Variation, Population Genetics and Natural Selection

Variation, Population Genetics and Natural Selection

1. Variation

   • Phenotype- an organisms physical appearance, constitution and behaviour
   • Genotype- an organisms genetic make up
   • Interspecific Variation- variation that occurs between different species
   • Intraspecific Variation- Variation that occurs within a species
     • e.g in snails; banding, colour of bands, colour of shell, size
   • Some characteristics fall into a limited number of distinct forms, with no intermediates e.g. the ABO blood group system in humans. This is due to genetic factors only, where only one or two genes act. This is discontinuous variation.
   • Certain characteristics only vary slightly between individuals in a population. This results in a range of values from one extreme to the other. Features which vary in this way are often controlled by many different genes, they are polygenic, and are also usually influenced greatly by the environment. e.g the height and mass of an organism shows this type of variation. This is continuous variation.
   • Causes of Variation
     • Environmental- such as disease, climate and diet.
     • Genetic
       • These are due to differences in genes in individuals which arise as a result of; meiosis, mutations and the random fusion of gametes.
         • Asexually reproducing organisms only show variation as a result of mutations.
         • Meosis occurs in the formation of gametes, cells of this type are genetically different from one another and so meiosis is a major cause of intraspecific variation.
           • Meiosis can introduce genetic variation into gametes by crossing over and independent segregation of chromosomes.
         • Mutations cause a change in the number or sequence of bases in the DNA of an organism. This produces a change in the characteristics of the organism which can be passed onto cells produced by the division of the mutated cell.
           • A frameshift mutation is an addition or a deletion as when an extra base is added or removed in the sequenced, all subsequent codons after the mutation are altered. Therefore a different amino acid sequence is coded for.
           • Duplication of Bases- this is when one or more bases are repeated, this produces a frameshift to the right.
           • Inversion of bases- a group of bases becomes separated from the DNA sequence and rejoin at the same position but in the inverse order. The base sequence of this portion is therefore reversed and effects the amino acid sequence coded for.
           • Translocation of bases- a group of bases becomes separated from the DNA sequence on one chromosome and then becomes inserted into the DNA sequence on the same or different chromosome. Translocations often have significant effects on gene expression leading to an abnormal phenotype.
           • Causes of Mutation- Mutagenic agents. Gene mutations can arise spontaneously during DNA replication, despite being random occurrences, mutations occur with a predictable frequency that varies from species to species. This basic mutation rate can be increased by outside factors known as mutagenic agents or mutagens. These include; high energy ionising radiation and chemicals.
         • Random fertilisation of gametes causes genetic variation as which gamete fuses with which at the point of fertilisation is a random process, and this produces a new combination of alleles and the offspring are therefore genetically different from the parents.
     • The phenotype of an organism is mainly determined by genetic factors, however the degree in which the genotype is expressed can be influenced greatly by the environment of the individual.
       • e.g conside human height, if you have inherited a number of alleles for tall height from your parents you have the potential to grow tall. However if your diet is poor while you are growing you are unlikely to fulfil this potential.
     • GENOTYPE + ENVIRONMENT = PHENOTYPE

2. Population genetics

   • A population is a group of organisms of the same species living in the same habitat/ ecosystem.
   • A gene pool is defined as all the alleles in a population at a particular time.
   • The Hardy- Weinberg Equation
     • For any particular gene there is usually more than one allele, therefore in the population, each allele represents a certain proportion of all the alleles in the population. This proportion s the allele frequency, the frequency with which an allele occurs has little bearing on whether it is dominant.
     • The general formula to represent the frequency with which the dominant and recessive forms of an allele occur in a gene pool of a population is: p+q=1
       • Where p represents the frequency of the dominant allele and q represents the frequency of the recessive allele.
       • p^2 +2pq + q^2 = 1.0
         • p^2 is the frequency of the homozygous dominant genotype, q^2 is the frequency of the homozygous recessive genotype and 2pq is the heterozygous genotype frequency.
     • The frequencies of the alleles of a particular gene in a population will stay constant from generation to generation.
       • However this only happens with certain conditions.
         • The populations must be large- in small populations, chance events can cause large swings in frequencies.
         • Mating between individuals must be random- this ensures an equal chance of each allele being passed on to the next generation.
         • No mutations must occur- this will introduce new alleles changing the allele frequencies.
         • There must be no migration into the population or out of the population- migration adds new alleles to or removes alleles from the population.
       • When the conditions are met and the allele frequency does not change, then the population is said to be in genetic equilibrium. The Hardy Weinberg equation only provides a reliable estimate of allele frequencies if this is the case. However the conditions listed never occur in real life as they go against natural laws.

3. Natural Selection

   • Identify the selection pressure/ change in environment. Identify potential beneficial phenotype and allele responsible (due to mutation. Individuals with beneficial phenotype are more likely to survive and reproduce and have more offspring. Thus the allele is more likely to be passed onto their offspring. Frequency of allele in population increases.
   • Types of Selection
     • Directional Selection
       • Results in an increase in the frequency of one phenotype relative to another. Directional selection acts against one of the extreme phenotypes and for the other. The mode changes, it happens in changing environments.
     • Stabilising selection
       • This type of selection acts against both of the extremes in a range of phenotypes, it favours the average phenotype and so acts to prevent change. The mode stays the same but the range in phenotypes decreases, it happens in an unchanging environment.
     • Disruptive Selection
       • In this type of selection both extremes of a phenotype have a selective advantage over those in the middle. It is the most important form of selection in bringing about evolutionary change and speciation. Can occur if there is too much competition in the middle region.
         • An example of this is the Pacific Caho salmon where large maels and small males have a selective advantage over middle sized males in passing on their alleles to the next generation by being more successful at fertilising females eggs on the riverbed. This is because the larger males can compete for females and small males can sneak up to the females in the spawning grounds without being seen.
   • Speciation
     • Selection changes gene pools causing a species to change greatly over time. The greatest change occurs when an existing species gives rise to two or more new species, this process is called speciation.
     • There are two types of speciation.
       • Allopatric Speciation
         • This is when two populations of the same species become geographically separated by a physical barrier and so are prevented from interbreeding. During the time where they are isolated genetic differences between the populations increase through selection due to different environmental conditions. Eventually the two populations are unable to interbreed and have become distinct species.
       • Sympatric Speciation
         • In this type of speciation, the two populations may inhabit the same area, but may be reproductively isolated from one another. Such as temporal isolation- members of the two populations reproduce at different times of the year. Mechanical isolation - anatomical differences may prevent mating. Behavioural isolation- members of the two populations have different courtship patterns.
     • The stages leading to speciation are usually as follows; Part of a population becomes isolated in some way. No interbreeding can occur between the two populations. Also different mutations may occur in each of the new populations, which will increase variation and may result in new beneficial alleles. The two isolated populations experience different selection pressure and environmental conditions. Different features and alleles will be an advantage in each environment, and so natural selection operates separately in the two populations, with different features selected for each case. In each population organisms with advantageous features will be more likely to survive and reproduce, than those without, and so the allele frequencies in each population will change in subsequent generations. Eventually individuals from each group wouldn't be able to interbreed to produce fertile offspring and so would be different species.
   • Genetic Drift
     • Genetic drift is the idea that allele frequencies can change simply due to chance, some individuals fail to survive or reproduce simply due to bad luck not because they are poorly adapted. In a small population these chance events can play a large part in changing allele frequencies.
     • Genetic drift may occur because of the founder effect or genetic bottlenecks.
       • The founder effect- sometimes happens when a few organisms from one population become isolated from the rest of the population. These individuals will carry with them only a small number of the alleles present in the parent population. This leads to a reduction in genetic diversity as there is a reduction in the variety of alleles.
       • Genetic Bottlenecks- A population is said to go through a genetic bottleneck when it is temporarily reduced to a very small number when the majority of individuals are killed. This could be due to a chance event or interference by man. The few survivors will only contain a small portion of the alleles present in the original population. This may result in a dramatic reduction in genetic diversity.