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 barriers:
- 2 Types:
- Extrinsic
- Ecological inviability: hybrids enoy normal development but suffer decreased viability, as they cannot find a suitable ecological niche. (Suspected increased fitness in intermediate environments, if they exist)
- Behavioral sterility: hybrids enjoy normal gametogenesis but suffer lowered effective fertility because they cannot obtain mates. Hybrids may present an intermediate courtship behavior or other phenotype that renders them unattractive to individuals of the opposite sex.
- Intrinsic
- Hybrid inviability: Hybrids suffer developmental defects causing full or partial inviability.
- Hybrid sterility:
- Physiological sterility: hybrids suffer developmental defects in their reproductive system causing full or partial sterility.
- Behavioral sterility: hybrids suffer a neurological or physical defect that renders them fully or partially incapable of courtship.
The hybrid is incapable of reproduction or is too developmentally flawed to live. Mules, for example – sterile, though viable.
So a hybrid is not suitable for a specific niche and cannot find mates, even though it is otherwise functional.
- Evolution of Extrinsic versus Intrinsic
- Extrinsic
- Extrinsic isolation may be a byproduct of adaptive radiation
- “additive gene interaction” – Dominant and recessive homozygotes both fit, but the heterozygous type is unfit due to intermediate phenotype / behavior.
- Intrinsic – Genetic Modes of Intrinsic Postzygotic Isolation: A Very Long Section
- Causes of Hybrid Difficulties
- Different ploidy levels: Polyploidy is more important in plant speciation than animal. Look in Ch. 9 for more about this.
- Different chromosomal arrangements: Structural changes in chromosomes might directly cause reproductive isolation. This can even cause semisterility within species.
The greater the reproductive isolation ultimately caused by a rearrangement, the more it is selected against when it appears. New arrangements are selected against until they reach intermediate frequencies. More likely to succeed if population size is small so that genetic drift can overcome natural selection. Common in plants, probably not in animals. - Different alleles that do not function together in hybrids: Between-locus incompatibilities (Gene at locus A from one species does not interact properly with gene at locus B from another species) often leads to hybrid sterility and inviability. Ancestor aabb diverged into aaBb and Aabb, then aaBB and AAbb. A and B are deleterious when together in a hybrid (AaBb), but are adapted to the alleles they normally occur with. Hybrid incompatibilities only occur at loci that have both experienced substitution.
- Cytoplasmic endosymbionts / cytoplasmic incompatibility: Wolbachia, a bacterial endosymbiont, is maternally transmitted. 15-20% of insects carry Wolbachia. Infected male x uninfected female = offspring that die as embryos. Reciprocal cross shows no lethality. Infected females can reproduce with any male, and uninfected females can only reproduce with uninfected males. Thus the bacteria is spread throughout the population. If two types of endosymbiont are present, the two infected types are usually incompatible; thus a population may not be overtaken by a single type. Natural selection at the level of parasites causes hybrid inviability at the level of hosts.
Can cause speciation, but is relatively uncommon.
Well, that’s interesting… I’d never heard of this sort of thing before.
I’d like to finish this book this week. I have about 160 pages left (assuming the remaining chapters are all relevant), and I’m getting a bit tired of this subject for now. It’s time to move on.
