GB 21- Speciation

Speciation

• process by which 1 species splits into 2; the appearance of new species involves mechanisms from last time 

• [5 mechanisms] + 4 billion years = biodiversity 

Biological Species Concept 

• Ernest Mayr, 1942 

• Species according to BSC: group of populations whose members have potential to interbreed in nature and produce viable offspring 

• Living, extent, sexual animals work well with this concept 

• Problems: can’t apply it to asexual, extinct, microbes, bacteria, viruses 

• Have to consider reproductive isolation 

• Generally useful for extant sexual species 

Morphological species concept:

• species distinguished by shape/structure 

• Strengths: Applies equally well to sexual, asexual, extinct organisms 

• Weaknesses: Disagreement over which features and how to divide

Ecological species concept:

• species defined by ecological niche: how individuals interact with living and non living parts of environment

• Works for sexual and asexual organisms, must be extant 

• Complicated if species can occupy more than one (diverse) niches 

Reproductive Isolation

• Mechanisms that prevent members of different species from interbreeding and producing viable fertile offspring 

• Prevents gene flow 

• Prevents formation of hybrids (offspring of 2 different species)

Prezygotic barriers

• Operates prior to zygote formation— prevent fertilization 

Habitat Isolation:

• overlapping geographic range, but live/breed in different areas —-> rarely interact, no opportunities to mate 

• Just live in a different place

• prezygotic barrier

temporal isolation

• prezygotic barrier

• overlap, but breed at different times —- different times of year season, day 

• Ex: dendrobium, genus of orchids – flower on different days, only open for 1 day 

• Usually things that can’t move 

• Spawning corals, 2 species 

behavioral isolation

• prezygotic barrier

• species-unique behaviors enable mate recognition 

• Ex: courtship: these 3 mechanisms prevent mating attempts 

mechanical isolation

• prezygotic barrier

• mating attempt but sexual structures are incompatible (this prevents fertilization when mating is attempted) 

gametic isolation

• prezygotic barrier

• molecular or chemical differences between species – egg and sperm incompatible (this prevents fertilization when mating is attempted) 

• Common in aquatic organisms – release gametes into water 

• But sometimes fertilization occurs between 2 species 

 Postzygotic Barriers 

• Operate after fertilization occurs (prevents hybridization from happening after fertilization)

reduced hybrid viability

• post zygotic barrier

•  embryo nonviable or offspring do not survive 

• Example: overlapping salamander species sometimes hybridize, rarely develop 

reduced hybrid fertility

• post zygotic barrier

• hybrid viable, but sterile or low fertility 

• Often due to problems during meiosis (parents have different numbers of chromosomes) 

hybrid breakdown

• post zygotic barrier

• 1st generation hybrids viable and fertile

• Hybrids can mate with each other or parent species 

• Subsequent offspring infertile or frail 

Reproductive isolation of populations of same species —>

• gene pools diverge 

Allopatric Speciation 

• Geographic isolation —> drift and/or selection —> divergence 

• Mechanisms of separation: 

1. Geographic barriers: mountain range, river, land, bridge, falling water level 

2. Migration: small offshoot population → isolated from parent population 

• Ex: crickets in Hawaiian Islands (get blown to diff islands and get stuck there and isolated) 

Sympatric Speciation

• Sym = together 

• Reproduction isolation without geographic isolation 

1. Sexual selection: selection for different traits in males and females 

• Can become a barrier to reproduction between subpopulations 

• Sexual selection can lead to weird stuff 

2. Habitat Differentiation: subpopulation uses habitat or resources not used by rest of population 

• Can lead to habitat isolation

• Ex: north american apple magot flies 

• Habitat isolation → temporal isolation → postzygotic barriers 

• Sympatric speciation can be rapid: 

• Ex: japanese snails 

• Single gene causes shell to spiral in different directions 

• Genitalia no longer orientated correctly —> mechanical barrier