Evolution - Week 7 Flashcards
What does extant mean?
modern-day, currently living (not extinct)
In a specific type of phylogenetic tree, what would the length of the branch correspond to?
the number of substitutions (mutations that made it into the next generation) that appear to have occurred along each branch
In a specific type of phylogenetic tree, what would a longer length branch represent?
more molecule evolution (more change in DNA sequences over time)
*notice how little evolution has occurred in some parts of the tree compared to others
What are phylogenetic trees used to represent?
relatedness between many different levels of taxonomic organization (ie. species, subspecies)
What is a taxon on a phylogenetic tree?
species, subspecies, genus, family, etc.
Where are taxa on a phylogenetic tree?
tips
they represent extant taxa (usually we only include extant taxa in trees)
What are sister taxa?
closest relatives in an evolutionary tree
What is a root on a phylogenetic tree?
common ancestor of the whole tree
Where is the root on a phylogenetic tree?
bottom of the phylogenetic tree
What are nodes on a phylogenetic tree?
common ancestors
they are NOT present-day species
Why are nodes on a phylogenetic tree NOT a present-day species?
- it is impossible for one extant species to be the common ancestor of another extant species
- any organism or population of organisms that was an ancestor of an extant species would itself now be extinct
remember that all extant species have undergone evolution compared to their ancestral species
ie. the common ancestor of the gibbon and the human was neither a gibbon, nor a human
How can you determine which individuals are more closely related?
sharing a more recent common ancestor = more closely related
What is a monophyletic group (clade)?
ancestor and all of its descendants
*remember that even the end of a single branch (lineage) can be considered a clade
What is an outgroup?
least related to all other taxa in the tree
Can you see outgroups in rooted and unrooted trees?
rooted trees have clear outgroups
unrooted trees do not have outgroups
What must you remember about the order of the branches (as read across the tips) when reading phylogenetic trees?
- when determining whether or not two trees show the same phylogenetic relationships between taxa, the order of the branches (as read across the tips) is not important
- branches can rotate
What must you remember about the order of the nodes (ie. from past (root) to present) when reading phylogenetic trees?
- important
- in order to show the same phylogenetic relationships, two trees must display the same monophyletic groupings
What is important when thinking about how the phylogenetic tree represents past to present?
- it is not accurate to think of a branch furthest from the root as being closer to the present than a branch closest to the root
- all branch tips (=extant species) are equally situated in the present day
What does it mean if a branch doesn’t stretch all the way to present-day?
the lineage/taxon went extinct
Does fewer offshoots on a tree mean the species is less evolved?
NO
just because a species branches off first, that does not mean that it is more ancestral or less evolved
Why are living species not a “sample” of the past?
- modern-day species have all undergone a considerable amount of evolution since branching off from closely related lineages
- modern-day species are not direct samples of the past
Do phylogenetic trees include all taxa?
NO
remember that these phylogenetic trees often do not include all species that actually exists – the data shown here depends entirely on what we are able to add as data
What came first, the chicken or the egg?
the egg is an ancestral trait here, and appeared before the evolution of chicken
How do you know which organisms are more closely related to each other on a phylogenetic tree?
more recent common ancestor = more closely related
How are phylogenetic trees constructed?
based on the similarity of traits
How do you determine which phylogenetic tree is the “best” tree?
count the number of changes (fewest number of steps), and the tree was the fewest amount of changes is the best tree
How likely is it that a trait evolved more than one separate times?
it is more likely that it evolved once
What is the principle of parsimony?
the simplest explanation is the most likely
- the tree with the smallest number of changes (derived traits) is the most likely
- if there are two competing trees that represent relationships between the same group of taxa, the tree that displays the fewest number of trait changes is considered to be the most likely
- gaining a trait or losing a trait (ie. because the trait turned into something else) are both considered to be changes in the trait
What is the relationship between octopus legs and tetrapod logs?
analogous traits (they fulfill a similar function), NOT homologous traits
What is a synapomorphies?
characteristics (traits) present in ancestral species and shared by its descendants
What do synapomorphies define?
clades
Does similarity mean relatedness?
NO
What do we use similarity to infer?
relatedness between lineages
- morphological similarity (but be careful!)
- similarity of DNA sequences
- based on homologous traits
How might 2 organisms be similar without being related?
convergent evolution
How might 2 organisms be closely related without being very similar?
divergent evolution
What happens when convergent evolution occurs?
- two organisms may be similar without being related
- homoplasies/analogous traits
What happens when divergent evolution occurs?
two organisms may be closely related without being very similar
What are the “best” phylogenetic trees based on?
- highly heritable traits (ie. DNA sequences)
- many different traits
What are homologies (homologous traits)?
traits that are similar because they were inherited from a common ancestor (ie. five digits)
What are homoplasy (analogous) traits?
trait shared, but not because of shared evolutionary history
What is convergent evolution?
independent evolution of similar (convergent) traits in multiple lineages
Why might traits be similar due to convergent evolution?
because they evolved under similar selective pressures, NOT because of inheritance
What is divergent evolution?
evolution of differences between closely related groups or lineages
Why might traits be different due to divergent evolution?
because they evolved under different selective pressures
What is an ancestral trait?
trait that is shared with the ancestor
What is a derived trait?
trait that has evolved in a lineage that is different from ancestral state
- a trait is derived if it represents a change from a relatively ancestral form of the trait (ie. no hair → hair)
What are similar traits due to?
- inheritance from a common ancestor
- convergent evolution
What is homology?
- inheritance from a common ancestor
- forms homologous traits
What do organisms with homologous traits reflect?
relatedness between lineages
What is homoplasy?
- convergent evolution
- forms analogous traits
What do organisms with analogous traits reflect?
evolution under similar selection pressures
How do most species arise?
- one existing species turns into another species
- one existing species splits into two species
- two existing species combine to create one new species
What is speciation through hybridization?
two existing species combine to create one new species
- possible, but requires a very special set of circumstances
What are the special set of circumstances required fro hybridization?
- reproductively isolated from either parental species in order to be considered a new species, AND
- be able to successfully produce their own offspring
Speciation Through Divergence
What is a speciation event?
what was initially one species is about to split into two independent lineages
- the two lineages diverge from one another through time until they are so different that we recognize them as a separate species
Speciation Through Divergence
What would happen in the case of a species that exists in multiple populations?
it seems unlikely that those populations would all evolve in exactly the same way at exactly the same time (although this might happen if the populations were all under the same type of selection, or if there was a lot of gene flow between the populations)
- what might be more likely is one population evolving into a new species, while the other population remains unchanged
- if the original species went extinct very soon after the speciation event, it would appear as though the entire species turned into a completely new species
Speciation Through Divergence
Describe the 3 steps of a speciation event.
- low gene flow between populations: populations are somehow prevented from interbreeding
- population allele frequencies begin to diverge due to evolutionary mechanisms
- populations diverge enough that they are considered different species: different species = evolutionary independent populations
How do we know if a ‘different’ species is different enough?
species concept
In what ways may asexual organisms be considered evolutionary independent from each other?
- in terms of the resources they use
- in terms of how they look (morphology)
- they belong to separate clades
- in terms of their DNA sequences
In what ways may sexual organisms be considered evolutionary independent from each other?
- in terms of the resources they use
- in terms of how they look (morphology)
- they belong to separate clades
- in terms of their DNA sequences
- they belong to separate gene pools (=reproductively isolated)
What are the pre-zygotic (pre-fertilization) mechanisms of reproductive isolation?
- spatial isolation (isolation by living in different habitats)
- temporal isolation
- behavioural isolation (different mating behaviour)
- mechanical isolation (different parts)
- gametic incompatibility
What are the post-zygotic (post-fertilization) mechanisms of reproductive isolation?
- hybrids are inviable
- hybrids are sterile
How can low hybrid fitness influence selection for pre-zygotic isolating mechanisms?
- if hybrid offspring have lower fitness than nonhybrids, individuals that avoid producing hybrid offspring will be more reproductively successful
- certain traits might influence how likely an individual is to produce hybrid vs. non-hybrid offspring
ie. some females may avoid mating with individuals that look/smell like they belong to a different species
- there is heritable variation in such traits, they could evolve due to natural selection
- post-zygotic isolating mechanisms (ie. low hybrid fitness) could result in natural selection for pre-zygotic isolating mechanisms (ie. behavioural isolation, in the form of different mating behaviours)
Spatial Isolation due to Vicariance
What is vicariance?
the physical splitting of one population into two or more smaller populations, due to a geographic barrier (ie. river, mountain range, etc.)
Spatial Isolation due to Vicariance
Describe the 3 steps of a speciation event.
- low gene flow between populations: populations can no longer interbreed (what kind of barriers are there?)
- population allele frequencies begin to diverge: what kinds of evolutionary mechanisms are involved? (ie. mutation, natural selection, genetic drift)
- populations diverge enough that they are considered different species: under which species concept?
Spatial Isolation due to Dispersal
What is dispersal?
movement of organisms from one location to another
Spatial Isolation due to Dispersal
Describe the 3 steps of a speciation event.
- low gene flow between populations: populations can no longer interbreed (what kind of barriers are there?)
- population allele frequencies begin to diverge: what kinds of evolutionary mechanisms are involved? (ie. mutation, natural selection, genetic drift)
- populations diverge enough that they are considered different species: under which species concept?
Distinguish between mechanisms resulting in physical isolation (ie. forming allopatric populations) vs. evolutionary mechanisms (ie. mutation, gene flow, genetic drift, natural selection).
mechanisms resulting in physical isolation: describe how those populations came about
evolutionary mechanisms: describe the changes in the allele frequencies on that population as a result of that event on the population(s)
- in step two of speciation, where the population allele frequencies begin to diverge, it may require more than just one evolutionary mechanism to be involved
What is speciation?
evolutionary independence of a group of organisms
What are the prerequisites for speciation?
evolution is the only thing absolutely necessary
- in order to become evolutionarily independent from one another, two groups of organisms must evolve along separate evolutionary paths
Why might physical separation of populations not be necessary for speciation?
it may be possible to have low gene flow between two populations and subsequent divergence and speciation, even if the two populations are in contact with one another
Why isn’t natural selection necessary for speciation?
other evolutionary mechanisms (ie. genetic drift) could also cause two populations to diverge
What is an allopatric population?
- found in separate geographic areas
- populations are physically isolated from one another
What is a sympatric population?
- exist in the same geographic area
- individuals regularly encounter one another
What mechanisms of reproductive isolation can possibly lead to sympatric speciation?
pre-zygotic:
- temporal isolation
- behavioural isolation (different mating behaviour)
- mechanical isolation (different parts)
- gametic incompatibility
post-zygotic:
- hybrids are inviable
- hybrids are sterile
What is the species concept?
different ways of defining species
What are the 4 species concepts?
- phylogenetic = smallest possible monophyletic group
- biological = interbreeding populations
- morphological = specific form & function
- ecological = specific niche (ie. habitat, food, predators)
Phylogenetic Species Concept
What is a species?
smallest possible monophyletic group
Phylogenetic Species Concept
Why is a single human population not a true monophyletic group on its own?
- high degree of interbreeding between human populations
- in order to create something resembling a monophyletic group, we would have to include all of humanity
- in other words, according to the phylogenetic species concept, all humans belong to the same species
Phylogenetic Species Concept
When is divergence between populations possible for sexually reproducing species?
only if there is a lack of gene flow between those populations
*a lack of gene flow does not cause divergence, it only makes divergence possible
Phylogenetic Species Concept
How does gene flow have a homogenizing effect on allele frequencies?
- if there is gene flow between two populations, then what happens to allele frequencies in one population will influence what happens to allele frequencies in the other population
- two lineages that have been sharing alleles with one another will not be evolutionarily independent, and they will not form separate monophyletic groups
- descendants in one population will have ancestors from both parent populations, and both populations will show much of the same variation in traits
- instead of being two separate branches on a phylogenetic tree, the populations will form one large branch
