Speciation

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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.

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

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.

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

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

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

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.

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!

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.

Reproductive barriers – a summary

https://www.youtube.com/watch?v=L3dkJVExfdE https://www.youtube.com/watch?v=TjhlkovEJqM