The Concept of Evolutionary Synthesis and Its Major Factors

The Evolutionary Synthesis

Introduction:

Evolution refers to the continuous and gradual process of change that occurs over a period of time. During evolution, characteristics of living organisms undergo changes and lead to the formation of new species. Scientists have predicted that the Earth is around 4.6 billion years old.

Around 3 billion years ago, the first life on earth appeared. It was a unicellular tiny prokaryote. Around 2 billion years ago, these tiny prokaryotic cells evolved into larger cells that contain a nucleus. Later on, smaller prokaryotic cells resided in larger cells and got transformed into organelles over the course of evolution (for example, mitochondria).

Around one billion years ago, multicellular organisms evolved. It is also anticipated that various animals and plants have been evolving for the past 500 million years.

Darwinism:

Darwinism is the most widely accepted theory of biological evolution. It was proposed by Charles Darwin (1809 – 1882), the British naturalist.  He coined the phrase “descent with modification”.  Darwinism states that every species has descended from pre-existing species by reproduction. It also states that species undergo change over time. 

The important concepts of Darwinism include overproduction, variation, natural selection, and adaptation.

Mendelism:

The passing of characteristics from one generation to the next generation is referred to as heredity. Genetics refers to the study of genes and heredity. Gregor Johann Mendel (1822-1884), an Austrian biologist is known as the “father of modern genetics”. He carried out many experimental studies on plant hybridization.

His experimental studies on pea plants (Pisum sativum) are very popular among biologists. His experiments provided a way for a clear understanding of the fundamental principles of inheritance. Mendel discovered three important laws of inheritance. Even before the discovery of genes and nucleic acids, Mendel described the transmission of genetic traits. His contributions laid the foundation of modern genetic studies.

Evolutionary Synthesis:

The term “evolutionary synthesis” was coined by Ernst Mayr and William B during 1930-1950.  Evolutionary synthesis represents the combined theory of evolution that includes the principles of natural selection, genetics, and biogeography.

In other words, the modern evolutionary synthesis is a unified theory that combines the concepts of Darwinism and Mendelism. This theory is also known as Neo- The Darwinian theory.  This theory explains the genetic basis of evolution. While other theories of evolution explain the origin of life, the theory of evolutionary synthesis aims to understand the pattern of life.

According to Darwin, evolution refers to the continuous and gradual process of change that occurs over a period of time. During evolution characteristics of living organisms undergo changes and leads to the formation of new species. The theory of evolutionary synthesis states that “evolution occurs as a result of changes in allele frequencies of a population”. Variations of allele frequency form the genetic basis of evolution.

Major Factors of Evolutionary Synthesis:

The theory of evolutionary synthesis states that genetic changes that take place in a population lead to the development of new species. Major factors that contribute to the changes in allele frequency are as follows.

1. Genetic recombination
2. Mutation

• Heredity

1. Natural selection
2. Isolation

Genetic Recombination:

Genetic recombination refers to the process of the formation of new allele combinations. Recombination occurs during gamete formation of sexual reproduction. During meiosis exchange of genetic material between non-sister chromatids (crossing over) leads to recombination. Recombination involves the production of new genotypes from existing genes.

Genetic recombination is an important cause of genetic variability in a population. Natural selection involves choosing among the existing population. Whereas genetic recombination involves making new genetic variations.

Mutation:

Mutations are very important factors in evolution. They are the major cause of diversity among organisms. Mutation refers to sudden changes that occur in genes. These changes are inheritable, and they could affect the phenotype of an organism. Mutations could be broadly classified into two types namely somatic and germline mutations.

Mutations that take place in body cells are known as somatic mutations. These mutations are not inherited by the next generation. Mutations that occur in gametes are known as germline mutations. These mutations are inheritable. Mutations may have a positive or harmful effect on the offspring.

Positive effects of mutation on the phenotype of an organism are referred to as adaptations. The harmful effects of mutations are referred to as genetic disorders.

Heredity:

DNA is the genetic material that carries hereditary characteristics. DNA transfers these characteristics from one generation to the next generation. During reproduction hereditary traits get inherited from parents to their offspring through genes (DNA).

This phenomenon is known as the “theory of inheritance”. Eye color, hair color, skin tone, blood group, etc. are some examples of inherited characteristics in humans. Mendelism is associated with inheritable traits in plants.

Natural Selection:

Natural selection refers to the process that causes populations to change over time. Darwin explained the mechanism of evolution through natural selection. According to Darwin, the evolution of living organisms takes place in such a way that over a period of time populations adapt themselves to their living environment.

He also states that natural selection is totally dependent on the specific environment. Living organisms with variations that are favorable for their survival in a particular environment continue to live. Whereas other organisms which do not have variations do not survive. This process of selecting living organisms with favorable variations is known as natural selection.

Through the process of natural selection, only those organisms with favorable variations continue to live in a particular environment. These favorable variations are transferred from one generation to the next generation. This procedure leads to the formation of new species.

Isolation:

Isolation is an important factor of evolutionary synthesis. Isolation plays an important role to prevent the interbreeding of organisms. Whereas hybridization leads to an increase in the genetic variability of the population. Different types of isolation are as follows.

1. Geographical isolation
2. Reproductive isolation

• Temporal or habitat isolation

1. Behavioral isolation
2. Mechanical isolation

Geographical Isolation:

Geographical isolation involves the separation of a population by a geographical barrier like sea, river, mountain, desert, and land. This prohibits interbreeding in an isolated population. The geographically isolated population is referred to as an allopatric population. These organisms evolve freely and are genetically different.

Example: Different species of finches identified in the Galapagos islands (archipelago). These islands are separated by the Pacific Ocean. Hence, the finch species of each island are distinct.

Reproductive Isolation:

The inability of related species to breed is referred to as reproductive isolation. When organisms are reproductively isolated from each other it is referred to as reproductive isolation. It prevents breeding among species that belong to the same group. Reproductive isolation helps in reducing gene flow between related species thereby maintaining the integrity of a species.

Example: Closely related species of fish living in a pond do not interbreed.

Temporal Isolation:

Temporal isolation is a type of reproductive barrier. It prevents interbreeding among species. Several factors such as physical hurdles, the difference in habitats, the difference in time of sexual maturity, etc. Temporal isolation prevents interbreeding among physically similar species that live in the same habitat.

Example: Two closely related species of flowering plants may be located in the same geographic location. Due to the difference in the time of sexual maturity, interbreeding between these species may be prevented.

Behavioral Isolation:

Behavioral isolation occurs among two populations that are capable of interbreeding. It occurs due to differences in mating rituals or other behaviors of organisms. The presence or absence of certain behavior prevents interbreeding among species.

Example: Male fireflies make use of specific light patterns to attract female fireflies.

Mechanical Isolation:

When interbreeding between species is prevented due to the physical incompatibility of their reproductive organs it is known as mechanical isolation. Mechanical isolation is characterized by any kind of physical barrier that prevents breeding. Mechanical isolation results in the co-evolution of two different species.

Example: Mechanical isolation between white sage and black sage. These two species do not interbreed as they depend on different pollinators.