Lec 10 Flashcards
Species are central in _________________
Evolutionary biology
What is a speies?
Certainly no clear line of demarcation has yet been drawn between species and sub-species - that is, the forms which come very near to, but do not quite arrive at, the rank of species. A well-marked variety may therefore be called an incipient species. From these remarks it will be seen that I look at the term species as one arbitrarily given.
-Charles Darwin, the Origin of Species
Darwin thought that species is arbitrary and difficult to define
Discrete clusters of sexually reproducing organisms do exist
Elephant clearly does not mate with shrew
Speciation
The process by which new species form
Challenges of studying speciation
Speciation is not instantaneous
-Takes a LONG time
It takes a long time for 2 lineages to accumulate differences
How long divergence takes depends on the interactions between migration, drift, selection, non-random mating, and mutation
If divergence is a process that takes a long time, how do we decide where to “draw the line” and define a species?
How do we reconstruct a process that happened in the past?
Species concepts
Different concepts for different divergence stages
At what stage along these 2 time scales officially species?
Species concepts:
- Phylogenetic
- Biological
- Ecological
- Morphological
- Genotypic
Species Cluster Concepts focus on differences in allele frequencies; declares in a species earlier on
Biological evidence:
- Monophyletic DNA
- Reproductively isolated
- Different habitat
- Morphological distinct
- Different allele frequencies
Gene flow cut off, 2 lineages diverge
Morphological or phenetic species concept
Species are recognized by their phenotype and cluster together in morphological space
How to determine which traits to use for definitions for species?
-I.e. smooth skin and live in water would make dolphins and sharks related species
Individuals or populations are highly clustered in phenotypic space
Use this pattern of clustering to assign species boundaries
Challenge: How do we determine which traits to use for our definitions? (Remember homoplasies, symplesiomorphies, and convergent evolution)
The biological species concept
“Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups”
-Ernst Mayr
The biological species concept focuses explicitly on __________-
Gene flow (migration)
Migration is the movement of alleles between populations
Focuses on whether or not interbreeding occurs in a population
Interbreeding = species
No interbreeding = not the same species
Gene flow occurs or could occur between some populations
Remember: Migration ___________ differences between populations, whereas drift and selection typically __________ differences between populations
DECREASES; INCREASES
Populations that exchange genes/migration are different species
Migration DECREASES differences; reason allele frequencies become more similar is because they are interbreeding
If you are NOT exchanging genes or migrating, more differences will accumulate
Both drift and selection make populations more different from each other
These are used for population divergence speciation
Use pattern of gene flow to assign species boundaries
The phylogenetic species concept
Smallest possible monophyletic group descending from an exclusive common ancestor and diagnosable by unique, shared, derived characters
Monophyletic group described by red character
Flower color is highly polymorphic and is not a shared derived trait of monophyletic clades
Flower shape is a shared derived trait of monophyletic clades, and thus it can be used to delineate species boundaries
The biological species concept
Clearly meaningful measure of “distinctness”
-Directly related to gene flow, drift, natural selection (evolutionary processes)
Challenging/impossible to test in:
- Geographically isolated populations
- -We don’t know if they can interbreed since they are too far apart
- Fossils
- -Don’t know because they’re dead
- Occasionally hybridizing taxa
- -Some different species can mate
- Asexually reproducing taxa
- -Some can clone
We focus on biological species concept for the rest of this module
The phylogenetic species concept
Easy to test
HOWEVER:
- Subjective and dependent on genetic marker and character choice
- May not be linked to reproductive isolation
- -Can’t really determine discrete entities
- Characters may not be important to natural selection
- -Can’t get broad understanding
All species concepts have flaws. When are most species concepts likely to AGREE (delineate the same species boundaries), and when are they more likely to DISAGREE?
Agree between deeply diverged groups, disagree between recently diverged groups
Reproductive isolation
Stopping gene flow between populations
Cessation of gene flow between populations
If populations no longer exchange genes between each other, they can diverge into other species
Classic approach to studying the speciation process: Focus on geography
Allopatric speciation: Occurs when there is NO gene flow (geographic barrier); divergence occurs
Parapatric speciation: Occurs when migration is greater than 0; LIMITED gene flow
-i.e. island separated from mainland, can still have some migration
Sympatric speciation: SAME PLACE; NO reduction in gene flow due to geography; only one proven example
Reproductive isolation in allopatry
Reproductive isolation evolves when populations are separated
In the absence of gene flow (no migration), divergence in allopatry can result from
- Genetic drift
- Selection
Reproductive barriers arise as a by-product of divergence in the absence of gene flow
-Species are change events
Allopatric speciation and the Isthmus of Panama
Isthmus of Panama formed ~3million years ago - this separated previously continuous populations in the Caribbean and Pacific
In Alpheus shrimp, multiple sister species live on either side of the Isthmus
Prediction: more divergent pairs should have stronger reproductive isolation
Divergence dates have a wide range (3-18 million years)
-Should see relationship between how long lineages have been diverging from each other and how strong reproductive isolation is
Brought shrimp into the lab at studied their behavioral interactions
More closely related species were more “tolerant” of each other
Only 1% of matings between sister species produced viable offspring
the ones 18Mya apart did not tolerate each other or interbreed (no gene flow - different species)
Results: Different species on opposite sides of the Panama
Reproductive isolation in parapatry
Adjacent species diverge in the absence of a geographic barrier
Divergence often occurs along some sort of cline: a geographic or ecological gradient
Occurs when migration >0
Ring species
It can be difficult to make the distinction between allopatric and parapatric speciation, especially when populations are distributed somewhat continuously across a large area
Ring species are a special case of parapatric speciation involving clinical divergence and geographic isolation
Different species connected by gene flow expand to either side of the lake
Populations 8 and 12 act as 2 distinct species, cannot interbreed
Reproductively isolated species in sympatry
Genetically and behaviorally different
Live on sides of mountains, forests
Expanded range above Himalayas
Parapatric divergence across habitat types
Selection in the high zinc area leads to local adaptation and selection against gene flow
Populations live in mineral waste soils and some that live in healthy soils
Limited gene flow between contaminated and uncontaminated soil because those from clean soil cannot survive to contaminated soil
Reproductive isolation in sympatry
Area of serious debate among evolutionary biologists
Strong disruptive selection leads to reproductive isolation in sympatry
New species arises with No barrier
Only way this can occur is with strong reproductive selection
If you have 2 extremes (tall vs. short) and interbreeding results in an intermediate with low fitness, there will be assortative mating with tall mating with tall (high fitness) and short mating with short (high fitness)
Resource competition can THEORETICALLY lead to sympatric speciation
Because beak size ad seed size distributions mirror one another, birds with different beak sizes get approximately the same amount of food
This is a MODEL; can theoretically happen
Because the frequency of medium-beaked birds is so much greater than the frequency of medium-sized seeds, birds with medium-sized beaks fare poorly
Distribution of seed size much wider; if all birds have intermediate sized beak to exploit intermediate seeds, deal with a LOT of competition
Sympatric speciation in Lake Micaragua cichlids
The only proven example of sympatric speciation
2 cichlids in lake Apoyo form a monophyletic clade, suggesting single colonization event
-Just got there once and diverged in the lake
Feeding specialization and habitat preference have resulted in the speciation of 2 populations within the same lake
Extensive debate about whether __________ is possible or whether allopatry is required for speciation
Sympatric speciation
Why might sympatric speciation be controversial and unlikely in nature?
Very strong disruptive selection and complete cessation of migration is necessary among individuals living in the same location, which is unlikely
Reproduction isolation in sympatry
Strong disruptive selection leads to reproductive isolation in sympatry
- Why might sympatric speciation be unlikely?
- Even limited migration homogenizes differences between populations
- For sympatric speciation to work there needs to be a complete or near-complete cessation of gene flow among individuals of the same species in the same location
Why might allopatry be necessary for speciation?
Some cessation of gene flow is likely necessary for genetic differences to accumulate
What does it mean if reproductive barriers break down in secondary contact?
Last step is 2 populations come together and exist in sympatry
Allopatric model is the most common form of speciation
What causes reproductive isolation?
Reproductive barriers
Prezygotic reproductive barriers
Occurs BEFORE fertilization in non-random mating
Habitat isolation
-Live in different locations, cannot physically mate
Temporal isolation
-Occurs when breeding seasons do not overlap
Behavioral isolation:
-Some overlap in range, use completely different songs
Mechanical isolation
- Physically cannot mate
- Occurs AFTER mating attempt
Things that prevent these individuals from mating
Behavioral and ecological isolation in sticklebakc
Stickleback inhabiting limnetic vs. benthic environments have different feeding morphology and don’t interact much
They also have different nuptial color on their throats, which is used in mate choice
Mate choice and habitat differences keep these two closely related morphs isolated in the same lakes
Fully capable of interbreeding, but each specialized for specific habitat and not good for fitness to create intermediate
What relationship do you see among spur length and pollinators, and how might this affect reproductive isolation?
Pollinators change as spur length increases; differences in pollinators likely increase reproductive isolation
Pollinator isolation in flowering plants
There is a tight correlation between flower morphology and the feeding morphology of pollinators
Closely related species with different pollinators often have very divergent morphology
Different pollinators means little gene flow between species
If a hummingbird goes to one flower and a bee goes to another, they are specialized for that flower and gene flow will be cut off to the flower
Postmating/postzygotic reproductive barriers
Barriers that come AFTER mating has occurred
Genetic isolation
- Occurs before fertilization but after mating
- Sperm meets egg but cannot fertilize (different speies)
Reduced Hybrid Viability
-Mate and produce offspring but offspring don’‘t survive long
Reduced Hybrid Fertility
-Offspring viable but infertile = genes not actually flowing
Hybrid breakdown
- Mating occurs, offspring survive and are fertile
- After a few generations, fitness decreases/breaks down
Reproductive Isolation via polyploidy
Duplication of all chromosomes
Occurs when a mutation results in the production of diploid (rather than haploid) gametes
Reproduction doesn’t work when ploidy levels don’t match (n can’t mate with 2n)
Frequency of polyploid speciation
Plants tolerage changes in ploidy better than animals
In animals, differences in ploidy level disrupt meiosis and lead to infertility
Polyploid plants can self-fertilize or mate with other polyploids
Polyploidy thus often means an instantaneous species barrier: the new polyploid plant can’t produce fertile offspring with members of its former species
Its’ estimated that 30% of fern and grass species originated form polyploidy
In animals, evidence for polyploid speciation in shrimp, frogs, insects, fish, bivalves, and oral
-Much less common
Why does postzygotic isolation occur?
Dobzhansky-Muller Model
Epistatic interactions between A and B loci result in a fitness cost to individuals with both the A2 and B2 alleles - selection AGAINST hybrids
1) Ancestral genotype
2) Mutations after allopatry (populations separated)
3) Mutations reach fixation due to natural selection (beneficial in environment) or drift
4) Come back together, hybrids reduce viability and fertility
Negative epistatic interactions between alleles that have evolved in isolation
Postzygotic isolation: Haldane’s Rule
If one sex is absent, rare, or sterile in hybrids, it is the HETEROGAMETIC sex
-Males (XY) in mammals and Drosophila, females (ZW) in birds and butterflies
Thought to occur because of negative interactions between recessive sex-linked genes and autosomes
Which type of barrier is stronger: pre- or post-mating isolation?
Generally good evidence that PREMATING isolation evolves first, and postmating isolation takes longer to arise… but not always
Postmating barriers may be necessary for the COMPLETION of speciation
Why might postmating barriers be important to the completion of speciation?
Because if premating barriers fail, gene flow can still occur, but postmating barriers can completely stop gene flow
Species
Can interbreed
Offspring can reproduce
Breeds are variations within a species
The process of speciation
1) Isolation: Gene flow cut off or restricted (except in rare cases of sympatric)
2) Divergence: Evolution in isolation caused by genetic drift, natural selection, mutation
3) Secondary contact: Divergent populations come back into sympatry. Reproductive barriers are tested by hybridization, reinforcement may evolve
Hybridization
The interbreeding of individuals from 2 populations, or groups of populations, which are distinguishable on the basis of one or more HERITABLE characters
-Woodruff
A natural test of reproductive barriers and species boundaries
We can better understand species boundaries by looking at the conditions under which we do and do not find hybridization
Hybrid zones
Interactions between genetically distinct groups of individuals resulting in at least some offspring of mixed ancestry. Pure populations of the two genetically distinct groups are found outside of the zone of interaction
Occur in SECONDARY CONTACT
- After evolving in isolation 2 populations may come back together
- -Point where they come back into contact has potential to become hybrid zone
- If the divergence is recent, these two species may hybridize, producing fertile offspring
- If the divergence is substantial, the 2 species may producing infertile offspring or may not hybridize at all
Hybrid zones are natural “__________” for studying species barriers
Laboratories
Outcomes in hybrid zones
Collapse and interbreeding (any differences obtained in allopatry are LOST = same species)
Complete isolation
Limited interbreeding/introgression
-Limited gene flow
Reproductive character displacement
-Limited gene flow
Reinforcement
-Limited gene flow
Adaptive introgression
Movement of beneficial alleles from one genetic background to another
Introduces BENEFICIAL alleles only
Females of both white-throated and gol-collared manakins prefer yellow throats to white throats = yellow throats selected for in hybrid zone
Altitude adaptation in Tibetans:
- Introgression of Denisovan DNA
- Allows them to survive at very high altitudes (no other humans have this)
If one species has a more beneficial allele and they interbreed, this beneficial allele is passed on through BOTH populations and into a different species; a way to introduce new alleles into populations
Reinforcement
Selection to maintain isolation in secondary contact
NOT selection for the formation of new species (species are a byproduct, not the end goal of selection)
Have some barrier between two populations
Hybrids are unfit, generating strong selection AGAINST interbreeding
2 populations might interbreed and hybrids have low fitness, selected against
2 populations don’t breed because of selection against hybrids
- Extrinsic selection against hybrids: Hybrids are maladapted to their environment
- Intrinsic selection against hybrids: Hybrids are genetically inferior (inviable, infertile)
Reinforcement may be important to the completion of the speciation process
Reinforcement selects against maladaptive hybridization
It can operate via:
- Prezygotic isolation: Mate choice changes in life history (timing of breeding), habitat isolation, etc.
- Postzygotic isolation: Genetically incompatibility, low hybrid fitness
Reproductive character displacement
Reproductive traits are more divergent in sympatry than allopatry
Selection favors better identification of your own species because hybrids are unfit
-Need to be good at distinguishing your species from others
Traits diverge where you overlap with close relatives
Can be part of reinforcement
Is hybridization “good” or “bad’?
At the POPULATION level, gene flow (migration, m) maintains genetic diversity and prevents inbreeding
Gene flow breaks down population differentiation
At the SPECIES level, hybridization may be maladaptive because it decreases individual fitness
When you interbreed, you break down linkage disequilibrium
CAN be a way to move genes around in beneficial way
Synthesis questions; Which statements about hybridization are true?
a) It is a form of migration that introduces new alleles and genetic variation into populations
b) It can lower the genetic fitness of parents if hybrid offspring are unfit
c) It can break up locally adapted linkage disequilibrium due to recombination
d) All of the above
d) All of the above
Ho new species are formed
1) Isolate
2) Diverge
3) Sympatry (come back together)
4) Reinforce (have secondary barriers to prevent breakdown of accumulated different traits)
5) Repeat
How (and why) are species lost?
Extinction
Extinction
When a species dies out leaving no living descendants
The idea of extinction was a matter of much debate until the turn of the ninteenth century. This is because extinction suggests a less-than-perfect world, which pre-Darwinians (especially religious) did not generally accept
Most species that have ever lives - but not all - have gone extinct
Extinction and the fossil record
The fossil record holds information about organisms that lived in the past but are gone today
The law of superposition tells us that fossils lower down in the sediment are older
Techniques such as radiocarbon dating use the fact that carbon 14 isotopes decay into nitrogen-14 at a steady rate to date fossils
Living organisms take in carbon 14, which stops when they die
Ratios of carbon14: nitrogen 14 can then tell us how old a fossil is
Other isotopes with longer half-lives (such as potassium-40 and uranium-235) can be used to date older fossils
However, there is often a time-lag effect in the fossil record: the last time a fossil shows up is NOT NECESSARILY the last living individual of that species - this sets extinctions older than they actually occurred
We can also date extinctions as more recent than they actually occurred when sediment gets disrupted (i.e. by burrowing organisms)
Big 5 Mass Extinctions
Late Ordovician (>80% species went extinct, major branches left intact)
Late Denovian
Late Permian
Late Triassic
Late Cretaceous
Not the only mass extinctions
Fossil record only captures small amount of actual specimens
More fossils from swamps because they support fossilization
Some mass extinctions worse than others
In a mass extinction, at least 75% of species must die out
Oxygen extinction also devastating, couldn’t tell how much since it affected microscopic organisms
Consequences of extinction
Impact o extinction on biodiversity depends on where on the phylogenetic tree of life extinctions occur
Extinctions at the tips of a tree preserve the overall structure of biodiversity
More basal extinctions alter the overall structure of biodiversity
Synthesis question: Why would we say that the extinction of the dinosaurs ‘paved the way’ for mammals?
Because the extinction of the dinosaurs removed competitors and opened up ecological niches, allowing for an adaptive radiation of mammals
Mass extinctions
Catastrophic events that wipe out many species at same time, normally caused by dramatic environmental change
Typically >75% of all species in a short geological time period
Responsible for only 4% of the total number of extinctions
-NOT the major source of extinction
Have huge effects on morphological diversity, behavior, niche occupation, developmental patterns
The Cretaceous (K-Pg) mass extinction
Lots of evidence suggests this extinction was due to asteroid impact
Dramatic increase in iridium in statigraphic layers at the K-Pg boundary (iridium is rare on earth but common in asteroids and meteors)
-High levels of iridium
Amino acids of extraterrestrial origin in clay near K-Pg boundary
Crater of predicted size located in Yucatan, along with high iridium and evidence of an ancient tsunami
The Permian extinction
Largest extinction on record -90% of all species extinct 250Mya
Many major groups wiped out
The extinction event is linked to low levels of O2 and high levels of CO2 in the atmosphere and oceanic waters, potentially due to Siberian Trap eruptions - volcanic events that continued for 2 million years
Organisms adapted to hypoxia survived the extinction event
Aftermath of mass extinctions: Extinction Debt
Clades that survive mass extinctions often go extinct shortly after
May be due to new, post-extinction environments to which clades fail to adapt
Could be due to altered/new ecosystemts post-mass extinction
The sixth extinction
Humans have been implicated in extinctions for a long time
There is evidence that human hunting - likely coupled with climate change - drove many of the Pleistocene megafauna to extinction
Similar proportions of groups endangered or threatened at present to those lost in “big 5” mass extinctions (current levels are lower, but approaching the 75% marker)
Which is true about mass extinctions?
They typically result from catastrophic events
