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Genome Evolution and the Molecular Basis of Adaptation - Coggle Diagram
Genome Evolution and the Molecular Basis of Adaptation
"Adaptation"
many aspects of genomes may not have arisen as a result of natural selection and instead arose from mutation and genetic drift
genome features that are not the result of natural selection are nonadaptive, but they are not necessarily bad for an organism
...from new mutation vs standing genetic variation
Adaptation from new mutation:
the alleles that encode a novel adaptive phenotype appear after a new selective challenge arises
newly mutated allele provides a fitness advantage and rises to fixation
Adaptation from standing genetic variation:
The alleles that encode a novel adaptive phenotype were present in the population before the novel selection challenge arose
Variation in Genome Size
C-Value: total amount of DNA found in a cell
prokaryotes usually have smaller c-values than eukaryotes
eukaryotes exhibit low correlation between DNA quanitity and morphological complexity
C-Value Paradox: lack of association between an organism's apparent morphological complexity and the size of its genome
arises from events such as whole genome duplications
large portion of the genome that are "functionless"
Mobile Genetic Elements
replicate and insert themselves in an organism's genome by hijacking the same cellular machinery that replicates and transcribes protein-coding DNA
effects:
rarely, disrupt function of protein coding genes and changes phenotype
commonly, have no effect on phenotype
Make up most of the intergenic regions
include introns
make up a high proportion of noncoding DNA in multicellular genomes
not considered intergenic
positions are well-conserved across distantly-related organisms
Genome Parasites
Includes...
Transposons: usually contain only the sequences required to transpose or move
Retrotransposons: Usually leave a copy of themselves behind when they move
Transposition events lead to an increase in element abundance within the host genome
Selection...
...at the level of hosts may select against mobile elements
...at the level of the elements favors their spread
spread within genomes, sometimes with deleterious consequences for the host
Defenses against these parasites include...
methylation: chemical modification of DNA via the addition of a methyl group
pre-transcription silencing
prevents transcription of DNA to RNA
common in regions with many mobile elements
prevents mobile elements from transposing
RNA interference (RNAi): insertion of RNA sequence that silences expression of a gene
post-transcriptional silencing
Can affect phenotypes
Oval tomato example illustrates three important features of gene and genome evolution:
The SUN locus was modified by a mobile element
The specific modification of SUN was a duplication that created an extra copy of a locus present elsewhere in the genome
The duplication of SUN resulted in a change in gene expression via the alteration of its regulation
Evolution of mutation rates
mutation rates vary among organisms
In simple organisms, such as viruses and prokaryotes and some unicellular eukaryoates, mutation rates have a negative relationship with genome size
In complex organisms, especially multicellular eukaryotes, mutation rates have a positive relationship with genome size
lower limit on mutation rates is set by the interplay between natural selection genetic drift
influence of genetic drift is stronger in smaller populations
phenotype favored by natural selection can be lost because of genetic drift
phenotype of interest: mutation rate
selection: low mutation rate
small population size: high influence of drift
Gene Duplication
the primary source of new genes
preservation of duplicated genes
After a gene is duplicated by a single individual, the new two-gene allele must achieve fixation within the population
most duplications have no selective advantage
it is rare that the duplicated allele will rise to fixation
Possible outcomes:
include:
New function
Loss of function
Nothing
mutations with small effects on phenotypes are difficult to find and study, so biologists have focused on mutations with large phenotypic effects
single mutations can have large phenotype effects
