Lecture 4 - Evolution and Speciation PDF

Lecture 4 – Evolution and speciation Three main mechanisms: Natural selection Genetic Drift Gene flow Part 1 – Testing Darwin’s postulates Rosemary and Grant (1976-78) Geospiza Fortis, medium ground finch shows variation in a beak shape....

Lecture 4 – Evolution and speciation Three main mechanisms: Natural selection Genetic Drift Gene flow Part 1 – Testing Darwin’s postulates Rosemary and Grant (1976-78) Geospiza Fortis, medium ground finch shows variation in a beak shape. shows the correlation between parent midbeak and offspring. In 1978 there were lost of finches but there was the same relationship.  In 1976 drought occurs and the number of seeds available which led many birds to died. The seeds that were produced were larger and harder in the drought than they were before or after. In 1978 only 90 birds hatched, and the population mean beak depth was increased. Therefore, there was selected for bigger birds with deeper stronger beaks. Therefore, natural selection change source of food and relationship between birds and their beaks. 1. Postulate - Individuals within a species are variable which means that particular traits such as beak will have different variants such as hard beak, shallow beak, and small beak. 2. Postulate 2 - Some of this variation is heritable which means that some of these traits will pass to another generation. 3. Postulate 3 – Reproduction is not random, ‘survival of the fittest which means that some of species which survive is certain conditions and some of will not survive. The species that will survive will reproduce more and will increase the traits and the population. Part 2 – Genetic drift Evolution is change in the inherited traits of a population through successive generations: the genetic content of a population changes over time. Mutations are the raw materials of evolution via natural selection Some increase ‘fitness’ - rarely Some decrease ‘fitness’ - often But many (probably the majority) have little, if any, effect on ‘fitness’, so they cannot be acted upon by natural selection. Most of mutation cannot change + or – but yet they still change in allele frequencies in a population and still they arise and lost. Mutation that has no selective value Not all genetic change (mutations) alter the phenotype. Some mutations in gene have no effect Some mutations in introns have no effect Most mutations between genes have no effect.  Yet the frequencies of these mutations can change in populations, even in the absence of natural selection. Modelling genetic drift by sample error Not all eggs and sperm contribute to the next generation. Only a sample of sperm and ova contribute and so there is a sample error. E.g. Two alleles (A and a) with a 50:50 frequency N = 20 where half the alleles are selected to determine the frequency. Attempt 1 = Allele A gets lots in generation 12 and allele a gets fixed. Attempt 2 = Allele A gets fixed in generation 8 and allele a gets lost. Thus, modelling shows that genetic can fix an allele in a population. Black line shows both heterozygote Aa and homozygote AA rises in frequency in the population (a dominant advantageous allele). Red line shows only homozygotes, and these take a long time to accumulate, so it takes long time to spread the traits (recessive advantageous allele). Richard Lenski: Long Term Evolution Experiment (LTEE) Day 1 – E.coli sample are put in 2 flasks labelled Ara+ and Ara- Day 2 – 12 flasks inoculated and grown in a medium of glucose and citrate Day 3 – Each flask is subculture. Every 75 days, sample fitness is estimated and samples are frozen. Timeline and Key findings In all flasks – Growth and cell size increased In all flasks DNA repair rates were increased, giving mutator phenotype with elevated mutation rate, thus they evolve more rapidly. In some flask E.coli was able to use citric acid as carbon source in aerobic conditions. Within a flask mutator phenotype evolved. How did citrate in aerobic conditions evolve. Anaerobic conditions - Expression of citrate transporter takes place and citrate is taken up from the growth medium. Aerobic condition - There is no expression of citrate transporter so E.Coli cannot use citrate as an energy source. Cit+ strain - The gene encoding citrate transported expression is duplicated. This deregulated expression so citrate transporters were expressed in aerobic conditions. This allows the Cit+ strains to use citrate and glucose as C and energy source which gives them a selective advantage. GENE FLOW It is the movement of alleles between previously separate populations Gene flow can occur by: Migration of adults and subsequent mating Migration of gametes (pollen) and subsequent fertilisation. Three main mechanisms: 4. Genetics drift can cause genetic variation in demes(sub population) and cause differentiation between demes due to random change in allele frequency. 5. Gene flow can lead new alleles into demes of a metapopulation and by itself cause homogeneity between demes.. 6. Selective and reproductive isolation. Genetic drift, gene flow and selective isolation can produce more diversity and lead to speciation. SPECIATION Species - (eukaryotic level) Group of organisms that can potentially or actually interbreed, giving viable, fertile offspring. Eukaryotic species are reproductively isolated. Thus, they cannot make offspring with any other species. TREE FROG AND CHROMOSOMAL DUPLICATION Argument - Instant speciation occurred when Hyla chrysocelis failed to sort 24 chromosomes during meiosis. Thus creating H. versicolor