6.3 Genetics and evolution Flashcards
Charles Darwin theory of evolution
Charles Darwin, as a result of observations on a round-the-world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection.
Darwin’s theory of natural selection states that:
individuals in a species show a wide range of variation caused by differences in genes.
individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce.
alleles that code for advantageous characteristics are therefore passed to offspring at a higher rate than those that code for characteristics less suited to survival.
over many generations these beneficial characteristics become more common in the population and the species changes, or evolves.
This idea of natural selection became known as survival of the fittest.
Darwin published his ideas in his famous book, On the Origin of Species (1859).
Acceptance of evolution
The theory of evolution by natural selection was only gradually accepted because:
There was much controversy surrounding these revolutionary new ideas.
The theory challenged the idea that God made all the animals and plants that live on Earth.
There was insufficient evidence at the time the theory was published to convince many scientists.
The mechanism of inheritance and variation was not known until 50 years after the theory was published.
The theory of evolution by natural selection developed over time and from information gathered by many scientists.
Lamarck’s theory of evolution
Another theory of evolution, developed at the start of the 19th century (before Darwin announced his theory), was that of French scientists Jean-Baptiste Lamarck.
Lamarck’s theory was based mainly on the idea that changes that occur in an organism during its lifetime can be inherited.
His theory involved two main ideas:
a characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn’t used gradually disappears.
the beneficial characteristics that are used frequently (and are improved as a result) are passed to offspring.
We now know that in the vast majority of cases this type of inheritance cannot occur and that Lamarck’s ideas were incorrect.
Alfred Russel Wallace
Alfred Russel Wallace was a scientist who, after conducting his own travels around the world and gathering much evidence, independently developed his own theory of evolution based on the process of natural selection.
He published scientific papers on this theory with Darwin in 1858 (Darwin published his book, On the Origin of Species, the following year.
Wallace is best known for:
His work studying the warning colouration of species (particularly butterflies) and how this must be an example of a beneficial characteristic that had evolved by natural selection, as the warning colouration helps to deter predators and Developing the theory of speciation.
Speciation
Speciation is a process that results in the formation of a new species.
When populations of the same species become so different that they are unable to interbreed and produce fertile offspring, they are considered different species and speciation has occurred.
Speciation can occur as a result of a combination of isolation and natural selection.
Populations of the same species can become isolated from one another due to the formation of a physical barrier – this is known as geographic isolation.
The environment will be different on either side of this physical barrier.
The environmental differences on either side will provide different selection pressures and natural selection will cause a different set of characteristics to become more common in the two isolated populations.
Over many generations, individuals from the two populations will have become so distinct (genetically, behaviourally, physically) that they will no longer be able to interbreed and produce fertile offspring.
The two populations are now separate species.
Mendel’s work on genetics
Gregor Mendel was an Austrian monk..
He was trained in mathematics and natural history at the University of Vienna.
In the mid-19th century, Mendel carried out breeding experiments on plants.
He studied how characteristics were passed on between generations of plants.
One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged.
His work eventually provided the foundation for modern genetics.
The importance of Mendel’s discovery was not recognised until after his death:
His studies were totally new to science in the 19th century.
There was no knowledge of the mechanisms behind his findings (DNA, genes and chromosomes had not been discovered yet).
Chromosomes behaviour
In the late 19th century and early 20th century, the behaviour of chromosomes during cell division was observed.
Scientists realised that chromosomes behaved in a very similar way to Mendel’s ‘hereditary units’.
Scientists believed Mendel’s ‘units’ must be located on chromosomes.
We now know that this is true and call these ‘units’ genes
Discovery of DNA structure
In the mid-20th century (1953 to be precise) the structure of DNA was determined and the mechanism of gene function worked out.
This scientific work by many scientists led to the gene theory being developed.
Evolution of bacteria
The theory of evolution by natural selection is now widely accepted and many sources of data are now available to support the theory of evolution.
One very clear piece of evidence for evolution is antibiotic resistance in bacteria.
Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span.
Like all other organisms, within a population, there will be variation caused by mutations.
A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin).
When the population is treated with this antibiotic, the resistant bacteria do not die.
This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead.
Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation.
Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment.
Evidence for Darwin’s theory
Darwin’s theory of evolution by natural selection is now widely accepted.
Evidence for Darwin’s theory is now available as it has been shown that characteristics are passed on to offspring in genes.
Our understanding of genetics has made clear the mechanism by which natural selection can occur.
There is further evidence for evolution in the fossil record and our knowledge of how resistance to antibiotics evolves in bacteria.
Formation of fossils
Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks
Fossils may be formed in several ways:
From parts of organisms that have not decayed because one or more of the conditions needed for decay are absent.
When parts of organisms are replaced by minerals as they decay.
As preserved traces of organisms, such as footprints burrows and rootlet traces.
Gaps in the fossil record
Many early forms of life were soft-bodied, which means that they have left few traces behind – soft tissues often decay fully, leaving no trace in the fossil record.
What traces there were have been mainly destroyed by geological activity (eg. tectonic plate movements may have crushed fossils that had already formed)
This is why scientists cannot be certain about how life began on Earth.
Changing planet
Evidence for early life forms on Earth can be found in the fossil record.
We can learn from fossils how much or how little different organisms have changed as life developed on Earth.
Evolutionary trees are diagrams that show the relationship between species over evolutionary time.
A new branch in the tree shows where speciation has occurred (when a new species has evolved).
Extinction
Extinctions occur when there are no remaining individuals of a species still alive.
Species that are poorly adapted to their environment are less likely to survive and reproduce compared to species that are well adapted to their environment.
If they are unable to survive and reproduce sufficiently to maintain their population numbers they will eventually go extinct.
Some factors which may contribute to the extinction of a species include:
New disease, new predators, new better adapted and more successful competitors, changes to the environment and catastrophic events.
Development of antibiotic resistance
This is an example of natural selection that humans have helped to develop due to overuse of antibiotics in situations where they were not really necessary, for example:
For the treatment of non-serious infections.
Routine treatment to animals in agriculture.
Failure to finish the prescribed course of antibiotics.
MRSA
Increases in the population of antibiotic-resistant bacteria cause infections and diseases which are harder to control as it is difficult to find antibiotics that certain strains of bacteria are not resistant to.
An example of this is MRSA, a very dangerous bacterial strain that is resistant to most antibiotics.
If someone gets infected with MRSA they cannot be treated easily.
Antibiotic development
The increase we are currently seeing in antibiotic resistance is encouraging drug companies to develop new antibiotics that are effective against these new resistant strains, such as MRSA.
However, the number of new antibiotics discovered has slowed significantly
Developing new antibiotics is also a very costly process.
Some scientists are worried we may not be able to keep up with the demand for new antibiotics, as more and more antibiotic-resistant strains evolve.
