BIOLOGY 3201 UNIT 4 EVOLUTION CH. 20 - MECHANISMS OF EVOLUTION
POPULATION GENETICS AND HARDY WEINBERG PRINCIPLE Population genetics: this is a study of the genes in a population and how they may or may not change over time. Recall: if there is a shift in the gene pool of a population then we know evolution is happening Population: a localized group of a single species occupying a particular area Gene pool: this is the total of all genes within a population
HARDY WEINBERG PRINCIPLE A principle proposed by English mathematician G. H. Hardy and German physician G. Weinberg. Is a model of a population that is NOT changing to help understand a population that is changing. Premise - the principle states that in a population, under certain conditions, the frequency of alleles will remain stable from generation to generation. In other words, under certain conditions, a populations genetic makeup will not change meaning it is not evolving. It is in genetic equilibrium. The principle explains why recessive alleles do not disappear in a population over time and it helps explain why dominant alleles do not become more widespread.
CONDITIONS NECESSARY TO ESTABLISH A POPULATION IN HARDY- WEINBERG EQUILIBRIUM 1. No mutations occur in the population 2. The population is large 3. Mating is random 4. There is no immigration or emigration from the population 5. No genotype has an advantage over another.
FORMULAS USED IN HARDY-WEINBERG EQULIBRIUM The Hardy-Weinberg principle uses formulas to help calculate the frequency of alleles and genotypes within a population. 1.p + q = 1 p = frequency of dominant allele (how often dominant allele shows up in the total population) q = frequency of recessive allele (how often recessive allele shows up in the total population) 1= 100%
FORMULAS USED IN HARDY-WEINBERG EQULIBRIUM 2. p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant genotype 2pq = frequency of heterozygous genotype q2 = frequency of homozygous recessive genotype
HARDY-WEINBERG EQUILIBRIUM - AN EXAMPLE Phenotype Black Black White Genotype BB Bb bb # of gerbils 196 168 36 Total # of gerbils Genotype frequency 400 400 400 196/400 = 0.49 168/400 = 0.42 36/400 = 0.09 Allele frequency B = (196 + 168)/800 = 0.7 b = (168 + 36)/800 = 0.3
HARDY-WEINBERG - EXAMPLE 2
Mechanisms of Evolution The following are mechanisms that cause genetic variation in a population and thus move it away from Hardy-Weinberg equilibrum: 1. mutations 2. genetic drift 3. gene flow 4. non-random mating 5. natural selection 6. sexual selection
Mutations These are changes in the DNA that can bring new alleles in a population. The new alleles provide variations that cause evolution Mutations can be harmful, beneficial, or neutral Mutations are beneficial if they provide a selective advantage which allows certain organisms to adapt to their environment.
Genetic Drift This is a change in allele frequencies in small populations caused by chance alone. Ex. In a small population mutations can cause the allele frequencies to change whereas in a large population the mutations may have little to no effect on frequencies. The gene pool will not be affected if the population is large. The allele frequencies in small populations can change over time and this can lead to evolution. Remember: In a non-evolving population (Hardy-Weinberg equilibrium) the allele frequencies remain unchanged
Causes of Genetic Drift 1. Bottleneck effect: a situation in which, as a result of chance, some alleles are overrepresented and others underrepresented because a population has been reduced through natural disasters, etc. Ex. Elephant seals have passed through a bottleneck. They have been overhunted causing their numbers to be reduced to about 20. Because of this, certain alleles have been eliminated (variety reduced). The population has since grown to 30, 000 having little variation. This has resulted in a change of allele frequency.
Causes of Genetic Drift 2. Founder effect: when a small amount of organisms (called a founder population) move into a new area, chances are they do not contain the entire genes representative in the parent population. This results in a change in the allele frequencies. Ex. Hawaiian Honeycreeper birds migrated from North America
Gene Flow This is a movement of genes into or out of a gene pool This causes a change in the gene pool resulting in evolution If gene flow happens enough between two neighbouring populations they may eventually merge into one population with a common genetic structure.
Non-Random Mating If a population mates on a random basis, genetic equilibrium is maintained and the population does not evolve Normal populations do not undergo random mating. For example, individuals will mate more with their neighbours rather than distant organisms. There are 2 types of non-random mating: Inbreeding Assortive mating
Inbreeding and assortive mating Inbreeding: Mating between closely related organisms Inbreeding will cause a loss of variety in the population and the allele frequencies will change Assortive mating: where organisms chooses mates similar to themselves Artificial selection (breeding of certain dogs) is an example of assortive mating. The dogs being mated are choosing (or are chosen from) mates that are similar to themselves Assortive mating causes a reduction in variety in the population and allele frequencies change
Natural Selection A population s characteristics can change because certain individuals within the population have heritable traits that allow them to adapt to and survive local environmental conditions There are 3 types of natural selection: Stabilizing selection Directional selection Disruptive selection
Stabilizing Selection This is natural selection where an intermediate or normal phenotype is favoured over the extremes Ex. Birth weight: most babies born today are of average or normal weight because the extreme (low or high) birth weight babies are selected against (we do not see many low birth weight or high birth weight babies anymore). The middle phenotype is favoured
Directional Selection Selection where one extreme is favoured over the other. This will cause a shift in the phenotypes in that direction. This type of selection is common during environmental change or when a population migrates to a new habitat Ex. The modern horse (adapted to a grassland habitat) has adapted from an ancestral horse (adapted to a forest habitat). Most horses today resemble a modern horse and not a forest horse.
Disruptive Selection Selection where both extremes of the phenotype are selected rather than the middle (intermediate phenotype) The intermediate phenotype may be eliminated from the population. Ex. Coho salmon. Males are either small or very large. No real medium size male salmon found in the population.
3 types of natural selection
Sexual Selection This is selection based on being able to find a suitable mate in which to produce offspring. Having the ability to choose a mate helps ensure genetic information is passed on as well as introduces variety into the population. Finding a suitable mate is based on 2 main characteristics: 1. Male competition male competition can determine who gets the chance to mate with a female 2. Female choice females choose who they mate with