Chapter 26 - Evolutionary Genetics Flashcards
What is evolution?
The genetic change in a group of organisms/population is biological evolution
Genetic variation arises
- Origins in mutation and recombination – both are random and continually produce variation
Change in frequencies of genetic variants
- Changes in allele frequencies arise via mutation, migration, drift, and natural selection (allow evolution to occur)
What is anagenesis?
Evolution taking place within a single lineage (a group of organisms connected by ancestry) with the passage of time
What is cladogenesis?
The splitting of one lineage into two
When it splits, the two branches no longer have a common gene pool and they evolve independently of each other
What is molecular variation?
Measures DNA, RNA, or proteins – the basis of evolution
Very versatile and can be quantifiably applied to all organisms
Applied to huge amounts of variation
All organisms can be compared using molecular data
Amount of data available to us is large and continues to expand
What is DNA sequence variation?
Analyzes genetic variation
Many different techniques to visualize this
What is population variation?
Measures of genetic variation between individuals
Explanation for high molecular variation through the neutral-mutation hypothesis (neutral theory)
What is the neutral-mutation hypothesis (neutral theory)?
Proposes that a lot of the molecular variation that we see is adaptively neutral (individuals with different molecular variance have equal fitness)
Because variants are functionally equivalent, natural selection doesn’t differentiate between them, so evolution is shaped by random processes (drift and mutation)
Doesn’t preclude the importance of natural selection – when selection occurs it will be directionally favored (selects “best” while eliminating others) – natural selection decreasing genetic variation
What balancing selection?
Balancing selection is selection that maintains genetic variation
How genetic variation can be maintained
Variance is not functionally equivalent and causes phenotypic changes that affect reproduction
What is speciation?
Occurs through cladogenesis
One population separates into two distinct evolutionary groups
How is a species defined?
A particular type of organism to which a unique name has been given (genus and species name)
An evolutionary independent group of organisms
What is a biological species concept?
Defines a species as a group of organisms whose members are capable of interbreeding with each other but are reproductively isolated from members of other species
Members of the same species have the biological potential to exchange genes
Because those species exchange genes between each other but not between other species, different species evolve independently of each other
What is reproductive isolation and its two types?
The existence of biological characteristics that prevent genes from being exchanged between different species
Two types: prezygotic and postzygotic
What is prezygotic isolation and its 5 mechanisms?
Isolation before fertilization - prevents gametes from 2 different species from forming a zygote
Ecological isolation – members of 2 different species don’t encounter each other because they inhabit different ecological niches or interact with their environment in different ways
Behavioral isolation – differences in behavior that prohibit breeding (ex. calling songs)
Temporal isolation – reproduction in a species takes place at different times of the year or in different seasons
Mechanical isolation – anatomical differences that prevent successful exchange of gametes
Gametic isolation – mating between individuals of different species can take place, but the gametes don’t fuse to form a zygote
What is postzygotic isolation and its 3 mechanisms?
Gametes from 2 different species can fuse and form a zygote but there is not gene flow between the species because the resulting hybrids are inviable or sterile or because reproduction breaks down in subsequent generations
- Often occurs when prezygotic mechanisms have not formed or fail
Hybrid inviability – incompatibility of genomes between 2 species prevent hybrid zygotes from developing
Hybrid sterility – hybrid embryos do form, but hybrid is sterile and gene flow doesn’t occur
Hybrid breakdown – 2 closely related species are capable of mating and produce viable F1 progeny, but further crossing of hybrid fertile F1 progeny results in hybrids that will eventually be produced that are sterile or inviable
What are the 2 modes of speciation?
Allopatric speciation
Sympatric speciation
What is allopatric speciation?
Occurs when some geographic barrier splits a population into two groups and blocks the exchange of genes between individuals that were in population
Interruption of gene flow between now isolated groups leads to the accumulation and evolution of genetic differences that further results in reproductive isolation that leads to speciation
If barrier disappears and individuals can disperse through it, populations will come back in contact (secondary contact)
What can occur is allopatric speciation when 2 populations are in secondary contact?
If limited genetic differentiation has occurred, reproductive isolation mechanisms may not be fully evolved and gene flow will occur again between those populations, eventually eliminating genetic differences that had accumulated before, and populations will go back to being a single species
If strong genetic differentiation occurred, strong reproductive isolating mechanisms will be in place and populations will remain 2 separate species
Some genetic differentiation occurred that leads to only postzygotic barriers – allows for reproduction, but no gene flow because of mechanisms
- Because selection favors mating with like individuals, eventually prezygotic barriers will arise between populations even though they are in secondary contact – called reinforcement – further isolates species
What is sympatric speciation?
Occurs in the absence of an external barrier to gene flow
Typically occurs when reproductive isolating mechanisms evolve within a single population
Can also occur through polyploidy
- Allopolyploidy – 2 diploid species hybridize to form a tetraploid through nondisjunction (not full separation of chromosomes)
- Because tetraploid contains 2 copies of each chromosome, it’s usually fertile, but it usually causes tetraploid to be reproductively isolated from parents
What is a phylogeny?
Representation of the evolutionary relationships among organisms
What are the 3 parts of a phylogeny?
Branches – evolutionary connections between organisms
- Sometimes length of branches represents different evolutionary amounts of divergence (time, number of mutations)
Nodes – point at which two branches diverge from each other
- Represents common ancestor that existed before divergence took place
Root – a node that is a common ancestor to all of the common ancestors that are being represented in the phylogeny
- Done by including organisms that are distantly related (out group) to the in group
What is a gene tree?
A phylogenetic tree that depicts the evolutionary relationship between genes
What is a homologous characteristic?
Characteristic that has evolved from the same character in a common ancestor
How can phylogenies be built from alignment of homologous DNA sequences?
Distance based approach – overall similarity between each organisms
Maximum parsimony – Fewest number of evolutionary changes since last common ancestor
Maximum likelihood and/or Bayesian methods – Infer phylogenies based on which phylogeny maximizes the probability of obtaining the set of characteristics exhibited by the organisms
How are the rates of molecular evolution determined?
Different genes or parts of genes can evolve at different rates
Usually measured by rate of nucleotide substitution – number of mutations occurring at a nucleotide site per year per organism
Rates of nonsynonymous and synonymous substitution
- Rate at which synonymous mutations occur is higher than nonsynonymous because synonymous mutations are not going to directly affect the fitness of the organism (selectively neutral mutation)
What are the substitution rates for parts of genes?
Highest rates of substitution are in sequences that have the least effect on protein function
3rd nucleotide of codon region, flanking regions, introns
Can affect gene expression and rate of evolution
What are the 4 ways genome evolution can occur?
Exon shuffling (exons of different genes are exchanged) creates new genes that are mosaics of other genes
Gene duplication can also give rise to new genes or new gene functions (multigene families – sets of genes that are similar in sequence but encode different products)
Whole genome duplication – creating 2 copies of every gene
- Genes can acquire new functions or be lost
Horizontal gene transfers – DNA is transferred between individuals of different species in the same generation
