Speciation
- Intro
- Species: latin for “kind”/”appearance”
- Speciation: processes where species splits into 2 new species
- Microevolution: changes over time in alleles in population
- Macroevolution: broad pattern where species are formed
- Biological species concept
- Species is group of population whose members have interbreed in nature/produce viable, fertile offspring/reproductively isolated/gene flow between populations keep genotypes similar
- Limitations of BSC
- Potential to interbreed in nature/produce viable, fertile offspring
- Sexual reproduction only (interbreed); alive today
- Other species concepts
- Morphological species concept: uses physical characteristics, works for extinct/asexual but must agree on characters
- Ecological species concept: species defined by niche (role in environment); works for extinct/asexual but have to understand complex interactions
- Reproductive isolation: prezygotic isolation mechanisms
- Reproductive isolation: barriers impeding 2 species from producing viable/fertile offspring
- Can be classified by whether factors act before/after fertilization and creation of zygote
- Hybrids: offspring of crosses between different species
- Prezygotic barriers block fertilization by impeding mating/preventing completion of mating/hindering fertilization if mating is successful
| How | Example | |
|---|---|---|
| Habitat (impedes) | 2 species encounter each other rarely/not @ all bc occupy different habitats | Birds live in USA and Australia |
| Temporal (impedes) | Species breed @ different times of day/season/years can’t mix gametes | Frog 1 breeds in spring, frog 2 breeds in summer |
| Behavioral (impedes) | Courtship rituals/other behaviors unique to species are effective barriers | Firefly A light pattern different than that of firefly B |
| Mechanical (impedes) | Morphological differences can prevent successful mating | PP of husky too big for chihuahua |
| Gametic (impedes) | Sperm of 1 species can’t fertilize eggs of another species | Coral A’s sperm can’t fertilize coral B’s eggs |
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- Postzygotic isolation mechanisms (postzygotic barriers): prevent hybrid zygote from developing into viable (living)/fertile adult
| How | Example | |
|---|---|---|
| Reduced hybrid viability | Genes of different parent species impair hybrid’s development | Homunculus between human and chicken can’t live long -ensation hybrid is tasty/obvious-Ex: Gerbil offspring is hybrid; add different gamete numbers of both parents (chrom. # doesn’t match parents) |
| Reduced hybrid fertility | Even if hybrids live, may be sterile | Mules are sterile (donkey and horse); all female |
| Hybrid breakdown | First generation hybrids are fertile but offspring of next generation are fertile/sterile (weaker and weaker lineage until can’t keep reproducing) | Rice plants become weaker each time they reproduce until can’t reproduce anymore |
Process of speciation
Allopatric speciation (different countries): gene flow interrupted/reduced by barrier; population isolated into subpopulations; barriers arise, migration/isolation by chance -> natural selection/genetic drift/sexual selection
- Barrier depends on ability of population to disperse; separate pop. may evolve independently through mutation/natural selection/genetic drift
Sympatric speciation (same countries): speciation takes place in geographically overlapping populations
- Disruptive selection: homozygous individuals have > fitness through natural/sexual selection (John Mayhard Smith)
- polyploidy: extra sets of chromosomes -> accidents during meiosis (3n/4n); mechanism for how plants evolve such as corn
- Sexual selection: females select males based on appearance (choosiness); preferences passed to offspring
- Habitat differentiation/host shift: results from appearance of new ecological niches (Ex: ants choose certain thing to grow on)
- Depending on species, speciation might require change of only 1 allele/many alleles (can be fast of slow--generations)
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