History of molecular evolution facts for kids
The history of molecular evolution is about how scientists started using DNA and proteins to understand how living things change over time. This field really took off in the 1960s and 1970s. Before that, scientists mostly studied how animals and plants looked different. But with new tools, they could look at the tiny building blocks of life.
One big idea was the "molecular clock". This is like a timer that uses the differences in DNA or protein sequences between two species to guess how long ago they shared a common ancestor. It was a bit controversial at first because many scientists believed that natural selection (where the strongest survive) was the *only* way evolution happened. Later, the neutral theory of molecular evolution came along, suggesting that some changes in DNA don't really affect survival, but still happen over time.
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How did molecular evolution begin?
Before the 1950s, only a few scientists thought about using tiny chemical differences between species to study evolution.
For example, Alfred Sturtevant in 1921 looked at changes in chromosomes in fruit flies. He helped create one of the first "family trees" (called phylogenies) based on these tiny changes. Later, Ernest Baldwin and Marcel Florkin also tried to build these family trees using other chemicals in living things.
In the 1950s, new methods made it easier to study these chemicals. Scientists like Alan Boyden and later Curtis Williams and Morris Goodman used special tests with blood to compare different species, especially primates (like monkeys and humans).
Linus Pauling, a famous scientist, and his students used a method called electrophoresis. This method separates proteins based on their size and charge, creating unique patterns. By comparing these patterns, they could see how similar or different proteins were between species.
Some naturalists, like Ernst Mayr and Charles Sibley, also started using these new molecular tools. Sibley used electrophoresis to study bird egg-white proteins, which helped him sort out how different bird species are related. He also used DNA hybridization, which compares how well DNA from two different species sticks together.
These early methods slowly gained acceptance, but they didn't immediately change how most scientists thought about evolution. That would happen as molecular biology grew even more.
Measuring genetic differences
In the mid-1960s, a new method called protein electrophoresis became very useful. It allowed scientists to measure how much genetic variation existed within natural populations.
Scientists like Jack L. Hubby and Richard Lewontin used this method to study fruit flies. They found that fruit flies had a surprising amount of genetic differences (about 12% of their genes were different, on average). This was a big discovery!
However, it was hard to explain these findings using only natural selection. Many scientists believed that most genetic changes were either good or bad for survival. But if so many genes were different, it was hard to see how natural selection could explain all of it. This puzzle helped set the stage for new ideas in molecular evolution.
Proteins and the molecular clock
While evolutionary biologists were starting to look at molecules, molecular biologists were also getting interested in evolution.
After figuring out how to "read" the sequence of insulin protein in the 1950s, Frederick Sanger and his team compared insulin from different species. This was a small start.
By the early 1960s, it became much easier to sequence proteins. Emanuel Margoliash sequenced a protein called cytochrome c from horses. This protein is found in many living things, making it great for comparisons. Soon, cytochrome c from many other species was sequenced.
In 1962, Linus Pauling and Emile Zuckerkandl came up with a revolutionary idea: the molecular clock. They suggested that if a protein changes at a steady rate over time, you could count the differences between the same protein in two different species to estimate how long ago they split from a common ancestor.
They first used hemoglobin (the protein in blood that carries oxygen) for this. The results matched what scientists already knew about how animals evolved. The molecular clock idea was very exciting, but also very challenging to the traditional view of evolution. It suggested that some changes happened at a regular pace, almost independently of what was happening in the environment.
The "molecular wars"
In the 1960s, some traditional evolutionary biologists, like Ernst Mayr and Theodosius Dobzhansky, felt that molecular biology was a threat. They thought molecular approaches were too simple and didn't explain the complex ways natural selection shaped life. The molecular clock, for example, didn't seem to care about how the environment affected evolution.
This disagreement became known as the "molecular wars." It was a struggle between scientists who focused on whole organisms and their environments (evolutionary biologists) and those who focused on tiny molecules like DNA and proteins (molecular biologists). Each group thought their way was the most important for understanding biology.
Dobzhansky famously said, "nothing in biology makes sense except in the light of evolution." He meant that understanding evolution was key to all biology, and he worried that molecular biology was ignoring this bigger picture.
Mayr and others attended conferences where molecular evolution was discussed. They argued that the molecular clock, which suggested constant rates of change, didn't fit with the idea that evolution speeds up or slows down depending on environmental pressures, like during adaptive radiation (when many new species quickly evolve from one ancestor). They wanted to keep Darwin's ideas of natural selection at the center of evolution.
Genes at the center
Around the same time, another idea became popular: the gene-centered view of evolution. This idea, pushed by George C. Williams, suggested that evolution is really about genes trying to make copies of themselves. This meant focusing on individual genes, not just whole organisms.
However, this didn't necessarily mean focusing on *molecular* evolution. In fact, this view often emphasized how genes adapted to their environment, which sometimes pushed the idea of non-adaptive molecular changes (like those studied by molecular evolutionists) to the side.
The neutral theory of molecular evolution
In 1968, Motoo Kimura introduced a very important idea: the neutral theory of molecular evolution. He looked at the molecular clock studies and realized something surprising. If all the changes in DNA and proteins were due to natural selection, there would be too many "bad" changes happening all the time. This would create a huge problem for populations.
Kimura suggested that most changes in DNA and proteins are actually "neutral." This means they don't really help or hurt an organism's survival. These neutral changes can become common in a population simply by chance, a process called genetic drift. This idea fit well with the high levels of genetic differences found in fruit flies by Hubby and Lewontin.
Soon after, Jack L. King and Thomas H. Jukes published a paper called "Non-Darwinian Evolution." This title was very provocative! They agreed with Kimura that neutral mutations were real and important. They also found that the rate at which proteins evolved was fairly constant, which was hard to explain without neutral changes.
King and Jukes' paper brought the neutral theory right into the middle of evolutionary biology. It offered a way to explain the molecular clock and helped connect molecular biology with evolutionary biology, even if it was a bit controversial at first.
In 1971, because of this growing interest, scientists like Emile Zuckerkandl started the Journal of Molecular Evolution, a scientific magazine dedicated to this new field.
Neutralists vs. selectionists
The neutral theory led to a big debate called the "neutralist-selectionist debate."
- Selectionists believed that natural selection was the main or only reason for evolution, even at the molecular level.
- Neutralists (like Kimura) argued that many mutations were neutral, and genetic drift (random chance) played a big role in how proteins evolved.
Kimura spent the rest of his career defending the neutral theory. He and his colleague Tomoko Ohta focused on how quickly neutral mutations could become common in a population and how proteins are limited in how much they can change without losing their function.
In the 1970s, Ohta developed the nearly neutral theory of molecular evolution. This theory suggested that many mutations are not strictly neutral, but "nearly neutral"—meaning they have a very small effect, either slightly good or slightly bad. For these mutations, both natural selection and genetic drift play a role.
Ohta also noticed that while protein evolution rates were fairly constant, the rates of change in noncoding DNA (DNA that doesn't make proteins) were different depending on how fast a species reproduces. This suggested that noncoding DNA changes were more neutral, while protein changes were still influenced by selection.
The tree of life
While early molecular evolution focused on proteins and recent evolutionary history, by the late 1960s, scientists started looking deeper into the "tree of life."
Carl Woese, a molecular biologist, began using ribosomal RNA (a type of RNA found in all living cells) to classify bacteria. He wanted to group them by their genetic similarities, not just how they looked.
By 1977, Woese and George Fox made a huge discovery. They found that some bacteria, like methanogens, were so different from other bacteria that they belonged to their own separate group. They called this new group "archaebacteria" (now called Archaea).
Woese's work led to the modern three-domain system of life:
This new system replaced the older five-kingdom system and completely changed how we understand the basic branches of life.
Studying tiny organisms also brought molecular evolution closer to cell biology and research on the origin of life. Woese's work supported the idea that early life might have been based on RNA, not DNA, a concept later called the "RNA world."
Later, in the 1990s, scientists found that genes could sometimes move between very different types of organisms, a process called lateral gene transfer. This, along with the idea that complex cells (eukaryotes) formed when one simple cell swallowed another (called endosymbiosis), showed that the early history of life was much more complicated than a simple branching tree.
