Fitness
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Defined as the reproductive success measured by the number of surviving offspring in
the next generation
o Ex. The most fit phenotype is simply the one that produces the most offspring
Scientifically, the most fit phenotype in a population is given a 1, and the others are
given a relative proportion
o Ex. If the best phenotype leaves an average of 4 offspring, then it is given a 4/4
(1.0 fitness value). If the next best phenotype leaves 2.5 offspring on average,
then it is given a 2.5/4 (.625 fitness rating)
 If these colors are produced genetically, then it is expected for an
evolutionary change to occur, if this remains unchanged, then it is
expected for the weaker phenotype to disappear
Selection will favor some that are more successful at attracting mates, this is called
sexual selection.
Additionally, the number of offspring produced is important
o Ex. Large female frogs lay more eggs than small ones, leaving more for the next
generation
Fitness, is a combination of mating success, survival, and offspring per mating
Selection favors phenotype with the greatest fitness, but predicting fitness is difficult
due to all of the components
Agents of Evolutionary Change
Mutation
the ultimate source of variation. Individual mutations occur so rarely that mutation
occur so rarely that mutations occur so rarely that mutation alone usually does not
change allele frequency much.
 Mutations happen maybe once per 100,000 cell division
 Have hardly any affect on Hardy- Weinberg
 Mutation is ultimate source of genetic variation and makes evolution possible
Gene Flow
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Movement of alleles from one population to another
Some gene flow isn’t obvious: drifting of gametes or immature stages of plants or
marine animals from on place to another
 For example, pollen, the male gamete of flowering plants, is often carried great
distances by insects like bees
Nonrandom Mating
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Individuals with certain genotypes sometimes mate with one another more commonly
than would be expected on a random basis
 Assortative mating, in which phenotypically similar individuals mate, is a type of
nonrandom mating that causes the frequencies of particular genotypes to differ greatly
from those predicted by the H-W principle.
 Doesn’t change frequency of individual alleles; increases proportion of homozygous
individuals
Genetic Drift
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Changes in allele frequencies that happen randomly is known as Genetic Drift.
Particularly likely in populations that were founded by a few individuals
Founder Effect: when one or more individuals disperse and become the founders of
a new, isolated population at some distance from their place of origin. *Some alleles
are lost during migration
 Bottleneck effect: When disasters occur and wipeout individuals, the surviving
individuals make random samples. The resulting alterations and loss of genetic
variability are known as the bottleneck effect. *Occurs when drastic reduce in
population
Selection
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Produces adaptive evolutionary change
These must happen to be natural selection: Variation must exist among individuals
in a population, Variation among individuals must result in differences in the
number of offspring surviving in the next generation, Variation must be genetically
inherited
Genetic Variation and evolution
Learning Outcomes
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Natural selection occurs when some individuals are better suited to their environment
than others. These individuals live longer and reproduce more, leaving offspring with
the traits that enabled their parents to thrive. In essence genetic variation within the
population provides the raw material on which natural selection can act.
Many process can leas to evolutionary change
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Genetic variation, that is, difference in alleges of genes found within individuals of a
population, provides the raw material for natural selection.
Natural populations contain a wealth of such variation. In plants, insects, and
vertebrates, many genes exhibit some level of variation.
The word evolution is widely used in the natural and social sciences. It refers to how an
entity-be it a social system, gas, or a planet changes through time.
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Natural selection produces evolutionary change when some individuals possess certain
inherited charecteristics.
Changes in Allele Frequency Allele frequency
2
2
2
(p+q) = P + 2pq + q
The 2’s on top are for squaring
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a
particular gene variant (allele). In other words, it is the number of copies of a particular allele
divided by the number of copies of all alleles at the genetic place (locus) in a population. It can
be expressed for example as a percentage. In population genetics, allele frequencies are used
to depict the amount of genetic diversity at the individual, population, and species level. It is
also the relative proportion of all alleles of a gene that are of a designated type.
Given the following: # a particular locus on a chromosome and the gene occupying that locus
a population of N individuals carrying n loci in each of their somatic cells (e.g. two loci in the
cells of diploid species, which contain two sets of chromosomes)
different alleles of the gene exist
one allele exists in a copies
then the allele frequency is the fraction or percentage of all the occurrences of that locus that is
occupied by a given allele and the frequency of one of the alleles is a/(n*N).
For example, if the frequency of an allele is 20% in a given population, then among population
members, one in five chromosomes will carry that allele. Four out of five will be occupied by
other variant(s) of the gene.
Note that for diploid genes the fraction of individuals that carry this allele may be nearly two in
five (36%). The reason for this is that if the allele distributes randomly, then the binomial
theorem will apply: 32% of the population will be heterozygous for the allele (i.e. carry one
copy of that allele and one copy of another in each somatic cell) and 4% will be homozygous
(carrying two copies of the allele). Together, this means that 36% of diploid individuals would
be expected to carry an allele that has a frequency of 20%. However, alleles distribute randomly
only under certain assumptions, including the absence of selection. When these conditions
apply, a population is said to be in Hardy–Weinberg equilibrium.
The frequencies of all the alleles of a given gene often are graphed together as an allele
frequency distribution histogram, or allele frequency spectrum. Population genetics studies the
different "forces" that might lead to changes in the distribution and frequencies of alleles—in
other words, to evolution. Besides selection, these forces include genetic drift, mutation and
migration.
Artificial selection- a breeder selects for the desired characteristics
Natural selection- environmental conditions determine which individuals in a population produce more
offspring
Variation must exist among individuals in a population
Variation among individuals must result in differences in the number of offspring surviving in the next
generation
Variation must be genetically inherited
Colias eurytheme usually exhibit a pale green color
Staphylococcus aureus causes staph infection