Ch. 21- Evolution of Populations

Ch. 21- Evolution of Populations

1. Microevolution

   • changes in allele frequency in a population over generations
     • is caused by genetic variation

2. Genetic Variation

   • 3 major sources:
     • natural selection
     • gene flow
     • genetic drift
   • Measuring it:
     • Gene variability
       • Whole-gene level
         • average % of loci that are heterozygouse
     • nucleotide variability
       • molecular level of DNA
         • not much variation, but even a single nucleotide difference can cause phenotypic differences
   • New alleles can arise by mutation
     • even a point mutation can have a profound impact
       • but mutations in introns do nothing to change phenotype
         • even mutations in exons may not have an impact on the protein's overall shape/function
   • Altering Gene #/ Position
     • Translocation
       • part of one chromosome translocated to a different chromosome could positively link them
     • sometimes, duplications of segments of DNA that are not harmful can persist, expanding the genome with new genes that take on new functions
   • sexual reproduction
     • Crossing over, independent assortment, random fertilization

3. Hardy Weinberg Principle

   • describes the gene pool of a population that is not evolving
     • the frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at play.
       • such a gene pool is in Hardy Weinberg equilibrium
   • Conditions for equilibrium
     • No mutations
     • Random mating
     • No natural selection
     • Extremely large population size
     • No gene flow

4. Changes in Allele Frequencies

   • Natural Selection
     • ONLY ADAPTIVE MECHANISM
   • Genetic Drift
     • chance occurances
       • cause unpredictable allele frequencies between generations
     • Founder Effect
       • A few individuals become isolated from the larger population. The smaller group may establish a new population within a different gene pool
     • Bottleneck Effect
       • With a severe drop in population size, by chance alone, some alleles may be overrepresented, underrepresented, or absent from survivors
     • Effects:
       • Genetic drift is significant in small populations
         • Genetic Drift can cause allele frequencies to change at random
           • genetic drift can lead to a loss of genetic variation within populations
             • genetic drift can cause harmful alleles to become fixed
   • Gene Flow
     • transfer of alleles between populations

5. Natural Selection and Adaptive Evolution

   • relative fitness is the contribution an individual makes to the gene pool of the next generation relative to the contribution of others
     • the relative fitness of a sterile mule is 0
   • Modes of selection
     • Directional
       • environmental conditions favor a certain extreme of phenotypic range
         • Ex: abundance of large seeds = directional selection of finches w/ large beaks
     • Disruptive
       • favors variants at both extremes of the phenotypic range
         • ex: abundance of small, hard to reach seeds AND large, hard to crack seeds
           • Birds with medium sized beaks are inefficient in getting both of these seeds and die
     • Stabilizing
       • removes extreme variants from the population and preserves intermediate types
         • ex: size of babies @birth --- too small= weak, too big = mom cant get it out
   • Sexual Selection
     • Results in sexual dimorphism- difference in secondary characteristics between males and females
     • certain inherited traits are more likely to make an individual obtain a mate
       • intrasexual: between members of the same sex (males competing against each other/ preventing each other from mating)
         • Intersexual: mate choice- individuals of one sex are choosy in selecting their mates (usually females)
   • Preservation of Variation
     • Diploidy- in diploid organisms, genetic variation is hidden from selection in the form of recessive alleles
     • Balancing Selection- Natural selection maintains 2 or more forms of an allele in a population
       • Heterozygote Advantage- Malaria/ Sickle Cell
   • Natural selection cannot produce perfect organisms
     • Selection only acts on existing variations
       • evolution is limited by historical constraints
         • adaptations are compromises
           • chance, natural selection, ant the environment interact