Evolution and genetics
Main contents
- Evolution and Genetics
- Facts and Definitions
- Evo Devo
- History of Genetics
- Genetics and the Tree of Life
Charles Darwin had one big problem in explaining evolution. He was sure parents passed traits on to their offspring, but how did this happen? What was it that carried this information?
In notes written 6 years after On the Origin of Species, Darwin proposes 'gemmules' as the means of heredity. These were atoms thrown off by developing cells, containing the detail of that cell. These gemmules would then form the sperm or eggs in the parent, and so carry the information for the next generation. He published these ideas 3 years later in a work on variation in domestic plants. For the next 30 years gemmules, or 'pangenes', would be viewed as the most likely mechanism for the transfer of information. Unlike the theory of evolution by natural selection, Darwin had little evidence to support this hypothesis, and it would soon prove incorrect.
In 1866, seven years after Darwin published On the Origin of Species, an Austrian monk published the results of his experiments on hybrid plants. The work was dismissed and forgotten for 35 years. Today, we regard his work as among the most important research of the modern era and the foundation of an entire branch of science. The monk was Gregor Mendel and the emerging science was genetics. Had the significance of Mendel’s work been realised at the time, Darwin would have had his answer.
Inheritance lies with the genes. From 1900 to 1953 scientists made a series of breakthroughs, unravelling the structure of genes until we were left with DNA, the molecule of life. Genetics has confirmed Darwin’s theory and taken us to the brink of understanding exactly how evolution works: how all life on Earth originated from simple inorganic molecules.
Facts and Definitions
Deoxyribonucleic Acid, or DNA for short, is the means by which evolution works. DNA allows traits to be passed on, and allows changes to be accumulated. Every change in an animal is a change in it’s DNA, and the DNA records the history of its changes.
DNA is written in code. A molecule of DNA is made of two linked strands, with the strands composed of just four molecules Adenine (A), Cytosine (C), Guanine (G) and Thymine (T). These molecules pair up to link the strands. A link is called a base pair. Adenine only joins to Thymine and Cytosine only joins to Guanine. There are 3 billion base pairs in the human genome. A genome is the complete set of DNA in an organism. The unique configuration of base pairs in DNA is what makes life so diverse.
Every cell in animals and plants contains DNA. The DNA is tightly bundled up to form Chromosomes in the cell nucleus. Unravelled, the DNA in a human cell would stretch for about 2 metres. If we did this with all the DNA in our bodies, that would go to the moon and back!
A Human is made up of around 1,000,000,000,000 cells
The DNA of a species contains the instructions to make proteins. Proteins perform the functions that make an organism 'alive'. A chunk of DNA coding for a specific protein is called a gene. An average gene consists of about 1,200 base pairs. We have about 20,000 genes, roughly the same number as an earthworm, but half that of the rice plant. Proteins are made from amino acids. An amino acid is coded into the DNA as a sequence of just 3 bases (not base pairs). This is called a triplet. There are 23 different amino acids.
Evo Devo
Earth today is inhabited by an estimated 20 million species, and it could be twice that number. Yet this is a tiny fraction, less than 1%, of all the life that has ever lived on Earth. What is it that makes one cluster of cells form a human and another form a cockroach? Genetics tells us how traits are passed on, but not why different plants and animals look the way they do. Where does that fabulous diversity, Darwin’s 'endless forms most beautiful', come from? The key lies in development, the process by which all organisms grow from a single cell. In that process lies all the diversity on Earth today, in the past, and in the future.
Biologists have been studying how plants and animals develop since the 19th century. By looking at the embryos of animals and comparing them to other animals, they hoped to work out how closely related different animals were. But this was an imprecise science.
Today, we have gone back to the embryo to learn how evolution works. But now we study the genes of these animals to see how the embryo forms.
The hero of this story is the humble Fruit Fly. Geneticists wanted to identify which genes controlled and shaped the fly’s development. The assumption was that different animals had different genes doing different things. The more different two animals were, the more different their genes. But what these scientists found was that many of the genes controlling development were the same in all animals, including humans. The genes for limbs, for eyes, for the heart, were pretty much the same in all animals, even though the end product was very different. As different genomes have been unravelled, we have discovered there is little genetic difference between organisms.
This became the science of Evolutionary Developmental Biology, or 'Evo Devo'. Today, a 'genetic tool kit' has been identified, with master genes shared between all animals that control how an animal forms.
Even though your DNA is huge, consisting of billions of molecules, only a tiny fraction is actually concerned with building you (around 2,000 of your 20,000 genes). And what’s more amazing, most of the genes that build you are the same genes that build every other animal on Earth today. We call these toolkit genes. There are about 500 toolkit genes common to all life on Earth. Study of these genes has shown they date back to the earliest forms of complex life more than 600 million years ago. This strongly supports the idea of shared ancestry. Yet these wonderful discoveries led to an uncomfortable question. If so much of the genetic material is the same, why do we see such amazing diversity?
It turns out that while organisms share so much, they use it in different ways. The genes that control when an animal activates a particular gene are the most important in understanding development, and in understanding evolution. These regulatory genes tell toolkit genes when and where to switch on or off. By changing these instructions, you change the animal. By simply turning genes on or off at different times, turning them on in different places, or not turning them on at all, very different animals develop.
Together, toolkit genes and regulatory genes make up less than 5% of your DNA, yet in that 5% we have all the variety of life on Earth today.
History of Genetics
In 1866 Gregor Mendel published the results of his investigations into inheritance. At the time it was assumed the offspring blended traits of the parents. This meant that if one parent had white flowers, and one purple, the offspring should have light purple/pink flowers. Mendel disproved this. Using pea plants, he showed that the characteristics of a parent were passed intact to the offspring. By repeating the experiments over several generations, he showed that unchanged traits passed down the generations. He also showed that some traits were more dominant than others. Unfortunately, his discoveries were dismissed and the work lost for 35 years.
In 1900 Hugo de Vries, Carl Correns and Erich von Tschermak discovered similar ideas, leading to a re-discovery of Mendel’s work. Five years later, two researchers worked out how sex was determined (The X and Y chromosomes). Researchers developed gene theory over the next 25 years, showing that mutations in the genes could be passed on and preserved.
The structure of chromosomes was established in the 1920s. Much to the surprise of researchers, they were made of four nucleic acids, much simpler molecules than originally suspected. The four were Adenine, Guanine, Cytosine and Thymine. In the late 1920s Frederick Griffith identified the 'transforming principle' through experiments with bacteria in mice. What he’d actually discovered was the DNA molecule, as would be shown in the 1940s.
In the 1930s Hans Spemann, by tying human hair round developing newt embryos, was able to establish that parts of the embryo were 'organisers', controlling development. He also discovered embryonic induction (this is where one part of the embryo causes another part to form a different structure). These discoveries would lead to Evo Devo.
In 1950 Erwin Chargaff showed that while the amount of the four nucleic acids varied in different organisms, there was always a 1:1 ratio of Adenine to Thymine and Cytosine to Guanine. This was a major step on the road to unravelling the structure of DNA. In 1952 Martha Chase and Alfred Hershey proved DNA, not individual proteins, was the molecule of heredity. A year later, Francis Crick, James Watson, Maurice Wilkins and Rosalynd Franklin revealed the structure of DNA, the famous double helix. By discovering this structure they allowed other researchers to understand the mechanism of replication and variation in genetics. During the 1960s a number of scientists worked on, and ultimately cracked, the genetic code. The genetic code is the way in which information on the genome is converted into proteins by living cells. This involves messenger RNA carrying the information from DNA to ribosomes in cells. All the pieces to understand the mechanism of evolution were now in place.
In the early 1970s scientists discovered the existence of regulatory genes. This showed that a change in just one gene could produce multiple changes elsewhere in the genome. Nadine McGinnis refined this idea in 1984 with the discovery of Hox genes, which are responsible for basic body structure in most animals. This gave the strongest indication yet of how complex evolutionary changes could occur so quickly; they may require very few mutations.
In 1972 the first entire gene was sequenced. Just 5 years later an entire genome, of a bacterium, had been sequenced. in 1988 The Human Genome Project began with the aim of mapping the entire human genetic code. In 2003 it completed it’s initial mapping; the results were surprising. As a complex animal, researchers had expected humans to have 40-50 thousand genes. But it was half that number. This meant there were many organisms that were much more genetically complex than us. They are now researching these genes in more detail to help understand how we work and hopefully provide new treatments for many conditions.
Genetics and the Tree of Life
One of the big questions in evolution is how closely related two animals are. Before the discovery of genetics as a tool for studying animals, we had to work this out on the basis of their physical similarities. While this works in some cases, we have seen that evolution can create very similar animals via very different pathways (convergent evolution). But genetics allows us to study the fine detail of an animal and make a much more precise diagnosis. There are three ways to do this:
How many genes two animals have in common
We can look at genetic relatedness, i.e. how much of two animals genome is the same. The more similarities, the more closely they are related.
Gene Function
It’s possible for small parts of a gene to change, as long as important parts of the code remain. By looking at the coding for a specific gene and comparing it to other animals, we can see which are more closely related. Essentially, the more letters the same in the same gene, the more closely they are related.
Mutations don’t just lead to variation, many mutations stop genes working.
If this change is harmful, it will be wiped out (the animal will die before reproducing, or the genetic code manages to fix itself). But if a gene ‘breaks’ and doesn’t affect the animal, this change will be kept (e.g. if a gene involved in colour vision breaks in an animal that lives in a cave, it doesn’t matter as the animal doesn’t need to see in colour). By looking at a group of animals and seeing which have a working copy of the gene and which do not, we can see which have common ancestors. This can also help us work out when changes happened.
By studying genes in this way, we have learned in much greater detail how life has evolved, and just how closely related some animals are. We have been able to test some of the predictions made by evolution and the fossil record. Genes have proved many evolutionary pathways, such as the evolution of whales from land mammals and the evolution of birds from dinosaurs. This method has also thrown up some surprises, with animals previously thought to be quite closely related turning out to be more distant than realised.
