Slide 1
In the previous tutorial we used the Hardy-Weinberg equation to determine expected allele and genotype frequencies in a population at a given time. The definition of evolution is a change in allele and genotype frequencies, or a change in the gene pool of a population. WHen population is not changing or not evolving it is in Hardy-Weinberg equilibrium. Biologists have determined 5 factors that maintain Hardy-Weinberg equilibrium in a population.
These 5 factors are:
No mutation
No gene flow
No genetic drift
Random mating
No natural selection
Slide 2
This means that the opposite five factors will result in evolutionary change.
They are:
Genetic drift
Gene flow
Mutation
Nonrandom mating
Natural selection
These five factors are known as the agents of evolutionary change. They cause populations to evolve. We will now talk a little bit about each of these factors.
Slide 3
We recognize two basic levels of evolution. Macroevolution refers to evolutionary changes that mark the appearance of a new species or a new genus or a higher level of biological classification. For example, birds evolving from a dinosaur ancestors is a example of macroevolution.
Microevolution, on the other hand, refers to evolutionary changes that lead to alterations of allele and genotype frequencies within a populations. In this tutorial we are focusing on microevolution changes.
Slide 4
Genetic drift is a phenomena of small populations. Please note genetic drift and gene flow which we will be talking about later, are two separate phenomena. People often confuse them. Genetic drift refers to the random fluctuations in allele and genotype frequencies that occur in populations because they are small.
Slide 5
This image illustrates the impact of genetic drift on a small population. On the left side of the diagram is a large population of penguins. Individuals in red carry the allele that we are monitoring. On the right side of the diagram is a small population of penguins with the carriers once again in red. If 50% of each population dies, the large population does not experience dramatic change in allele frequency. The frequency of the allele goes from 10% to 9% which is a rather small change considering the dramatic decrease in size the population has experienced. In the small population however, the loss of 50% of the population has a much greater impact on the allele frequency. By change the single carrier of the allele was lost in the population decline and the frequency of the allele has gone to zero. Small populations experience changes in allele and genotype frequencies more dramatically than large populations.
Slide 6
There are two examples of how a population can become small so that genetic drift is a factor. These are the founder effect and the bottleneck effect.
Slide 7
A bottleneck effect occurs when a catastrophe decimates a large population and the population becomes small. Northern elephant seals have reduced genetic variation because of a population bottleneck humans inflicted on them in the 1890s. Extreme hunting pressure reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 individuals but their genes still carry the marks of this bottleneck. They have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
Slide 8
The founder effect refers to the loss of genetic information when a new colony is formed by a very small number of individuals from a larger population. The old order Amish of Pennsylvania are an example of a population that has experienced a founder effect. Because of their closed population stemming from a small number of German immigrants, about 200 individuals, the Amish carry unusual concentrations of gene mutations that cause a number of otherwise rare inherited disorders. Including forms of dwarfism such as Ellis-van Creveld syndrome, seen here. Polydactyly, the presence of extra fingers or toes is an other aspect of this condition