What are the defining features of the biological species concept? What is the rule, and what exceptions might exist? Think of the Canidae and Equidae examples.
- a group of interbreeding (or potentially interbreeding) individuals, reproductively isolated from other such groups
- Subspp, races, and varieties named based on the degree of repr. isolation, which is often a function of geographic isolation
- However, the boundaries are often unclear when it comes to domestic dogs being able to hybridize with wolves, jackals, dingos, coyotes, etc.
- Equines can hybridize, although the offspring is typically sterile
What are the limitations of the BSC?
- Would allopatric populations interbreed if they were to encounter eachother?
- Extinct forms are solely classified on morphology
- Asexual species never mate with eachother (must use biochemical similarities to classify)
- 100% reproductive isolation not required (some species share genes to an extent, but are still considered separate spp)
How has re-evaluation of the Linnean taxonomic system using the BSC changed our understanding of systematics?
What is a premating isolating mechanism? Be able to discuss different examples of each type
- *note-allopatry is NOT an isolation mechanism
- Premating isolating mechanism: impede gamete transfer to members of the other spp
- ecological incompatibility - potential mates do not meet
- habitat isolation- diff spp occupy diff habitats within the same area
- EX: frogs in same area in NZ, but one in streams while other is in trees)
- temporal isolation- spp have different activity patterns (nocturnal vs dirunal) or breed in different seasons
- EX: butterfly are sexually active during different times of the year
- potential mates meet, but do not mate
- behavioral isolation-
- EX: morphologicaly indistinguishable green lacewing spp produce different songs (mating requires females to sing back)
- EX: male hummingbirds have different color patterns (females tend to be similar to eachother)
What is a post-mating pre-zygotic isolating mechanism? Be able to discuss different examples of each type
- Postmating, pre-zygotic isolation mechanisms: mating or gamete transfer occurs, but zygotes are not formed
- Mechanical isolation- copulation occurs, but not transfer of male sperm takes place due to reproductive structures not fitting together
- EX: almost the only morphological trait that differs in males of 3 drosophila spp is the posterior lobe of their genital arch
- Gamete isolation- failure of proper transfer of gametes for fertilization
- EX: among abalones, sperm only enters egg is protein lysin (from sperm) can dissolve egg membrane
What is a post-zygotic isolating mechanism? Be able to discuss different examples of each type
- Post-zygotic isolating mechanism: hybrid zygotes form, but have reduced fitness
- Hybrid inviability- mortality occurs in embryonic stages due to failure of proper development.
- F2 breakdown- first generation fertile, but subsequent generations are infertile or non-viable
- Hybrid sterility- structural differences between chromosomes or in genes from 2 parents; offspring unable to breed
- *haldane's rule: Often limited to the heterogametic sex (it is difficult to determine the appropriate homologue location between two species which leads to disjunction)
How might character displacement in reproductive isolating mechanisms be positively selected in sympatric sister species, but under neutral selection in allopatric sister species?
- If hybrids are at a disadvantage (lowered fitness) then it would be of benefit to prevent hybridization through character displacement in reproductive isolating mechanisms
- Allopatric sister species would not be under this same stress, because hybridization would not occur in the first place
Why is prezygotic isolation a stronger barrier to gene exchange than postzygotic isolation?
In prezygotic isolation there is literally no genes exchanged under any circumstances. If two species are completely unable to mate then there is a direct barrier against gene exchange. In post-zygotic isolation the barrier to gene exchange often involves a reduction in fitness AFTER the exchange has already occurred.
Why does postzygotic isolation evolve more rapidly in males than females? (think about Haldane's rule)
- Haldane's rule states that hybrid sterility is often limited to the heterogametic sex.
- Since the sex chromosomes are often dramatically different in size it is diffiduclt to determine the actual homolog location between them, and thus disjunction events occur during meiosis which lead to sterility.
- Females that contain two X chromosomes would be less likely to suffer from these disjunction events.
How does a hybrid zone form? How is the steepness of the genetic cline related to the environmental cline and the rate of dispersal?
- Hybrid zones are formed in regions of contact between formerly allopatric populations
- Gene flow introduces alleles between hybridizing populations, and forms clines of varying steepness
- The steepness depends on the dispersal rate, distance, and strength of selection in a hybrid zone
What are the potential fates of hybrid populations in hybrid zones?
- Barrier to gene flow may break down
- The hybrid zone may persist indefinitely
- NS may favor stronger prezygotic isolation
- Hybrids may become isolated from parent forms and become a third species
What are the two basic patterns of divergence leading to speciation?
- Splitting: two populations evolve differently until they become separate species (common in allopatric speciation over a large geographical area)
- budding: subpopulations "bud off" from main population and evolve to form new species while leaving most of the larger population unchanged (common in allopatric speciation when small subpopulation is separated)
What are the four modes of speciation?
- Allopatric speciation: geographic barrier leads to reproductively isolated subpopulations; dispersal over barrier can re-establish sympatry
- Sympatric speciation: genetic differences within a single (panmictic) population results in reproductive isolation
- Parapatric speciation: range expansion by partially distinct populations (w/ some gene flow) leads to sympatry
- Peripatric speciation: divergence of a small population from a widely-distributed ancestral species re-establishes sympatry
What is a ring species? Discuss an explicit example.
- Ring species provide strong evidence that one species can split into two
- They occupy a wide geographical area, where individuals at the end of a cline can be very different from one another (and unable to breed)
- EX: Herring gulls and lesser black-backed gulls
- In certain regions gulls are recognized as herring gulls, there is a sort-of hybrid zone where they intermingle, but then as the distance continues to grow the birds are recognized as lesser black-backed gulls.
- The two extremes CANNOT mate
What occurs in the process of allopatric speciation? How do environmental components and reproductive isolating mechanisms interact in this process? What explicit evidence exists for this process?
- Gene flow between populations is reduced by geographic or habitat barriers, allowing genetic divergence by NS, GD, or both
- Stage 1: Moving to new environments
- Parent pop expands its range
- Pop has common gene pool with regular gene flow
- Any individual has potential access to opposite members of sex for mating
- Stage 2: geographic isolation
- gradual formation of physical barriers isolates sub population at extremes of range
- gene flow between isolated populations is prevented
- Stage 3: Formation of a sub species
- isolated pops may be under diff selective pressures
- selection favors ind. w/ traits suited to each env
- allele freq can change for certain genes
- pops take on status of sub spp (isolation not yet fully established)
- Stage 4: reproductive isolation
- divergence in each separated subspp results in mutations for isolating mechanisms; prevents mating with individuals from other populations
- Each subspp gene pool becomes repr isolated from others and attain spp status
- even if geographic barriers are removed to allow pop. mixing genetic isolation is complete
- EVIDENCE: Dusky salamanders in E. US - more geographically distant populations were more genetically different, and less likely to mate
- EVIDENCE: 6 freshwater fish spp in SE US - DNA sequences are from 2 distinct clades (E and W) which shows that isolation occurred ~3-4MYA followed by range expansion and secondary contact
What occurs in the process of sympatric speciation? How do genetic components and reproductive isolating mechanisms interact in this process? How might ecologically divergent traits play direct role in reproductive isolation of sympatric populations? What explicit evidence exists for this process?
- Ecologicaly divergent traits may play a role in reproductive isolation
- EX: sexual isolation in leaf beetle (more isolation between ecologically-divergent population (different trees) in the same area than between groups from Canada and Georgia!)
- EX: ecomorphs of stick insect (body form/color pattern closely match foliage of the 2 different food plants. Pairs from diff plants are repr isolated)
- Stage 1: reproductive isolation usually occurs first within an initially randomly-mating population
- Often a result of disruptive selection, for change in habitat preference (insects conditioned to lay eggs on a preferred plant, if that plant isn't available they have to pick a different plant- those eggs will prefer the new plant)
- Stage 2: Establishing complete reproductive isolation
- further adaptation in each subpopulation leads to genetic differences
- Ultimately, 2 groups will diverge and be recognized as 2 spp
- Stage 3: Polyploidy involves multiples of whole sets of chromosomes
- Common in plants and some animal groups (rotifers, earthworms)
- When such individuals spontaneously arise, they are instantly reproductively isolated from their parent population
- As many as 80% of flowering plant spp may have originated as polyploids
- EVIDENCE: apple maggot fly vs hawthorn fly in US have 3 week difference in mating time, but the hawthorn-exclusive fly (ancestor) in Mexico does not
- EVIDENCE: palms on island in Australia exhibit isolation by soil type (nutrient availability) they can grow in and different flowering seasons
What is the BSI? What is the range of BSI in animal species? Which process is generally faster, speciation by polyploidy or by mutation/drift?
- Biological speciation interval: the meant time that has elapsed between two sequential forks in a phylogeny
- BSI in animals ranges from ~.3my (ciclid fishes) to 3my (horses) to 6-11my (bivalve mollusks)
- TFS (time for speciation) is shorter for polyploids than mutation/drift