Exam 4 (Parapatric Speciation - Human Evolution) Flashcards
How does parapatric speciation occur?
A population doing well expands its geographic range and experiences a stepped cline and a hybrid zone is formed
What is a cline?
A range of area where the environment rapidly changes and there are different selective pressures on either side
What is a hybrid zone?
A primary contact zone where the populations have never been separated by physical barriers
What is a tension zone?
A zone where the hybrids are less fit than their parents and sets up conditions for reinforcement to be maintained
What is reinforcement?
When hybrids are less fit than their parents because they are not suited to either parental environment and selection will favor assortative mating
What is a primary contact zone? What about secondary?
Primary = the populations have never been separated
Secondary = populations separated at first then populations meet
** most hybrid zones have been secondary; it is hard to determine whether hybrid zones are either or
Why do we expect to find greater prezygotic isolation closer to the hybrid zone than away from it?
We would expect to find a selection for a greater amount of assortative mating near the step because individuals are hybridizing with each other more than the individuals farther from the step who are interbreeding amongst themselves
Why should selection for assortative mating be strongest at the cline?
The individuals farther away from the step are mating with those in their own environments so there is no selection on them for assortative mating; they do not experience any fitness advantage because they have been selected to live in that environment; individuals at the step can hybridize and have less fitness if they do because they are not suited for either environment
What do we actually see regarding prezygotic isolation and the hybrid zone?
In reality, we do not often find greater prezygotic isolation closer to the hybrid zone than away from it
What can we conclude about parapatric speciation?
It is possible but evidence suggests it is not common
What is sympatric speciation?
2 populations of the same species stay in the same environment and still speciate
What are the 2 types of sympatric speciation?
Gradual and Instantaneous
What is required for gradual sympatric speciation?
A population has to have at least 2 morphs and a patchy environment
What is a patchy environment?
Places within the environment where one morph does better than the other
Why is this not small scale allopatry?
There is no physical barrier just different habitat preferences and the different morphs can still breed with each other
Why are hybrids less fit?
They are not suited to either parental environment
Why does that favor the evolution of assortative mating?
Individuals mating with other like individuals will mean their offspring will be suited for the parental environments (greater fitness) compared to a hybrid individuals not suited for either one (lower fitness)
What is a host shift?
The movement of a parasite, disease, insect herbivore from one host to another
Explain an example of a host shift & how it relates to sympatric speciation.
The tephritid fly (R. Pomonella) originally lived on hawthorns (native to U.S.), then it was seen on other species such as apples, pears, cherries and have genetically differentiated on their different hosts since apple-morph maggots do best on apple while hawthorn-morph maggots do best on hawthorn
Why do the developmental biases of species like these promote speciation?
Females prefer to lay eggs in the same fruit type as they developed and in turn their female offspring do the same
The pre-exisiting genetically-based behaviors quickly create high levels of pre-zygotic isolation since maggots in apples attracted to the apple smell as adults, they find mates there, and won’t usually interact with adults that were hawthorn maggots
What are the 2 types of instantaneous sympatric speciation?
Allopolyploid Hybrids and Diploid Hybrids
What are allopolyploid hybrids?
When 2 different hybridize but their offspring inherit the complete diploid nuclear genomes of their parents; have a higher ploidy level
** N1 != N2 (ploidy levels of parents)
Why is this instanteous?
A backcross to either parent will produce a triploid offspring which will be completely sterile if not inviable due to inability to assort properly during meiosis
What are diploid hybrids?
The normal haploid gametes from 2 species combine to produce a diploid hybrid
** N1 == N2
Why is speciation not instantaneous for diploid hybrids?
The hybrid can backcross to its parents
How does the hybrid become isolated then?
The combination of the parental genomes creates a hybrid with new phenotype combinations and there is selection for survival in a different habitat than either of the parental species OR chromosomal rearrangements promote post-zygotic incompatibility
What chromosomal rearrangements are most important in this process? Why?
Inversion but mostly multiple translocations can cause multivalents to form during meiosis
What were the pre-biotic conditions of early earth like?
There was volcanic activity, the atmosphere was primarily composed of 80% nitrogen and reducing
Why did oxygen take so long to accumulate?
Oxygen was reacting with other elements instead
What was the Miller-Urey experiment?
They recreated the conditions of early earth: added energy (lightning/UV) to a reducing atmosphere
What were the results of their experiment?
Were able to produce 17/20 amino acids, produced the nucleic acids: AUCG, T was not produced but it is just a methylated form of U
What does the RNA world theory hypothesize?
RNA came before DNA so RNA was the first genetic material and RNA sequences are the catalysts instead of proteins (ribozymes), some are self-catalytic for their own replication
What evidence is there for the RNA world theory?
The presence of uracil and very little thymidine in the Miller-Urey experiment, modern ribozymes such as self splicing introns & ribosomes, and experimental production of artificial ribozymes that self-reproduce
When does life start?
Precambrian about 3.6 bya - 541 mya
What evidence do we have for the first life?
Best fossil evidence = fossil stromatolites, bacteria like cells that were mostly unicellular and probably heterotrophic due to lack of oxygen
Why are fossils rare?
Fossils form easier in marine environments than terrestrial ones and due to subduction zones
What are subduction zones?
When marine and continental plates collide, marine plates are pushed underneath the continental ones because they are more dense while the continental one is pushed up
The fossils go under as well and are lost
Why did the accumulation of oxygen lead to profound changes in life?
Oxygen at first acted as a poison to the aerobic forms of life (also highly combustible); Selection for aerobic life forms that could tolerate it
Allowed greater metabolic efficiency (Anaerobic = 2 ATP/ glucose vs. 30-32 ATP/glucose
Made the complex multicellular life forms that require much higher levels of energy possible
Development of ozone layer which protects against UV radiation (allowing terrestrial life)
Decrease the greenhouse effect from CO2
What were the first eukaryotes like?
No evidence for eukaryotic cells until 1.8 bya
Mitochondria acquired from purple line of Eubacteria
Chloroplasts acquired from cyanobacteria
Development of nucleus
Precursor for multicellular life
What was multicellular life like?
The first organism (a fungus) appears about 2.4 bya
First animals appear about 670 - 550 mya: mostly soft bodied animals
What was the Cambrian explosion?
Sudden increase in fossil diversity
Evolution of hard parts likes bones, exoskeletons
Why does speciation produce nested sets of species?
Most speciation occurs when a single interbreeding group gets divided into two groups that stop interbreeding with one another; This process will necessarily create a nested set/hierarchical set of relationships among species
What are the 2 classification scheme criteria?
Objectivity Criterion & Naturalness
What is the objectivity criterion about?
Reasoning for grouping allows other people to have access to the same characters and apply it themselves and the characters are directly measured and observed
What is naturalness?
characters not used to determine the common features of a group will produce the same classification as the characters used to determine the group
What is character conflict?
Different sets of characters indicate different groupings (unnaturalness)
Why do we use these classification criteria?
They allow rational people to independently discover the same classification scheme; the classification is stable and repeatable
What is a sister species?
Species derived from the same most recent common ancestor (MRCA)
What is a homologous character?
Character shared between species because of common descent (inherited from the ancestral species)
EX: tetrapod forelimbs have the same bone structure
What is an analogous character?
Character shared due to convergence (similarity due to either chance or common selective pressures)
EX: Cacti & Euphorbs (also have spines)
What is a similarity?
Two character appear alike without making a claim about common descent or convergence
What is a symplesiomorphy?
A trait inherited from a MRCA of a group BUT NOT ALL descendants have the character (shared ancestral homology)
What is a synapomorphy?
A character derived in multiple lineages
What is a monophyletic group?
All descendants from a single common ancestor (a clade) which is created using synapomorphies and only used for cladistic classification
What is a paraphyletic group?
Some but not all descendants of a MRCA which is created using symplesiomorphies
What is a polyphyletic group?
Species not grouped due to the same CA which is produced using analogies and not used in any classification
Why can’t symplesiomorphies be used for classification?
They don’t include all members of the clade defined by the first CA with the trait
Different symplesiomorphies can indicate different groupings
Why can analogies never be used to make inferences about relationships?
Traits evolved independently, groupings won’t reflect a single clade
Does the phylogenetic approach meet the classification criteria?
YES
What is a polytomy? What does it indicate?
More than 2 branches coming from the same root (star tree)
Used when you know nothing about the species relationships
If a tree has a polytomy, it is not resolved
What does a fully resolved tree have?
Only 2 branches from each node
What does a phylogram show?
Topology and evolutionary distance
What does a cladogram show?
Only topology (branching order)
What is a parsimony? How is it used to construct phylogenetic trees?
The simplest explanation is best
A tree that has the least number of changes for characters
When is parsimony violated?
When there is too much change because we can’t really discover what actually occurred
What does parsimony work well for? Why?
Complex organs because it is unlikely to have unrisen repeatedly in the same way
What doesn’t parsimony work well for? Why?
DNA sequences because a nucleotide of DNA has a 25% chance of being similar to one in another species just by chance
What are the 2 steps to produce a tree?
Find the shortest possible unrooted tree (requires fewest changes) then root the tree
How do you unroot a tree?
Remove time from the tree
Take 2 branches coming from the root and make them a single branch
Rearrange the branching pattern so that time is not reflected in the unrooted tree
** repeat for all possible unrooted trees
Count the number of evolutionary events & choose the most parsimonious
What is the process for morphological characters?
Choose and measure good characters
Find true homologies
Figure out character polarities
Make & root the tree
How do you determine good characters?
Good characters are complex ones unlikely to have evolved the same way twice and heritable (homologies)
What are the 3 principles to determine homologies?
Homologous characters show the same fundamental structure
Homologies have the same fundamental relationships to surrounding characters
Same embryonic development in different groups
What are some dangers in determining homologies?
Mistakenly classifying analogies as homologies can introduce character conflict and useful info can be lost
What are the 3 methods determine synapomorphies to figure out character polarity?
(Did A or A’ come first)
Outgroup comparison
Embryological criterion
Paleontological criterion
What is outgroup comparison?
Use a close relative (sister) of the species being studied to determine the ancestral character state using parsimony to judge the most likely set of events
What are the shortcomings of outgroup comparison?
Different outgroups can sometimes give conflicting info about which character state is ancestral
We must already have some phylogenetic knowledge to be able to select the sister group
What is the embryological criterion?
The general features of a large group of animals appear earlier in the embryo than the special features
What are shortcomings of the embryological criterion?
Some derived features get inserted into an older developmental sequence
Ex: metamorphosis- caterpillar state was inserted
What is the paleontological criterion?
Any recorded characters in the fossil record
Features in earlier fossils are ancestral to ones found in later fossils
What are the shortcomings of the paleontological criterion?
The incompleteness of the fossil record can give a misleading view of what characters are ancestral or derived
What should you do even if there is still character conflict after the methods?
Go to parsimony anyways or continue to collect data & refine analysis
** once the phylogeny is constructed, we may reassign conflicting characters to symplesiomorphic or analogous status
What criterions are needed to root a tree?
Outgroup Comparison: can use for almost any group
Embryological Criterion: only useful for multicellular organisms
Paleontological Criterion: mostly only for organisms with hard body parts
How do you root a tree?
Take the branch to be the most parsimonious for adding the outgroup and break it into 2 to make the root
Rearrange the tree to be a normal rooted representation
What are good characters for the molecular method?
Nucleotides and amino acids in a sequence is a character
What are some advantages to 4 bases?
There are millions or billions of characters available for analysis for many organisms
What is the process for molecular characters?
Collect characters
Align character for positional homology
Go straight to parsimony
Choose the most parsimonious unrooted tree
Root the tree
How do the 3 principles for determining homologies apply to molecular characters?
Homologous characters show the same fundamental structure: all molecules of the same name have the same structure
Homologies have same fundamental relationships to surrounding characters: same as above
Same embryonic development in different groups: nucleotides and aa’s don’t have development
What is positional homology?
History of the nucleotide or amino acid at that position
Explain why getting the correct alignment of a set of DNA sequences is essential for using them for phylogenetic inference.
Each column of nucleotides/AAs in an alignment needs to represent the actual history of the nucleotide/AA at that position (positional homology)
Otherwise, you’ll be inferring the phylogeny using sequence histories that didn’t really happen and you will infer the incorrect phylogeny
Why do we go straight to parsimony?
There is no means to determining homologies & analogies
How do we determine the most parsimonious unrooted tree?
Only outgroup comparison is available because molecular fossils are rare and there is no embryological development of NT’s and aa’s
What are some issues with phylogenetic inference?
The number of combinations is enormous even with a relatively small number of taxa (we can only exhaustively search through trees for about 30 species)
Need at least as many synapomorphies as taxa to be resolved (can have several results that are compatible with the evidence due to not enough sites to distinguish them)
Need to pick a molecule that evolves at an appropriate rate (too slow = not enough info to resolve the phylogeny; too fast = too many analogous changes to make sense of the relationship)
What is coevolution?
When 2 or more species act as reciprocal selective agents/ forces on one another, selection on each other
What is needed for coevolution?
The biota acts as a selective pressure on itself
Selection must be reciprocal
There can be feedback loops
Which types of species interactions could lead to coevolution?
Consumer-Resource: predation, herbivory, parasitism (+,-)
Mutualism (+,+)
Why do commensalism (+,0) and amensalism (-,0) not lead to coevolution?
There is no selective affect for one of the species in both cases
Why does competition (-,-) not lead to coevolution?
It is rare that both species are equally competitive so the less competitive one is driven to extinction or has to switch to a different resource and therefore no longer in competition
What is a component of coevolution?
Coadaptation: mutual adaptation of 2 species to one another
Cospeciation: when a pair of species that are associated with each other speciates at the same approx. time
What are the 2 types of coadaptation?
Positive (mutualism)
Negative (arms race)
What is positive coadaptation?
A coadaptive relationship where 2 or more species experience increased fitness from the mutual adaptations
What is negative coadaptation?
A coadaptive relationship where one species experience increased fitness and the other experiences decreased fitness
Explain an example of positive coadaptation?
Pseudomyrmex ants and tropical Acacia trees
Acacia benefits the ants by swollen thorns providing shelter, beltian bodies provide food
Ants benefits the trees by defending them against herbivore and disease (attack organisms & ants lower the amount of pathogenic bacteria
How do we know that the ant-plant mutualism is an example of positive coadaptation?
The tree evolved in response to the ants because not all species of acacia tree have the swollen thorns and beltian bodies
How does cospeciation work?
When one of the species undergoes a speciation event, association is maintained since they are adapted to each other
As a consequence, the other member also undergoes speciation
Why is cospeciation needed?
What appears to be coevolution could just be association by chance
Could be preadapted to each other
The role of history important
Why are synonymous mutations more clocklike?
There is a constant rate of substitutions occurring in a region of DNA like a clock because the only thing acting on them is drift not selection since neutral
How does negative coevolution work?
The feeding of insects on plants exerts a selective pressure on the plants for antiherbivore defense
The plant evolves a chemical defense against the plants (taste nasty, make sick)
That defense creates a selective pressure on the insects for resistance (will have higher fitness & replace non-resistant ones)
Process repeats
How does negative coevolution work in the context of disease and hosts?
The host would be under selective pressure to evolve resistance to a parasite because of improved fitness for the host
The disease would evolve to better exploit its host (increased population growth & increased virulence)
What is virulence in this context?
The degree to which a parasite reduces the hosts fitness (taking more resources from the host)
What is virulence limited by?
A disease can’t be so virulent that its host dies before it moves on to the next host
What is the exception?
Extremely virulent diseases that can stay in the environment for a long time after the host dies
Why do we expect diseases to evolve to be more virulent over time?
Selection doesn’t look ahead
More rapid reproduction = using more of hosts resources
Provides immediate fitness increase over avirulent types
How do so many diseases actually evolve to be less virulent over time?
Mode of transmission and Rate of infection
What are the 2 modes of transmission?
Vertical and horizontal
What is vertical transmission?
From parent to offspring
The juvenile must live long enough after to grow and reproduce
Promotes lower virulence
What is horizontal transmission?
Movement from one host to another without respect to the genetic relationships of the hosts
Only needs to live long enough to for the disease to be transmitted
Doesn’t favor lower virulence
How does high rate of infection work?
If there are individuals with both types of genotypes: avirulent and virulent
Virulent is reproducing at higher rate, the lesser competitor dies out and the other dominates
Virulent type has a selection advantage
Avirulent type only has as much time to reproduce as the virulent type gives it before the host dies
How does lower rate of infection work?
Individuals only affected with one genotype at a time
Virulent type kills its host but this time the avirulent isn’t dying off
Avirulent type has more time to reproduce since virulent type eventually dies off
Explain an example of the evolution of lower virulence.
Control of rabbit populations with myxoma virus
At first, the introduction of the virus to the pests killed off 100% of rabbits affected
Over time virulence declined because the density of rabbits lowered the rate of infection (changed as it progressed)
How do we know coevolution occurred in the above example?
The lab strain of rabbits was less affected by later genotypes of the virus
Wild rabbits were also less susceptible to the original strain stored in the lab
