Evolution describes how particular or accidental gene changes have occurred over time to give the living organism characteristics they need to survive better.
During this mind-blowing topic, you will learn about natural selection and how this causes genetic variation, including how bacteria can become resistant to certain antibiotics.
In addition, this fascinating module will cover selective breeding, i.e. causing changes to genes on purpose to improve crops or livestock. As a direct result of human intervention, selective breeding causes new varieties of species to be born and is therefore referred to by some as artificial selection.
With a focus on continuous and discontinuous variation, you will also find out what types of characteristics fall into which category as well as exploring the effects of gene mutation.
A child is unlikely to look identical to their mother or father respectively because they inherit genes from both. Photo by beltz6 on Visualhunt
The paragraphs that follow will shed light on just some of the different elements of evolution to give you a better understanding of the topic, mainly in relation to humans and animals.
To help you to follow the biological parts, processes and functions described below, we have provided a glossary of terms, put together by the experts at BBC Bitesize, which can be referred to whilst you read this text, or can be saved for future reference. Go to their website for tonnes more guides, tests and other useful references during your course.
Note: Having a list of key terminology can be very useful when you come to revise for your exam, as this topic is likely to come up in at least one question on your paper.
(Extracted From BBC Bitesize Evolution)
“Acidic: Having a pH less than 7.
Alkaline: Having a pH greater than 7.
Alleles: Different forms of the same gene.
Antibiotics: Substance that controls the spread of bacteria in the body by killing them or stopping them reproducing.
Bacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium.
Chromosome: A structure made of DNA that codes for all the characteristics of an organism.
Evolution: The change of inherited characteristics within a population over time through natural selection, which may result in the formation of a new species.
Fertilisation: The joining of a male and female gamete.
Gene: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell’s chemistry – particularly protein production.
Genetic: To do with inheritance because of genes.
Haploid: A sex cell (gamete) that contains one set of chromosomes.
Homozygous: This describes a genotype in which the two alleles for the characteristic are identical.
Meiosis: Reduction division in a cell in which the chromosome number is halved from diploid to haploid.
MRSA: Methacillin-resistant Staphylococcus aureus, a strain of bacterium resistant to an important antibiotic.
Mutagen: A physical or chemical agent that can induce or increase the frequency of mutation in an organism.
Mutation: A random and spontaneous change in the structure of a gene, chromosome or number of chromosomes.
Natural selection: The natural process whereby the best-adapted individuals survive longer, have more offspring and thereby spread their characteristics. Sometimes referred to as ‘survival of the fittest’.
Neutral: A neutral mutation is a change in the DNA of an organism that has no effect on the organism itself.
Radiation: Energy carried by particles from a radioactive substance, or spreading out from a source.
Recessive allele: Alternative form of a gene that is expressed only if a dominant allele of that gene is not present. An organism must have two copies of a recessive allele for that allele to be expressed.
Selective breeding: An artificial process in which organisms with desired characteristics are chosen as parents for the next generation.
Species: A type of organism that is the basic unit of classification. Individuals of different species are not able to interbreed successfully.
Variation: Difference between individuals; distance from the norm.
Zygote: A fertilised egg cell.”
With our planet being approximately 4.5 billion years old, there is evidence to prove that life was first detected around three billions years ago. However, Earth would be very far from the environment we know and live in today.
Evolution is the theory that species have developed over time from alternative life forms, like humans evolving from ape-like ancestors.
The idea behind evolution is that it happens naturally, so those with characteristics most suited to a particular environment survive and reproduce, which eventually causes the unsuitable population to diminish. This is called natural selection.
A good example of natural selection is giraffes: those with longer necks were better adapted to survive. Photo on Visualhunt.com
Genes and the environment alike can cause variation, yet only genetic variation can be passed on.
Selective breeding, on the other hand, relies on human intervention and is therefore occasionally referred to as artificial selection. This is the traditional method used by farmers to improve crops and livestock by carefully selecting cows that produce higher quantities of milk, chickens that produce larger eggs and wheat plants that produce more grain.
The new super-efficient varieties can be very important to the economy as they could be responsible for improving the quality of our diets. That said, domestic pets are often the outcome of selective breeding to either make them look cuter or with the purpose of crossbreeding species to produce animals that are better suited to specific lifestyles and environments.
One of the biggest problems with selective breeding is that, by making future generations very similar in terms of their gene structure, diseases could have dangerous effects on the entire population of the species and may not be controlled as easily. Also, recessive alleles can lead to a number of genetically inherited diseases.
Species within a kingdom are often quite similar but have their own unique traits. While some of the differences are caused by genes, some are a result of the environment in which they live. Furthermore, a combination of both might come into play.
Take, for example, gun dogs. The genetic make-up of breeds of dogs like retrievers means that they are instinctively driven to retrieve game for hunters. However, domestic retrievers differ from working dogs in their temperament and playfulness because of the different environments in which they have been brought up.
Although still instinctively a gun dog due to the genetic make-up, do,estimated retrievers will differ from working dogs because of the environment they are exposed to. Photo on Visualhunt.com
While most children resemble their parents, they are unlikely to look identical to one of them because they inherit half of their genes from one parent and the other half from the other. Yet, they will neither look 50% like their mother and 50% like heir father because of genetic variations that occur at fertilisation.
Genetic variations in humans can include blood group, skin colour, eye colour and more and are caused by haploid cells joining at fertilisation to create new diploid cells.
Environmental variation, which includes things like language and religion in humans, are characteristics that are affected by factors like culture, lifestyle, climate and diet.
Variation can be described as continuous or discontinuous, with the first having no limit on the value that can occur win a population (i.e. height, weight, leaf length, etc…) and the latter having distinct groups for organisms to belong to (i.e. fingerprints, blood group, eye colour, etc…).
In some cases, mutation can occur which can have no effects at all, or can be beneficial to the species. For example, carriers of the sickle cell allele are more resistant to the tropical disease, malaria, than those who are homozygous for the gene.
On the other hand, gene mutation can result in changes that will affect the carrier’s development and lifestyle. Down’s syndrome is a mutation caused by the presence of three (or almost three) particular chromosomes (known as chromosome 21), instead of just two. This happens when the sperm or egg cell forms abnormally during meiosis.
Evolution, as you can probably tell, is like a never-ending story. With so much evidence and so many theories to get to grips with, evolution explains to us how many species, including humans, came about. However, Evolution cannot accurately tell us how life will change in hundreds of years, as this is still very much unknown.
Additional subtopics covered by Evolution are:
Genetic modification and cloning
Old and new species
Check out our other blogs covering more topics on the GCSE Biology curriculum:
GCSE Biology Syllabus: Topics At A Glance
GCSE Biology Revision: Reproduction In Humans
GCSE Biology Revision: Living Organisms
GCSE Biology: Digestive System
GCSE Biology Revision: Inheritance and Genetics