The Origin of the Species Chapter 22 Mom, Dad… There’s something you need to know… I’m a MAMMAL!

“That mystery of mysteries…” Darwin never actually tackled how new species arose… Both in space and time, we seem to be brought somewhat near to that great fact —that mystery of mysteries— the first appearance of new beings on this Earth.

Defintions Macroevolution: origin of new taxonomic groups Speciation: origin of new species Anagenesis (phyletic evolution): accumulation of changes associated with transformation of one species to another Cladogenesis (branching evolution): budding of new species from parent species

Biological Species Concept Species: pop. or group of pop. with potential to interbreed & produce fertile, viable offspring Reproductive isolation (barriers) exist between different species Two types

Same Species?????? YES!

Prezygotic Barriers Habitat isolation Behavioral isolation Temporal isolation Mechanical Isolation Gametic Isolation

behavioral isolation geographic isolation ecological isolation temporal isolation mechanical isolation gametic isolation Habitat isolation

Geographic isolation 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 Ammospermophilus spp

Ecological isolation 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 lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest

Temporal isolation Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Behavioral isolation Bowerbirds make display bowers and present species- specific “gifts” at the front

Mechanical isolation Damsel fly penises

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse. Gametic isolation

Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

Reduced hybrid viability Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

Mules are vigorous, but sterile Reduced hybrid fertility Donkeys have 62 chromosomes (31 pairs) Horses have 64 chromosomes (32 pairs) Mules have 63 chromosomes!

Hybrid breakdown In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species. In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species.

Problems with B.S.C. Does not account for asexual species Can’t use for extinct species Many other species concepts exist

Modes of Speciation Allopatric Speciation: a population forms a new species while geographically isolated from its parent population Sympatric Speciation: happens without geographic isolation

Allopatric Speciation Increases when population is small & isolated ◦ More likely to be influenced by genetic drift & natural selection

Allopatric Speciation Depends on if they become so different over time that they can no longer interbreed

Adaptive Radiation Evolution of many diversely adapted species from a common ancestor Island chains – movement between islands could result in allopatric speciation

Sympatric Speciation Plants ◦ Autopolyploids (4n) can mate with other polyploids (or themselves), but not the typical diploid plant ◦ Allopolyploid (contribution of two different species to make a polyploid hybrid)  Infertile, but can reproduce asexually

Sympatric Speciation Animals ◦ Polyploidy is rare

Punctuated Equilibrium Sudden appearance of new species Spurts of rapid change, followed by long periods of little to no change

Mechanisms of Macroevolution Genes for controlling development ◦ Allometric growth: relative growth rates of body parts during development ◦ Modification (evolution) of this: heterochrony  Ex: Paedomorphosis: retention of juvenile features of ancestral species in adult  Due to change in rate of development

Mechanisms of Macroevolution Changes in genes that control placement & spatial organization of body parts ◦ Ex: homeotic genes (control location of basic features)  Hox genes Hox genes  (11:54)

FYI… Evolution is not predicting the future Evolution does not look for the “best way” It is not always a linear pathway from ancestor to current organism