15. Genes in two species that are derived from the same ancestral gene in their most recent common ancestor are called
(A) analogs
(B) heterologs
(C) orthologs
(D) paralogs
Genes Derived from the Same Ancestral Gene in Two Species: Orthologs Explained
Correct Answer: (C) Orthologs
Genes present in two different species that originated from the same ancestral gene in their most recent common ancestor are called orthologs or orthologous genes. Therefore, the correct answer is option (C) orthologs.
Orthologous genes arise when a single ancestral species separates into two or more descendant species through speciation. The ancestral gene is inherited by the newly formed species, and over evolutionary time, the corresponding gene copies may accumulate mutations and become somewhat different in their DNA sequences. Despite these differences, the genes can still be traced back to the same gene present in the most recent common ancestor.
Orthologs are extremely important in evolutionary biology, molecular genetics, bioinformatics, and comparative genomics because they frequently retain similar biological functions across species. For example, a gene involved in an essential cellular process in one species may have an ortholog with a closely related function in another species.
The central concept is straightforward: when homologous genes are separated by a speciation event, they are orthologs.
What Are Orthologs?
Orthologs are homologous genes found in different species that originated from the same ancestral gene through speciation. The term describes an evolutionary relationship between genes rather than simply a similarity in their DNA or protein sequences.
Suppose an ancestral species contains a gene called gene X. If that ancestral species divides into two descendant species, each species may inherit a copy of gene X. The gene in species 1 and the corresponding gene in species 2 are orthologs because their evolutionary separation occurred when the species themselves separated.
In simple evolutionary form:
Ancestral gene → Speciation → Orthologous genes in different species
The definition given in the question closely matches this concept. The words “two species,” “same ancestral gene,” and “most recent common ancestor” directly indicate an orthologous relationship.
How Are Orthologous Genes Formed?
The formation of orthologous genes is closely linked to the process of speciation. Before speciation, the ancestral population contains a particular gene. When the ancestral lineage divides into separate species, the descendants inherit copies of that ancestral gene.
After the species become evolutionarily independent, their genes also evolve independently. Mutations, natural selection, and genetic drift may gradually produce sequence differences between the genes. However, their evolutionary origin remains connected to the same ancestral gene.
For example, imagine an ancestral species containing gene A. A speciation event produces species X and species Y. Species X inherits gene A₁, while species Y inherits gene A₂. Gene A₁ and gene A₂ are orthologs because their separation corresponds to the separation of the two species.
Thus, orthology is fundamentally associated with speciation rather than gene duplication.
Why Is Option (C) Orthologs Correct?
Option (C) is correct.
The question asks specifically about genes in two species that are derived from the same ancestral gene in their most recent common ancestor. This is the precise definition of orthologous genes.
The evolutionary event responsible for producing orthologs is speciation. When an ancestral species divides into separate descendant species, corresponding copies of the ancestral gene become separated into different evolutionary lineages.
These genes may remain highly similar because they often continue to perform related functions. However, functional similarity is not the fundamental criterion used to define orthology. The decisive factor is their evolutionary history.
Two genes are orthologs when their evolutionary divergence can be traced to a speciation event. Therefore, option (C) provides the most scientifically accurate answer.
Detailed Explanation of Option (A): Analogs
Option (A) is incorrect.
Analogs, or analogous features, have similar functions but do not share the same evolutionary origin. The concept of analogy is commonly used when organisms independently evolve similar adaptations because they experience similar environmental conditions or selective pressures.
A classic example is the evolution of wings in birds and insects. Both structures are involved in flight, but they evolved independently and have different evolutionary origins. Their functional similarity does not indicate descent from the same ancestral structure.
At the molecular level, analogous proteins or genes may perform similar functions without being derived from the same ancestral gene. Such similarities can arise independently through convergent evolution.
The question clearly states that the genes are derived from the same ancestral gene. This indicates shared evolutionary ancestry and therefore excludes analogy. For this reason, option (A) is incorrect.
Detailed Explanation of Option (B): Heterologs
Option (B) is incorrect.
The term heterologous is generally used to describe genes, proteins, DNA sequences, or biological components that originate from different sources or species. It does not define the specific evolutionary relationship described in the question.
For example, when a gene from one species is introduced and expressed in another species, it may be described as a heterologous gene, and the process may be called heterologous expression. A bacterial cell expressing a gene obtained from a eukaryotic organism is a common example of a heterologous expression system.
However, the term heterolog does not specifically mean that genes in two species originated from the same ancestral gene in their most recent common ancestor. The scientifically precise term for that evolutionary relationship is ortholog.
Therefore, option (B) is incorrect.
Detailed Explanation of Option (D): Paralogs
Option (D) is incorrect.
Paralogs are homologous genes that originate through a gene duplication event. This is the major difference between paralogs and orthologs.
Suppose an organism initially contains one ancestral gene. If that gene is duplicated, two copies are produced within the genome. These copies can evolve independently and may gradually acquire different functions. The resulting genes are called paralogs.
In evolutionary form:
Ancestral gene → Gene duplication → Paralogous genes
Paralogs may occur within the same species, although duplication events that occurred in the distant past can also result in paralogous genes being found across different species. Therefore, simply finding genes in different species does not automatically make them orthologs. The evolutionary event responsible for their divergence must be identified.
If the genes separated because of speciation, they are orthologs. If they separated because of gene duplication, they are paralogs.
Since the question describes genes derived from the same ancestral gene in the most recent common ancestor of two species, the relationship is orthology rather than paralogy.
Orthologs and Paralogs: The Most Important Difference
The distinction between orthologs and paralogs is one of the most important concepts in molecular evolution and comparative genomics. Both are types of homologous genes, meaning they share evolutionary ancestry. However, they are produced by different evolutionary events.
Orthologs arise through speciation, whereas paralogs arise through gene duplication.
Consider a single ancestral gene present in an ancestral species. If the species divides into two descendant species and each inherits the gene, the corresponding genes are orthologs. In contrast, if the gene is duplicated within a lineage, the resulting copies are paralogs.
Therefore:
Speciation → Orthologs
Gene duplication → Paralogs
Orthologs often retain similar biological functions because they descended from the same gene while the species diverged. Paralogs are more likely to develop specialized or novel functions because duplicated genes can accumulate changes independently.
What Are Homologous Genes?
Homologous genes are genes that share a common evolutionary origin. Homology is the broad evolutionary category under which orthologs and paralogs are classified.
When homologous genes are separated by a speciation event, they are called orthologs. When homologous genes are separated by a gene duplication event, they are called paralogs.
Therefore, the relationship can be summarized as:
Homologous genes + speciation = orthologs
Homologous genes + gene duplication = paralogs
This classification is based on evolutionary history rather than merely on sequence similarity. Two genes can have similar sequences without necessarily being orthologs, and true orthology must ultimately be interpreted in the context of gene and species evolution.
Why Are Orthologs Important in Comparative Genomics?
Orthologous genes are widely used to compare genomes and study evolutionary relationships among species. Because orthologs originate from the same ancestral gene, researchers can compare their sequences to determine how genes have changed after species divergence.
Orthologs are also valuable for predicting gene function. If a gene has a known function in one species and a closely related ortholog is identified in another species, the known function can provide important evidence about the probable role of the corresponding gene.
However, orthologs are not guaranteed to have exactly identical functions. Evolutionary changes can modify gene regulation, expression patterns, biochemical activity, or interactions with other molecules. Therefore, shared ancestry defines orthology, while functional conservation is a common but not absolute consequence.
Role of the Most Recent Common Ancestor in Identifying Orthologs
The phrase “most recent common ancestor” is crucial in this question. The most recent common ancestor is the latest ancestral population or species from which the two species being compared descended.
If that common ancestor contained a particular gene and the two descendant species inherited corresponding versions of that gene, those gene copies are orthologous, provided their separation is associated with the speciation event.
This evolutionary approach is essential because gene relationships cannot always be classified correctly by sequence similarity alone. Scientists often compare gene trees with species trees to determine whether gene divergence resulted from speciation or duplication.
Therefore, the reference to the most recent common ancestor strongly supports orthologs as the correct answer.
Relationship Between Speciation and Orthology
Speciation is the evolutionary process through which one ancestral lineage gives rise to two or more independently evolving species. When a speciation event occurs, the ancestral genome is inherited by the descendant lineages.
Genes that were present in the ancestral genome may therefore continue in both descendant species. As the species evolve independently, the corresponding gene copies also accumulate evolutionary differences.
The resulting genes remain connected by their shared ancestry. Because their divergence reflects the separation of the species, they are classified as orthologs.
This relationship makes orthologs valuable for reconstructing evolutionary history and understanding how biological functions have been conserved or modified across different organisms.
Final Answer
The correct answer is (C) Orthologs.
Genes in two different species that are derived from the same ancestral gene in their most recent common ancestor are called orthologs. Orthologous genes are produced when gene lineages separate as a result of a speciation event.
The key distinction is that orthologs arise through speciation, whereas paralogs arise through gene duplication. Analogs do not share the same evolutionary origin, and heterologs do not describe the specific ancestral relationship given in the question.
Correct Answer: (C) Orthologs
