Lecture 14/15/16 - speciation

Question 1

Species definition

Answer 1

-groups of populations that are evolutionary independent of other populations - little to no gene flow. In other words it is the smallest unit in which gene flow, drift, selection, mutation, and recombination operate independently

Question 2

Species concepts (2)

Answer 2

-three are 26 (and counting) different concepts to define a species but we wil focous on the 3 most common ones:

-morphological / morphometric

-phylogenetic

-biological species concepts

Question 3

Morphospecies concept (MSC)

Answer 3

-Assumes that independent populations will accumulate morphological differences

Question 4

Morphospecies concept advantages (3)

Answer 4

-partical use with keys and field guides

-works well with living, preserved, and fossil species

-has the longest running history and therefore cultural recognition

Question 5

Morphospecies concept disadvantages (6)

Answer 5

• not all differences are species-level differences:

-sexual dimorphism (male / female peacocks for example)

-Life cycle differences (caterpillar -> butterfly are very different looking but same species)

-environmental effects (coat morphs of arctic foxes in the tundra being brown and white in the arctic)

-polytheisms (different phenotypes from single genotype ie soldier vs worker vs queen ants of the same species)

-Some species are cryptic

Question 6

Cryptic species (3)

Answer 6

-species that look the same but are separate gene pools (different species)

-often uncovered by DNA barcoding

-ex the skipper butterfly thought originally to be a single species with subtle phenotypic differences in adults actually is at least 10 different species.

Question 7

Phylogenetic species concept (PSC) (4)

Answer 7

-where a species is the smallest identifiable unit that is diagnostic and monophyletic

-PSC detects species that have been evolutionarily independent long enough for diagnostic traits to emerge

-Imagine a phylogenetic tree ending with species A, B, C where each letter is a different species

• Some species have no diagnostic characters and are labeled so B becomes B1 B2 B3 all of which are the same species

Question 8

Diagnostic character

Answer 8

-phenotypic trait or genetic marker we can use to tell species apart

Question 9

monophyletic

Answer 9

lineages that contain all known descendants of a common ancestor

Question 10

Advantages of PSC (4)

Answer 10

-Can be applied to any type of organism

-can be tested more formally than MSC

-Can be based on morphological and genetic characters

-Requirement for monophyly means that species that appear similar due to convergent evolution are recognized as separate species

Question 11

Biological species concept (BSC) (3)

Answer 11

-Defines species through the requirement of reproductive isolation where organisms are Separate species if they do not hybridize regularly in nature or if they fail to produce viable fertile offspring when they do

-Uses a lack of gene flow as a “litmus test” for evolutionary independence (IE it has gene flow = not a species, no gene flow = species)

-Is widely accepted since Ernst Mayr “championed it”

-Is the legal definition of species used in the US endangered species Act

Question 12

Problems with BSC (4)

Answer 12

-Only accounts for current reproductive isolation, so hypothetically if gene flow were to resume between two “different” species what then?

-Some species hybridize such as the mule (male donkey X female horse) does that mean that horses and donkeys are not their own species?

-is difficult to test and apply to populations that don’t interact with each other, fossils and is completely irrelevant in asexual organisms

-Have to make assumptions about whether a population is reproducibly isolate

Question 13

Comparing Species Concepts

Answer 13

-In elephants there traditionally are two species - African and Asian

-Two different African types, Forrest and grassland (MSC)

-Because the two African types don’t interact BSC cant be tested

-PSC showed them to be different species

Question 14

Allopatric speciation (3)

Answer 14

-Physical barriers to gene flow can lead to allopatric speciation – geographical isolation leads to loss of gene flow

-Two physical mechanisms: dispersal/colonization and vicariance

-Populations can evolve independently by virtue of it being impossible to exchange individuals

Question 15

General stages to speciation (3)

Answer 15

1. Isolation: Populations become isolated due to physical, behavioral, or ecological barriers
2. Divergence: Once isolated, evolutionary processes can occur independently
3. Reproductive isolation (RI): Mechanisms develop that prevent gene flow between populations (can be pre and or post zygotic). Note this does not always develop (i.e. mules)

Question 16

Gene flow

Answer 16

Movement of Alleles between populations

Question 17

Mechanisms of isolation (2)

Answer 17

Allopatric (physical)

Intrinsic (natural)

Question 18

Dispersal / colonization

Answer 18

-Considered common mechanism for initiating speciation especially in non-motile (non moving) animals and plants

-Ex: Plant seeds getting carried to a new location where the population is not established yet

Question 19

London underground mosquitos (5)

Answer 19

-Example of speciation through dispersal/colonization

-above ground, bird specialized mosquitos moved into the subway tunnels as they were being dug

-They have since evolved to be mammal (human / rat) specialists

-Impossible for them to mate with the above ground mosquitos in the same area so good BSC species

-genetic differences between under and above ground mosquitos so good PSC species

Question 20

Vicariance (5)

Answer 20

-Can be a slow process (ex rise of a mountain range or glaciers) or rapid (ex lava flow or human caused habitat fragmentation)

-Ex snapping shrimp on either side of panama became isolated 3m years ago when a land bridge formed

-Morphologically sisters (bad MSC Species)

-Confirmed through DNA that they were once the same species

-Mating study shows they can no longer interbreed (good BSC Species)

Question 21

intrinsic speciation (2)

Answer 21

two forms:

parapatric speciation

Sympatric speciation

Question 22

Parapatric speciation (5)

Answer 22

-para = beside / near

-when a population enters a new habitat within rage of parent species

-No physical separation between populations. Instead, speciation results from evolution of other mechanisms that reduce gene flow between populations

-Outcome is dependent on balance between gene flow and selection –
no divergence if there is little or no selection happening to drive it

-Speciation most likely when gene flow is low and selection for divergence is strong.

Question 23

Sympatric Speciation (3)

Answer 23

-Occurs entirely within range of parent species. New species evolve from a single ancestral species while inhabiting the same geographic region.

• Usually occurs due to chromosome changes producing a
  barrier to gene flow such as Polyploidy (Whole genome duplication) or Chromosomal mutations: (aneuploidy, inversions / translocations)
• Can also arise from Other mechanisms that promote assortative mating (like with
  like phenotypes)

Question 24

Polyploidy

Answer 24

heritable condition where an organism posses more than two complete sets of chromosomes in their genome

Question 25

Aneuploidy

Answer 25

when an organism has an abnormal number of chromosomes

Question 26

Speciation through polyploidization (5)

Answer 26

-Whole genome duplication (e.g., 2N → 4N) -Largest scale of mutation possible. Errors in meiosis produces diploid (unreduced) gametes * Gametes with different chromosome numbers normally incompatible (e.g., 3N low fertility) * Immediate reproductive isolation between parental and daughter populations (2N vs 4N), in other words divergence leads to isolation rather than vice-versa * Very common in plants, not so much in animals

Question 27

How do species form (summary)

Answer 27

Allopatric: * Physical barriers causes reproductive isolation. * Isolation is followed by divergence Parapatric / Sympatric : * Speciation in the presence of gene flow * Reproductive Isolation requires assortative mating due to chromosomal or ecological divergence * Isolation and divergence probably occur in tandem

Question 28

Mechanisms of Divergence

Answer 28

-Natural selection and drift becoming recognized as most important promoting divergence not just random differences accumulating over time. This is because of Different selection pressures on each population in different habitats, using different resources -Can also be caused by sexual selection where assortative mating due to direct mate choice (not a product of ecological selection) can contribute to reproductive isolation leading to speciation.

Question 29

Apple and hawthorn maggot flies (5)

Answer 29

-Host trees occur together and flies search widely for host * Flies not morphologically distinct * But strong host preference (hawthorn vs apple) * Therefore, assortative mating since they mate on host * Different host preference (in sympatry) leads to isolation

Question 30

Mechanisms that reduce interbreeding and hybridization (2)

Answer 30

1. prezygotic isolation: temporal isolation (flowers blooming at different times), anatomical reproductive incompatibility (their parts don't fit together anymore), mate choice (assortative mating) 2. postzygotic isolation: hybrid or backcross leading to inviability, sterility, or reduced fitness

Question 31

Fate of hybrids and speciation

Answer 31

-Fate of hybrids will determine the course of speciation -If hybrids are less fit, selection for reinforcement through prezygotic isolation -When hybrids are equally fit, divergence between populations is erased -when hybrids are more fit it will result in either a stable hybrid zone or a new species

Question 32

Reinforcement

Answer 32

selection that results in assortative mating

Question 33

Species collapse (4)

Answer 33

- “Despeciation” can be due to decreased selection against hybrids -Ex beak size in Darwin's finches was selecting against intermediate phenotypes (disruptive selection) and leading to divergence due to a lack of seeds fit for the intermediate. -human activity on the island created feeders with rice that can be eaten by all phenotypes -no longer selection against intermediate beak sizes = breakdown of hybridization and divergence (species collapse)

Question 34

Hybrid zones (2)

Answer 34

-Stable hybrid zones can form when hybrid fitness depends on environments ex bushes on a mountain with 2 subspecies, one on the mountain one at the foot of it. The two subspecies hybridize at intermediate elevations. The hybrids were more fit at their elevation, but were outcompeted at higher and lower elevations resulting in a small but stable hybrid zone

Question 35

What degree of genetic differentiation is required to create a new species

Answer 35

-Previously thought that radical reorganization required = “genetic revolution” (Mayr 1963) * Reproductive isolation due to genome-wide divergence in allopatry * Sequential fixation of large number of genetic changes, each with very small effect (Fisher’s geometric model) * More recent research suggests that few major changes may be possible to cause divergence