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Evolution > Lec 10 > Flashcards

Lec 10 Flashcards

1
Q

Species are central in _________________

A

Evolutionary biology

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2
Q

What is a speies?

A

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

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3
Q

Discrete clusters of sexually reproducing organisms do exist

A

Elephant clearly does not mate with shrew

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4
Q

Speciation

A

The process by which new species form

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5
Q

Challenges of studying speciation

A

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?

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6
Q

Species concepts

A

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

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7
Q

Morphological or phenetic species concept

A

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)

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8
Q

The biological species concept

A

“Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups”

-Ernst Mayr

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9
Q

The biological species concept focuses explicitly on __________-

A

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

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10
Q

Remember: Migration ___________ differences between populations, whereas drift and selection typically __________ differences between populations

A

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

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11
Q

The phylogenetic species concept

A

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

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12
Q

The biological species concept

A

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

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13
Q

The phylogenetic species concept

A

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
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14
Q

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?

A

Agree between deeply diverged groups, disagree between recently diverged groups

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15
Q

Reproductive isolation

A

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

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16
Q

Classic approach to studying the speciation process: Focus on geography

A

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

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17
Q

Reproductive isolation in allopatry

A

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

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18
Q

Allopatric speciation and the Isthmus of Panama

A

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

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19
Q

Reproductive isolation in parapatry

A

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

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20
Q

Ring species

A

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

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21
Q

Reproductively isolated species in sympatry

A

Genetically and behaviorally different

Live on sides of mountains, forests

Expanded range above Himalayas

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22
Q

Parapatric divergence across habitat types

A

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

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23
Q

Reproductive isolation in sympatry

A

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)

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24
Q

Resource competition can THEORETICALLY lead to sympatric speciation

A

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

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25
Q

Sympatric speciation in Lake Micaragua cichlids

A

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

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26
Q

Extensive debate about whether __________ is possible or whether allopatry is required for speciation

A

Sympatric speciation

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27
Q

Why might sympatric speciation be controversial and unlikely in nature?

A

Very strong disruptive selection and complete cessation of migration is necessary among individuals living in the same location, which is unlikely

28
Q

Reproduction isolation in sympatry

A

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
29
Q

Why might allopatry be necessary for speciation?

A

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

30
Q

What causes reproductive isolation?

A

Reproductive barriers

31
Q

Prezygotic reproductive barriers

A

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

32
Q

Behavioral and ecological isolation in sticklebakc

A

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

33
Q

What relationship do you see among spur length and pollinators, and how might this affect reproductive isolation?

A

Pollinators change as spur length increases; differences in pollinators likely increase reproductive isolation

34
Q

Pollinator isolation in flowering plants

A

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

35
Q

Postmating/postzygotic reproductive barriers

A

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
36
Q

Reproductive Isolation via polyploidy

A

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)

37
Q

Frequency of polyploid speciation

A

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

38
Q

Why does postzygotic isolation occur?

A

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

39
Q

Postzygotic isolation: Haldane’s Rule

A

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

40
Q

Which type of barrier is stronger: pre- or post-mating isolation?

A

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

41
Q

Why might postmating barriers be important to the completion of speciation?

A

Because if premating barriers fail, gene flow can still occur, but postmating barriers can completely stop gene flow

42
Q

Species

A

Can interbreed

Offspring can reproduce

Breeds are variations within a species

43
Q

The process of speciation

A

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

44
Q

Hybridization

A

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

45
Q

Hybrid zones

A

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
46
Q

Hybrid zones are natural “__________” for studying species barriers

A

Laboratories

47
Q

Outcomes in hybrid zones

A

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

48
Q

Adaptive introgression

A

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

49
Q

Reinforcement

A

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
50
Q

Reproductive character displacement

A

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

51
Q

Is hybridization “good” or “bad’?

A

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

52
Q

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

A

d) All of the above

53
Q

Ho new species are formed

A

1) Isolate
2) Diverge
3) Sympatry (come back together)
4) Reinforce (have secondary barriers to prevent breakdown of accumulated different traits)
5) Repeat

54
Q

How (and why) are species lost?

A

Extinction

55
Q

Extinction

A

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

56
Q

Extinction and the fossil record

A

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)

57
Q

Big 5 Mass Extinctions

A

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

58
Q

Consequences of extinction

A

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

59
Q

Synthesis question: Why would we say that the extinction of the dinosaurs ‘paved the way’ for mammals?

A

Because the extinction of the dinosaurs removed competitors and opened up ecological niches, allowing for an adaptive radiation of mammals

60
Q

Mass extinctions

A

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

61
Q

The Cretaceous (K-Pg) mass extinction

A

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

62
Q

The Permian extinction

A

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

63
Q

Aftermath of mass extinctions: Extinction Debt

A

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

64
Q

The sixth extinction

A

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)

65
Q

Which is true about mass extinctions?

A

They typically result from catastrophic events

Evolution (15 decks)

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