Chapter 26 - Evolutionary Genetics Flashcards
What is evolution?
- The genetic change in a gropu of organisms/population is biological evolution
1. genetic variation arises
2. change in frequencies of genetic variants - changes in allele frequencies arise via mutation, migration, drift, and natural selection (allowing evolution to occur)
- Evolution can take place via anagenesis or cladogenesis
Does not take place on an individual. Must be a group of organisms.
What is important to genetic variation?
- Genetic variation must be present for evolution to take place
x genetic variation tends to occur at a high rate in natural populations
x forces that affect evolution
1. natural selection
2. drift
3. ?
4. ? - genetic variation was first through variations in phenotypes
What are the different types of genetic variation?
- Molecular variation
- DNA sequence variation
- Population variation
What is molecular variation?
- It is versatile and can be quantifiablty applied to all organisms
- molecular data are genetic
- DNA, RNA, proteins
- can be used over huge amounts of variation as we can generate so much data now
- easily quantified
What is DNA sequence variation?
- It analyzes genetic variation
- There are many techniques to visualize this
- individuals within a population are not necessarily identical in DNA sequences
What is population variation?
- this measures genetic variation between individuals
- explains higher molecular variation through the neutral-mutation hypothesis
- variation is maintined via balancing selection
What is speciation?
Cladogenesis occurs through speciation, the process by which one population separates into two distinct evolutionary groups
How do we define a species?
- there are seven general models that we use to define a species
- is a human construct to help differentiate/categorize living things into similar groups
- a construct for us to organize organisms in a specific way that is similar throughout
- some species are really easy to differentiate, others are rather complicated
- is a particular type of organism to which a unique name has been given (genus and species name)
- a species is also considered an evolutionary independent group of organisms
Carolus Linneaus, is the father of modern taxonomy
How much genetic differentiation is required for reproductive isolation to take place?
Reproductive isolation: existence of biological characteristics that prevent genes from being exchanged between speciess
What are the different reproductive isolating mechanisms?
- Prezygotic Isolating Mechanisms: prevent gametes from 2 different species from forming a zygote
-ecological isolation
-behavioral isolation
-temporal isolation
-mechanical isolation
-gametic isolation - Postzygotic Isolating Mechanisms
-hybrid inviability
-hybrid sterility
-hybrid breakdown
Ecological isolation
members of 2 different species do not encounter each other, as they interact with their environment differently
behavioral isolation
differences in behavior that prevent interbreeding
* calling songs
temporal isolation
reproduction in a species takes places at different times of the year or in different seasons
mechanical isolation
result of anatomical differences that prevent exchange of gametes
* male and female genetalia match in a species
gametic isolation
mating between individuals of different species can take place, but the gametes don’t fuse to form a zygote
* often sperm is killed by the female reprodutive tract as they do not match the egg
hybrid inviability
incompatability of the genomes from the 2 species prevent the zygote from developing (embryo does not fully develop)
hybrid sterility
embryo does fully develop, but the offspring is sterile
hybrid breakdown
typically occurs between similar species
* they produce viable F1 progeny, but further reproducing fertile F1 to F1 can lead to sterile F2 generations
What are modes of speciation?
- the process by which new species arise
1. allopatric speciation -
occurs when some geographic barrier splits a population into two groups and blocks exchange of genes between two groups
2. sympatric speciation - occurs in the absence of an external barrier to gene flow
allopatric speciation
occurs when some geographic barrier splits a population into two groups and blocks exchange of genes between two groups
* interruption of gene flow leads to accumulation and evolution of genetic differences that further result in reproductive isolation, which eventually leads to speciation
* if the barrier disappears and the populations come back into contact (secondary contact) there can be several outcomes
i. limited genetic variation has occured, reproductive isolating mechanisms may not have fully evolved, leading to gene exchange and eliminate genetic differences causing them to revert to the same species
ii. genetic differentiation when separated lead to strong reproductive isolation and the populations will remain seperate and distinct.
iii. some differentiation leading to genetic incompatibilies, and postzygotic barriers will have formed. Hybrid offspring possible, but offspring cannot pass on hybrid genomes/genes. Remain seperate populations. Natural selection will favor same species mating.
Darwin’s Finches (beaks (not a mating change), mating calls)
sympatric speciation
occurs in the absence of an external barrier to gene flow
* typically occurs when reproductive isolation occurs within the original population
* without reproductive isolation how can gene exchange be interrupted ?
* how can reproductive isolation occur within a population that freely exchanges genes ?
* can occur through polyploidy
i. allopolyploidy: 2 diploid species hybridize to form a tetraploid through nonadjunction (not full separation of chromosomes)
apple maggot flies (evolve on different host plants) hawthorne and apple trees, they fruit at different times of the year, full speciation has not occured here yet
What is a phylogenetic tree?
- These consist of a root, branches, and nodes.
- Show taxa and outgroup.
- They can be created to depict the evolutionary relationships between genes (gene tree).
- Branches: evolutionary connection between organisms
- nodes: the point at which two branches diverge from each other. Represents a common ancestor that existed before the divergence took place
- If nodes are a common ancestor to all of the species we are looking at we call it a rooted tree.
- outgroups are a “control group” in the phylogeny.
What is important about phylogenies?
There is no hierarchy in a phylogeny. Branches can be rotated at nodes. Show changes in relative time (older on the left). Evolutionary relatedness is based on common ancestry.
* we use phylogeny because evolution takes a really long time to occur
* homologous characteristic: one that has evolved from the same character in a common ancestor
* Nodes on a tree are really important as the tree itself can be rotated aroung the nodes. Does not change the evolutionary relationships
* though taxa lie adjacent to eachother they are not more closely related to each other. The branches are what we use to determine relatedness
How are phylogenies built?
- often done through a database
- determine where nucleotide strands are similar or different
- looking for the most parsimonious relation (least amount of steps between each species) makes them more closely related and builds nodes
- number of species on the tree changes the number of trees you get from the combination
- can use distance, parsimony, maximum likelihood/Bayesian methods
- change taking place at the nucleotide level
Molecular evolution
- measured by rates of nucleotide substitution
- nonsynonymous and synonymous substitution
-synonymous mutation does not have an affect on the fitness of the organism
-nonsynonymous are often detrimental and natural selection handles these - substitution rates for parts of genes
-relationship between the function of DNA sequence and ots rate of evolution - differnet genes, or even different parts of genes may have evolved at different times
synonymous mutations occur at a higher rate than the nonsynonymous
noncoding portions, introns, regulating regions, and third codon position as they do not cause a change/funcitonally change the gene, but can affect expression of the genes
What is evolution by gene regulation?
It is hypothesized that most adaptations are not from evolution of new genes, but from smalll DNA differences that alter the expression of existing genes.
What is genome evolution?
- the exchange of different exons through exon shuffling to create new genes that are mosaics of other genes
- gene duplication can also give rise to new genes or new gene functions (creating multigene families)
- whole genome duplication (creating two copies of the same gene)
- horizontal gene transfer occurs when DNA is transferred between individuals of different species
exon shuffling, where exons are shared between genes
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)
* natural selection will be largely directional “best allele” while eliminating others
* natural selection will decrease genetic variation
* some genetic variation is maintained by natural selection (through fitness differentials, where phenotypic changes cause alterations during reproduction)
balancing selection
is balancing between the two alleles
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 another 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 are evolutionarily independent of each other
