Species & Speciation Flashcards
w10
nomialism
only individuals exist, species are man-made constructs
Ernst Mayr
- lived with tribe in Papua New Guinea
- “These superb woodsmen had 136 names for the 137 sp of birds which I distinguished (confusing only 2 non-descript sp of warblers)”
- ^ powerful evidence that sp exist!!
diffs among sp are a reflection that speciation is a process
phenotypic sp concept
- sp are groups of organisms that are more similar phenotypically to 1 another than they are to other groups
- usually measure many traits at once
-> individs that consistently cluster together are classified as a sp
problems:
- how do we decide which diffs (traits) are important?
- geographic races -> so there’s a diff in outward appearance & iindivids they can mate with
- cryptic sp
ways to categorise sp
- phenotypic sp concept
- ecological sp concept
- biological …
- cladistic …
- recognition …
ecological sp concept
- sp are groups sharing a common niche
- diffs among sp relate to ecological resources they exploit
- competition causes character displacement
- individs that avoid competition with another group have adv
problems:
- complex life cycles eg. flatworms
- which niche should we focus on?
cladistics sp concept
- groups of organisms on single branch of a phylogeny
- defines sp through time
problems:
- hard concept to apply in practice (only really useful intellectually)
recognition sp concept
- sp are groups with common method of mate recognition
- emphasis on sp cohesion
problems:
- need thorough understanding of a sp mate recognition system to use it
biological sp concept
(most widely accepted concept)
- sp are groups of interbreeding pops that are reproductively isolated from other pops (Mayr, 1963)
- emphasis on what keeps species separate
- sp share a common gene pool
-> this def explains why members of a sp resemble each other
- important concept as it merges taxonomy with genetics
problems?
- what to do with extinct sp? they haven’t got a gene pool!
- clonal organisms -> gene pool isn’t mixing things up like sexua gene pool
all sp concepts agree on…
- there are barriers to gene exchange between sp (they may be porous)
- sp are ecologically distinct (to large degree)
- sp have distinct character states (to large degree)
- sp have evolved along independent trajectories (to large degree)
Gene flow acts to merge sp
so what keeps them apart?
speciation is a process
- smt must happen to restrict gene flow
- smt needs to drive divergence
- reproductive isolation needs to evolve
what can cause restriction of gene flow?
-
change in chromosome no.
-> polyploidy can result in instant speciation (2-4% of land plants) -
physical barriers
-> dispersal & colonisation (eg. Hawaiian drosophila)
eg. of polyploids
barbus fish (sexual)
3 separate polyploid speciation events
vicariance
process by which a physical barrier separates pops
Allopatric speciation
- geographic isolation
- most common form of speciation
- evidence from biogeography
-> closely related sp with non-overlapping ranges occur near each other
eg. Kaibab squirrel geographically isolated from common ancestor of its closest relative (Abert’s squirrel) in North Rim of Grand Canyon 10,000 ya. Since then, several distinguishing features, (black belly and forelimbs), have evolved
Parapatric speciation
speciation resulting from environmental gradient within a species’ range
eg. Fire Belly Toads - Bombina
Bombina bombina
- Lowlands of C. & E. Europe
- Mainly aquatic
- Breeds in permanent waters
- Prolonged territorial breeder
Bombina variegata
- Uplands of W. & S. Europe
- Mainly terrestrial
- Breeds in temporary pools
- Explosive breeder
^have an overlap of the 2 sp in C. Europe, where there is a hybrid zone (region of overlap between 2 divergent groups where gene exchange can occur)
SUMMARY ON BOMBINA:
- B. bombina & variegata diverged >2 MYA
- gene-flow still occurs between 2 sp via Hybrid zone
- there is selection against Hybrids
- increased mortality in embryos
- increased deformities in tadpoles and adults
Fire Belly Toards hybrid zone
- doesn’t follow an environmental control
- no located in any ecotone
- not product of human intervention
- not changing rapidly
Bombina summary
- Parapatric speciation is mainly of interest because of existence of hybrid zones & possibility of reinforcement
- difficult to determine whether hybrid zone is 1º (parapatric) or 2º (allopatric)
- conclusion: parapatric speciation is certainly possible, but there are limited no. of egs.
sympatric speciation
- controversial
- difficult to prove
- comes in 2 flavours:
-
‘True sympatry’
-> strong disruptive selection
-> polyploidy - Host shifts
