Adaptations Flashcards
What does the evolution of senescence involve?
random effects (drift and mutation) and trade-offs
What is extrinsic damage?
(bad luck)
death/injury/illness from causes for which your genes can’t help much (ie. accidents, overwhelming predators or diseases, weather, etc.)
selection cannot help with this
What is intrinsic damage?
(your genes’ fault)
death/injury/illness from developmental or survival issues that genes could have solved (ie. cancer, heart attacks)
selection can help with this – but didn’t
What is the senescence theory?
if extrinsic damage tended to kill your ancestors when they were old, then intrinsic damage will have evolved to kill you when you are old also, even if you are now protected from extrinsic damage
bad luck tripped up your ancestors, then their own genes evolved to kick them when they were down
What is the mutation accumulation theory to why we have genes that make us senesce?
(random effects)
deleterious mutations that act in old age passively accumulated because selection against them was weak, because frequent extrinsic damage meant that individuals were unlikely to be alive then anyway
involves only random effects (mutation and drift), and failure of selection to act
What is the antagonistic pleiotropy theory to why we have genes that make us senesce?
(trade-offs)
deleterious mutations that act in old age were selected for because of their beneficial effects earlier in life
What are somatic mutations?
mutation may have an effect on the body, but won’t get passed to offspring because it’s somatic – not in the germ-line that would make gametes
What are germ-line mutations?
mutation that happens in the germ-line likely won’t have any effect on that body, but it could get passed to offspring where it would act
What is pleiotropy?
gene has more than one effect
Why might pleiotropic genes be selected for?
high rates of extrinsic damage
- early selection is much stronger than late selection, as old-age individuals are so likely to die anyway
advantage of early reproduction
- better to reproduce early than late (assuming one’s body can be ready early) because early reproducing genes get a shorter generation time and therefore multiply faster
- gaining early babies more than compensates for losing late ones
- reproducing earlier (having a shorter generation time) is advantageous – genotype that starts reproduction earlier will outcompete a slower one, because it will multiply faster if all else is equal
What is a counter-adaptation?
adaptation that counters change to organism’s environment
What is another term for coevolution?
reciprocal adaptation
What is entangled fates?
association that persists through offspring generation after generation that selects for cooperation (inhibits cheating) between lineages – within species, or between species
What are the two consequences of vertical transmission?
- co-speciation (parallel phylogenies)
- evolution of cooperation because of entangled fates
What are transposable elements?
sequences of DNA that can move within and between genomes
persist in genome merely because they have succeeded in replicating themselves – not necessarily intrinsically good or bad
Are transposable elements selfish?
yes – do not normally perform useful functions for the organism
What deleterious effects can transposable elements have?
- metabolic cost of maintaining extra DNA
- introduce mutations when they move
- counter-adaptations to suppress their activity
- can disrupt gene expression
What is isogamy?
same-sized gametes – ie. single-celled alga
What is anisogamy?
some gametes are really big (eggs), some are really small (sperm) – ie. animals, land plants
differences in gametes size means:
- eggs (and consequences of making them) are costly
- sperm are cheap
What is the operational sex ratio?
ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate
Is variance in reproductive greater for males or females? Why?
males
- females are more or less assured of some offspring (if they are healthy)
- some males get a lot of offspring, others don’t – males are competing
(roles are reversed in some species – if males invest more in caring for offspring, they are the choosier)
What is intrasexual selection?
males compete amongst each other, and females mate with winner
What is intersexual selection?
females choose among males according to their qualities
Female Intersexual Selection
-
What are the costs of sexual reproduction?
- cost of producing males – by dividing resources between daughters and sons, sexual females should be quickly outcompeted by asexual variants (all else being equal)
- destruction of favourable gene combinations
- cost of finding mate
- risk of not finding mate
- risk of disease transmission during matching
What is Fisher’s sex ratio?
total reproductive value of males in a population is exactly equal to total value of all females, because each sex must supply half the ancestry of all future generations of the species
sex ratio will so adjust itself, under the influence of natural selection, that the total parental expenditure incurred in respect of children of each sex, shall be equal
Evolution of Sex Ratios
Which sex has the advantage?
the rarer sex typically contributes more to next generation, selecting for mutations that equalize sex ratio
Evolution of Sex Ratios
What type of ratio would reduce the cost of sex?
female-biased sex ratios – populations with these ratios would be able to grow faster
When do trait values show a continuous distribution?
when many genetic loci contribute equally to a trait
What is the breeding value of a genotype?
tells you how much that individual contributes to the trait mean of its offspring, when assessed against a variety of genetic backgrounds
measures degree to which individual’s phenotype can be expected to be transmitted to that individual’s offspring
NOT a direct measure of the phenotype of an individual
Why does breeding value differ from direct measure of the phenotype?
because it is a function of additive effects of alleles
What are additive effects?
determines how much the mean of a trait changes given that an individual carries a particular allele
What does the additive effect of an allele depend on?
- h (dominance coefficient)
- frequencies of alleles at that locus
- s (selection coefficient) – matter if the trait is under selection
What are additive effects of alleles (A)?
average effect of all alleles that contribute to a phenotype
What are dominance effects (D)?
effects of dominance interactions among alleles at each locus
What are epistasis effects (I)?
effect of interactions among alleles at different loci – reflects reshuffling of alleles over a set of loci
Why does the breeding value differ from the direct measure of the phenotype?
when two individuals produce offspring, they can only contribute 1 allele
- that allele in the offspring may find itself paired in a new dominance relationship
- across loci, that allele will find itself in new epistatic relationships
Why don’t the other components of phenotypic variation matter (much) for QT evolution?
- non-additive effects (D, I) are disrupted every generation, and therefore do not have a predictable effect on offspring
- environmental effects (E) are (by definition) not genetically based
- these three types of variation affect phenotypic trait values (P), but are not predictably passed onto offspring
What is additive genetic variance (VA)?
variance in breeding values within a population
What does evolution by natural selection require?
- variation in a trait
- variation in fitness associated with different phenotypes
- that the trait is heritable
What is narrow sense heritability?
describes proportion of phenotypic variance due to additive genetic variance among individuals, OR extent to which we expect trait variation in to be passed from parents to offspring
What is broad sense heritability?
describes proportion of phenotypic variance due to total genetic variance among individuals
Are heritability (h^2) estimates for populations or individuals
populations – measure of the properties of a trait in a particular population
Does heritability indicate the degree to which a trait is genetically based?
no
rather, it measures proportion of phenotypic variance that is the result of (additive) genetic factors – ie. trait may be under genetic control, but if there is no additive genetic variance associated with the trait, h2 will be 0
Estimating Heritability in Nature Using Parent-offspring Correlation
h^2 = slope of line describing relationship between parent trait and offspring trait values
Estimating Heritability: Parent-offspring Regression
What is the midparent value?
average of phenotypic value of parents
Estimating Heritability: Parent-offspring Regression
What is the midoffspring value?
mean phenotypic value of offspring
Estimating Heritability in Nature Using Other Forms of Relatedness
heritability can also be estimated using other measures of relatedness, and then associating relatedness with trait variation
information about relatedness can come from pedigrees, or from analysis of genomic markers segregating in populations (ie. SNPs)
What would you need in order to predict how much the mean trait value would shift under directional selection?
- starting mean trait value
- heritability of the trait
- strength of selection
Is the selection differential (S) larger or smaller with stronger selection?
larger
What are the possible reasons for limits in long-term selection experiments?
- biological/physiological limits (ie. below a certain level of seed oil, plants are not viable)
- pleiotropy: genes that contribute to traits under selection may have other roles, with negative effects on survival or reproduction
- linkage: genes that contribute to traits may be tightly linked to loci with negative effects on fitness (recombination between these loci would solve this problem)
- lack of variation: if all loci that contribute to a trait go to fixation (= no additive variance) – this is less common than we might expect
What are the possible sources of more extreme traits compared to the original population?
- new environments change the selective value of traits so that traits that were previously neutral (or even deleterious) become advantageous
- new combinations of alleles that were not previously found together or expressed are favoured
- new ‘beneficial’ (desirable) mutations that arise by chance, and then are favoured by selective breeding (longer term)
- in some cases, new environments can reveal different phenotypes and make them available for selection
Building evidence for adaptation involves providing evidence of evolution by natural selection by….
- establishing an association between trait values and fitness values
- establishing that there is a genetic basis for the trait / knowing the genetic basis of the trait (which genes and alleles contribute to the phenotype)
- understanding the mechanism of selection (demonstrating with realistic tests that exposure to a specific setting / ecological context leads to evolution in the trait values)
What are the two possible sources of adaptive alleles (new adaptations)?
- new mutations arising in species or populations as they adapt
- standing variation – alleles already present in species (as neutral or deleterious alleles), which become favoured in new selective context
(traits with complex genetic underpinnings (ie. quantitative traits) also are adaptive)
What is likely to happen to novel mutations that are neutral?
likely to be lost due to drift (even beneficial mutations may be lost)
What type of dynamic would affect novel mutations?
context – if novel environment is so harsh that it requires truly novel traits, and individuals can’t escape the conditions, mutation is unlikely to solve the challenge because relevant mutations are unlikely to appear at just the right moment
What populations risk extinction when the environment changes?
populations that are small and have low additive genetic variation
large additive genetic variation and/or large population can allow evolution to rescue declining populations
What is QTL mapping?
(mapping of quantitative traits) scores set of F2 individuals for random genetic markers, and builds linkage maps
- variation in traits of these same F2 is then statistically associated with the markers
- loci explain small/large proportions of phenotypic variance
How can we uncover the genetic basis of adaptive traits?
- QTL mapping
- simple plots that show distribution of phenotypic trait scores in F2 population (as well as parental and F1 values), give some hint about genetic basis and degree of dominance of traits
Are mutations of very large effect common?
relatively uncommon, but many effect sizes are also not super small
Small effect loci/mutations have lower potential to…
`yield large, rapid changes in trait means in response to selection
Large effect loci/mutations have potential to…
be highly disadvantageous, if they take you away from the optimum that selection is favouring
but these mutations are quickly removed by selection, and therefore we should still see evidence of large effect beneficial mutations
Do large populations change more slowly or quickly than smaller populations due to drift?
slowly
When can gene flow act?
if two populations differ in frequency or presence/absence of different alleles, then gene flow can change their allele frequencies
What type of populations share more gene flow?
nearby populations
What type of populations have the highest number of migrants leaving them?
large populations
What types of populations are more likely to have similar allele frequencies at their neutral loci?
nearby populations
Migration is a powerful force that affects what?
how neutral and non-neutral variation is distributed in population
What is isolation by distance?
pattern of increasing genetic distance with geographic distance
- represents outcome of two forces: gene flow moving and mixing alleles, while genetic drift produces random divergence between populations
- BUT this pattern is not always found, or what we expect
When is isolation by distance not expected?
- when gene flow is very high
- when system has recent signature of founding of populations, frequent losses of populations, of signatures of chance long-distance dispersal
When might isolation by distance not govern patterns of variation on all traits and loci?
when selection is acting differently in different populations
How might the outcomes of selection look similar to patterns of isolation by distance?
- populations that have greatest geographic separation are also most climatically different
- for alleles related to temperature adaptation, we expect biggest difference in frequencies between extreme North and South
How can we distinguish between drift and selection?
using QST
What is QST?
analog of FST that measures differentiation in quantitative trait values
like FST, large values indicate strong differentiation among populations
What are some negative effects of gene flow?
- when different alleles are favoured in different locations, gene flow will counteract selection, contributing to lowering mean fitness of population
- populations that are well-suited to their local environments may be negatively impacted by gene flow from populations experiencing different conditions
What are some positive effects of gene flow?
- reduces effects of drift (especially in small populations) that would lead to loss of alleles, increased homozygosity, and potential negative effects of inbreeding
- allows alleles that arise (by mutation) in one population to spread to other populations – adaptations spread via gene flow, wherever trait was advantageous, it would have risen in frequency by natural selection
- populations connected by gene flow may be better at responding to changing conditions, because they contain more potentially relevant variation
What do we need to determine to decide that this is really a new species?
- genetic analysis
- phylogenetic analysis
- is it morphologically distinct
- can it interbreed with other ‘species’
- does it have distinct range and ecology
What is involved in making new species ‘official’?
- come up with name
- write species description (in line with other species descriptions in this group)
- publish that description in accordance with rules governing plant species
- wait to see whether others ‘accept’ the species
What is the work of taxonomy?
to provide broad catalogue of Earth’s biodiversity (past and present) in a way that promotes understanding of history of life on Earth (patterns, outcomes), and processes that generate biodiversity
How are new species discovered?
- through work on museum collections
- through genetic/phylogenetic studies
- through insights from traditional knowledge
- through field work
What are the 3 arguments for species being real (considering sexually-reproducing eukaryotes)?
- discreteness argument
- accumulating evidence argument
- origin of species argument
What is the origin of species argument?
- species arise by speciation
- species must be real because they are products of a real process
What is the accumulating evidence argument?
long recognized species mostly stand up to further scientific study
What is the discreteness argument?
- variation in nature is often noticeably discontinuous
- statistical analysis of variation often identifies separate clusters
Do taxonomists think species are real?
sometimes, taxonomists take explicitly operational approach to defining species
- but this cut-off is based on comparing sequence divergence for well-characterized species groups – it’s a biologically-based cut-off
- most taxonomists surely aim to have their species reflect natural breaks in diversity – units that share fundamental properties, even if they don’t explicitly state a species concept in their work
- species concept generalizes our ideas of what we think it means to be a species
Does Darwin think species are real?
yes – species is real thing with respect to contemporaries
- fertility must settle it
- species may be good ones and differ scarcely in any external character
- we do not know what amount of difference prevents breeding
What are species concepts?
set of ideas about the way that evolutionary forces act to divide biological diversity into discrete units
What are the two arguments to why there are so many species concepts?
- ‘different situations call for different concepts’ argument
- ‘different groups of scientists use different approaches and tools, and therefore favour concepts emphasizing different criteria’ argument
What is the ‘different groups of scientists use different approaches and tools, and therefore favour concepts emphasizing different criteria’ argument?
- people who study speciation have traditionally focused on reproductive isolating barriers (BSC)
- those who think about diversification processes focused on phylogenetic data (PSC)
What is the ‘different situations call for different concepts’ argument?
- evolution is ongoing process, and speciation is therefore ongoing process
- some groups of populations will be somewhere along the way towards (possibly) becoming clearly distinct species
- some concepts rest on criteria that cannot be evaluated in some groups
What is the evidence for species, which we could use to build a species concept?
- forces that establish and reinforce boundaries between species
- history of connections among individuals and populations
- traits that distinguish different species
Species Concepts
–
What might a potential unifying species concept (apply to nearly all situations) look like?
- would explain discontinuities among lineages
- would be consistent with many of the criteria previously described
- would allow for variation in ways in which species arise and persist
- would help us understand how evolution produces diversity
What is interbreeding between species?
interspecific mating or hybridization
How are isolating barriers evaluated?
by comparing success of interspecific mating relative to success of intraspecific mating
What do pre-mating barriers do?
reduce likelihood/probability that individuals exchange gametes (mating)
What are the 3 types of pre-mating barriers?
- geographical isolation
- behavioural isolation – ie. may overlap in range, but don’t recognize each other’s songs
- mechanical isolation – genitalia of different species may not fit
Mating Preference vs. Behavioural Isolation
- if females PREFER males with longer tails, this suggests directional selection for longer tails (or maybe stabilizing selection if long tails are costly for male survival)
- reproductive isolation could come about if there is divergence in mate preferences – ie. if some females prefer shorter tails
- this could lead to disruptive selection on tail length, that could ‘set the stage’ for isolating barrier to arise
What do post-mating (pre-zygotic) barriers do?
reduce likelihood that individuals that mate produce zygote (occurs after mating, but before zygote formation)
What is the one type of post-mating (pre-zygotic) barrier?
failure of fertilization – sperm may not have correct protein to fertilize eggs, or gametes may not survive in reproductive system
What do postzygotic barriers do?
reduce likelihood that zygotes will live or reproduce
What are the 3 types of postzygotic barriers?
- intrinsic
- extrinsic
- BDM incompatibilities
What are intrinsic postzygotic barriers?
independent of environment
What is intrinsic hybrid inviability?
hybrids are more likely to die at early life stages, regardless of environment
What is intrinsic hybrid sterility?
hybrids are viable, but have reduced or zero fertility that does not depend on environment
What are extrinsic postzygotic barriers?
dependent on environment
What is extrinsic hybrid inviability?
hybrids are more likely to die at early life stages, but this varies with environment, and might do worse in environment of one or both parents
What is extrinsic hybrid sterility?
hybrids are viable, but have reduced fertility due to environmental factors – ie. hybrid plant might produce normal gametes but fail to attract pollinators
What are BDM incompabilities?
predicted to arise as byproduct of overall divergence when populations are isolated
because novel alleles arise (and go to fixation) in each population, alleles that accumulate in different populations may not function well together (in hybrids), contributing to postzygotic isolation
What do postzygotic barriers focus on?
hybrid fitness in viability and fertility, over one or many generations
Where are postzygotic barriers seen? What are they measured as?
seen in offspring of matings between species, therefore are measured as relative disadvantages of hybrid offspring relative to non-hybrids
What is total reproductive isolation?
measures overall strength of various barriers that contribute to isolation between pair of species
Does the order in which isolating barriers act affect their contribution to total reproductive isolation?
yes – not all isolating barriers (of equal strength) are equivalent
earlier acting barriers can prevent loss of reproductive effort that comes from choosing wrong mate, and producing less fit offspring
Example of Order of Isolating Barriers
- ecogeographic isolation
- pollinator isolation between parental species (how often each type of pollinator visits wrong species)
- pollen precedence (given both types of pollen transferred to flower, how big is advantage to “right” type)
- % germination of F1 interspecific seeds (first generation hybrids)
- survival of F1 hybrids to adulthood
- F1 percent of individuals flowering
- F1 biomass at flowering (includes measure of overall size and flower production)
- F1 pollen viability (measures quality of pollen of hybrids)
- mass of seeds produced by F1 individuals
Can reproductive isolation be asymmetrical?
yes – with gene flow more common into one species than the other
Does hybridize produce equal or unequal frequencies?
unequal frequencies
each event of hybridization generates specific discordant tree – one discordant tree will likely be more common than the other
For barriers of equal strength, do early or late acting barriers have a larger impact on total isolation?
earlier
What is an advantage of earlier acting isolating barriers?
have advantage of reducing parental investment in hybrid offspring that have potential to be of poor quality
ie. producing inviable hybrids is likely more costly than avoiding mating with incompatible individuals of another species
What is homoploid hybrid speciation?
hybrid gives rise to new species
What is divergent speciation?
one species splits into two
What types of additional isolating barriers (other than the geographic barrier itself) would you expect to evolve most often between species that arise in allopatry?
no specific type of barriers should be especially common
Can we find evidence that reproductive isolating barriers accumulated as a by-product of overall divergence in allopatry?
if reproductive isolation accumulates over time in allopatry, then species that have been separated geographically for longer should be more strongly isolated
prezygotic isolation accumulates gradually with overall divergence
What are the two pathways to geographical isolation?
- dispersal
- vicariance
What is the dispersal pathway to geographical isolation?
part of population crosses barrier that already exists
What is the vicariance pathway to geographical isolation?
population that is already widely distributed is divided by the appearance of a usually physical barrier (ie. mountain range, river) – diverge into separate sister species
What are evidence for allopatric speciation?
Are current distributions non-overlapping? – weakest evidence
Do speciation events within clades coincide with (rare) dispersal events across geographic barriers?
Do multiple (unrelated) groups have sister species on either side of a geographic barrier (supporting vicariance)?
Does reproductive isolation appear to accumulate as a function of time since divergence? – this is a prediction
hybrid zones ≠ hybrid species
- hybrid zones involve ongoing (or recent) gene flow between two ‘parental’ species to generate hybrids
- hybrids are NOT reproductively isolated from parents
- hybrids in hybrid zones are often less fit than their parents
- hybrid zone formation is far more common than hybrid species formation
if two species overlap in some places but not in others, we predict stronger pre-zygotic barriers will be present in areas of sympatry
–
When is sympatric speciation most likely to occur?
if there is strong selection (to overcome gene flow):
- ecological selection: shift in habitat preference, resource use
- link between ecology and mating preferences
if there is rapid evolution of reproductive isolation, preferably through pre-zygotic barriers
Is allopatric or sympatric speciation easier?
allopatric speciation is ‘easier’ – reproductive isolation can just accumulate over time
What is reticulate speciation?
two species come together to produce new third species
What is the strong ‘in sympatry’ element of reticulate speciation?
in order for species to originate this way, it must occur near one or both of its parents, and some of the challenges of sympatric speciation apply here as well
What is the first step in origin of new species in reticulate speciation?
hybridization between divergent species
What is homoploid hybrid speciation?
hybrid species is at same ploidy level as parents
Why is hybrid speciation rare? What are the challenges?
- parents have to hybridize (without merging)
- hybrids must be viable, and have some fertility
- hybrids must be/become reproductively isolated from their parents, and do so ‘quickly’
- hybrids must overcome/recover from low fertility and be able to propagate themselves
What is polyploid hybrid speciation (allopolyploidy)?
hybrid species is at higher ploidy level than parents
- relatively common, especially in plants (especially relative to homoploid hybrid speciation)
- (probably) most rapid mode of species birth – within ~2 generations, novel polyploid is produced that is highly reproductively isolated from its parents
Instant Post-zygotic Isolation via Allopolyploidy
crosses between ploidies have very low success
Polyploid Hybrid Speciation (Allopolyploidy)
What are the challenges of being new?
- finding mate
- avoiding gene flow with parents
- early persistence of new polyploid is unlikely without pre-zygotic isolation because new polyploid surrounded by its parents will produce triploid (sterile) offspring
For any new species that arises in sympatry with its progenitor, what is the immediate challenge to establishment?
difficulty of finding compatible mate
With new polyploids and new hybrid species, the numerical disadvantage in early generations is always severe. It appears that successful establishment of hybrid species and polyploid species is most likely when…
new species have habitat preferences or other traits that allow them to overcome early numerical disadvantages that limit establishment
What is microevolution?
evolution within species
- population genetics
- adaptation
- sexual selection
What occurs ‘at the boundary’?
- speciation
- ILS
What is macroevolution?
evolution among species
- phylogeny
- species selection (diversification patterns)
What can explain why the same (or similar trait) appears in many species?
trait has evolved repeatedly, but there is no common underlying selective mechanism, and without notable impact on shape of tree of life (sort of null hypothesis)
trait evolves repeatedly, associated with particular situations or selective pressures
- understanding conditions that favour trait’s evolution reveals something about how evolution works
- [study of adaptation, parallel and convergent evolution]
in lineages where trait evolves, it is associated with higher rates of speciation or lower rates of extinction
- patterns of evolution of trait alter shape of tree of life
- [focus of macroevolution]
How does individual selection oppose species selection in the evolution of sex?
- within species, selection favours spread of asexuality
- among species, there is selection for sex
in evolution of sex, selection among species appears to dominate
What can we do to test for species selection?
use sister group comparisons
ie. compared species numbers in sister clades with radial vs. bilateral flower shapes – is one shape associated with having more species per clade?
What is diversification?
represents accumulation of species within and among lineages
What is adaptive radiation?
when extraordinary diversification in lineage is driven by specific environmental or trait change
What is adaptive radiation by key innovation?
key innovation: trait that promotes diversification – this term is applied only when diversification is especially impressive
What is adaptive radiation by ecological opportunity?
relatively rapid diversification into distinct niches, in response to ecological opportunity – being in the right place at the right time
ie. arriving to Hawaii, tarweeds apparently found open environment without competitors, enabling them to radiate into habitats they would otherwise have been excluded fr
What explains rapid diversification?
adaptive radiations, either linked to ecological opportunity or to specific traits (key innovations)
What is a weakness of sister group analysis?
cannot distinguish whether differences between sister lineages in species richness are caused by more speciation events in one lineage, or by more extinction events in the other
Microevolution
What is an organism’s fitness dependent on?
ability to survive
ability to have babies
Macroevolution
What is a species’ fitness dependent on?
ability to avoid extinction
ability to speciation
Why is it difficult to place extinctions on the tree of life?
would require fossil record far richer than what we have, and ability to use these fossils to infer relationships
because of these limitations, other approaches are used:
- neontological
- paleontological
What is the neontological approach?
inferring past extinction from patterns of diversity in the present
What is the paleontological approach?
using groups for which there is good fossil record to infer past changes in diversity
What is background extinction?
focuses on patterns of extinction in ‘normal’ times
What is mass extinction?
periods of extreme, high rates of extinction, during which there is catastrophic loss of species in ‘short’ period of time
What are boundaries of most geological periods associated with?
- big shifts in fossil record
- often represent mass extinctions
What are Earth’s 5 mass extinctions?
end Ordivician end Devonian end Permian end Triassic Cretaceous-Paleogene
What happened during the Cretaceous-Paleogene mass extinction?
extinction/death of big dinosaurs, more than half of land plant species, pterosaurs, ammonites (shelled ‘squids’), strange asymmetrical clams
Do most species extinctions occur as part of mass or background extinctions?
background extinctions
Why is extinction understudied?
difficult to study due to many biases and gaps in the fossil record
