Module 6: Evolution Flashcards
(31 cards)
What is Evolution?
It is the change in allele frequencies over time.
Define gene pool and allele frequency.
Gene pool = the complete range of alleles present in a population.
Allele frequency = how often an allele occurs in a population.
Usually given as a percentage of the total population.
What are the 6 factors that can affect the evolution of a species?
Natural Selection.
Stabilising Selection.
Directional selection.
Genetic Drift.
Genetic bottleneck.
Founder effect.
Explain how does evolution take place by the process of natural selection.
1) Individuals within a population vary due to having different alleles. (new alleles are generated by mutations in genes)
2) Selection pressures such as disease, predation and competition create struggle for survival. (some individuals are better adapted to the selection pressures than others)
3) People who have an allele that increases their chance of survival - an advantageous allele- are more likely to survive than people with other alleles.
4) Therefore a greater proportion of the next generation inherit the advantageous allele. So they are more likely to survive, reproduce and pass on their genes.
5) frequency of the advantageous allele increases from generation to generation.
An allele is only advantageous with the right selection pressure- without it, natural selection wont take place.
Define what Stabilising Selection is.
This is when the environment is not changing that much. (environmental conditions remain stable)
Alleles for an average phenotype are more likely to be selected for as they are more likely to survive and reproduce.
Reduces the range of possible phenotypes.
Give examples of stabilising selection.
Human birth weight.
-Underweight babies are less likely to survive as they lose a lot of body heat.
-Overweight babies are more likely to lead to childbirth complications.
-Alleles for a mean birth weight are more likely to survive and reproduce, passing on their average weight alleles onto their offspring
Fur length:
- Having short or long length of fur reduces chance of survival as it is harder to maintain the right body temp.
-Mammals with alleles for average fur length are more likely to survive, reproduce and pass on their alleles - have a selective advantage.
(results in a taller graph in average region)
Define what Directional Selection is.
Alleles for an extreme phenotype are more likely to be selected for
Usually triggered by a change in the environment.
Give examples of directional selection.
Development of antibiotic resistance in bacteria:
A mutation in a bacterium enables it to develop resistance to an antibiotic. When antibiotic is used (the selection pressure), the resistant bacterium survives while the rest are killed. The bacterium can grow without competition and reproduce, passing on its resistance allele to its offspring. This produces a population of bacteria which carry the antibiotic resistance allele
Fur length:
-If the environment becomes too cold, mammals with alleles for long fur length will find it easier to maintain the right body temp compared to mammals with short fur length.
-Overtime frequency of alleles for long fur length increases and pass on their alleles.
(results in graph moving towards extreme end)
Explain how evolution occurs due to genetic drift.
Genetic drift = A change in a population’s allele frequencies that occurs due to chance rather than selective pressures. This means that chance dictates which alleles are passed on.
1) Individuals within a population show variation in their genotypes (A or B)
2) By chance, the allele for one genotype (e.g. B) is passed on to the offspring more often than others. So the number of individuals with this allele increases.
3) If by chance the same allele is passed on more often again and again - this leads to evolution as the allele becomes more common in the population.
What is a strength and weakness of Evolution by genetic drift?
Strength:
- has a greater effect in smaller populations where chance has a greater influence.
Weakness:
- tends to cause the genetic diversity of a population to decrease. Lack of genetic diversity may make species less able to adapt to changes in their environment.
What is a genetic bottleneck?
A genetic bottleneck occurs when a population is greatly reduced in size e.g., if a natural disaster/disease/habitat destruction kills most of the population.
It has a detrimental effect on genetic diversity because it reduces the number of different alleles in the gene pool.
The remaining population reproduce a re-populate the area, creating a new population with less genetic variation.
Describe the effect of evolution by genetic drift is there is a genetic bottleneck and give an example.
Evolution by genetic drift has a greater effect is there is a genetic bottleneck.
Example:
- Mice in a large population are either black or grey (coat colour does not affect survival/reproduction).
- A large flood hits the population and only grey mice and one black mouse survive.
- The grey mice become more common due to genetic drift.
What is the founder effect?
Describes what happens when just a few organisms from a population start a new population and there are only a small number of different alleles in the initial gene pool.
Describe how the founder effect works.
1) Individuals within a population show variation in their genotypes.
2) Some of these individuals start a new population. By chance these individuals mostly have one particular genotype.
3) The new population will grow with reduced genetic variation. As the population is small, it is more heavily influenced by genetic drift that an a larger population.
Why does the founder effect occur?
It occurs as a result of migration leading to geographical separation.
Or if a new colony is separated from the original population for another reason such as religion.
E.g. Amish people.
What is the Hardy-Weinberg principle?
It is a mathematical model which predicts that the frequencies of alleles in a population won’t change from one generation to the next.
What conditions are required for the Hardy- Weinberg prediction being true?
1) It has to be a large population where there is no immigration, emigration, mutations or natural selection.
2) There need to be random mating where all possible genotypes can breed with all others.
What are the equations for the Hardy-Weinberg equation?
p + q = 1
p2 + 2pq + q2 = 1
p= the frequency of the dominant allele.
q= the frequency of the recessive allele.
p2= frequency of the homozygous dominant genotype
2pq= frequency of heterozygous genotype
q2= frequency of homozygous recessive genotype.
What are the rules for the Hardy-Weinberg equations?
p + q = 1 —> for allele frequency: the frequencies of the individual alleles (dominant and recessive) must add up to 1.0.
p2 + 2pq +q2 =1 —> for genotype frequency: the frequencies of the individual genotypes must add up to 1.0.
What is artificial selection?
This is when humans select individuals in a population to breed together to get desirable traits.
It is also called selective breeding.
Explain how a high milk yield from modern dairy cows has been produced by artificial selection.
- Farmers select a female with a very high milk yield and a male whose mother has a very high milk yield and breed them together.
-Then they select the offspring’s with high milk yield and breed them together.
- This is continued for many generations until a very high milk yielding cow is produced.
Other characteristics include:
-High milk quality.
-Long lactation period.
-Large udders.
Explain how a high yield of wheat from bread wheat has been produced by artificial selection.
Bread wheat is the plant which flour is produced for bread- making.
Wheat plants with a high wheat yield (e.g. large ears) are bred together.
The offspring with the highest yields are then bred together.
This is continued over many generations to produce a plant that has a very high yield.
Other characteristics include:
-Higher tolerance of the cold.
-Short stalks - don’t collapse.
How has artificial selection been made easier
- Artificial insemination and IVF give farmers more control over which cows reproduce.
- Animal cloning - farmers produce genetically identical copies of their best cows.
What are the disadvantages of using artificial selection?
-Leads to a reduction in the number of alleles in the gene pool. This can cause future problems (disease), there is less chance of the alleles that could offer resistance to that disease being present in the population.
- Potentially useful alleles are accidently lost from the population when other alleles are being selected for.
-Can exaggerate certain traits which can lead to health problems for the organisms involved. Reduced gene pool results in an increased risk of genetic disease. (therefore it is important to preserve the original ‘wild type’ organisms that have not undergone any artificial selection)
-Ethical issues - many people do not think it is fair to keep artificially selecting traits in dogs that cause them health problems. COUNTERPOINT: but makes food production more efficient = more people are fed.
Makes food production more cheaper.
