How Populations Evolve

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Siobhan
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146753
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How Populations Evolve
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2012-04-10 11:19:23
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How Populations Evolve
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How Populations Evolve
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  1. Genetic variation
    • Variation in heritable traits is necessary for evolution- cannot happen without it
    • Genetic variation among individuals is caused by differences in genes or other DNA segments
    • Phenotype is the product of inherited genotype and environmental influences
    • Natural selection can only act on variation with a genetic component- has to be passed on for future generations or it is not significant

  2. Phenotype
    • Phenotype is the product of inherited genotype and environmental influences
    • Natural selection can only act on variation with a genetic component- has to be passed on for future generations or it is not significant

  3. Gene Pools and Allele Frequencies
    how to see if population is evolving

    • Population:
    • a localized group of individuals, same place
    • Same species
    • interbreed and produce fertile offspring
    • gene pool:
    • Populations genetic make up
    • consists of all the alleles for all loci in a population
    • Look at their differences
    • Loci- where genes are located on chromosomes

    A locus is fixed if all individuals in a population are homozygous for the same allele- if all of organisms in population are green, it is fixed because no matter how much they interbreed they will have green offs[ring

    • The Hardy-Weinberg equation can be used to test whether a population is evolving
  4. Population
    • a localized group of individuals, same place
    • Same species
    • interbreed and produce fertile offspring

  5. gene pool:
    • Populations genetic make up
    • consists of all the alleles for all loci in a population
    • Look at their differences
    • Loci- where genes are located on chromosomes
  6. Hardy-Weinberg equation
    • The Hardy-Weinberg equation can be used to test whether a population is evolving
    • The Hardy-Weinberg principle describes a population that is not evolving
    • If not evolving, no change in allele frequency

    If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving


  7. Genetic variation can be measured:
    • as gene variability
    • or nucleotide variability
    • For gene variability, average heterozygosity
    • = % loci that are heterozygous in a population. Measures the chance of getting variety in new population

    • Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals
  8. phenotype, genotype, alleles, homozygous
    • Phenotype= what you see
    • Genotype= what is in genes
    • Alleles-specific gene

    • Homozygous- chromosomes are the same

  9. The five conditions for non-evolving populations
    (rarely met in nature):

    • No mutations (no alleles altered)

    Random mating (no preference, no inbreeding)

    • No natural selection (all have equal reproductive success)
    • No pressure to compete for resources ect

    • Extremely large population size (no genetic drift)
    • 1,000,000 wouldnt be affected if one had mutation compared to group of 3 duck
    • No gene flow (alleles do not move in or out of the population)
  10. Mutations
    • Mutations are the original source of genetic variability
    • Mutations are rare changes in the base sequence of DNA in a gene
    • They usually have little or no immediate effect and can be passed to offspring only if they occur in cells that give rise to gametes
    • They are the source of new alleles and can be beneficial, harmful, or neutral
    • Mutations arise spontaneously, not as a result of or in anticipation of, environmental necessity

  11. Allele frequencies
    • For diploid organisms, the total # of alleles at a locus = the total number of individuals X 2
    • If there are 2 alleles at a locus,
    • p and q are used to represent their frequencies
    • p could be 40% of population are red while q could be 60% of population are green

    • The frequency of all alleles in a population will add up to 1
    • p + q = 1
    • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
    • p2 + 2pq + q2 = 1
    • where p2 and q2 represent the frequencies of the homozygous genotypes,
    • and 2pq represents the frequency of the heterozygous genotype

  12. Genetic Drift
    • chance events in a small population that eliminates part of population- volcano eruption ect.
    • causes allele frequencies to change at random
    • tends to reduce genetic variation through losses of alleles
    • can cause harmful alleles to become fixed- everyone left in population has it- everyone who had good genes got wiped out
    • Founder effect- child with six fingers in amish country. What is evident in founders is seen in remaining offspring
    • Bottleneck effect-




  13. Frequency-Dependent Selection
    • the fitness of a phenotype declines if it becomes too common in the population
    • Selection can favor whichever phenotype is less common in a population
    • For example, right-mouthed and left-mouthed scale-eating fish
    • Fish ate with right side of mouth but then prey because aware, so it became more of an advantage to be left sided
  14. Natural selection
    • The fittest individuals are those that not only survive, but are able to leave the most offspring behind
    • Ultimately, it is reproductive success that determines the future of an individuals alleles
    • Natural selection acts on phenotypes
    • Natural selection does not act directly on the genotypes of individual organisms
    • Instead, natural selection acts on phenotypes, the structures and behaviors displayed by the members of a population
    • Natural selection acts on the genotype only indirectly, but crucially, through the phenotype by determining which organisms survive to pass their alleles on
  15. Competition
    • Competition is an interaction among individuals who attempt to utilize a limited resource
    • The competition may be among individuals of the same species or of different species
    • It is most intense among members of the same species because they all require the same things
  16. Predation
    • Predation is an interaction in which one organism (the predator) kills and eats another organism (the prey)
    • Coevolution between predators and prey is akin to a biological arms race
    • Wolf predation selects against slow, careless deer
    • Deer evolve to get faster, and wolf has to evolve to get faster to keep up- evolve together

    • Alert swift deer select against slow, clumsy wolves
  17. Sexual selection
    • natural selection for mating success
    • Sexual selection is a type of selection that favors traits that help an organism acquire a mate
    • Examples of traits that help males acquire mates include the following
    • Conspicuous features (bright colors, long feathers or fins, elaborate antlers)
    • Bizarre courtship behaviors
    • Loud, complex courting songs
  18. sexual dimorphism
    • marked differences between the sexes in secondary sexual characteristics- guppies, cardinals
  19. Intrasexual selection:
    • competition among individuals of one sex (often males) for mates of the opposite sex. Intra means between. Male tigers fighting
  20. Intersexual selection
    • Intersexual selection: mate choice- occurs when individuals of one sex (usually females) are choosy in selecting their mates
  21. Risk/benefit analysis
    • Risk/benefit analysis: showiness can increase a males chances of attracting a female, while decreasing his chances of survival. Risk- although opposite sex notices, so would predator

  22. Directional, Disruptive, and Stabilizing Selection
    • Three modes of selection:
    • Directional selection: favors individuals at one end of the phenotypic range. Many red or white flowersà many R or W alleles
    • Disruptive selection: favors individuals at both extremes of the phenotypic range. Both R and W alleles popular
    • Stabilizing selection: favors intermediate variants and acts against extreme phenotypes- Mostly Pink (RW) flowers

  23. Balancing Selection- heterozygous advantages
    • heterozygotes have a higher fitness than do both homozygotes
    • Natural selection will tend to maintain two or more alleles at that locus
    • Example: The sickle-cell
    • normal and sickle-cell hemoglobin alleles coexist in malaria-prone regions of Africa


  24. Why Natural Selection Cannot Fashion Perfect Organisms
    • Selection can act only on existing variations- no molding/predicting.
    • Evolution is limited by historical constraints, ancestry- if ancestors survived sickle cell anemia, might inherit sickle cell
    • Adaptations are often compromises
    • The giraffe couldn’t have a neck that would be too short or long
    • Chance, natural selection, and the environment interact. chance- where you are when a volcano erupts, and the fastest survive

  25. mutation
    • creates new alleles
    • increases variability
  26. gene flow
    icreases similarity of different populations
  27. genetic drift
    • causes random change of allele frequencies
    • can eleminate alleles
  28. nonrandom mating
    changes genotype frequencies but not allele frequencies
  29. natural and sexual selection
    • increases frequency of favored alleles
    • produces adaptions

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