1. Speciation

2. Macroevolution and speciation
Evolutionary theory must explain macroevolution, the origin of new taxonomic groups
Speciation, or the origin of new species, is central to macroevolution since all higher taxa originate with a new species which is novel enough to be the first member

3. Anagenesis and cladogenesis
Fossil record provides evidence for two patterns of speciation: A B A B C D E F
Anagenesis (phyletic evolution) – transformation of an unbranched lineage of organisms to a different state (the new species)
Cladogenesis (branching evolution) – budding of one or more species from a parent species that continues to exist

4. What is a species? Species = Latin for “kind” or “appearance”
Linnaeus described species in terms of their morphology
Modern taxonomists also consider genetic makeup and functional and behavioural differences when describing species

5. So…what is a species? Biological species concept defined by Ernst Mayr
population whose members can interbreed & produce viable, fertile offspring
reproductively compatible
Distinct species: songs & behaviors are different enough to prevent interbreeding
Eastern Meadowlark
Western Meadowlark

6. The biological species concept emphasizes reproductive isolation
In 1942, Ernst Mayr proposed the biological species concept
A biological species is defined as a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring but cannot do so with members of other species
The species is the largest unit of population in which gene flow is possible
It is defined by reproductive isolation from other species in natural environments (hybrids may be possible in the lab or in zoos)

7. How and why do new species originate?
Species are created by a series of evolutionary processes
populations become isolated
geographically isolated
reproductively isolated
isolated populations evolve independently
Isolation
allopatric
geographic separation
sympatric
still live in same area

9. Speciation Original Species 1st Step of Speciation
Allopatric
Speciation
Peripatric
Speciation
Parapatric
Speciation
Sympatric
Speciation
Original Species
1st Step of Speciation
Barrier
Formation
New
Niche
New
Niche
Genetic
Polymorphism
Evolution due to reproductive Isolation
Isolation
Isolation
Adjacent Niche
Within Population
New Distinct Species

11. Gene pools of biological species are isolated by pre- and post-zygotic barriers
Any factor that impedes two species from producing viable, fertile offspring contributes to reproductive isolation:
Most species sequestered from others by multiple barriers
Reproductive barriers prevent interbreeding between closely related species
Various barriers classified by whether they function before or after zygote formation:
Pre-zygotic barriers impede mating between species of hinder fertilisation of the ova by sperm from another species
If fertilisation does occur, post-zygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult

12. Pre-zygotic barriers: habitat isolation
Two species living in different habitats may not encounter each other:
Two species of garter snake (Thamnophis) occur in the same area but one species lives in water and the other is terrestrial
Since they live in separate habitats, the two seldom come into contact as they are ecologically isolated

13. Pre-zygotic barriers: behavioural isolation
Species-specific signals and elaborate behaviour to attract mates e.g different flashing patterns in fireflies
Many animals recognise mates by sensing pheromones:
Female Gypsy moths emit a volatile compound to which olfactory organs of male gypsy moths are specifically tuned
Males of other moth species do not recognise this chemical as a sexual attractant
Other behavioural isolating mechanisms:
Eastern and western meadowlarks only recognise songs of the same species
Specific courtship rituals

14. Other pre-zygotic isolating mechanisms
Temporal isolation:
Two species that breed at different times of the day, seasons or years cannot mix gametes
Brown trout breed in the autumn whereas rainbow trout living in the same streams breed in the spring
Mechanical isolation:
Anatomical incompatibility may prevent sperm transfer
Clasping appendages in dragonflies
Floral anatomy corresponding to specific pollinator
Gametic isolation:
Sperm of one species may not survive internal environment of female reproductive tract in another species
Lack of gamete recognition in external-fertilising species

15. Geographic isolation Species occur in different areas physical barrier
Ammospermophilus spp
Geographic isolation
Species occur in different areas
physical barrier
allopatric speciation
“other country”
Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel

16. Ecological isolation
Species occur in same region, but occupy different habitats so rarely encounter each other
reproductively isolated
2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
lions & tigers could hybridize, but they live in different habitats:
lions in grasslands
tigers in rainforest

17. Temporal isolation
Species that breed during different times of day, different seasons, or different years cannot mix gametes
reproductive isolation
sympatric speciation
“same country”
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

18. sympatric speciation?
Behavioral isolation
Unique behavioral patterns & rituals isolate species
identifies members of species
attract mates of same species 
courtship rituals, mating calls
reproductive isolation
Blue footed boobies mate only after a courtship display unique to their species

19. Mechanical isolation Plants
sympatric speciation?
Mechanical isolation
Morphological differences can prevent successful mating
reproductive isolation
Plants
Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

20. Mechanical isolation Animals
For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer
lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals!
I can’t even imagine!
Damsel fly penises

21. sympatric speciation?
Gametic isolation
Sperm of one species may not be able to fertilize eggs of another species
mechanisms
biochemical barrier so sperm cannot penetrate egg
receptor recognition: lock & key between egg & sperm
chemical incompatibility
sperm cannot survive in female reproductive tract
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

22. PRE-zygotic barriers
Obstacle to mating or to fertilization if mating occurs
geographic isolation
ecological isolation
temporal isolation
behavioral isolation
mechanical isolation
gametic isolation

23. Post-zygotic isolating mechanisms
Reduced hybrid viability:
Genetic incompatibility may abort development at embryonic stage
Several species of the frog Rana live in the same habitats but hybrids do not complete development
Reduced hybrid fertility:
Species mate and hybrid is viable but sterile e.g. mule
If chromosome numbers are different, meiosis cannot produce normal gametes
Hybrid breakdown:
First generation hybrids are fertile but subsequent generations are defective

24. POST-zygotic barriers
Prevent hybrid offspring from developing into a viable, fertile adult
reduced hybrid viability
reduced hybrid fertility
hybrid breakdown
zebroid

25. Reduced hybrid viability
sympatric speciation?
Reduced hybrid viability
Genes of different parent species may interact & impair the hybrid’s development
Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

26. Reduced hybrid fertility
Even if hybrids are vigorous they may be sterile
chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes
Mules are vigorous, but sterile
Horses have 64 chromosomes
(32 pairs)
Donkeys have 62 chromosomes
(31 pairs)
Mules have 63 chromosomes!

27. sympatric speciation?
Hybrid breakdown
Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile
In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.
On path to separate species.

28. Reproductive barriers – a summary