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Evolution
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- 1In what two ways can DNA be transferred between organisms?Vertically and horizontally.
- 2Explain vertical transferThe transfer of genomic sequences from parent(s) to offspring. Sexual reproduction produces offspring that show genetic variation. Asexual reproduction produces offspring that do not show genetic variation.
- 3Explain horizontal transferThe transfer of genomic sequence between members within a population. In prokaryotes, DNA can be passed vertically (through asexual reproduction) but also horizontally.
- 4State an advantage and disadvantage of horizontal transferThe advantage of this is that it can bring about rapid evolutionary change.
The disadvantage is that it can be risky, as there is no guarantee that the genetic information received will benefit the recipient. It could be harmful. - 5What can some prokaryotes and viruses do?Transfer genes to eukaryotes (also horizontal), particularly plants. This means we can use bacteria as vectors to insert desired genes into the genomes of a plant species during genetic modification.
- 6Define evolutionThe result of changes in organisms over generations caused by genomic variations.
- 7Define natural selectionThe non-random increase in frequency
of DNA sequences that increase survival. These changes depend on random alterations in the genome due to mutations. The mutations can be advantageous or detrimental to the survival of the organism. - 8Give a summary of natural selection.Within a population, there is variation and competition. Depending on the selection pressure, some members will have advantageous mutations that help them to survive and reproduce the frequency of the gene. Those with detrimental mutations don't have a selective advantage and are less likely to survive.
- 9Into what ways can selection be categorised?By the impact it has on the population. There are three types (stabilising selection, directional selection, and disruptive selection).
- 10Describe the effect of stabilising selectionIt selects against the extremes of a trait, reducing the amount of variation within a population- phenotypes in a range become more aligned with a mean value.
- 11Describe the effect of directional selectionIn directional selection, one extreme of a trait is selected against. This means the average phenotype moves towards an extreme value in a range.
- 12Describe the effect of disruptive selectionIn disruptive selection, selection pressures act against individuals in the middle of the trait distribution- it favours two extreme phenotypes, and results in two or more common phenotypes.
- 13Define sexual selectionThe non-random increase in frequency of DNA sequences that increase reproduction. This means that it favours characteristics related to sex, which then lead to better reproductive success.
- 14It what two main ways does sexual selection work?Through choice by members of one sex for certain members of the other sex (display). Through competition among members of one sex for access to members of the other sex (combat).
- 15What is genetic drift?Genetic drift is the random increase or decrease in frequency of DNA sequences. It can occur due to sampling error, neutral mutations or founder effects. These changes in frequency, especially in small populations, are called genetic drift.
- 16Sampling errorIn a small population, not all alleles are passed onto the next generation simply through random chance. Because of this, alleles may be ‘lost’ from the gene pool.
- 17Founder effectIf a small sample of a population becomes isolated it may not be large enough to represent the entire gene pool. Gene frequencies will therefore be different in that population.
- 18Neutral mutationsThese change the nucleotide sequence of a gene, but do not change the amino acids coded for. They are not subject to natural selection, because they have no effect on phenotype. They do contribute to genetic drift, because they create new gene sequences.
- 19Define speciationThe formation of a new species, brought about by evolution. (A species is a group of similar organisms that can interbreed to produce fertile offspring).
- 20Allopatric speciation (summary)A populations of one species occupies an environment where they share a gene pool and interbreed freely. They become split by a geographical barrier that prevents interbreeding and gene exchange. Random mutation within each sub-population can result in new variation. Because there is no interbreeding between the sub-populations new variation is not shared. Different selection pressures act on each sub-population. Natural selection acts according to these selection pressures. The frequency of the most beneficial alleles will increase. Over a long period, the groups become more genetically distant.
- 21When is a new species produced?If interbreeding to produce fertile offspring could no longer occur it is concluded that two distinct species have evolved.
- 22What is sympatric speciationThis is where different sub-populations become genetically isolated, despite still living in close proximity. This can be because of a behavioural barrier, an ecological barrier or polyploidy (in plants).
- 23Sympatric speciation (summary)The initial population interbreeds freely and shares a gene pool. Some members may begin to exploit a new ecological niche or show different mating preferences. The two populations no longer interbreed because of the ecological or behavioral barrier. Gene flow between the two populations ceases. Sub-populations are subjected to different selection pressures, so different characteristics are selected for. Mutations within each population also produce new variation. Natural selection is able to act separately on them. Over many generations the populations become more genetically distant, and can no longer breed to form fertile offspring.
- 24Sympatric speciation due to polyploidyIf a polyploid plant is produced within a population, this plant is unable to interbreed with the plant(s) that produced it because of having a different number of sets of chromosomes. It is therefore genetically isolated and from the rest of the population, even though it is living in the same area. If the polyploid plant finds another polyploid to mate with that has the same number of sets of chromosomes, a new polyploidy species can evolve ‘overnight’ by sympatric speciation.
- 25What is a hybrid zone?A region in which frequent interbreeding between two very similar and closely related species occurs. The offspring is usually either infertile or survives less well compared with non-hybrids. The zones persist because of repopulation with pure forms which then hyberdise.
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- CfE Higher (DNA and the Genome) CfE Higher Biology Fichas sobre Evolution, creado por NinaJasT el 23/09/2015.
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