Understanding Theories of Evolution and Population Genetics
Current Theories of Evolution
The Neo-Darwinism Theory or Synthetic Theory
This theory states:
- The evolutionary process is based on Darwinist principles of the variability of the offspring and natural selection.
- De Vries found that mutations in the offspring produced variability due to mutation and genetic recombination. Mutations create new genes, while recombination only creates new combinations of genes.
- Hardy and Weinberg showed that populations are evolving, not individuals. Populations persist and change as they vary their gene frequencies, while individuals die with the same genotype they were born with.
- Fisher, Haldane, and Wright proved that the factors that change gene frequencies in a population are natural selection, migration, mutation, and genetic drift.
- For two populations to evolve differently and give rise to two species, they must be kept isolated from one another.
The Neutral Theory of Molecular Evolution
Kimura proposed the neutral theory of molecular evolution. From 1980, when scientists began to deduce the nucleotide sequence of nucleic acids, it was observed that individual differences in DNA are significant.
According to the neutral theory of molecular evolution, most mutations are neither favorable nor unfavorable; that is, mutations are neutral, so natural selection does not affect them. The major part of the molecular evolutionary changes would not be adaptive.
The Theory of Punctuated Equilibrium
This theory contrasts with the neo-Darwinian model of phyletic gradualism. The characteristics of phyletic gradualism are:
- All species form a single, consecutive evolutionary line from the ancestral species.
- The transformation is slow and continuous.
- The transformation occurs in the whole population.
The theory of punctuated equilibrium hypothesizes that a small ancestral population of a species was isolated from the rest in a small area and therefore evolved differently until a new species emerged.
Introduction to Population Genetics
The neo-Darwinism or synthetic theory explains the mechanisms of evolution, which are:
- The population’s gene pool with corresponding genotypic and gene frequencies.
- The factors that can alter gene frequencies: natural selection, mutation, migration, and genetic drift.
- The geographical isolation of subpopulations.
Population Genetics and Gene Frequencies
A population is the set of individuals of the same species inhabiting the same place and therefore can interbreed. They share a set of genes called a common genetic background.
Genotypic frequencies are the proportions of each genotype. In a population of diploid individuals, each locus is occupied by two alleles, the same or different.
Gene frequencies are the proportions for each allele of a character. They can be calculated from the genotypic frequencies.
Hardy-Weinberg Law
According to Hardy and Weinberg (1908), in a sexually reproducing population where all individuals cross randomly (called a panmictic population), and where there is no mutation, migration, genetic drift, or selection, the gene and genotypic frequencies will remain constant from generation to generation. Therefore, the population is not evolving, and the gene frequencies are not changing.
Mutations
Mutations are random changes in the genetic information. They are preadaptive, meaning they do not exist to be beneficial but occur in both directions. Natural selection favors beneficial mutations, allowing carriers to survive. Mutations that tend to occur repeatedly are called recurrent mutations and are important in evolution, while others have no evolutionary significance.
Migration
Migrations are the arrivals of individuals from other populations (immigrants) or the departure of individuals (emigration). The gene pool in a population can vary due to the contribution or subtraction of alleles through migration. This variation is called gene flow.
Genetic Drift
Genetic drift is the change in gene frequencies that occurs when the number of breeding individuals forming subsequent generations is lower than what is necessary for genetic frequencies to be well-represented.
Founder effect: The fewer individuals there are, the more influence their characteristics will have, and therefore their progeny will be more similar to the founding population.
Bottleneck effect: When events like droughts, plagues, or fires periodically occur, there are drastic reductions in the number of individuals, leading to significant genetic drift.
Natural Selection
Natural selection is the elimination of individuals less fit, with lower biological efficiency. This causes adaptations.
The observed gene frequencies are the result of the balance between selection and mutation. There are three types of selection:
- Directional selection: An external genotype is favored, such as increasing the average size of a species.
- Stabilizing selection: An intermediate phenotype is favored, such as maintaining the average size.
- Disruptive selection: Both extreme phenotypes are favored. For example, small and large individuals may escape predation, leading to two subpopulations.