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Evolutionary Biology

Speciation 3

QuestionAnswer
Causes for speciation Macro(mutation) Genetic drift Natural selection (a) Directly Reinforcement (b) Indirectly In allopatry In sympatry Sexual selection
1. (Macro)mutation as a cause of speciation Speciation via polyploidy Polyploidy – doubling of chromosomes either through hybridisation or spontaneous doubling within a species Allopolyploidy = hybridisation Autopolyploidy = Same species doubling
1. (Macro)mutation as a cause of speciation Estimated 30-70 % plants are polyploids Also occurs in animals – e.g. salmonids, cyprinids, catfish…. i.e. Speciation by polyploidy in plants e.g. Goatsbeards (Tragopogon spp) in N. America
2. Genetic drift as a cause of speciation Proposed to occur under peripatric mode Also known as founder effect speciation (Mayr)
2. Genetic drift as a cause of speciation A small subpopulation becomes geographically isolated from parent population, and becomes reproductively incompatible through genetic drift Reproductive incompatibility may also be aided by indirect effects of natural selection
Example of speciation by genetic drift via peripatric mode Hawaiian drosophila New species vary across island habitats
Evidence for founder effect, speciation via genetic drift Bottlenecking experiments in Drosophila (experiment separating putative effects of genetic drift from those of natural selection)
Evidence for founder effect, speciation via genetic drift Rundle passed >40 experimental Drosophila populations through bottlenecks In mating experiments, no significant sexual isolation from parent population
3. Natural selection as a cause of speciation Natural selection as a direct cause of speciation (reinforcement) Reinforcement = selection for prezygotic isolation, arising from reduced fitness in hybrids
(b) Natural selection as an indirect cause of speciation: Populations adapt to different environments or niches (in (i) allopatry or (ii) sympatry), reproductive isolation follows as a by-product
Speciation in the laboratory 1 e.g. Fruitfly Drosophila pseudoobscura (Dodd 1989 Evolution) 8 populations: 4 reared on starch-based medium 4 reared on maltose-based medium Result = positive assortative mating: individuals from each group prefer to mate with opposite sex from same group, even across cages
3. Natural selection as a cause of speciation (b) Natural selection as an indirect cause of speciation: (i, ii) In sympatry (within lakes) / allopatry (between lakes): Ecological character displacement within lakes (natural selection for habitat specialisation):
‘Ecological character displacement’ Competition (e.g. for food) between diverging species may be important in driving adaptive radiation (i.e. a series of rapid speciation events)
‘Ecological character displacement’ Evidence that competition is important comes from a number of studies supporting the idea of ‘ecological character displacement': i.e. the influence of one species on the evolution of another as aconsequence of competition for resources
‘Ecological character displacement’ The idea is that (i) competition (e.g. for food) plays a critical role in divergence and that (ii) character displacement occurs following competition for similar ecological niches
(b) Natural selection as an indirect cause of speciation: (ii) In sympatry: e.g. Host race formation in phytophagous (plant-eating) insects: case of the Apple Maggot Fly (Rhagoletis pomonella) in N. America
(b) Natural selection as an indirect cause of speciation: (ii) In sympatry: Indirect natural selection (associated with specialisation on different host plants) has caused speciation to occur sympatrically
4. Sexual selection as a cause of speciation e.g. Cichlid Fish species flocks of the African Great Lakes Features predisposing cichlid fish to adaptive radiation 1) feeding biology (pharyngeal jaw) 2) reproductive biology
Three stages in speciation of African Great Lake cichlids 1) Habitat choice (water column, rock, sand) 2) Ecological diversification 3) Colour diversification through sexual selection (via female choice of male colour differences)
Evidence for microallopatry in Great Lake Cichlids Geological evidence shows water levels have fallen and risen, so lakes may have been formerly subdivided Few species have lake-wide distributions
Evidence for sexual selection causing speciation in African cichlids If sexual selection has driven speciation, expect allopatric, related colour forms (putative incipient species) to show assortative mating
Summary Evolutionary causes of speciation include macromutation (polyploidy), genetic drift, natural selection (directly, indirectly) and sexual selection These processes are not mutually exclusive and may act in combination
Summary In most cases of speciation, including some well-studied ones, the mode and/or cause of speciation remain unknown and/or uncertain Indirect natural selection can cause speciation in sympatry (as well as in allopatry)
Summary Host race formation in phytophagous insects is probably a major precursor to sympatric speciation as a result of indirect natural selection The Cichlid fish of the African Great Lakes demonstrate extraordinarily rapid and prolific speciation
Summary This probably involved a mixture of indirect natural selection (ecological selection) and sexual selection operating in allopatry
Created by: reub8n
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