Evolution through natural selection


Biologists’ understanding of evolution since the 1940s is called the modern (or neo-Darwinian) synthesis, the synthesis being between evolution through natural selection and the science of genetics.1)Or between morphology and genetics.

Biologists like to study what is called a population, the collection of those members of a particular species living in a specified area. Population genetics considers the gene pool of such a population, i.e., the sum of all alleles – versions of genes2)More specifically, gene variants at the same positions on homologous – corresponding – chomosomes. All this will be explained in a later chapter. – within the population. A given gene may have more than two alleles, as in the case of human blood (A, B and O). In this context, evolution describes the changes in distribution (frequency) of the alleles in the gene pool. The link with what we actually see comes about because alleles are expressed as phenotypes, i.e., observable traits in the organisms of the species. The underlying genetic composition of the phenotype is the genotype.

The theory of evolution says that all living organisms of one species have evolved from organisms of a different species, and those from still different species, all the way back to the original ur-form of life, the first cell or amino acid or whatever.3)More on this in the geology chapter. Evolution can give rise to two kinds of similar structures.

  1. Analogous structures are due to convergent evolution, when different species confronted to similar challenges respond by evolving similar solutions.
  2. Homologous structures in different species are similar because the two species evolved from a common ancestor,

Biologists recognize four mechanisms of evolution.4)Sometimes, they may mention non-random mating as a fifth mechanism.

  1. Natural selection, is due to random changes in individuals which improve those individuals’ ability to survive and reproduce successfully, thus passing the changes on to their offspring. The superior survival rate of these individuals will assure that over time, their relative numbers will increase. In terms of population genetics, the frequency of their alleles will increase in the gene pool, to the detriment of those of others.
  2. Mutation is due to a change in DNA, the genetic material. Mutations are small and slow. They may disappear quietly, or lead to the death of the organism (and the allele). Or they may accumulate very slowly. Actually, mutation is the ultimate generator of variation in alleles.
  3. Genetic drift is due simply to chance. A chance variation (due to mutation) may be reproduced and then gradually become important in the gene pool. Of course, this happens more in a smaller (therefore, less statistically significant) population. Genetic drift may originate with the migration of one sub-group of a population to a different environment, physically separated from the original one.
  4. Gene flow is similar to genetic drift, but involves the migration of individual orgtanisms or alleles from one population to another.

Speciation (formation of new species from an older one) may take place when groups of members of one species occupy different niches or habitats and each group evolves to fit into its niche, a phenomen called adaptive radiation.

In all these cases, change is a random, chance affair, whereas natural selection is a law which always operates. In spite of the terms often used to express evolutionary change (“Flowers have colors in order to attract bees.”), it is not teleological (goal-oriented). (“Colored flowers attract bees and so survive better.”)

Now, back to basic theories.

Notes   [ + ]

1. Or between morphology and genetics.
2. More specifically, gene variants at the same positions on homologous – corresponding – chomosomes. All this will be explained in a later chapter.
3. More on this in the geology chapter.
4. Sometimes, they may mention non-random mating as a fifth mechanism.

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