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Exam question(2008). During the process of meiosis, independent assortment and crossing over (recombination) can occur. This results in genetic variation in the offspring of sexually reproducing individuals Describe what happens during independent assortment
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Exam question(2008) • During the process of meiosis, independent assortment and crossing over (recombination) can occur. This results in genetic variation in the offspring of sexually reproducing individuals • Describe what happens during independent assortment • Explain how crossing over (recombination) can contribute to the genetic variation that results from sexual reproduction. You may wish to draw a diagram. • Genetic variation can be a results from mutation. Explain the results of mutation sin somatic and gametic cells
EVOLUTION MAH – Year 12
DO NOW • What is the gene pool? • Where does the variation originally come from? • What is gene flow?
Evolution Intro • ...process by which new species of plants and animals develop from earlier forms • ...is the change in the gene pool of a population over periods of time and occurs whenever the frequency of alleles in the gene pool changes • ... is the change in the inherited traits of a population of organisms through successive generations
Gene Pools and Allele Frequencies • What is the gene pool? • Is the total number of genes and alleles present within a population at a given point in time
Allele Frequency • Frequency of allele = Occurrence of that allele Total number of alleles Take two bits of paper either a pink/green (or beans) We will use this to show how allele frequencies can change
Frequency of allele = Occurrence of that allele Total number of alleles
Simulation • Sickle Cell and Malaria Simulation
Gene Pools and Allele Frequency VARIATION IN GENE POOL DUE TO MUTATIONS AND RECOMBINATION IMIGRATION AND EMIGRATION Gene Pool 1 GEOGRAPHICAL BARRIERS THAT AFFECT GENE FLOW Gene Pool 2 GENE FLOW: Genes are exchanged with other gene pools as individuals move between them. This is a source of new genetic variation and tends to reduce differences between populations due to natural selection or genetic drift
Evolution, Alleles, and Gene Pools • Variation within a gene pool comes from mutation or recombination • Changes in allele frequencies proceed EVOLUTION • Changes in allele frequency come from immigration and emigration (gene flow), genetic drift (CHANCE), natural selection, sexual selection
Gene Migration • Is the introduction or loss of alleles into or out of a population • Immigration: when individuals migrate INTO a population they bring new alleles into the gene pool • Emigration: When individuals migrate OUT OF a population they remove some of the alleles from the gene pool • New alleles introduced into a gene pool INCREASE S THE GENETIC VARIATION of a population more likely to survive changes in the environment
Genetic Drift • ...is the change in allele frequency in a gene pool in a small population by CHANCE alone • e.g. The death of an individual carrying a rare allele will make the frequency of that allele less or removed all together Special cases: Founder and Bottleneck
Founder Effect ...occurs when a few individuals migrate from a larger population to establish a new population in a new area Consequence: The colonising population may evolve differently from that of the parent population In some cases: it may be possible for certain alleles to be missing altogether from the individuals in the isolated population.
Polydactyly Fingers • Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect."
Why?? • Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community. • This prevents new genetic variation from entering the population. • Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.
Founder and Huntington’s • Huntington’s is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and dementia • The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. • Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. • This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.
Another example • Milk A1/A2 Protein
Bottleneck Effect ...occurs when a population is reduced to low numbers by predation, disease, periods of climate change, or catastrophe (selection pressures of natural disasters) The populations recover after being squeezed through a ‘bottleneck’ of low numbers Results in some alleles being above normal number or lost altogether (loss of genetic variation) Pg. 105 Biozone
Example of Bottleneck • Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. • Hunting reduced their population size to as few as 20 individuals at the end of the 19th century.
Example of Bottleneck • Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.
Non-random Mating • Nearby mating lead to inbreeding (song) • The ultimate of inbreeding is self-fertilisation (occurs in plants) • Inbreeding • Results in homozygous alleles – heterozygous advantage lost • Chance of unfavourable recessive genes turning up in the offspring is enhanced – humans have strict rules against incest
Natural Selection ...is the environment selecting FOR or AGAINST certain PHENOTYPES phenotypes: structure, metabolism, behaviour, physiology environment factors: predators, competition, disease, lack of food, light, water, climate factors ..possess adaptations such as ability to avoid predators or to compete strongly for food BEAN EXPERIMENT
Natural Selection ...can act on phenotypes in different ways that will either maintain or change the genotypes present in a species
Allopatric speciation • Is just a fancy name for speciation by geographic isolation • Speciation: The formation of new and distinct species in the course of evolution
Allopatric speciation • In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. • Isolation might occur because of great distance or a physical barrier, such as a desert or river.
Allopatric speciation (cont) • In order for a speciation even to be considered “allopatric,” gene flow between the soon-to-be species must be greatly reduced—but it doesn’t have to be reduced completely to zero.
Steps Involved: • A single randomly mating species is widely distributed through a geographical region. • Quite suddenly, a natural barrier forms, dividing the species into two separate groups. • Over the succeeding generations, either through selection or random events, the two populations come to differ. • When the barrier is removed, the populations are now so different that there is no reproduction between them: they have become two distinct species
Example: • Geographic patterns: If allopatric speciation happens, we’d predict that populations of the same species in different geographic locations would be genetically different. There are abundant observations suggesting that this is often true. • For example, many species exhibit regional “varieties” that are slightly different genetically and in appearance, as in the case of the Northern Spotted Owl and the Mexican Spotted Owl.
Question? • On the Galapagos islands Charles Darwin noticed a large variety of finches which differed in beak shape and overall size. WHY WOULD INDIVIDUALS ON THE SAME ISLAND OR NEARBY ISLANDS HAVE DIFFERERNT BEAK SHAPES AND SIZES?
Question? • On the Galapagos islands Charles Darwin noticed a large variety of finches which differed in beak shape and overall size. WHY WOULD INDIVIDUALS ON THE SAME ISLAND OR NEARBY ISLANDS HAVE DIFFERERNT BEAK SHAPES AND SIZES? THIS IS ADAPTIVE RADIATION AND NATURAL SELECTION AT WORK
Adaptive radiation (defined…) • This is where species all deriving from a common ancestor have over time successfully adapted to their environment via natural selection. • The development of many different forms from an originally homogeneous group of organisms as they fill different ecological niches
This is where species all deriving from a common ancestor have over time successfully adapted to their environment via natural selection
Example: Finches • Finches managed to occupy the Galapagos islands, over 600 miles away from their original location. They occupied an ecological niche with little competition. • As the population began to flourish in these advantageous conditions, intraspecific competition became a factor, and resources on the islands were squeezed and could not sustain the population of the finches for long. • Due to the mechanisms of natural selection, and changes in the gene pool, the finches became more adapted to the environment. • As competition grew, the finches managed to find new ecological niches, that would present less competition and allow them, and their genome to be continued. • The finches adapted to take advantage of the various food sources available on the island, which were being used by other species. Over the long term, the original finch species may have disappeared, but by diversifying, would stand a better chance of survival. • All in all, the finches had adapted to their environment via natural selection, which in turn, has allowed the species to survive in the longer term, the prime directive of any species.
