Speciation and its Consequences

Ch. 18 – Sympatric Speciation and Darwin’s Finches
All allopatric speciation invokes an imporobable number of range splitting and rejoinings – this leads some to consider parapatric and sympatric speciation. Sympatric speciation is a good way to account for presence of large number of sister species in environment with many niches
1st Step: A stable [...]

Read Full Post »

Speciation and Macroevolution

Speciation Rate:
Speciation can occur rapidly no matter what the driving force. Transition times are generally shorter whenever the process involves an unstable intermediate stage; the forming species must either cross that unstable stage quickly or dissolve (involves sympatric phase or crossing adaptive valley). Transition times for allopatric speciation can be longer since the [...]

Read Full Post »

Selection vs. Drift

This chapter’s question: Does natural selection or genetic drift play a larger role in the origin of species?
A lovely table directly from the book summarizes definitions nicely (p. 384):
Selection
Direct: direct natural selection for reproductive isolation. Direct selection characterizes models of sympatric speciation and reinforcement.  — Definitely not ubiquitous, especially in allopatric speciation.
Indirect: [...]

Read Full Post »

Reinforcement

Reinforcement: enhancement of prezygotic isolation in sympatry by natural selection – Two taxa diverge in allopatry; upon secondary geographic contact, hybridization occurs at some rate, yielding unfit hybrids. Because production of hybrids is maladaptive, individuals who only mate with their own taxa enjoy a fitness advantage.
Evidence: Reveals that sympatry can enhance prezygotic [...]

Read Full Post »

Recombinational Speciation

Recombinational Speciation: hybridization between 2 species gives rise to a new lineage that is both fertile and true breeding, but is reproductively isolated from both parental species.
Forms novel genotype that is homozygous for several chromosome arrangements that differentiate the parental species
Certain rare hybrid genotypes can be fitter than parentals, though most hybrids are less [...]

Read Full Post »

Polyploidy

Polyploid Classification:
Autopolyploids: result from an increase in ploidy level within a species (AAAA)
Allopolyploids: Result from hybridization between species

Genomic allopolyploids: carry entire chromosome sets from 2 or more species. Chromosomes from different species do not pair during meiosis. (AABB)
Segmental allopolyploids: some chromosomes do not pair, others do. Often unstable [...]

Read Full Post »

Genetics of Intrinsic Postzygotic Isolation

Haldane’s rule, paraphrased: The heterogametic sex is more likely to be absent, rare, or sterile in the offspring of two species; the homogametic is less likely to be affected by these things. This rule holds in most animals possessing sex chromosomes. Obviously this wouldn’t hold true for sex determined by temperature during [...]

Read Full Post »

Postzygotic Isolation

I’ve found that I remember what I read better if I use this blog for note taking and then observations on what I’ve read. I’ll most likely use this approach for the remainder of this course unless there are objections. My thoughts on the reading will be in italics.
Notes!
Classification of postzygotic reproductive isolating [...]

Read Full Post »

Behavioral and Nonecological Isolation

Behavioral Isolation: may be more of a species identification mechanism?
Mechanical Isolation: square peg, round hole. It just doesn’t work.
Gametic Isolation: sperm competition / inactivation in heterospecific genital tract, and / or incompatible binding proteins on gamete surfaces. — I wondered how that worked.

Read Full Post »

Ecological Isolation

Habitat Isolation: often a direct byproduct of adaptation to the environment. Seems based on relatively few genes. “Habitat” can include host preference in addition to soil type preference and other habitats.
Pollinator Isolation: gene flow between flowering plants is reduced if the species use different pollinators. Strong dependence on a particular [...]

Read Full Post »