Viral evolution

Tiny germs called viruses change fast.

They make mistakes when they make copies. These mistakes change their genes.

Some changes help them stay alive. These changes help them hide. They can even hide from medicine.

Viruses can also swap parts. They do this inside a cell. This makes new kinds of germs.

It is hard to stop them. They change to fit new places. Scientists study how they change.

Viruses change very fast. This is called viral evolution. Many viruses have a high mutation rate. A mutation is a tiny change in a virus's genes. Most changes do not help the virus. Some changes even hurt it. But some changes help a virus adapt. This means it can fit into a new home.

When a virus makes copies, it makes mistakes. RNA viruses make many mistakes. This happens because they lack ways to fix errors. One virus can make millions of copies very quickly. This lets new changes spread fast. Some changes help viruses hide from the body. Other changes help them hide from drugs.

Viruses can also swap genes. This happens when two similar viruses are in one cell. This can make new and strong types of viruses. Scientists study these changes to fight diseases. They look at old genes left in our own DNA. This is called paleovirology. It helps us learn about the history of life.

Viruses are tiny things that change over time. This process is called viral evolution. It is a major part of how viruses adapt to their surroundings. Many viruses, especially RNA viruses, have a very high mutation rate. A mutation is a tiny mistake or change in a virus's genes. Most of these changes do not help the virus at all. Some changes can even be harmful to the virus. However, a few changes might help a virus survive better. This allows them to adapt quickly to new environments.

Evolution happens through a step-by-step way it works. First, a virus makes many copies of itself inside a host. During this copying, mistakes in the genes often happen. RNA viruses are very prone to these mistakes. This is because they lack the tools that cells use to fix errors. One single virus particle can produce millions of new viruses in one cycle. If a mutation helps the virus, that version will spread fast. This is similar to how natural selection works in other living things.

Scientists have many ideas about where viruses first came from. One idea is the regressive hypothesis. This suggests viruses were once small cells that lost genes over time. Another idea is the cellular origin hypothesis. This says viruses might have started as bits of DNA that escaped from larger organisms. Barbara McClintock discovered "jumping genes" in maize in 1950, which are called transposons. A third idea is the co-evolution hypothesis. This suggests viruses and cells appeared around the same time on Earth.

There are many different types of viruses with different speeds of change. Some viruses, like HIV, evolve very rapidly. Others, like GB virus C, evolve much more slowly. Viruses can also change through a process called genetic shift. This happens when two similar viruses swap genes inside one cell. This can create new and strong strains of a virus. Scientists also study the hepatitis delta virus. This is a defective virus that needs hepatitis B to help it work.

We can learn about viral history even without fossils. Viruses are too small to leave traditional fossils in rocks. Instead, scientists use a field called paleovirology. They look for endogenous viral elements, or EVEs, in the DNA of living things. These are old virus genes left behind in a host's genome. Most vertebrate species have hundreds or thousands of these ancient sequences. These tiny traces act like a map of the past. They show us how viruses and cells have interacted for a very long time.

Viral evolution is a specialized field within evolutionary biology and virology. It focuses on how viruses change over time to adapt to their environments. Viruses are unique because they often have very short generation times. Many types, specifically RNA viruses, possess high mutation rates. A mutation is a change in the genetic code. In some RNA viruses, there is one or more mutations per genome during every round of replication. While most mutations are useless or even harmful to the virus, natural selection allows beneficial changes to spread. This rapid adaptation helps viruses survive in changing host environments.

The mechanism of viral evolution relies on how viruses replicate. When a virus infects a host, it makes many copies of itself. During this process, errors occur in the genetic material. In host cells, there are tools to fix mistakes in DNA replication. However, these mechanisms do not work for RNA. Consequently, RNA viruses are much more prone to errors. A single virus particle can produce millions of progeny in just one replication cycle. Most of these mutations are "silent," meaning they cause no obvious change. However, some mutations increase a virus's fitness. These changes might help a virus hide from the immune system. They can also make antiviral drugs less effective.

Viruses change through different processes like antigenic drift and genetic shift. Antigenic drift involves the gradual accumulation of mutations in viral genes over time. In contrast, genetic shift occurs when two similar viral strains infect the same cell. These two strains can "shuffle" or swap their genes. This sudden change can result in new and more virulent strains. Different viruses also evolve at different speeds. For example, HIV evolves very rapidly. Other viruses, such as GB virus C, evolve much more slowly. Even within a single virus, different genomic regions may evolve at different rates due to varying selective pressures. This was observed in the SARS-CoV-2 virus during the COVID-19 pandemic.

Scientists use several classical hypotheses to explain how viruses first emerged. The regressive hypothesis suggests viruses were once small cells that lived as parasites. Over time, they lost genes that were not needed for parasitism. The bacteria rickettsia and chlamydia provide support for this idea. The cellular origin hypothesis, or "escape hypothesis," suggests viruses evolved from genetic material that escaped larger organisms. This material could come from plasmids or transposons. Transposons are mobile genetic elements often called "jumping genes." Barbara McClintock discovered these in maize in 1950. The co-evolution hypothesis, or "virus-first hypothesis," proposes that viruses and cells evolved from complex molecules at the same time.

A newer theory called the chimeric-origins hypothesis was proposed in 2019. This model suggests that different parts of a virus have different origins. The replication modules likely came from the primordial genetic pool. However, the structural proteins likely evolved from diverse host proteins. This theory combines features of both the escape and virus-first hypotheses. Some viruses also exist as "satellites" or defective viruses. The hepatitis delta virus is a defective virus with an RNA genome. It requires the hepatitis B virus to provide a protein coat for transmission. Similarly, the sputnik virophage depends on the mimivirus to function.

Because viruses are too small to form traditional fossils, scientists use paleovirology to study their history. They look for endogenous viral elements, known as EVEs. These are remnants of ancient viral genes that invaded a host's germline. Most vertebrate species contain hundreds or thousands of these sequences. These EVEs allow researchers to reconstruct the evolutionary history of viruses. Scientists also use a "molecular clock" to estimate when different viruses diverged. By measuring mutation rates, they can infer dates of divergence from contemporary genomes. This provides a window into the deep past of the virosphere.

Understanding viral evolution is critical for modern medicine and epidemiology. Rapid mutations cause significant challenges in developing vaccines and antiviral drugs. Resistant mutations often appear within weeks or months of starting a treatment. This is especially true for diseases like influenza, AIDS, and hepatitis. One theoretical model used to study this is the quasispecies model. This model defines a viral quasispecies as a group of closely related strains competing in an environment. Studying these patterns helps scientists prepare for emerging viral threats.