the total.

Comparative between Evolutionary Theories

Several evolutionary theories emerged, highlighting, among them, the theories of Lamarck and Darwin. Currently, the Synthetic Theory of Evolution, also called Neo-Darwinism, has been formulated, which incorporates the modern concepts of genetics to Darwin's essential ideas about natural selection.

Lamarck's theory

Jean-Baptiste Lamarck (1744-1829), French naturalist, was the first scientist to propose a systematic theory of evolution. His theory was published in 1809, in a book called Zoological Philosophy.

According to Lamarck, the evolutionary principle would be based on two fundamental laws:

Law of use or disuse: the use of certain parts of the body of the organism causes them to develop, and disuse causes them to atrophy.

Law of transmission of acquired characters: changes caused in certain characteristics of the organism, through use and disuse, are transmitted to descendants.

Lamarck used several examples to explain his theory. According to him, water birds became waders due to the effort they made to stretch their legs to avoid wetting their feathers while moving in the water. With each generation, this effort produced birds with taller legs, which passed this trait on to the next generation. After several generations, the current wading birds would have originated.

Lamarck's theory is currently not accepted because his ideas have a basic error: acquired characteristics are not hereditary.

It was found that alterations in individuals' somatic cells do not alter the genetic information contained in germ cells, thus not being hereditary.

Darwin's theory

Charles Darwin (1809-1882), English naturalist, developed an evolutionary theory that is the basis of modern synthetic theory: the theory of natural selection. According to Darwin, organisms that are better adapted to the environment have a greater chance of survival than those that are less adapted, leaving a greater number of descendants. The best adapted organisms are therefore selected for that environment.

The basic tenets of Darwin's ideas can be summarized as follows:

Individuals of the same species show variations in all characters, and therefore are not identical to each other.

Every organism has a great ability to reproduce, producing many offspring. However, only a few of the descendants reach adulthood.

The number of individuals of a species is kept more or less constant over generations.

Thus, there is a great "struggle" for life among the descendants, because despite the birth of many individuals, few reach maturity, which keeps the number of individuals in the species constant.

In the "struggle" for life, organisms with favorable variations to the conditions of the environment where they live are more likely to survive, when compared to organisms with less favorable variations.

Organisms with these advantageous variations are more likely to leave offspring. As there is transmission of characters from parents to children, they present these advantageous variations.

Thus, over the generations, the action of natural selection on individuals maintains or improves their degree of adaptation to the environment.

The synthetic theory of evolution

The Synthetic Theory of Evolution or Neo-Darwinism was formulated by several researchers during years of studies, taking as essence Darwin's notions of natural selection and incorporating current notions of genetics. The most important individual contribution of Genetics, extracted from the works of Mendel, replaced the old concept of inheritance through the mixing of blood with the concept of inheritance through particles: genes.

The synthetic theory considers, as Darwin had already done, the population as an evolutionary unit. The population can be defined as a group of individuals of the same species that occur in the same geographic area, in the same time interval.

To better understand this definition, it is important to know the biological concept of species: grouping of natural populations, actually or potentially interbreeding and reproductively isolated from other groups of organisms.

When, in this definition, it is said to be potentially interbreeding, it means that a species can have populations that do not naturally interbreed because they are geographically separated. However, artificially placed in contact, there will be crosses between individuals, with fertile offspring. Therefore, they are potentially intercrossing.

The biological definition of species is only valid for organisms with sexual reproduction, since, in the case of organisms with sexual reproduction, since, in the case of organisms with asexual reproduction, the similarities between morphological characteristics are what define the groupings in species.

Observing the different populations of individuals with sexual reproduction, it can be seen that no one individual is the same. Exceptions to this rule could be identical twins, but even they are not absolutely identical, despite the initial genetic heritage being the same. This is because somatic changes can occur due to the action of the environment.

The enormous diversity of phenotypes in a population is an indicator of the genetic variability of that population, and it can be noted that this is usually very wide.

Understanding the genetic and phenotypic variability of individuals in a population is fundamental for the study of evolutionary phenomena, since evolution is, in reality, the statistical transformation of populations over time, or even changes in the frequency of genes in that population. population. The factors that determine changes in gene frequency are called evolutionary factors. Each population has a gene pool, which, subject to evolutionary factors, can be altered. The gene pool of a population is the set of all genes present in that population. Thus, the greater the genetic variability.

The evolutionary factors that act on the gene pool of the population can be grouped into two categories.

Factors that tend to increase the genetic variability of the population: gene mutation, chromosomal mutation, recombination;

Factors that act on the already established genetic variability: natural selection, migration and genetic oscillation.

The integration of these factors associated with geographic isolation can lead, over time, to the development of reproductive isolation mechanisms, when, then, new species emerge. In the following chapters, these topics will be covered in more detail.

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