Question

Why is the definition of species as "successfully interbreeding organisms" not satisfactory for most microorganisms?

Short answer

The definition of species as 'successfully interbreeding organisms' is not satisfactory for most microorganisms because they primarily reproduce asexually, and thus do not 'interbreed'. Furthermore, microorganisms often participate in horizontal gene transfer which can occur between organisms of different 'species' as per the interbreeding definition.

Step-by-step solution

Understanding species definition

Firstly, it's important to understand the given definition of species - 'successfully interbreeding organisms'. This generally means that two organisms belong to the same species if they can mate and produce fertile offspring.

Consider the case of microorganisms

Microorganisms primarily reproduce asexually, meaning they do not engage in sexual reproduction (which involves the combining of genetic material from two parents to produce offspring). Instead, they replicate their own DNA and then divide, making a copy of themselves. Thus, the concept of 'interbreeding' does not really apply to them in the same way.

Horizontal Gene Transfer

Additionally, many microorganisms participate in what is known as 'horizontal gene transfer' - the transfer of genetic material from one organism to another that is not its offspring. This can occur between microorganisms that would not be considered the same species under the interbreeding definition, further complicating the matter.

Key concepts

Asexual Reproduction in Microorganisms

Microorganisms, such as bacteria, archaea, and some fungi and protists, predominantly reproduce through asexual processes. This means only one parent is involved. The most common form is binary fission, where a single organism divides into two identical offspring. Unlike sexual reproduction, there is no combination of genetic material from two parents.
Other forms of asexual reproduction can include budding, where a new organism grows on the parent before detaching, and sporulation, where spores are produced. A key feature of asexual reproduction is the rapid multiplication rate. Microorganisms can quickly expand their populations when conditions are favorable. This method of reproduction does not involve meiosis, a type of cell division that halves the number of chromosomes, unlike in sexual reproduction. Relying exclusively on asexual reproduction means genetic diversity is primarily driven by mutations rather than parental genetic recombination.

Horizontal Gene Transfer

In addition to reproducing asexually, microorganisms possess a unique ability to exchange genetic material through a process known as horizontal gene transfer (HGT). This is a crucial factor for their adaptability and evolution.
HGT allows for the movement of genes between different branches of the microbial world, not just from parent to offspring as seen in vertical gene transfer. This can happen through several mechanisms:

• Transformation: Uptake of naked DNA from the environment.
• Conjugation: Transfer of DNA through direct contact between bacterial cells.
• Transduction: Transfer of DNA mediated by viruses.

Through HGT, microorganisms can quickly acquire new traits, such as antibiotic resistance or new metabolic capabilities, that aid in survival and adaptation. This gene exchange challenges traditional concepts of species boundaries since even distantly related microorganisms can share genetic material.

Species Concept Limitations for Microorganisms

The traditional species concept, where species are defined based on the ability to interbreed and produce fertile offspring, does not suit microorganisms well. This definition hinges on sexual reproduction, which is rare or absent in many microorganisms. Since they reproduce asexually, they do not fit neatly into this reproductive framework. Moreover, horizontal gene transfer obscures these definitions even further. When microorganisms acquire genetic material from unrelated organisms, the lines between different species blur. They can obtain traits from other species, which may lead to significant differences even among organisms typically considered the same species. These limitations indicate that microbial species classification requires alternative methods. Scientists often turn to genetic, physiological, or ecological characteristics to define species among microorganisms. By looking at DNA sequences or ecological roles, a more accurate picture of microorganism biodiversity and classification emerges.