Gametic isolation in sea urchins Fig. 24-4k (g) Sea urchins
Reduced hybrid viability - Genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring.
Reduced hybrid fertility - Even if the hybrid offspring are vigorous, the hybrids may be infertile and the hybrid cannot backbreed with either parental species.
Reduced hybrid breakdown – In some cases, first generation hybrids are viable and fertile.
However, when they mate with either parent species or with each other, the next generation is feeble or sterile.
Reduced Hybrid Fertility Fig. 24-4m (i) Donkey
Interfertility concept does not apply to
Asexually reproducing organisms
Modes of speciation
Allopatric – geographical separation
Sympatric – biological barriers prevent gene flow in overlapping populations as in autopolyploidy, allopolyploidy, mate preference, etc.
Figure 24.6 Two modes of speciation
Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon
Figure 24.8 Has speciation occurred during geographic isolation?
Autopolyploidy – more than two sets of chromosomes; meiotic failure, in self-pollination in plants
Allopolyploidy * – interspecific hybrid may become fertile due to nondisjunction
* more common
Sympatric speciation by autopolyploidy in plants
Figure 24.15 One mechanism for allopolyploid speciation in plants
Fig. 24-UN2 Ancestral species: Triticum monococcum (2 n = 14) AA BB Wild Triticum (2 n = 14) Product: AA BB DD T. aestivum (bread wheat) (2 n = 42) Wild T. tauschii (2 n = 14) DD
Around 1870, a new species of grass turned up at the salt marches near the coast of the English Channel: Spartina townsendii . It was taller than the indigenous Spartina alternifolia .
Another relative, Spartina stricta, inhabits the North-American east coast. It was brought in to Europe and began to occupy the sites of Spartina alternifolia.
It was now suspected that Spartina townsendii was a hybrid of the two original species. The fact that Spartina townsendii has 2n = 126 chromosomes, Spartina alternifolia has 2n = 70 and Spartina stricta has 2n = 56 chromosomes makes this suggestion seem likely.
Where divergent allopatric populations come back and interbreed
Biologists look for patterns to study reproductive isolation
Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Strengthening of reproductive barriers
Weakening of reproductive barriers
Continued formation of hybrid individuals
Fig. 24-13 EUROPE Fire-bellied toad range Hybrid zone Yellow-bellied toad range Yellow-bellied toad, Bombina variegata Fire-bellied toad, Bombina bombina Allele frequency (log scale) Distance from hybrid zone center (km) 40 30 20 20 10 10 0 0.01 0.1 0.5 0.9 0.99
The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully.
Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species
Fig. 24-15 Sympatric male pied flycatcher Allopatric male pied flycatcher Pied flycatchers Collared flycatchers Number of females (none) Females mating with males from: Own species Other species Sympatric males Own species Other species Allopatric males 0 4 8 12 16 20 24 28
Fusion: Weakening Reproductive Barriers
If hybrids are as fit as parents, there can be substantial gene flow between species
If gene flow is great enough, the parent species can fuse into a single species
Fig. 24-16 Pundamilia nyererei Pundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water
Stability: Continued Formation of Hybrid Individuals
Hybrids continue to be produced between the two species in the area of their overlap, but the gene pools of both parent species remain distinct.
Fig. 24-14-4 Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid Possible outcomes: Reinforcement OR OR Fusion Stability
Timing of evolution?
Gradual (Lyell, Darwin)
Punctuated equilibrium (Gould) – evolution occurs in spurts and interspersed within long periods of stasis
In the punctuated
Species undergo most morpholo- gical modifications when they first bud from their parent population.
After establishing themselves as separate species, they remain static for the vast majority of their existence.
The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid
The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years