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Selection and evolution, Individuals showing 1 or more desired features…
Selection and evolution
Genetic variation is caused by:
- a 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 1st 4 reshuffle existing alleles in the population to give phenotypic variation No. 5 creates completely new alleles
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Artificial Selection
Describe how selective breeding (artificial selection) has been used to improve the milk yield of dairy cattle
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Forms of reproductive isolationPrezygotic (before a zygote is formed) isolating mechanisms:
- individuals not recognising one another as potential mates/not responding to mating behavior
- animals being physically unable to mate
- incompatibility of pollen + stigma in plants
- inability of a male gamete to fuse with a female gamete
Postzygotic isolating mechanisms:
- failure of cell division in the zygote
- non-viable offspring (offspring that soon die)
- viable, but sterile offspring
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What is genetic drift?
A change in allele frequency that occurs by chance, because only some organisms of each generation reproduce.
What is the founder effect?
When a small number of individuals are separated from the rest of a large population.
They form only a small sample of the original population so they have different allele frequencies from the larger population.
Further genetic drift in the small population will alter the allele frequencies more and evolution of this population will cause a significant difference with the larger population.
Molecular evidence that reveals similarities between closely related organisms (with ref. to mitochondrial DNA (mtDNA) & protein sequence data)
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Comparing amino acid sequences of proteins
When the amino acid sequence of a particular protein is compared in different species, the # of differences gives a measure of how closely related the species are.
Sickle cell anaemia Having 2 copies of the Hbs allele obviously puts a person at a great selective disadvantage. People who are homozygous for the sickle cell allele are less likely to survive and reproduce. The sickle cell allele is most common in parts of the world where malaria is found.There are 2 strong selection pressures acting on these 2 alleles:
- Selection against who are homozygous for the sickle cell allele, Hbs Hbs, is very strong, because they become seriously anaemic
- Selection against people who are homozygous, HbA HbA, is very strong because they are more likely to die from malaria
People who are heterozygous for the sickle cell allele (Hbs) are less likely to suffer from a serious attack of malaria and only some of their red blood cells are sickle cells
Antibiotic resistance
There are 1 or more individual bacteria with an allele giving resistance to penicillin - such as Staphylococcus - by producing penicillinase, which inactivates penicillin. These bacteria can survive & reproduce, while others will die (selection pressure is antibiotics)
Industrial melanism
- Speckled moths produced by recessive allele, c
- Black moths produced by dominant allele, C
- The selection pressure causing the the change of allele frequency in industrial areas is predation by birds
- in industrial areas air pollutants cause the trees to have darker bark => black moths are better camouflaged
- in non-industrial areas with unpolluted air, speckled moths have a higher chance of survival
- the frequency of allele C increased in industrial areas
- the frequency of allele c remained the more common allele in non-industrial areas
Natural Selection - Environmental factors
Biotic factors = caused by other living organisms e.g. predation, competition for food, infection by pathogens
Abiotic factors = caused by non-living components of the environment e.g. water supply, nutrient levels in the soil
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Individuals showing 1 or more desired features (high milk yield) are chosen for breeding. Some of the alleles conferring these features are passed on to the individual's offspring. Again, the 'best' animals from this generation are chosen for breeding.2 problems of selective breeding of cattle
- the animals take time to reach maturity
- the gestation period is long + the # of offspring is small
How can a bull be assessed for the sex-linked milk allele? A bull cannot be assessed for milk production since this is a sex-linked trait. Instead the performance of the bull's female offspring is looked at to see whether or not to use the bull in selective breeding.Selective breeders have to consider the whole genotype of an organism, not just the genes affecting the desired trait e.g. increased milk yield. Within each genotype are all the alleles of genes that adapt it to its particular environment. These genes are called background genes. Selective breeders have to consider the background genes.
Suppose the chosen parents come from the same environment & are from varieties that have already gone through artificial selection. The parents will share a large # of alleles of background genes so the offspring will be adapted for the same environment.
But suppose instead that 1 of the chosen parents comes from a different part of the world. The offspring will inherit the appropriate/needed alleles from only 1 parent. It may show the trait being selected for, but it may not be well adapted to it's environment.
- the introduction of disease resistance to varieties of wheat & rice
- the incorporation of mutant alleles for gibberellin synthesis in dwarf varieties increasing yield by having a greater proportion of energy put into the grain:
Most of the dwarf varieties of wheat carry mutant alleles of 2 reduced height (Rht) genes.
These genes code for DELLA proteins which reduce the effect of gibberellins on growth.
The mutant alleles cause dwarfism by producing more of, or more active forms of, these transcription inhibitors.
Wheat plants now having shorter stems makes them easier to harvest and they have higher yields (because they put more energy into making seeds than growing tall). The shorter stems make the plants less susceptible to being knocked flat by heavy rains and produce less straw.
- inbreeding and hybridisation to produce vigourous, uniform varieties of maize:
Inbreeding and producing heterozygosity and uniformity - If maize plants are inbred (crossing with other plants with genotypes like their own), the plants in each generation become progressively weaker => inbreeding depression - because homozygous maize plants are less vigorous (weaker/unhealthy) than heterozygous ones.
Crossing homozygous maize plants produces F1 plants that all have the same genotype. Hybridisation of maize - Different crosses between different homozygous maize plants can produce different hybrids for different purposes e.g. high yield, resistance
- Observation 1 Organisms produce more offspring than are needed to replace the parents
- Observation 2 Natural populations tend to remain stable in size over long periods
- Deduction 1 There is competition for survival
- Observation 3 There is variation among the individuals of a given species
- Deduction 2 The best adapted variants will be selected for by the natural conditions operating at the time. In other words, natural selection will occur. The 'best' variants have a selective advantage; 'survival of the fittest' occurs
mtDNA only passed by mother - why?
mtDNA is inherited by the female line because a zygote contains the mitochondria of the ovum, not the sperm.
Why can the changes in the nucleotide sequence only arise by mutation?
The mtDNA is circular + cannot undergo any form of crossing over.
Why does mtDNA mutate faster than nuclear DNA?
mtDNA isn't protected by histone proteins and oxidative phosphorylation in the mitochondria can produce forms of oxygen that act as mutagens.
What does the 'molecular clock' hypothesis assume?
That there is a constant rate of mutation over time & that the greater the # of differences in the sequences of nucleotides, the longer ago those individuals shared a common ancestor.
How is the 'clock' calibrated?
By comparing nucleotide sequences of species whose date of speciation can be estimated from fossil evidence
Cytochrome c is a component of the ETC in oxidative phosphorylation in mitochondria. A protein with such an important function is expected to have a similar sequence of amino acids in different species.When the sequences of cytochrome c humans, mice & rats were compared, it was found that:
- all 3 molecules consist of 104 amino acids
- the sequences of mouse & rat cytochrome c are identical
- 9 amino acids in human cytochrome c are different from the mouse or rat sequence
- most of these substitutions in human cytochrome c are of amino acids with the same type of R group
This comparison suggests that mice & rats are closely related species, sharing a more recent common ancestor than with humans