Ernst Mayr (1904–2005) was one of the 20th century’s most influential evolutionary biologists. Originally a renowned ornithologist (bird expert), Mayr was a key architect of the Modern Evolutionary Synthesis (often called the Neo-Darwinian Synthesis) in the 1930s and 1940s. This synthesis unified Darwin's theory of natural selection (Article 42) with Mendel's genetics (Article 84). Mayr's most critical contribution was defining the Biological Species Concept and clarifying the role of geographical isolation in the formation of new species.

By the early 20th century, the fields of Mendelian genetics (studying discrete inheritance) and Darwinian natural selection (studying gradual change) were largely separate and sometimes seen as contradictory. The Synthesis was the movement that reconciled these two fields, demonstrating that evolution is a two-step process:

1. Genetic Variation: Mutations and genetic recombination create random variation within a population (Mendel's laws).

2. Selection: Natural selection acts on this variation, favoring traits that improve survival and reproduction (Darwin's theory).

Mayr, along with figures like Theodosius Dobzhansky and George Gaylord Simpson, integrated population genetics, taxonomy, and paleontology into a cohesive, universal theory of evolution.

Before Mayr, species were often defined simply by their physical appearance (the morphological species concept). Mayr argued that this definition was flawed because individuals within a species can look very different (polymorphism), and individuals from different species can look identical.

Mayr proposed the Biological Species Concept (BSC), which defines a species based on its potential to interbreed:

"Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

• Reproductive Isolation: The core criterion is the ability to produce fertile offspring. If two populations cannot produce viable offspring together, they are separate species, even if they look similar.

• Gene Flow: The BSC emphasizes that a species is a real entity defined by gene flow—the exchange of genetic material—which keeps the population unified. Reproductive barriers prevent gene flow between species.

Mayr championed the idea that the most common mechanism for the formation of a new species (speciation) is allopatric speciation (meaning "other country").

• Geographic Isolation: The process begins when a population is split into two geographically isolated groups (e.g., by a mountain range, a river, or an ocean).

• Divergence: Because the two isolated populations face different environmental pressures and experience different mutations, they evolve independently.

• Reproductive Barriers: Over time, the accumulated genetic differences lead to the development of reproductive isolation (barriers like behavioral differences or hybrid infertility). Even if the geographical barrier is later removed, the two populations can no longer interbreed, and they are officially two separate species.

This model is a cornerstone of evolutionary biology, explaining the often rapid branching of the evolutionary tree.

In Conclusion: Ernst Mayr was a central architect of the Modern Evolutionary Synthesis, successfully merging Mendelian genetics with Darwinian selection. His most lasting conceptual contribution is the Biological Species Concept, which defined species not by appearance but by reproductive isolation. Furthermore, his work on allopatric speciation established geographic separation as the primary driver for the origin of new species, providing a clear mechanism for the branching pattern of life on Earth.

Ernst Mayr (1904–2005) was one of the 20th century’s most influential evolutionary biologists. Originally a renowned ornithologist (bird expert), Mayr was a key architect of the Modern Evolutionary Synthesis (often called the Neo-Darwinian Synthesis) in the 1930s and 1940s. This synthesis unified Darwin's theory of natural selection (Article 42) with Mendel's genetics (Article 84). Mayr's most critical contribution was defining the Biological Species Concept and clarifying the role of geographical isolation in the formation of new species.

By the early 20th century, the fields of Mendelian genetics (studying discrete inheritance) and Darwinian natural selection (studying gradual change) were largely separate and sometimes seen as contradictory. The Synthesis was the movement that reconciled these two fields, demonstrating that evolution is a two-step process:

1. Genetic Variation: Mutations and genetic recombination create random variation within a population (Mendel's laws).

2. Selection: Natural selection acts on this variation, favoring traits that improve survival and reproduction (Darwin's theory).

Mayr, along with figures like Theodosius Dobzhansky and George Gaylord Simpson, integrated population genetics, taxonomy, and paleontology into a cohesive, universal theory of evolution.

Before Mayr, species were often defined simply by their physical appearance (the morphological species concept). Mayr argued that this definition was flawed because individuals within a species can look very different (polymorphism), and individuals from different species can look identical.

Mayr proposed the Biological Species Concept (BSC), which defines a species based on its potential to interbreed:

"Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

• Reproductive Isolation: The core criterion is the ability to produce fertile offspring. If two populations cannot produce viable offspring together, they are separate species, even if they look similar.

• Gene Flow: The BSC emphasizes that a species is a real entity defined by gene flow—the exchange of genetic material—which keeps the population unified. Reproductive barriers prevent gene flow between species.

Mayr championed the idea that the most common mechanism for the formation of a new species (speciation) is allopatric speciation (meaning "other country").

• Geographic Isolation: The process begins when a population is split into two geographically isolated groups (e.g., by a mountain range, a river, or an ocean).

• Divergence: Because the two isolated populations face different environmental pressures and experience different mutations, they evolve independently.

• Reproductive Barriers: Over time, the accumulated genetic differences lead to the development of reproductive isolation (barriers like behavioral differences or hybrid infertility). Even if the geographical barrier is later removed, the two populations can no longer interbreed, and they are officially two separate species.

This model is a cornerstone of evolutionary biology, explaining the often rapid branching of the evolutionary tree.

In Conclusion: Ernst Mayr was a central architect of the Modern Evolutionary Synthesis, successfully merging Mendelian genetics with Darwinian selection. His most lasting conceptual contribution is the Biological Species Concept, which defined species not by appearance but by reproductive isolation. Furthermore, his work on allopatric speciation established geographic separation as the primary driver for the origin of new species, providing a clear mechanism for the branching pattern of life on Earth.