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Population Genetics & Patterns of Evolution

Population Genetics & Patterns of Evolution. Are these organisms the same species?. Same column, same species! The same species can be born different colors, depending on the season they were born. Can we use appearances to determine whether organisms are the same species?.

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Population Genetics & Patterns of Evolution

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  1. Population Genetics & Patterns of Evolution

  2. Are these organisms the same species? Same column, same species! The same species can be born different colors, depending on the season they were born. Can we use appearances to determine whether organisms are the same species?

  3. Are these organisms the same species?

  4. What makes organisms the same species? • Species: a group of similar organisms that are capable of producing fertile offspring A donkey & horse can produce offspring (a mule), but mules are NOT fertile! So, donkeys & horses are considered separate species!

  5. Species vs. Population • Population: a localized group of a species in a defined area Individuals do NOT evolve; a population evolves! True or False – individuals within a population can evolve. FALSE!

  6. An individual giraffe stretches its neck until it’s taller. That’s Lamarck’s theory & it’s wrong!

  7. How do we study evolution? We study evolution as genetic changein a population! • Evolution is change over time, which means it occurs within a group whose individuals are actually breeding with each other. • Therefore, we study evolution by examining genetic change within a population. Individuals cannot change their genes & therefore cannot evolve!

  8. With natural selection, only the giraffes that have the beneficial genes get to reproduce & pass on the good genes. The giraffes with bad genes do not reproduce & the bad genes disappear over time. That’s genetic change in a population over time! In order for evolution to occur, there has to be some genetic variation within the population.

  9. Genes & Variation • Inheritable traits are coded for by genes, & the different forms of a gene are called alleles. • There exists variation within a population for many of these alleles! In this population of rabbits, there is a wide variety of fur color. There are several different alleles for fur color in this population of rabbits!

  10. Genes & Variation We can figure out what the frequency of a particular allele is by calculating the number of times that the allele appears in a population compared to others in the entire gene pool. Basically, we are trying to answer this question: What percentage of the time does the brown allele occur in a population compared the black allele?

  11. The Gene Pool • Consists of all the alleles for a gene present in a population • The relative frequency of an allele in a population is often expressed as a percentage. The gene pool here consists of 20 alleles! 13/20 (65%) carry the “A” allele. 7/20 (35%) carry the “a” allele.

  12. This gene pool consists of 72 alleles. What is the frequency of A? 24/72 = .33 or 33% What is the frequency of a? 48/72 = .67 or 67%

  13. How many alleles are in the gene pool? 50 • What is the frequency of the black allele? 20 out of 50, or 40% • What is the frequency of the brown allele? 30 out of 50, or 60%

  14. In this sample population, is the most common allele the dominant one? NO! The most common allele does NOT have to be the dominant allele!

  15. Has evolution occurred in this population?

  16. If the frequency of an allele changes, is that considered evolution? • When a change in the relative frequency of an allele occurs in a population, “change over time” has occurred! • This is evolution on a small scale, or microevolution!

  17. Microevolution • Consider alleles for polydactyly in the gene pool. • The allele coding for extra digits, the polydactyly allele (P), occurs in 1% of the population (the frequency is .01). • The allele for 5 fingers & toes (p) occurs in 99% of the population (the frequency is .99). What do you think would happen if polydactyly became an advantage? • If over time, extra fingers was an advantage, & natural selection selected FOR individuals with extra digits, a shift in that allele frequency might happen, & evolution on a small scale would occur!

  18. Remember! Evolution is a genetic change in a population. In order for that genetic change to occur, there has to be some genetic variation within the population.

  19. Sources of Genetic Variation • Mutations: new alleles can arise only if a mutation occurs & a new sequence of DNA makes a new form of a gene Blue eyes is caused by a mutation that originally occurred 6-10 thousand years ago!

  20. Sources of Genetic Variation • Gene Shuffling: independent assortment of chromosomes & crossing over during meiosis produce millions of possible arrangements of your genes There’s not a change of frequency when genes are shuffled, but that’s why there is so much variation.

  21. Meiosis can produce egg/sperm that carry many different combinations of alleles!

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