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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
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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
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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:
- Population’s 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
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Population
- a localized group of individuals, same place
- Same species
- interbreed and produce fertile offspring
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gene pool:
- Population’s 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
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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
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Genetic variation can be measured:
- as gene variability
- or nucleotide variability
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- 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
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phenotype, genotype, alleles, homozygous
- Phenotype= what you see
- Genotype= what is in genes
- Alleles-specific gene
- Homozygous- chromosomes are the same
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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 wouldn’t 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)
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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
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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
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- 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
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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-
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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
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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 individual’s alleles
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- 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
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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
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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
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Sexual selection
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- 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
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sexual dimorphism
- marked differences between the sexes in secondary sexual characteristics- guppies, cardinals
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Intrasexual selection:
- competition among individuals of one sex (often males) for mates of the opposite sex. Intra means between. Male tigers fighting
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Intersexual selection
- Intersexual selection: mate choice- occurs when individuals of one sex (usually females) are choosy in selecting their mates
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Risk/benefit analysis
- Risk/benefit analysis: showiness can increase a male’s chances of attracting a female, while decreasing his chances of survival. Risk- although opposite sex notices, so would predator
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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
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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
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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
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mutation
- creates new alleles
- increases variability
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gene flow
icreases similarity of different populations
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genetic drift
- causes random change of allele frequencies
- can eleminate alleles
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nonrandom mating
changes genotype frequencies but not allele frequencies
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natural and sexual selection
- increases frequency of favored alleles
- produces adaptions
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