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Genetics and Neutral evolution
The Blending Theory of Inheritance
This was a late 19th century idea that was held up as evidence that natural selection could not work
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The contributions are halved at each successive
generation
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Gregor Mendel 1822-1884
He was an Austrian monk and scientist and he performed various experiments in the monastery gardens
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Around the 1900, his ideas were
rediscovered by William Bateson to illustrate the rules of inheritance
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The plants in the first cross are called the P (parent) generation
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When he allowed the plants of
the F1 generation to self-fertilize and produce a second generation (referred to as the F2 generation), he
found that 3/4 were yellow and 1/4 were green
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This meant that one expression of the trait
(green) had completely disappeared in the F1 generation and then reappeared in the F2 generation
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The genetic material is particulate (does not
blend)
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He
realised that somehow the pair of units separated into different sex cells and were
then again united with another member during fertilisation of the egg
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Paired chromosomes (and the genes they carry) are
separated and distributed to different gametes
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It involves a single replication step and two cell divisions
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s
**see CBG notes for detailed description of Meiosis**
The modern synthesis
It’s the reconciliation of Mendel’s principles of inheritance and Darwin’s idea on natural selection to create a
joint mathematical framework that established evolution as a central paradigm in biology
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Each organisms traits affect its survival and reproductive success
compared with other individuals
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Population evolve, individuals do not
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There are three main mechanisms by which allele frequency change occurs
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However, natural selection is the only one the consistently improves the match
between organisms and their environment (adaptation)
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Individuals often reflect genetic variation
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Without genetic variation, evolution cannot occur
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Discrete characters are often determined by a single gene locus with different alleles
that produce distinct phenotypes
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Genetic variation at a whole gene-level is called gene variability while that at the level of DNA is called
nucleotide heterozygosity
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Heterozygosity can be tested by gel electrophoresis, however this approach cannot detect silent
mutations that alter the DNA sequence but not that of the amino acid
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The Hardy Weinberg Equilibrium
A population is a group of individuals of the same species that live in the same area and interbreed,
producing fertile offspring
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If only one allele exists for a
particular locus in a population, it is said to be fixed in the gene pool
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The Hardy-Weinberg Principle assesses whether natural selection or other factors
are causing evolution at a particular locus by determining what the genetic
makeup would be if it were not evolving at that locus
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No differences mean that the population is
evolving, however if there are, then it can be argued that the real population may
be evolving
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A gene pool is in Hardy-Weinberg equilibrium when the frequencies of alleles
and genotypes in the population will remain constant from generation to
generation, given that only Mendelian segregation and recombination of alleles
are at work
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The effective population size = Ne; the number of total breeding individuals in the population
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There is a probability that either the sperm of egg has either of these alleles
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From the previous statements, it is given that the expected genotype frequencies are as
follows; (this is the probability that the allele you inherit from your mother and father of the combination AA,
Aa or aa)
AA = p x p = p2
aa = q x q = q2
Aa = (p x q) + (q x p) = 2pq
2 or 2pq or q2) x Ne will give you the expected number of offspring in the population with the genotypes
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If
the population exists in equilibrium, the members will continue to mate randomly generation after generation
and allele and genotype frequencies will remain constant
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No mutations - mutations alter alleles, and genes can be deleted or duplicated resulting in changes to the
gene pool
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Random mating - the random mixing of gametes do not occur if there is a preferential mating, resulting in
changes in the genotype frequency
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No natural selection - the difference in survival and reproductive success of individuals can influence the
allele frequencies in the population
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Extremely large population size - small populations are more likely to have greater fluctuations from one
generation to the next (genetic drift)
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It is important to remember that natural populations can exist in a state of both evolutionary change and
equilibrium as the population may be evolving at some loci, yet remaining in equilibrium at another
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Populations are not ideal however, and these conditions don’t usually hold
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Nonrandom mating can cause changes in the homo/heterozygous allele frequencies but won’t usually have
an affect on allele frequencies in the gene pool
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Factors affecting gene flow
Natural selection
Natural selection is based on differential success in survival and reproduction
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Natural selection can also result in adaptive evolution where its outcomes are a better match
between the organisms and its environment
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If populations are small
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Some of these
unpredictable changes in allele frequencies can be caused by chance events associated with survival and
reproduction
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The founder effect is a type of genetic drift that occurs when a few individuals are isolated from a
population, and this smaller group establishes a new population whose gene pool differs from the source
population (for example, when members of a population are blown by a storm to another population, or in
case of survivors from a war or famine)
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The bottleneck effect is when a factor such as a fire or flood causes a drastic reduction in the population size
after genetic drift
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As a
result of chance, some alleles may be over represented, under represented or absent
in comparison to other survivors
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Despite the recovery, the population may still
experience low genetic variation for a long period of time as an outcome of this
genetic drift
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From the Prairie chicken bottleneck event (1993), it
was suggested that the genetic drift caused a loss in
genetic variation and that there was an increase in the frequency of harmful
alleles
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The negative

alleles had an negative effect on the hatching rate of the eggs
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Gene Flow
Gene flow are the transfer of alleles between populations, or the exchange of genes between populations due
to the movement of fertile individuals or their gametes
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It is the migration followed by the event of mating of individuals
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Gene flow can be positive such that it can transfer
alleles that improves the ability of populations to adapt to local conditions (example is gene flow in the
spread of insecticide resistance in Culex pipiens)

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