gen bio - lecture 21: speciation

species

latin for “kind” or “appearance”

speciation (and factors)

process where one species splits into two new species

factors:

• genetic divergence

• reproductive isolation

biological species concept

• made by ernst mayr

• states that… species is a group of populations whose members…

  • interbreed in nature

  • produce viable, fertile offspring

  • are reproductively isolated

• suggested gene flow between populations keeps genotypes similar

limitations to biological species concept

• one of the criteria of the biological species concept is “potential to interbreed in nature + produce viable, fertile offspring”

  • sexual reproduction only applies to this!!

  • must be alive today! not extinct

morphological species concept

• uses physical characteristics

• works for extent or asexual populations

• consensus based in characters

ecological species concept

• species is defined by its niche (ex. location, diet)

• works for extinct or asexual populations

  • complicated to use if species can occupy diverse niches

reproductive isolation

barriers that impeded two species from producing viable, fertile offspring

hybrid

offspring of crosses between different species

pre-zygotic isolation mechanisms (definition + LIST 5 mechanisms)

• prezygotic barriers operate PRIOR to zygote formation to prevent fertilization

  • help impede mating, prevent completion of mating, or hinder fertilization if mating is successful

• 5 mechanisms:

1. habitat isolation

2. temporal isolation

3. behavioral isolation

4. mechanical isolation

5. gametic isolation

habitat isolation

overlapping geographic range but live/breed in different areas

• rarely interact + have no opportunities to mate

temporal isolation

species that breed @ diff times of the day, diff seasons, or diff years

behavioral isolation

courtship rituals + other behaviors unique to a species are effective barriers

mechanical isolation

morphological differences can prevent successful mating

gametic isolation

sperm of one species may not be able to fertilize egg of other species

post zygotic isolation mechanisms (definition + LIST 3 mechanisms)

• goal is to prevent the hybrid zygote from developing into a viable + fertile adult

• 3 mechanisms:

1. reduced hybrid viability

2. reduced hybrid fertility

3. hybrid breakdown

reduced hybrid viability

genes of the diff parent species impair the hybrid’s development

reduced hybrid fertility

hybrids are viable but sterile/low fertility

• often due to problems during meiosis

hybrid breakdown

• 1st generation hybrids are viable + fertile

• hybrids can then mate with each other or parent species

  • subsequent offspring (F2) is frail or infertile

allopatric speciation (+ mechanisms)

• gene flow is interrupted or reduced by a barrier

• population is isolated into subpopulations

mechanisms:

• migration: small offshoot population, isolated from parental population

• natural selection

• genetic drift (ex. founder effect)

sympatric speciation (+ mechanisms)

• speciation takes place in geographically overlapping populations

mechanisms:

• disruptive selection: homozygous individuals have greater fitness either through natural or sexual selection

• sexual selection: females (typically) select males based on their appearance

  • usually in insects

  • can pass sexual preferences on

• habitat differentiation/host shift: results from the appearance of new ecological niches

  • ex. north american maggot fly