Evolution driven by 4 process

Evolution driven by 4 process

1. Natural selection

   • increases freq. of alleles tht contribute to reproductive success in particular environment
   • occurs in 5 modes/patterns
     • directional selection
       • changes avg phenotype
       • reduces genetic diversity
         • If it continues, favored alleles will approach frequency of 1.0 while disadvantageous alleles will approach frequency of 0.0
           • Alleles that reach frequency of 1.0 are termed fixed and those that reach 0.0 are termed lost
       • EX: avg beak depth in finches increased
     • stabilizing selection
       • reduces variation in trait
       • EX: Birth weight
         • EX: Alleles associated with high birth weight or low birth weight were subject to purifying selection, and alleles associated with intermediate birth weight increased in frequency.
       • favoring phenotypes near the average value and eliminating extreme phenotypes
     • disruptive selection
       • increases variation in trait
       • eliminates phenotypes near the average value and favors extreme phenotypes
       • EX: Whitefish & Gill Rackers
         • Whitefish with relitavily low or high number of gill rackers are most likely to survive & reproduce
           • cause of this pattern? Food availability. whitefish w/dense gill rakers are efficient at filtering tiny plankton. The whitefish w/ sparse gill rakers are efficient at capturing small animals w/o getting their rakers clogged up. Whitefish with an intermediate number of gill rakers are inefficient at gathering both types of food.
       • can play a role in speciation
     • balancing selection
       • maintains variation in trait
       • no phenotype has a distinct advantage
       • Frequency-dependent selection
         • Occurs when certain alleles are favored when they are rare but not when they are common
           • EX: Guppies
             • Rare alleles responsible for coloration favored: – Predators learn to recognize common color patterns – Alleles for common colors get eliminated – Alleles for rare colors increase in frequency
     • Sexual selection
       • Darwin concluded peacock tails must be adaptive to help attract mates, resulting in increased reproductive success
         • Intrasexual selection: Where indvdls of same sex compete w/1 another to get mates
           • Elephant seals
         • Intersexual selection: Mate choice
           • Birds with long tails (clipped vs. unclipped
       • The Fundamental Asymmetry of Sex
         • Bateman–Trivers hypothesis
           • Pattern: Traits that attract opposite sex are more elaborate in males.
           • Process: Eggs are more energetically costly to create than sperm
         • Bateman–Trivers – Predictions
           • Female fitness limited by ability to gain resources to produce more eggs and healthy offspring, so females produce few offspring during lifetime
           • Male fitness limited by # of females can mate with, & he can mate freq b.c sperm energetically inexpensive
       • sexual dimorphism
         • intersexual and intrasexual selection help explain why traits differ between sexes of the same species
         • Traits Can Help Attract Mates and/or Deter Competitors for Mates

2. Genetic Drift

   • causes allele freq to change randomly

3. Gene Flow

   • occurs when idvdls leave one pop, join another & breed

4. Mutation

   • modifies allele freqs by continually introductions new alleles

5. Gene Pool Concept

   • def: alleles from all gametes in ea. generation go into single group and then combine randomly
   • calc. predict genotypes of offspring that pop. would produce as well as freq. of ea. genotype

6. Hardy Weinberg Principle

   • Mathematical null hypothesis for study of evolutionary process
     • null hypothesis: Specifies what the results of an experiment will be if the main hypothesis is wrong. Often states there will be no difference between experimental group
   • Predicts the genotype frequencies and allele frequencies in the next-generation. If the frequencies can conform to the predictions, then frequencies are in Hardy Weinberg equilibrium and population isn’t evolving.
   • P^2 + 2pg + q^2 = 1.0 AA + Aa + aa = 1.0
     • p= A% q= a%
     • freq of 2 alleles which sum to 1 or 100% of the population
   • Assumptions
     • Random Mating
     • No natural selection
     • no genetic drift
     • no gene flow
     • No mutation

7. Nonrandom mating

   • In nature mating might not be random w/ respect to any particular gene
   • Interbreeding
     • form of nonrandom mating.
     • Homozygotes that breed w/ea. other & decrease genetic diversity
       • Interbreeding depression
         • decline in avg fitness. common in small pop & first cousin marriage. increased mortality rate.
           • Florida panthers
             • Florida x Florida decreased fitness so to increase it Texas x Florida to increase fitness
     • increases homozygosity & decreases heterozygosity
     • can speed the rate of evolutionary change
     • increases rate @which natural selection eliminates recessive deleterious alleles (i.e lower fitness) from pop.
     • Self fertilization
       • extreme form of interbreeding
       • Many flowering plants contain both male and female organs and self-pollinate
         • Homozygous parents that self-fertilize produce all homozygous offspring
         • Heterozygous parents that self-fertilize produce homozygous and heterozygous offspring in a 1:2:1 ratio