Speciation

I. The species concept

A. Definitions - advantages and disadvantages

1. Biological species

a. interbreeding population capable of producing fertile offspring which is genetically distinct from other populations
b. can't apply to fossils

2. Morphospecies

a. morphologically distinct, widely used to classify fossils
b. miss extant cryptic species, may underestimate amount of morphological variation in fossil populations

3. Phylogenetic species

a. use gene geneologies to infer speciation events
b. problems arise when gene flow occurs between species, e.g. hybridization events

B. Problem cases

1. Asexual lines, e.g. Cneimodophorus lizards
2. Persistent hybrids - Bullock's and Baltimore Orioles = Northern oriole, oak trees
3. Bacteria

II. Allopatric speciation - occurs by isolation, genetic change, gene flow between daughter populations, reinforcement (hybrids selected against) and evolution of premating isolating mechanisms

A. Isolation

1. dispersal - to island or habitat isolate
2. vicariance

a. river forming - salamanders
b. mountain range - black-billed and yellow-billed magpies
c. island forming - sea level change after glaciation event, Sumatran rhinos

B. Genetic changes - usually accumulate with time

1. due to drift caused by a founder event
2. due to selection in different habitats
3. due to mutations - chromosomal rearrangements can cause infertility or inviability

C. Reproductive isolation

1. postzygotic isolating mechanisms - hybrid fitness differs from parents

a. embryo death - sheep and goats
b. hybrid inviability
c. hybrid sterility - female horse and male donkey = mule
d. hybrid has lowered fitness - selection acts against, determines hybrid zone

2. Prezygotic mechanisms are thought to evolve rapidly to reinforce postzygotic mechanisms - data on allopatric vs sympatric Drosophila

3. prezyotic isolating mechanisms

a. habitat isolation - toads use ponds or streams where ranges overlap
b. seasonal isolation - crickets, Rana frogs
c. behavioral isolation - fiddler crabs, drosophila courtship song
d. mechanical isolation - flower parts
e. gametic isolation - sperm are killed before fertilization can occur

III Parapatric speciation

A. Thought to begin with clinal variation

1. Range expansions often occur with adaptations to the local environment
2. often altitudinal or latitudinal and follow environmental gradient
3. Maintained by reduced gene flow

B. If environmental gradient is sufficiently strong, hybrids may be unfit and selection for assortative mating will occur

C. Or, hybrid has increased fitness

Unique species have arisen in environmental gradients, such as heavy metal tolerance

IV Sympatric speciation

A. Autopolyploids - e.g. 2n -> 4n, triploids cannot produce viable gametes

1. Evening primrose - 14 chromosomes - changed to 28 chromosomes, triploid can't undergo meiosis

2. polyploids are typically larger, often more vigorous - domesticated strawberry

3. 70% of flowering plants evolved by polyploidy - most crop plants are polyploids

B. Imprinting

1. habitat - Rhagoletis hawthorne/apple flies

a. find oviposition site by visual and odor cues
b. apples were planted 300 years ago, now race prefers apples, but does less well
c. apple race persists due to escape from parasitoid that hunts on hawthorne

2. sexual - if bird learns its mate's song in the nest, can lead to reproductive isolation. Brood parasites are good example (didn't discuss this possibility)

V. Rapid speciation - species flocks

A. Cichlid fishes of African rift lakes

1. Tankanikya - 2 MYR - 171 endemics
2. Malawi - 1-2 MYR - 500 endemics
3. Victoria < 1 MYR - 300 endemics
4. mtDNA shows that each lake is monophyletic
5. Victoria may have dried up completely 12400 years ago - 300 spp in 10,000 years or 33 yrs/speciation event!!! - little sequence variation among species in Victoria
6. Lake Nabugobu is 4000 years old and has 3 endemic species!
7. Tremendous morphological change in feeding adaptations and body color, indicating that character displacement has occurred, possibly involving mate selection

B. Hawaiian Drosophila

1. 500 spp
2. exhibit tremendous variation in body size, bristle and wing patterns, etc., often in characters related to courtship suggesting sexual selection is important
3. Genetic data indicate they may have evolved over 40 MY