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
Why is genetic drift in large populations not noticeable overall
The randomness of inheritance of alleles is not noticeable because the proportion of alleles that are affected is a low
Species
organisms with similar characteristics that are capable of interbreeding and producing fertile offspring
Speciation
If two populations are isolated for a very long period of time, and the environmental influences on each are different enough, major changes in the allele frequencies within each population could occur. In this situation, the members of these populations may become so different that even if the barriers to reproduction were removed, interbreeding would no longer be possible. The two populations would be regarded as seperate species.
what are the 4 steps involved in speciation
- Variation: A range of variations exists within the population, which shares a common gene pool
- Isolation: A barrier has formed, dividing the population into two. No interbreeding occurs between the two populations. Each population has a seperate gene pool
- Selection: Different selection pressures act on each of the two populations over a number of generations. This brings about a change in the gene frequencies of each gene pool. Such changes lead to the evolution of seperate subspecies
- Speciation: Over a long period of time the changes in the gene frequencies may be great enough to prevent the production of fertile offspring by interbreeding between the two populations from every occurring again. When this happens, two species exist.
What are four examples of natural selection
Body stature, Sickle Cell anaemia, Tay Sachs disease and Thalassemia
what is Sickle Cell anaemia
An inherited disease causing early death. Results from the inheritance of two alleles for sickle cell anaemia that distorts the shape of the red blood cell
how did malaria spread
The mosquito which transmits the malarial parasite needs quiet, stagnant pools of water for breeding sites. This habitat is more often found in open areas. As humans began to clear the forests of Africa for agriculture, they changed the environment, creating additional breeding areas. Human population increased, providing more bodies for the mosquitoes feed on, thus the incidence of malaria increased
complications that result from sickle cell aneamia
The sickle shaped red blood cells often die early, resulting in anaemia. They are also inflexible and can become stuck in the blood vessels, causing a blockage. Other complications includes fatigue, organ damage, high BP and heart failure
sickle cell anaemia and natural selection
A mutation established a new allele in the population. Having one of these alleles gave individuals living malarial prone areas a survival advantage. This is known as a heterozygote advantages. Individuals with two sickle cell alleles (with sickle cell anaemia) usually die. Those who are homozygous for normal haemoglobin are more susceptible to malaria. Therefore, the presence of malaria acted as a selective agent for the sickle cell allele.
what is tay sachs
A lethal condition caused by a mutation that results in the absence of the enzyme beta-hexosaminidase. This conditions leads to a deterioration of the nervous system, death usually occurs at a young age
tay sachs and natural selection
People who are heterozygous have a reduced amount of beta-hexosaminidase. It appears that these individuals have some protection from tuberculosis (infectious disease that affects lungs) this means that people who are:
- Homozygous recessive die before reproducing. do not pass on recessive allele
- Heterozygous survive tuberculosis, reproduce and pass on both alleles
- Homozygous dominant are affected by tuberculosis and may die prior to reproducing
what is thalassemia
people with thalassemia have less haemoglobin in their red blood cells and therefore cannot carry as much oxygen in their blood. The severity varies depending on number of affected genes, ranging from mild anaemia and fatigue to enlarged liver and heart
What are the two forms of thalassemia
- Alpha thalassemia is due to a mutation in the HBA gene on chromosome 16. This reduces the level of alpha globin in haemoglobin
- Beta thalassemia is due to a mutation in the HBB gene on chromosome 11. This reduces the level of beta globin in haemoglobin
thalassemia and natural selection
Malaria can act as a selective agent, resulting in an increased frequency of the alleles of alpha thalassemia. The lower amount of haemoglobin gives some protection against malaria
