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Germs: small organisms, including bacteria, viruses and funghi. They can be passed from individuals to others. The body can defend itself with the immune system.
Ignaz Semmelweis
A lot of women in the ninetheenth century died after giving birth. Ignaz Semmelweis saw that medical students dissected a dead body and then helped giving birth without washing their hands. A colleague of him died of the same symptoms as the women after cutting himself. Semmelweis found the disease was caused by an infectious agent. The death rate was decreased enormously, but still he couldn’t convince others because:
It was hard to admit
It became a part of giving birth
Washing your hands was odd in that time
Semmelweis found the rejection unbearable.
Cholera in London was very easily spread by poor people with poor sanitation. Most doctors thought it was spread by touching or bad air.
John Snow: doctor in Newcastle, interested by cholera. His conclusion: cholera was caused by poison which reproduced in the human body, found in vomit & diarrhoea. He was ignored.
When another cholera outbreak hit London, Snow found that two different parts of London had a big difference in deaths. The most deaths were in the part that used the water of the river that went through London, the other river went around it and was cleaner.
The work of Semmelweis and Snow led to acceptance, but it took long before it was widespread.
Pasteur: studied fermentation -> he became interested in microbes. People thought living things arised from dead things (spontaneous generation). Pasteur showed an experiment to disapprove this: microorganisms were already in the air. He also found that silkworms were infected by leaves used by infected worms -> important influence on his thinking about infectious diseases, he saw that some infected worms died and some didn’t. He didn’t know how to identify different germs.
1866: Koch became interested in examining microbes with more powerful microscopes. He grew bacteria on agar jelly. He discovered 11 diseases (anthrax, TB, cholera) and tried to stop them.
He got the Nobel Prize for medicine. By the end of the 19th century the idea that infectious diseases were caused by germs was generally accepted.
Things that helped to decrease the deaths by infectious diseases: relieved poverty, improved housing, installing piped water supplies and enclosed sewers, diet guidelines.
Everybody knew small pox didn’t come back once you had had it. Therefore, they tried to immunise people (in prison). It worked but not with everybody, so it was illegalised.
Jenner knew that girls working with the cows suffered from cowpox but rarely from smallpox. He wondered if infecting people with cowpox would help them not to get smallpox. He tested his idea in 1974: he took pus from cowpox spots and scratched it into the arm of a healthy boy, who recovered. Two months later he scratched smallpox into the boy’s arm. The boy didn’t develop smallpox. The pus was the first form of vaccination (vacca = cow)
Pasteur wondered if vaccination could help to prevent other diseases. In 1879 he studied chicken cholera and managed to grow a cell culture. They showed microbes outside a chicken’s body infected them and they died. An assistant injected chickens with cultures that had been standing for several weeks, so they had weakened. The chickens got ill but recovered. The second time, using fresh microbes, they stayed healthy. This is the basis of vaccination.
Making an anthrax vaccination was more difficult. Pasteur used the theory of Koch to grow anthrax cultures. He and his team found that the anthrax was weakened by keeping it warm. Rossignol challenged Pasteur to convince his ideas. They vaccinated half of the sheep, then they were injected with anthrax spores. The half that was injected survived, the other didn’t.
He invented a vaccination for dogs that had rabies. He dried out parts of the spinal nerves from rabbits infected with rabies. It was experimented on a boy that was bitten by a dog with rabies and it worked. It was a laboratory-based medical breakthrough. It led to the Pasteur Institute.
Anw hoofdstuk 2
To prevent a disease you need to know which microbes cause the disease.
Vaccination: effective, but new diseases appear constantly.
They only work if nearly all babies are immunised, otherwise an epidemic may appear
Antibiotics: gave the doctors power to treat the symptoms of bacterial diseases, but they develop resistance
WHO: works with governments to develop strategies to protect people from infection and fight the spread of disease.
Toxins: poisons produced by living cells or organisms
The work of scientists led to the general acceptance of the germ theory of disease.
Infectious diseases are widespread, affecting humans, plants and animals. People catch them when their body is invaded by an infectious agent/microbe invades their body. They reproduce themselves in the body. Sometimes the damage and symptoms are caused by a toxin, not the microbe (cholera)
Living organisms are build up by cells, carrying out the basic functions of life, similar in all organisms. (new proteins, transferring energy, replicate)
Bacteria: a single-celled organism, a microbe, can cause disease (tonsillitis, TB, cholera), mostly harmless or beneficial
Viruses can only be seen since the electron microscope. It are packages of DNA. They multiply by invading a healthy cell, taking it over and producing new copies of itself. The cell destruction and the bodies reaction cause the symptoms.
Fungal infections: mostly superficial but can also cause serious disease: Aspergillus (long infection)
They can be very harmful for people who are already weakened by other disease.
Culture: growing a microbe in the laboratory
It’s easy to tell them apart with microscopes because they are very different
In the past, people thought that infection happened through the air. They tried to avoid contact between people and put people with infectious disease into quarantine. Later people understood it was more effective to improve public health measures (better housing, sewage disposal, drinking water)
Insect vector: an insect species which transits human disease. E.g. the mosquito
Immune system: when a foreign microbe enters the blood, cells start to destroy them, by attacking them and making antibodies. White blood cells that fought the microbe stay in the body for years, so after infection a body stays immune. Natural defences are weaker in younger people.
Vaccination became popular after Jenner’s demonstration. In 1853 it became compulsory for infants under three months. Compulsory vaccination ended in 1946.
By 1971 routine children vaccination in Britain was no longer recommended. In May 1980 the WHO declared the world free of the scourge of smallpox. Greatest triumph so for of vaccination programmes. Between 1967 and 1977 many dollars were spent in a worldwide vaccination programme but this is cheaper than curing.
Vaccination against other diseases:
As long as enough people are immune, immunisation helps to prevent the spread of disease. The necessary level of vaccination depends on how infectious the disease is. At least 90% of children must be vaccinated year by year to stop for example the highly infectious measles.
Risks of vaccination:
There are no perfect vaccines and parents may ask the following questions before having their children being vaccinated:
- Is the vaccine really effective in preventing disease?
- What are the possible side effects and what is the chance that my child will be affected?
- What will the authorities do to compensate if my child suffers lasting damage from a vaccination recommended as part of public policy? Some effective vaccines can produce side effects which are not serious and which clear up quickly.
Medicines to treat and cure disease:
Remarkable developments in techniques of analysis and synthesis allow chemists to model and manipulate the detailed structure of complex molecules. Until the mid-30’s doctors relied on immune systems of patients by making simple chemicals made from plants. One or two of the older medicines were effective like vitamins and foxgloves to treat heart failure. Quinine was an anti-malaria drug found in tropical rainforests. In 1922 insulin was successfully isolated and tested in the pancreas of a dog. The use of chemicals to treat disease started in 1935 with sulfonamide drugs. This was followed by the discovery of antibiotics in the 40’s.
Drugs are the active ingredients in medicines used for the treatment, relief or prevention of disease. People also take drugs for pleasure, stimulation and relaxation.
Medicines normally consist of one or more drugs, mixed with other inert materials, and combined in a way which makes the treatment available as pills to swallow, ointments to rub onto the skin, powders or vapours to inhale, solutions to inject or drops for the eyes or ears.
The pharmaceutical industry is the part of the chemical industry which makes drugs and medicines.
Antibiotics:An antibiotic is a chemical produced by a microbe that kills or limits the growth of another microbe.
The discovery of Penicillin:
When species collected themselves on for example an old slice of bread they start to prevent others from trespassing and then they start to release chemicals to kills invaders which are called antibiotics. In 1928 Alexander Fleming was examining Staphylococci. He saw a mold on a dish around which this bacteria was dying. This led him to conclude that the mould produced a substance which killed the bacteria. The mould released chemicals into the solution which could kill bacteria that cause human diseases. To show it did not harm he inserted it into mice. The mould was identified as Penicillium notatum and Fleming named the anti-bacterial substance it produced penicillin. He found the first antibiotic.
Large-scale production:In 1938 Ernst Chain and Howard Florey at Oxford University took up the investigation. They established penicillin’s effectiveness against different bacteria. By 1941 they made methods to produce enough penicillin for clinical trials. 100 litres of the mixture were needed for one person on one day. Penicillin was very effective for treating infections. During WO2 there was an urgent need for penicillin as untreated wounds often became infected with bacteria, causing fatal diseases. Large-scale manufacturing processes started in the USA. By 1944 the production of penicillin was sufficient to treat all victims. New high yielding strains of Penicillium mould ensure today’s massive worldwide demand. Genetic engineering has also helped to improve. Penicillins are produced in huge containers(fermenters) which hold up to 200000 litres of mould and culture solution. The mould is filtered off and the penicillin extracted from the solution. The drug weakens cell walls of the bacterial cells so that they burst and die. Human cells do not have cell walls so they are not affected by the antibiotic.
Antibiotic restistance:With time, antibiotics become less effective because bacteria develop resistance. Since the introduction the 40’s strains of bacteria have emerged which produce the enzyme penicillinase. The enzyme beaks down the penicillin, making the drug ineffective. Populations of bacteria always contain a few individuals with gened for penicillinase, making them resistant to the antibiotic. These individuals survive the onslaught of penicillin when a patient is treated with the drug, and reproduce new individuals which inherit the genes for penicillinase. The offspring are resistant, and because bacteria multiply very quickly resistance spreads quickly. As resistance becomes more widespread the dosage of drug has to be increased step by step until the drug becomes ineffective or so poisonous to the patient that an alternative has to be found. Incorrect diagnosis, patients not finishing a course of antibiotics and the use of antibiotics in animal feed all contribute towards development of resistance. Limit the use of antibiotics to people who really need them. Superbug infections in hospitals involve bacteria which have developed resistance to a wide range of antibiotics. These microbes are a real threat to life in situations where antibiotics are used constantly and extensively. Keeping hospital-acquired infections at bay is possible through careful use of antibiotics and strict hygiene procedures when caring for patients. Hospitals routinely test new patients to identify those carrying dangerous bacteria; these people can transfer the bacteria to others even though they may not have symptoms themselves.
Tuberculosis
The Disease
Turberculosis(TB) is an infectious disease which is most commonly caused by infections of the bacterium Mycobacterium tuberculosis. By coughing, sneezing, talking or spitting TB is spreaded. Crowded living or working conditions add to the ease with which it spreads. However the immune system give protection and the bacteria may lie dormant in the body(latent TB). Only the minority of infected people fall sick or become infectious.
Symptoms of TB
The most common forms of TB affect the respiratory system. It doesn’t only damage and destroy lung tissue, it also sppresses the action of the immune system, which makes the body less able to fight the disease. Fever, night sweats, inability to eat and loss of weight are typical symptoms. Mostly mucus is contained because the lungs are damaged which can culture M. Tuberculosis. Improving living standards is most effective way of controlling TB. Vaccination, less crowded conditions and generally healthier and better-fed people help.
Immunisation:
Every bacterium is different so vaccination is difficult. There is no fully active vaccine against TB. The BCG(Bacillus Calmette-Guerin) vaccine was invented in 1921 and is made from a weakened form of a bacterium closely related to human TB. Only available vaccination.
It reduces the likelihood and severity of TB in young children. In areas were TB is highly prevalent this is especially important. Costs for research are not likely to be recouped as the disease is mostly common in developing countries. You are likely to get it if you have another disease, don’t eat well or live in overcrowded or sub-standard housing.
BCG is also used for healthcare workers and people immigrating to countries with high TB-levels. The BCG vaccine does not continue to adults. BCG does not not the reactivation of latent TB so it can still be spread.
Incidence:
The frequency of a disease in a particular population or area is called the disease’s incidence and it is expressed as the number of newly diagnosed cases during a specific time period.
Drug treatments for TB:
The antibiotic of streptomycin was discovered and was promised to provide an effective treatment for TB. Austin Bradford Hill tested this drug by giving some people the drug and let others stay in bed and the drug worked better. Because this drug has harmful sides it has been superseded by new antibiotics as it can affect the nerves from the ears to the brain. Today treating TB is very effective by the means of 4 antibiotics and they have to be taken regularly for 6 to 12 months. The disease has to be under control as fast as possible otherwise drugs resistance may occur. A combination of drugs is used because it is unlikely for a drug to be resistant to the whole drug cocktail. Most deaths from multi-drug resistance have been patients infected with HIV.However, there are strains of bacteria resistant to all of the known anti-TB drugs. Better living conditions, vaccinations and drug treatment led to a dramatic fall in the cases of TB in the UK.
TB on the rise:
There has been an increase in notifications of TB since 1990 with people in close contact with a person with TB or people who have lived in place where TB is still common. Weakened immune systems caused by HIV or other medical conditions, people with chronic poor health as a result of homelessness, alcoholism or drug abuse and the very young and very old are most susceptible.TB is the leading cause of death for people who are HIV-positive. Badly managed TB treatments make TB incurable through the development of drug resistance. This is accelerated if doctors prescribe the wrong treatment or patients taking the full dose and stope when they feel better.
TB worldwide:
According to the WHO 90% cases of TB are in developing countries and 75% of those affect people in the 15-54 range when they are most productive which is economically serious.
Children have to leave school, women may be abandoned and a adult may miss three to four months of work. Multidrug resisantce caused by poorly TB treatment is a growing problem. The WHO has developed a strategy to detect and cure TB if applied systematically. Key elements:
- Political Commitment
- Microscopy servises to detect the TB bacteria in samples of saliva
- Steady drug supplies
- A drug regime which has proved effective
- Monitoring of patients by a health worker to ovserve and record patients swallowing the full course and the correct doses of drugs.
It also reduced new cases of drug resistance. Ambitious targets were to detect 70% of the cases and a target of 85% succesfull treatment. In the Western Pacific region this was accomplished but overall not. A crucial problem is some parts of the world is the lack of laboratory services with the capacity to confirm diagnosis and monitor treatments.
Influenza
Hundreds of millions of people suffer from an influenza infection every year.
The disease
Influenza (flu) is a relatively common respiratory disease casued by the influenza virus. There are several strains of the virus. The disease is highly infectious and has a very short incubation period.
Symptoms:
- Fever (often accompanied by shivering and sweating)
- Feeling very unwell and unable to do anything
- Loss of appetite
- Aching muscles and painful joints
Simple influenza: 5-7 days till fever goes down and convalescence begins. But the exhaustion which follows can last from 6-12 weeks.
Influenza infects the cells lining the tubes leading to the lungs, causing them to die. This leaves the airways open to infection, and many of the deaths associated with influenza are from severe secondary bacterial infections on top of the original viral invasion. The people most likely to die of the disease are the elderly and anyone who is prone to asthma or heart disease.
Treatment for influenza: rest, warmth, plenty of fluids and mild painkillers.
Immunization
Each year the various strains of the influenza virus are subtly different; the change is usually quite small, so having influenza one year leaves people with some immunity against infection for the next.
Antiviral drugs
Two antiviral drugs used to treat influenza patients are: oseltamivir (Tamiflu) and zanamivir (Relenza). These drugs are designed to halt the virus spreading in the body. The main effects of the drugs are to reduce the symptoms of the disease and make it less likely that the patient will suffer from complications.
Developing effective antiviral drugs is proving to be very difficult. Viruses constantly change as a result of mutations in the viral genetic code. This means that antiviral drugs effective against a particular form of a virus can quickly become useless against the different mutated version.
Pandemics of influenza
If a new influenza virus appears against which the human population has no immunity, there is a risk of simultaneous epidemics of the disease all over the world, leading to a huge number of deaths. This is a pandemic.
Influenza infects many animals as well as humans. Poultry farming is particularly vulnerable to some forms of ‘bird flu’ which can spread very rapidly where chickens or turkeys are crowded together on intensive farms. The disease can kill all birds in just a couple of days.
Millions of birds have been killed in attempts to control the disease, but scientists believe that an influenza pandemic is inevitable in the near future.
Chapter 9 - Evolution
The science behind the issues
If biologists are correct, all forms of life are sharing the same common ancestor some 3500 million yrs. ago. Many theories are controversial and the general public in some countries is divided over whether evolution really took place. Some believe it hasn’t and they believe the world is just 10000 years old. Scientist now have information about how genetics are passed to offspring. Changes start with mutations and constantly change. Natural selection is widely accepted as a major element of the process of evolution. Evolution is a scientific explanation for the diversity of life on Earth. The processes of fossilization and natural selection are open to experiment.
Charles Darwin
Charles Darwin is one of the most famous scientists of all time for two reasons: Het produced a hypothesis that succeeded in convincing scientists of the theory of evolution. Secondly, he came up with natural selection. He was born in 1809. Darwin as a student became interested in beetles, which lead to his ideas of natural selection. He was sent with the HMS Beagle that was to sail around the world to make world maps. The ships’ leader, Robert Fitzroy, doubted the truth of the biblical flood when Darwin still believed in that. During the five year sail, Darwin was not sure about the similarities and differences that occurred between animals and plants. He slowly became convinced that evolution explains the huge diversity of living species on Earth.
His evidence for evolution
During the years, Darwin got convinced by the fact that species weren’t fixed, but that they evolved from their ancestors over a huge period of time. This was lead by the example that fossils often resembled species currently alive in the same place, but were clearly different form them. Another reason for Darwin’s change of mind came from study of living species. The Beagle spent a month in the Galapagos, and the island provided Darwin with evidence that isolated groups of organisms can change. Darwin noted that one of the most striking things about the organisms on the Galapagos Islands is that few are found on the mainland of South-America yet all resemble organisms found on the mainland. On the first island, Darwin found a mocking bird like the South-Americans ones he knew but clearly different. On the second island he found another kind of mocking bird. On island three, he found yet another and labeled that.
Natural selection
Once home, Darwin tried to come up with a mechanism that could account for evolution. For scientist, a new period in the history of evolution starts here. Darwin once read an essay about human populations (Thomas Malthus), and saw how it might apply equally to animal populations, so there is your explanation. Eventually, Darwin came up with 4 conclusions:
- Individuals within a species differ from one another.
- Offspring generally resemble their parents.
- In all species more offspring are born than can survive to adulthood and reproduce.
- Some individuals are better suited to their environment than others, and they survive and reproduce more effectively than others.
By means of the following statements Darwin completed his theory of natural selection:
- Individuals that survive to reproduce pass onto their offspring the characteristics that enabled them, the parents, to survive.
- Over time, a group of individuals belonging to a single species may give rise to tow different groups that are sufficiently distinct to belong to two separate species.
The theory of natural selection was Darwin’s attempt to explain the observations he made. He spent years to strengthen his theory. Scientific theories need creative thinking. The best evidence for natural selection is ‘artificial selection’. Artificial selection is intentional breeding for particular characteristics. Darwin became so interested that he built a pigeon house, bought several different pigeons and let them breed. He found surprisingly large differences between the pigeons he bred. These differences were not just the visual ones; the insides also had changed. Red blood cells had mutated.
Jean-Baptiste de Lamarck
The ancient Greeks had been thinking about evolution as well. But Darwin wasn’t the first one to come up with the theory of evolution. Jean-Baptiste de Lamarck had published his very different theory 50 yrs. earlier. Lamarck stated that organisms changed as their environment changed, and thereby also changing inside body structures. They physically adapted to the environment. These adaptations could also be passed onto their offspring. He backed his theory up by means of the following example: a son of a blacksmith had stronger arms than the son of a weaver, a trait that was given to them by their fathers. Lamarck’s theory is much like Rudyard Kipling’s Just so stories, in which an elephant gets its trunk by having its nose pulled up by a crocodile. Lamarck’s theory suggested that giraffe’s reached up to eat leaves growing on higher trees. Their necks stretched as a result. Better said: characteristics which were vital were acquired during an animal’s lifetime so the animals progressed to an ideal form. A then acquired charac. could then be passed on to the offspring. Darwin’s explanation was different. It would state that there are always more giraffes than the food supply can support. As a result, those giraffes happen to have taller than average necks would be most likely to survive. This trait would be passed on to their offspring which will increase the length of the neck over the years. Lamarck is credited with attempting to explain the evolution of higher species from simpler ones from Darwin, even though his explanation didn’t match with the evidence from simple observations of inheritance.
Alfred Wallace
On 18th June 1858 Darwin got a letter from a relatively unknown naturalist named Alfred Wallace. The letter contained Darwin’s theory of evolution. This may seem strange, but when looking at the history of both, we can see that they both studied and thought about Malthus’s arguments (which were the fact that there would never be enough food to keep up with the world’s population). Wallace had been studying the rainforests in the Amazon and came up with 8000 new animal species. Then bad luck struck him; His ship caught fire and all (15000) animal species were lost including lots of his notes. After this he set for Asia -an 8 yr. trip- and he got the flu there. While laying on bed he thought about Malthus’s theories. Why do some die and some live? The effects of disease etc. Wallace then wrote his theory and sent it to Darwin. Darwin and Wallace joined up their theories and this became the generally accepted evolution theory.
How long does evolution take?
Offspring of dogs, sheep, pigeons etc. differ greatly from the ancestors a century ago. However, wall paintings from 30000 yrs. ago show recognizable differences from today’s species. Evolutionary biologists think that it takes millions of years for species to evolve into new species. Darwin had no idea, but he knew that his theory only could be valid if the world was very old. For evolution this meant that it only could have started 700 million yrs. ago. This is because only then multicellular organisms appeared. William Thompson, a well respected scientist in the age of Darwin, calculated that the Earth was 100 million yrs. old. Darwin changed his theories in order to let them come true with Thompson’s theory. Scientists today have corrected Thompson’s calculations so Darwin doesn’t need to worry about his theory anymore.
Reactions to Darwin’s theory
Darwin’s theories had by 1859 been accepted by critical scrutiny. One thing struck the people in those days; In the bible it says that humans descend from Adam and Eve, who were created by God. This was in conflict with Darwin’s statements, which say that we descend from worm-like creatures. Theologians of those days were quicker to accept Darwin’s theories than scientists. This is interesting. To this day, the theory of evolution remains controversial .
A human need to make sense of the organization of living organism on Earth is seen right back in ancient writings. In Aristotle’s view, the Great Chain of Being was perfect; there were no empty links in the chain an no link was represented by more than one species. There is still a cultural argument about the origins of life on Earth. On the one hand there are people who believe in the creation of God. On the other hand there are those who see scientific theories as distinct from religious belief.
The genetic basis for evolution, Gregor Mendel
Gregor Mendel was born in 1822. From 1856 to 1866, Mendel bred, examined, described and counted over 28000 garden peas. He discovered that the various characteristics of the peas (height, color, shape of seeds) were determined by factors or particles that passed unchanged down the generations and didn’t blend. We now know Mendel’s factors as genes. Mendel’s explanation of inheritance was right; in most species there is just a single copy of each gene in the sex cells, whereas all other cells have two copies of each gene. Lots of scientist, including Darwin were unaware of Mendel’s thoughts at that time. After Mendel’s theory became public, the hunt for gene understanding was on. Scientists soon appreciated that Darwin had been right when he saw variation between individuals as lying at the heart of evolution. Mutations are changes in the structure of an organism’s genetic material. They are caused by spontaneous errors, cell division, certain chemicals and radiation. If a mutated gene happened to be in a sex cell, copies of it were handed on to the offspring. It is only these mutations that occur in the sex cells that have a role in natural selection. Sometimes, mutations are very harmful. Usually they are beneficial.
The theory of evolution by natural selection
The fundamental principle behind the theory of evolution is that all the Earth’s present-day life forms have evolved from common ancestors, and ultimately from self-replicating molecules which happened to develop under the conditions then prevailing on Earth. The mechanism that best explains evolution is natural selection. Within a species, some individuals are better able to survive and reproduce in the environment in which they find themselves than are others. Over time, changes to the genetic make-up of individuals in a species accumulate, and can lead to the evolutions of new species. Among the implications of the Darwin-Wallace theory of natural selection is the idea that the process is lead by random events and doesn’t have any overall direction at all. Evolutionary biologists believe that all the species which now exist, and all those which have existed, can be linked in a single ‘branching tree’ structure. The popular view of life as progress, with humans at the top of the evolutionary tree, is misleading and certainly Darwin didn’t believe in this ranking of species. In evolution, there is no upward inevitable ascent towards perfection. Instead, species merely respond to the environmental pressures they face. A person’s ideas about animal rights and our responsibility to other species may reflect their views on the relationship between species.
Environmental pressure
The theory of natural selection relies on the existence of variation among individuals in a population, but also on environmental pressure. Competition for resources is just one type of environmental pressure. The end and result of natural selection is often extinction, as new species evolve and eliminate their relatives. Ecosystems are often able to withstand minor disruptions that are due to extinctions and environmental changes, but the knock-on effect of a single species becoming extinct is difficult to predict due to the complex interdependence of species.
Does natural selection explain all of evolution?
Generally, scientists are more confident about theories which provide a good explanation for what is being observed. Most biologists today accept that natural selection is the prime mover behind evolution. Modern evidence for evolution uses changes in DNA and proteins in cells as evidence for natural selection. Much of the modern debate about the mechanisms of evolution is around the unit of selection, whether this is genes, cells, individuals, or whole species.
