9.2 onwards extended response Flashcards
Gene flow
The transfer of alleles from one population to another through migration
migration
The movement of people from one area to another with the intention of settling permanently. If immigrants to a certain country bring alleles that are not already in the population, the frequencies for that allele of that gene will be altered
What are the two barriers to gene flow
- Geographical barriers: A feature of the landscape that prevents populations from interbreeding. Includes oceans, mountain ranges, larger lake systems, deserts and expensive ice sheets
- Socio-cultural barriers: Barrier to interbreeding that is due to social or cultural factors such as economic status, educational background and social position
What is an example of how the distribution of ABO blood groups has been influenced by migration
The change in the frequency of the (I B) allele across Europe and Asia. The inhabitants of East Asia (mongols) have a proportionately higher frequency of the (I B) allele than those living to their west in Europe. It is thought that most western Europeans originally did not have the (I B) allele at all until the mongols Invaded Europe, spreading not only their culture but their genes as well. Today, There is a steady decrease in this area from central Asia to western Europe
Special creation
The belief that a god created all species
Evolution
The process of cumulative, gradual, and heritable change in a population of organisms that occurs over many generations
Who put forward the theory of evolution
Charles Darwin and Alfred Russel Wallace in 1858 however Darwins name is usually associated with the theory because of the massive amount of supporting evidence he collected
on the Galapagos islands, Darwin was able to observe the differences and similarities between animals separated by what
- Geography: those living on the mainland of South America and those on the various islands
- Time: Animals recently extinct and species still alive
who was Darwin greatly influenced by
Carolus Linnaeus, Charles Lyell (the principles of geology), Thomas Malthus, Alfred Russel Wallace
Natural selection
The process by which a species becomes better adapted to its environment. Those individuals with favourable characteristics have a survival advantage and so pass those characteristics on to subsequent generations
what are the three observations natural selection is based on
- Variation: Darwin noted that all members of a species vary. These variations were passed on from one generation to the next, with characteristics displayed by the parents being passed on to their offspring
- Birth rate: All living organisms reproduce at a rate far greater than that at which their food supply and other resources increase. Results in overcrowding
- Nature’s balance: Although the birth rate of organisms was very high, each species’ numbers tended to remain at a relatively constant level
struggle for existence
A principle where the number of organisms is greater than the resources in the environment can support, therefore there is competition between the organisms for these resources
Survival of the fittest
A principle whereby organisms with favourable characteristics survive, but organisms with unfavourable characteristics die before they have a chance to reproduce. Possible because there is variation within any species
Variation
The differences that exist between individuals or populations of a species
Why do individuals not adapt
Instead the species adapts to its environment by natural selection, and the process of adaptation takes many generations
Selective agent
Any factor that causes the death of organisms with certain characteristics, but which has no effect on individuals without those characteristics
What are the principles of evolution through natural selection can be summarised
- There is variation of characteristics within a species
- More offspring of a species are produced that can possibly survive to maturity
- Because of excessive birth rate and limited resources, there is a struggle got existence or competition for survival (struggle for existence)
- Individuals with characteristics best suited to the environment have more chance of surviving and reproducing (survival of the fittest)
- Favourable characteristics are passed on to the next generations
- in the gene pool, the proportion of alleles that produce favourable characteristics gradually increases
random Genetic drift
A change in the gene pool of a population as a result of chance over time. Effects strongest in a small population. Occurs when a random, non-representative sample from the population produces the next generation and therefore overtime the proportion of an allele can drift up or down also called the Sewall Wright effect
Human example of random genetic drift
The isolated populations of aboriginal Australians on the Bentinck and Mornington Islands in the Gulf of Carpentaria. Originally these islands were part of mainland Australia but rising sea levels cut them off and they became isolated. However, the Mornington islanders maintained some contact with the mainland. The occupants of bentinck island show allele frequency values for blood groups that fall outside the range for Aboriginal people in the rest of Australia. They show a very high proportion of the I B allele and a complete absence of the I A allele, unlike the mainlaind population, which has a low proportion of the I B allele and a relatively high proportion of the I A allele
what are the two extreme examples of genetic drift
Founder effect and bottleneck effect
Founder effect
A type of genetic drift where a new population is formed by a small number of individuals. The small sample size can cause marked deviations in allele frequencies from the original population. Because of its small size, chance can cause the new groups to have a different allele frequency from the original population and decreased genetic variation. This means that the new population may show a frequency of features that are not typical of the original population
example of the founder effect
The incidence of Tay-Sachs disease in the population of Ashkenazi Jews. Approximately 1 in 27 Ashkenazi Jews carries the altered allele, compared to 1 in 300 in non Ashkenazi Jews. Ashkenazi Jews descended from a small number of individuals from central or eastern Europe. This group was initially isolated geographically but also through endogamy (only marrying within the community). This created a founder effect. The incidence of the mutated allele for Tay sachs disease was higher in the ancestors, and hence genetic drift is responsible for the prevalence of the disease in the current Ashkenazi Jews population.
Bottleneck effect
occurs when the size of a population is severely reduced due to a catastrophic event (natural disaster). The allele frequency of survivors may not reflect that of the original population
Example of the bottleneck effect
When a typhoon reduced the population of Pingelap, an island in Micronesia, to only 20. These survivors formed the founding population for the current inhabitants. Among the survivors was a person heterozygous for achromatopsia (inherited form of total colour blindness). The allele for achromatopsia is recessive. Today, after a number of generations, the incidence of achromatopsia on Pingelap is 5% of the population. In other parts of the world it is 0.0033%. Furthermore, 30% of the Pingelap population are carries
