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- individuals DO NOT evolve
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-it acts on individuals but the evolutionary impact of evolution is only seen in a population of organisms over time!
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-individual variation occurs in al species and often reflects genetic variation, differences among individuals in the composition of their genes or other DNA segments
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- not all phenotypic variation is heritable. Phenotype is the prodcut of an inherited genotype and environmental influences. Only the genetic component of variation has evolutionary consequences
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- genetic variation provides the raw material for evolutionary change, without genetic variation, evolution cannot occur
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population
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-localized group of individuals that belong to the same species
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species
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a group of populations who have the ability to interbreed and produce fertile offspring in nature
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gene pool
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a collection of alleles within the population
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allele frequency
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how common is that allele with the population
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think: A vs a
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1. No mutations
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2. Random mating
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3. No natural selection
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4. Extremely large population size
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5. No gene flow
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- chance events that cause allele frequencies to fluctuate unpredictably from one generation to the next especially in small populations
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- 2 ways genetic drift can occur in small populations:
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1. Founder Effect
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2. Bottleneck Effect
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the transfer of alleles into and out of a population due to the movement of fertile individuals or their gametes
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- tends to reduce differences between populations
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directional selection
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favors one extreme
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disruptive selection
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favors the 2 extremes and not the middle
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stabilizing selection
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favors the middle
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occurs when the heterozygote genotype has a higher relative fitness than either the homozygous dominant or recessive genotype
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EXAMPLE: Malaria and Sickle Cell
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Homozygous dominant (normal)- die of malaria (HbHb)
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Homozygous recessive- die of sickle cell anemia (HsHs)
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Heterozygote Carriers (HbHs)- relatively free of malaria sick cell anemia. They survive more, therefore are more common in the population
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the smallest scale we can define evolution occurs as changes in allele frequencies in a population over time
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- allele: the different versions of the SAME GENE
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discrete characters
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classified on an either-or basis
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usually determined by a single gene locus
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quantitative characters
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most traits vary along a continuum within a population
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usually result from the influence of 2+ genes on a single trait
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population geneticists measure genetic variation in a population by determining the amount of heterozygosity at the gene level and the molecular level of DNA (nucleotide variability)
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- describes the gene pool of a population that is not evolving
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- states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation as long as ONLY Mendelian segregation and recombination of alleles are at work
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- although new mutations can modify allele frequencies, because mutations are rare, the change from generation to generation is very small
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- mutation can ultimately have a large effect on allele frequencies when it produces new alleles that strongly influence fitness in a positive or negative way
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- nonrandom mating can affect the frequencies of homozygous and heterozygous genotypes, but it usually has no effect on allele frequencies in the gene pool
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1. significant in small populations
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2. causes allele frequencies to change at random
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3. leads to a loss of genetic variation within populations
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4. can cause harmful alleles to become fixed
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natural selection consistently increases the frequencies of alleles that provide reproductive advantage and thus leads to adaptive evolution!!
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-although we may refer to the relative fitness of a genotype, the entity that is subjected to natural selection is the whole organism, not the underlying genotype
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-natural selection acts on the genotype indirectly, via how the genotype affects the phenotype
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Natural Selection cannot create a perfect organisms because...
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1. selection can act only on existing variation
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2. evolution is limited by historical constraints
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3. adaptations are often compromises
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4. chance, natural selection, and the environment interact
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1. Natural Selection
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2. Genetic Drift
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chance events that alter allele frequencies
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3. Gene Flow
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the transfer of alleles between populations
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- mutations
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- a change in the nucleotide sequence of an organisms DNA
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* some mutations can alter gene number or position
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= deletions
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= duplications
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=translocations
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=inversions
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sexual reproduction
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- through the process of sexual reproduction we are mixing up the genes (DNA) of 2 parents to create 1 offspring with a mixture of the parental traits
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- useful in determining how fast a population is changing
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-based on a simple Punnett square where p is the frequency of the dominant allele and q is the frequency of the recessive allele
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-frequency of dominant and recessive alleles MUST equal 1 (100%)
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p + q = 1
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frequency of the genotypes must equal 1 (100%)
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p2 + 2pq + q2 = 1
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Charles Darwin was the first scientist to investigate sexual selections, which is selection for mating success
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-sexual dimorphism
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differences in secondary sex characteristics
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-intrasexual selection
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competition among individuals of one sex for mates of the opposite sex
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-intersexual selection
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mate choice- one sex is choosy in selecting mates from the other sex
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-based on differential survival and reproductive success
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-individuals in a population vary in their heritable traits
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-individuals with variations better suited to the environment tend to produce more offspring than those with variations that are less well suited
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-as a result of selection, alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population
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-by consistently favoring some alleles over others, natural selection can cause adaptive evolution (evolution that results in a better match between organisms and their environment)
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