evolution

• Population Genetics and Speciation


  The Evolution of Populations
• In any given population, there are chance individual variations
• The number of individuals that survive and reproduce in each generation is small compared to the number produced
• Natural Selection


  The process in which individuals with characteristics well suited to their environment survive and reproduce more successfully, passing these favorable traits on to offspring
• Darwin didn't know how hereditary and genetic mechanisms worked
• Darwin provided abundant evidence that life evolves over time and proposed Natural Selection as the primary mechanism for that change
• Darwin's theory of natural selection was missing an understanding of inheritance
• What was missing in Darwin's work was a basic understanding of inheritance - how do chance variations occur and how are these variations passed from parent to offspring
• Gregor Mendel


  Published groundbreaking paper on the principles of inheritance
• Mendel and Darwin were contemporaries, but Darwin was never aware of Mendel's work that would resolve Darwin's problem of explaining natural selection
• The importance of Mendel's work wasn't recognized for decades
• Connecting the Dots


  Biologists connected Mendel's work to Darwin's work in the 1930's
• Genes


  Heritable traits that can produce variation in offspring
• DNA model


  Provided by Watson and Crick in the 1950's, explained how DNA could carry information and how this information could be passed on
• Population Genetics


  The branch of science that emerged from the synthesis of Darwinian evolution and Mendelian genetics, studying populations from a genetic point of view
• Composition of genetic material


  • Many genes found along chromosomes in a cell
  • A gene is a segment of DNA that contains instructions for a single trait
• Chromosomes


  Occur in pairs, with each parent contributing one of their genetic material to offspring
• Alleles


  The alternative forms of a gene, with two or more alleles for each gene
• Population


  An inbreeding group of organisms of the same species that breed and produce fertile offspring, located in the same geographic area
• Populations are important to the study of evolution as they are the unit in which evolution occurs
• Individuals in a population vary in both observable and unobservable ways
• Sources of genetic variation

  • Mutation
  • Recombination
  • Random pairing of gametes
• Mutation


  A change in the DNA sequence that can be passed on to future offspring
• Recombination


  The reshuffling of genes that occurs during meiosis, producing new combinations of genes in offspring
• Independent assortment


  Genes for different traits segregate independently during gamete formation, leading to many genetic variations in offspring
• Crossing-over


  The process where homologous chromosomes exchange portions of their chromatids during meiosis, further increasing the number of genetic variations that can appear in offspring
• Gene pool


  The total sum of all alleles of all genes in a population
• Allele frequency


  The number of times a particular allele occurs compared to the number of times other alleles for that trait occur in a population
• Evolution is any change in the relative frequency of alleles in a population, and is the result of changes in the gene pool
• Calculating allele frequency


  Divide the number of a certain allele by the total number of alleles for that trait in the population
• Even though phenotypic ratios can change from one generation to the next, the allele frequencies tend to remain the same
• Genetic equilibrium


  A situation where allele frequencies remain constant from one generation to the next
• Hardy-Weinberg principle


  Describes a hypothetical population that is not evolving, where allele frequencies remain constant from one generation to the next unless acted upon by outside forces
• Conditions for Hardy-Weinberg equilibrium


  • Very large population size
  • No migration in or out of the population
  • Random mating
  • No natural selection
  • No mutations
• Hardy-Weinberg equation


  p^2 + 2pq + q^2 = 1, where p is the frequency of one allele and q is the frequency of the other allele
• The Hardy-Weinberg equation allows scientists to detect changes in allele frequencies and determine if evolution is occurring in a population
• How the Hardy-Weinberg equation is derived

  1. Consider a population of sexually reproducing diploid organisms

  2. The symbol "p" represents the frequency of the "A" allele

  3. The symbol "q" represents the frequency of the "a" allele

  4. 100% of the population will have either "AA", "Aa", or "aa" (1 being 100%)

  5. If males and females mate at random, the frequency that gametes carrying "A" will combine with gametes carrying "A" will be p²

  6. The frequency that male gametes carrying "A" will combine with female gametes carrying "a" will be pq

  7. The probability that a male gamete carrying "A" will combine with a female gamete carrying "a" is p

  8. The probability that a male gamete carrying "a" will combine with a female gamete carrying "a" is q

  9. Therefore, the probability of the "Aa" combination is 2pq
• The Hardy-Weinberg equation represents:
• Solving the attached earlobes problem

  1. p² = frequency of homozygous dominant individuals

  2. 2pq = frequency of heterozygous individuals
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