Macroevolution: the origin of new taxonomic groups

Speciation: the origin of new species 1- Anagenesis (phyletic evolution):

accumulation of heritable changes – evolution within a lineage

2- Cladogenesis (branching evolution):

budding of new species from a parent species that continues to exist (basis of biological diversity) – evolution that results in splitting of a lineage

Origin of Species

It’s all about reproductive Isolation!

What is a species?

Biological species concept:

a population whose members can interbreed and produce viable, fertile offspring

Reproductively compatible

How do new species originate?

Populations must become isolated

geographically isolated reproductively isolated

isolated populations evolve independently

Isolation allopatric

geographic separation “other country”

sympatric still live in same area “same country”

PRE-reproduction barriers Obstacle to mating or to fertilization if mating occurs

(prezygotic)

geographic isolation ecological isolation temporal isolation

behavioral isolation mechanical isolation gametic isolation

Geographic isolation

Species occur in different areas physical barrier allopatric speciation

Ecological isolation

Species occur in same region, but occupy different habitats so rarely encounter each other reproductively isolated sympatric speciation

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

Temporal isolation Species that breed

during different times of day, different seasons, or different years cannot mix gametes reproductive isolation sympatric speciation

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

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

Behavioral isolation

Unique behavioral patterns & rituals isolate species identifies members of

species attract mates of same

species courtship rituals, mating

calls reproductive isolation sympatric speciation?

Blue footed boobies mate only after a courtship display unique to their species

Blue footed boobies mate only after a courtship display unique to their species

Mechanical isolation Morphological differences can

prevent successful mating Even in closely related

species of plants, the flowers often have distinct appearances that attract different pollinators.

For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer

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.

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

POST-reproduction barriers Postzygotic

barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult

Reduced hybrid viability- homologs don’t line up; abort at embryonic stage. (frogs; zygotes fail to develop or reach sexual maturity)

Reduced hybrid fertility- usually have an uneven number of chromosomes in their gamates. (mule; horse x donkey; cannot backbreed)

Hybrid breakdown-first couple of generarations work, but successive generations become more and more feeble. (cotton: 2nd generation hybrids are sterile)

Rate of Speciation Gradualism: gradual

divergence over long spans of time

(Darwin was more of a gradualist)

Punctuated Equalibrium:

Niles Eldredge and Stephen Jay Gould (1972);

helped explain the non-gradual appearance of species in the fossil record.

“Spurts”of relatively rapid* change followed by long periods of little or no change.

* Keep in mind that in geologic time “rapid” still refers to 1000s of years….

Modes of speciation (based on how gene flow is interrupted)

Allopatric: populations

segregated by a geographical barrier; can result in adaptive radiation (island species)

Emergence of mountains/valleys Movement of glaciers Land bridges or lake subsidence Founder effect

Peripheral populations are usually extreme representations of the group, so are already more likely to speciate.

Modes of speciation continued

Sympatric: reproductively isolated subpopulation in the midst of its parent population.

Uncommon for animals Quite common for plants Often results from non-disjunction or cross-

species mating. AUTOPOLYPLOIDY: when 2 or more

chromosome sets all derived from a single species stay together.

ALLOPOLYPLOIDY: results from contributions from 2 different species.

animation

What accounts for Evolutionary TRENDS? 1. Gradual Refinements of

existing structures: EXAPTATION: when structures

evolved in one context becomes functional for something else. Ex. Bird Wings

2. Magnification of slight genetic changes:

Sometimes just a few mutations can cause major modifications.

HETEROCHRONY- evolutionary changes in the timing or rate of embryological development.

HOMEOSIS- Alteration in placement of body parts by just a few gene changes.

(Remember the legs in the fruitfly head?)