Different Origins of Viruses

What does viral diversity suggest about their origins?

Viruses likely evolved multiple times from different cellular origins.

Why do viruses share some features despite diverse origins?

Convergent evolution.

What does the shared genetic code between viruses and cells support?

A post‑cellular origin for viruses.

What is the progressive (escape) hypothesis?

Viruses evolved from mobile genetic elements like retrotransposons.

What process is shared by retrotransposons and retroviruses?

Reverse transcription.

How might retrotransposons have become viruses?

By gaining capsid proteins.

What example supports the progressive hypothesis?

Retrotransposons and HIV both use reverse transcription and genome integration.

What is the regressive (reduction) hypothesis?

Viruses were once free‑living cells that became parasitic and lost genes.

Which virus supports the regressive hypothesis?

Mimivirus.

Why does Mimivirus support the regressive hypothesis?

It retains metabolic and enzyme‑related genes.

What cellular example parallels the regressive hypothesis?

Parasitic bacteria like Chlamydia.

What is convergent evolution?

Independent evolution of similar traits in unrelated organisms.

What core features do all viruses share?

A genome, a capsid, and obligate parasitism.

Why are shared viral features considered convergent?

They evolved due to similar selective pressures, not shared ancestry.

What does obligate parasitism mean?

Viruses cannot replicate without a host cell.

What misconception do students often have about viral genomes?

That all viruses have DNA genomes.

What analogy helps explain convergent evolution in viruses?

Viruses are like different smartphone brands with similar essential features.

What is the likely origin of RNA viruses?

Progressive (escape) hypothesis.

What evidence supports a progressive origin for RNA viruses?

Similarity to mobile RNA elements and retrotransposons.

What is the likely origin of large DNA viruses?

Regressive (reduction) hypothesis.

What evidence supports a regressive origin for large DNA viruses?

Their genomes contain remnants of metabolic and replication genes.

What explains traits shared across all viruses?

Convergent evolution.

Why does viral diversity suggest multiple origins?

Extreme variation is unlikely from a single ancestor.

How do retrotransposons support the progressive hypothesis?

They share reverse transcription and integration mechanisms with retroviruses.

How do Mimivirus and Chlamydia support the regressive hypothesis?

Both show reduced metabolic independence.

How does convergent evolution explain shared viral features?

Different origins but similar selective pressures produced similar structures.