Cycle 9 - Species and Speciation

Question 1

Why is the idea that “humans are descended from chimps” inaccurate.

Answer 1

• We aren’t descended from chimps, they are our cousins
• Our MRCA was neither a human or chimp/bonobos

Question 2

State the criteria used by the morphological, biological and phylogenetic species concepts to define species

Answer 2

1. Morphological: individuals of a species share similar traits/physical characteristics
2. Biological: two organisms belong to the same species if they can produce fertile offspring
3. Phylogenetic: a species is the smallest population that can be united by derived characteristics (evolutionary history)
   
   • Thus, a phylogenetic species comprises populations that share a recent evolutionary history.

Question 3

State the weaknesses/limitations of these different species concepts

Answer 3

1. Morphological: we cannot generalise the traits; sometimes different species look very similar
2. Biological: this method cannot be applied to asexual species, extinct organisms or geographically separated organisms
3. Phylogenetic: detailed evolutionary histories have only been described for a few groups of organisms, so this concept cannot be applied to all forms of life

Question 4

What are ring species?

Answer 4

Ring species: a species with a geographic distribution that forms a ring around uninhabitable terrain

Question 5

What is clinical variation?

Answer 5

Clinal variation: a pattern of smooth variation in a characteristic along a geographic gradient

Question 6

Explain prezygotic and postzygotic isolating mechanisms + examples

Answer 6

Question 7

Compare allopatric and sympatric speciation

Answer 7

Allopatric speciation occurs when a physical barrier subdivides a large population (ex., hurricane, uplifting mass) or when a small population becomes separated from a species’ main geographical distribution

• Small populations separated create species clusters: a group of closely related species recently descended from a common ancestor
• Leads to reproductive isolation because the populations are separated

Sympatric speciation is when reproductive isolation evolves between distinct subgroups that arise within one population (no barrier needed)

• Sympatric speciation often occurs in plants through a genetic phenomenon, polyploidy, in which an individual has one or more extra copies of chromosomes

Question 8

Explain the roles of secondary contact and reinforcement in allopatric speciation

Answer 8

Populations separated by allopatric speciation may re-establish contact when a geographical barrier is eliminated or breached (secondary contact)

1. If the populations can breed successfully, species fusion: merging of two populations into one after the establishment of secondary contact (usually, the physical barrier didn’t last very long)
2. If not, they will remain separate species. Hybrid zones can occur where the two populations reproduce (due to weak prezygotic isolating mechanisms since the two populations are likely newly separated)

• However, the hybrids are not especially adapted to either environment so they are usually unfit (not always). Reinforcement occurs; mating with one’s own species

Question 9

Explain how genetic divergence in allopatry causes speciation

Answer 9

• Geographically separate populations accumulate differences (mutation, genetic drift, natural selection)
• Some mutations can establish postzygotic and prezygotic reproductive isolation (ex., 2 alleles from both species come together to produce tumours)
• Leads to speciation

Question 10

Explain how polyploid mutations in sympatry causes speciation

Answer 10

• Autopolyploidy: having more than two sets of chromosomes from the same parent species can occur through error in replication
  
  • Common in plants
  • Leads to reproductive isolation; plant can only replicate itself to produce sterile offspring or breed with another autoploid plant
• Allopolyploidy: having two or more complete sets of chromosomes from different parent species
  
  • Replicates like autopolyploidy
  • Very rapid speciation
  • Polyploid hybrids are often robust and large and thus important to agriculture
  • Leads to speciation

Question 11

Explain how chromosome alterations cause speciation

Answer 11

Chromosome alterations (inversions, translocations, deletions, duplications) may result in postzygotic isolation

Once a chromosome rearrangement becomes established within a population, that population will diverge more rapidly from populations lacking the rearrangement. The genetic divergence eventually can cause reproductive isolation.

Ex., the fusion of two chromosomes in chimp, gorilla, and orangutan species with 48 chromosomes likely resulted in reproductive isolation and perhaps given rise to humans (46 chromosomes)