Selection and Evolution Flashcards
What is genetic variation caused by?
- Independent assortment of chromosomes, and therefore alleles, during meiosis
- Crossing over between chromatids of homologous chromosomes during meiosis
- Random mating between organisms within a species
- Random fertilisation of gametes
- Mutation
- The first four of these processes reshuffle existing alleles in the population
- Offspring have combinations of alleles which differs from those of their parents and from each other
- This genetic variation produces phenotypic variation
What does a mutation do?
- Mutation does not reshuffle alleles that are already present
- Mutation can produce completely new alleles
- E.G a mistake occur in DNA replication so that a new base sequence occurs in a gene
- Such a change in gene, which is quite unpredictable is called a gene mutation
What is a gene mutation like?
- The new allele is often recessive and so does not show up in the population until some generations after the mutation occurred
- When by chance two descendants of organisms in which the mutation happened mate and produce offspring
When is a mutation passed on?
- Mutations that occur in somatic cells often have no effects at all on the organism
- Somatic mutations can be passed on to the offspring by sexual reproduction
- Mutations in cells in the ovaries or testes of an animal, or in the ovaries or anthers of a plant may be inherited by the offspring
How is a mutation passed on?
- If a cell containing a mutation divides to form gametes, then the gametes may also contain the mutated gene
- If such a gamete is one of the two which fuse to form a zygote, then the mutated gene will also be in the zygote
- The single cell then divides repeatedly to form a new organism, in which all the cells will contain the mutated gene
What is genetic variation?
- Genetic variation, whether caused by the reshuffling of alleles during meiosis and sexual reproduction or by the introduction of new alleles by mutation, can be passed on by parents to their offspring, giving differences in phenotype
- Genetic variation provides the raw material on which natural selection can act
What does variation within a population mean?
- Some individuals have features that give them an advantage over other members of that population
- Variation in phenotype is also caused by the environment in which organisms live
- E.G some organisms might be larger than others because they had access to better quality food while they were growing
- Variation caused by the environment is NOT passed on by parents to their offspring
What are examples of quantitate and qualitative phenotypic differences?
- Qualitative: blood groups
- Quantitative: height and mass
What is discontinuous variation?
-Qualitative differences fall into clearly distinguishable categories, with no intermediates, for example you have one of four possible ABO blood groups A, B, AB or O
What is continuous variation?
- Quantitative differences between your individual heights or masses may be small and different to distinguish
- When the heights of a large number of people are measures, there are no distinguishable height classes, instead there is a range of heights between two extremes
What are the similarities of continuous and discontinuous aviation?
- Both qualitative and quantitative difference in phenotype may be inherited
- Both may involve several different genes
What happens in discontinues (qualitative) variation?
- Different alleles at a single gene locus have large effects not he phenotype
- Different genes have quite different effects on the phenotype
What happens in continuous (quantitative) variation?
- Different alleles at a single gene locus have small effects on the phenotype
- Different genes have the same, often additive, effect on the phenotype
- A large number of genes may have combined effect on a particular phenotypic trait; these genes are known as polygenes
What are examples of discontinuous variation?
- Inheritance of single cell anaemia and haemophilia
- Flower colour in snapdragon and stem colour of tomato plants and feather colour of chickens
What does gene interaction and dominance do?
Reduce variation
What are the effects of the inheritance of continuous variation?
- The small effects of the different alleles of one gene on the phenotype
- The additive effect of different genes on the same phenotypic character (hypothetical is organisms height)
How can height be determined?
- Height controlled by two unlinked (on different chromosomes) genes A/a and B/b and a and b add xcm and A and B add 2x cm
- Effect of genes additive, aabb 4x cm and AABB potentially 8xcm so other genotypes will fall between two extremes
What do these hypothetical results rely on?
- Assuming that two unlinked genes, each with two alleles, contribute to the height of the organism
- The number of discrete height classes increases as more genes are involved and the differences between these classes get less
- Even if two or more of the genes are liked on the same chrome, potentially reducing the number of classes of offspring an increasing the difference between them, crossing over in meiosis will restore the variation
- The differences between different classes will be further smoothed out by environmental effects
- NOTES?
How might environment stunt growth?
- Less food, less nutritious food than another with the same genetic contribution
- Plant may be in lower light intensity pr in soil with fewer nutrients than another with the same genetic potential height
What is another example of environmental effects?
- Development of dark tips to ear, nose and paws, and tail in Himalayan colouring or rabbits or Siamese and Burmese cats
- Allele allow formation of dark pigment only at low temperature (extremities coldest part)
- Area other placed of fur, new fur will be black
What is the experiment for genetic variation?
- Crossed two varieties of maize which differed cob length both of the parental varieties were pure bred lines
- Cob lengths of the plants used as parents and the first and second generations of offspring resulting form the cross were measured to nearest cm
- Pure bred so were homozygous at a large number of loci and first generation genetically different from the parents but were genetically the same as one another
1. The phenotypic variation that you can see within the two parental varieties and also within the first generation of offspring shows the effect of the environment
2. The second generation of doddering shows a much wider variation in cob length and this is both genetic and environmental
What is genetic and environmental variation?
- Just as genetic variation provides the raw material on which natural selection can act, so in selective breeding it is important to know how much of the phenotypic variation is genetic and how much is environmental in origin
- There is no point in selecting parents for a breeding programme on the basis of environmental variation
What happens to the population if it is left unchecked by environmental factors?
Number in a population may increase exponentially
What is an example of when the population can increase exponentially?
- Rabbits, fed on abundance of vegetation
- Few predators
- Numbers so great affected availability of grazing for sheep
What are the different types of environmental factors?
- Biotic: caused by other living organisms such as through preditation, competition for food or infection by pathogens
- Abiotic: caused by non-living components of the environment e.g. water supply, nutrient levels in the soil
What might an increase in rabbit population cause?
1 .Eat increasing amount of vegetation until food in short supply
- Larger population of rabbits may allow populations of predators such as foxes, stoats and weasels to increase
- Overcrowding occur increasing ease disease e.g. myxomatosis may spread which is transmitted by fleas so closer together virus pass more easily
- These environmental factors reduce the rate of growth of the rabbit population
- Of all rabbits born many die lack of food, killed by predator or myxomatosis
How does the population of rabbits fluctuate?
- Only a small proportion of young grow into adulthood and reproduce so population growth is slow
- Population size decreases if environmental factor pressure increase
- Population only grow when only when numbers of rabbits falls a lot
- Over period of time population oscillate about a mean value
What are some describable characteristics that rabbits may have?
- Number of young produced much greater than number survive to adulthood so many die before reaching reproductive age
1. Coat colour; e.g. if white not brown stand out so predator easier to spot so chances of reproducing of white is low so allele for white coat remain rare in population
What is fitness?
Capacity of an organism to survive and transmit its genotype to its offspring
What is an example of a selection pressure?
Predation by foxes
What does a selection pressure do?
Increases the chances of some alleles being passed on to the next generation and decreases the chances of others
What is natural selection?
- Effects of such selection pressures on the frequency of alleles in a population
- Raises the frequency of alleles conferring an advantage and reduces the frequency of alleles conferring a disadvantage
What is directional selection?
-If a new environmental factor or a new alleles appears then alleles frequencies may also change