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Evolution Main Concepts
Natural selection, variation - what are the main concepts related to Evolution? Find out more and gain tips for your essays and coursework with our analysis here.
The differences between individuals in a population are called variation. The differences between members of the same species are called intra-specific variation. These differences may be in observable characteristics such as hair colour, or differences in behaviour, such as intelligence or temperament.
The differences may also be seen in the biochemical molecules found within the cells of the organism such as DNA, which will give rise to differences in the amino acid sequence of proteins.
Variation will also occur between members of different species. This is called inter-specific variation. The number of differences will tend to be smaller in closely related species and greater in species that are not closely related.
The environment, in which they are found, will also influence each individual.
There are some characteristics that have no distinct groups or categories, such as the height of a plant or man, or the mass of a person. There is a complete range of measurements between two extremes. These variations can be measured and the results then plotted in the form of a histogram. The feature is therefore quantitative. This histogram will have a characteristic bell- shaped curve known as a normal distribution curve. Continuous characteristics are controlled by a large number of genes. This is called polygenic inheritance.
With a large number of genes involved in determining the characteristic there can be a very large number of possible combinations of alleles during meiosis and this gives a very large number of different phenotypes. Environmental factors can also contribute to continuous variation. For example, plants may be genetically identical but could grow to different heights due to differences in soil mineral availability, or the light intensity in the growing region. Continuous variation is the product of the polygenes and the environment.
Some characteristics fit into very distinct forms or types e.g. biological sex blood group -there are no ‘in-between’ characteristics. This is called discontinuous variation.
The ABO blood group system gives rise to four different blood groups, A, B, AB and O. This is because the characteristic is controlled by one single gene that has three different alleles. The different combinations of alleles gives rise to the four different blood groups. Discontinuous characteristics are not measured, but the numbers falling into each group are counted. The data are then plotted as a bar chart showing the numbers of individuals in each group. The discontinuous characteristic is qualitative meaning it cannot be measured. Environmental factors have no, or very little effect, on discontinuous characteristics.
Causes of variation
There are two causes of variation – genetic and environmental. The genotype of the individual is the genes they possess, and which in particular which alleles, or forms of these genes, are present in the DNA. The genotype can vary as a result of mutations, crossing over and independent assortment during meiosis, the random pairing and mating of individuals and the random fusion of gametes. These variations can be passed from one generation to the next. Genetic variation is heritable. Examples would include eye colour, colour of the seed in peas and coat colour in cattle.
Environmental factors can affect the way that the alleles are expressed in the individual. There are a large number of factors that can affect characteristics such as temperature, food availability, light intensity, soil pH and rainfall. Some environmental factors can also cause genetic mutations, or can cause certain genes to actually be switched on or off. Environmental variation is not passed from one generation to the next. It is non-heritable.
In any environment there will always be competition for the available resources such as food, water, light, territory and nutrients. The population may also have to survive poor conditions such as drought, disease, heat and cold. These factors are called selective agents. Darwin observed that there are variations in each population, but a tendency for the population size to remain fairly constant, even though large numbers of offspring are produced. It is this variation in the population that allows some members to survive unfavourable conditions, or to compete for resources more successfully than others. All members of a species will have physiological, anatomical or behavioural adaptations enabling them to live in their chosen environment.
If the environment changes then the organisms living there may no longer be fully adapted, and this places a selection pressure on the population.
This is the pressure of competition to survive and have surviving offspring. The word ‘selection’ refers to the fact that some variants (the ‘fittest’) are better at doing this than others. Fitness in this case means reproductive success. Darwin proposed that members of a species that are most suitably adapted to the environment have a greater survival and reproduction rate than others that are less well adapted. This is his theory of natural selection. Over time natural selection causes changes in the frequency of different alleles in the gene pool of the population, and this is the basis of evolution.
This occurs when there has been an environmental change, which causes a selection pressure. In a mixed population of brown and white rabbits, the white rabbits would be at an advantage if there were a new Ice Age but are at a disadvantage in temperate climates, as predators easily see them. White rabbits may survive, reproduce and pass on the alleles for this characteristic to the next generation if a new Ice Age occurred.
This type of selection maintains the variety in the population but tends to select against the extremes, especially in a stable environment. In animal populations extremely large or extremely small individuals are less likely to reproduce so the alleles for this characteristic are not passed on. Extremely large and extremely small babies also have a lower survival rate.
This type of selection tends to favour the extremes. The middle of the range is selected against. This was seen in some of the species of Galapagos finches. During a drought those birds in the population with larger, stronger beaks could break open seeds and use these as a source of food. The birds with smaller, thinner beaks could retrieve small insects from cracks in plants as an additional food source. These options were not available to birds with the medium sized beaks and so these birds did not survive. Each extreme passed the alleles on for their beak type to their offspring this type of selection causes divergence – there are two distinct forms of the species. Although uncommon, this type of selection may be more likely to result in the formation of new species.
It is usual to define a species as a number of individuals that share similarities, and reproprodue fertile offspring. If a male horse mates with a female donkey then the offspring is called a mule. The mule is a sterile hybrid. For this reason the horse and the donkey are considered to be separate species. New species must evolve slowly over time from pre-existing species.
Evolution of a new species
There are three conditions that may lead to the development of a new species:
Isolation - Part of the population becomes cut off or isolated so it cannot breed with the rest of the population. In allopatric speciation rivers, mountains or roads physically separate the populations. In sympatric speciation the populations can be in the same place but do not interbreed due to different mating seasons or incompatible mating behavior, or genitalia.
Natural selection - a change in the environment causes certain individuals to not survive and this changes the frequency of certain alleles in the population.
Speciation – the genetic differences between the populations become greater with each generation and may cause them to be unable to breed together.