Outstanding Origin of Species Ch 24

Vocabulary  1. Macroevolution – origin of new taxonomic groups (new species, genera, families etc)  2. Speciation – origin of new species – key process in macroevolution

 3. Biological species concept- a species is a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring but cannot produce viable, fertile offspring with members of another species.  *Hinges on reproductive isolation – each species is isolated by barriers that prevent interbreeding

Same or different?

Same or Different?

Pre-zygotic Barriers to reproduction  impede mating or hinder fertilization of the egg  1. Habitat Isolation – two species that live in different habitats within the same area may encounter each other rarely, if at all. (i.e. parasite on different host or garter snake – one lives in water, other on land)

Pre-zygotic Barriers to reproduction  2. Behavioral Isolation – sexual signals that attract mates and elaborate behavior unique to a species (songs, mating dances etc. specific to a species)  3. Temporal Isolation – two species that breed during different times of day, different seasons or different years can’t mate together

Pre-zygotic Barriers to reproduction  4. Mechanical Isolation – anatomical differences might prevent closely related species from mating  5. Gametic Isolation – even if gametes from different species meet, they rarely fuse to make a zygote (i.e. sperm of different species can’t survive internal conditions of female)

Post-zygotic Barriers  occur when a sperm from a different species does fertilize the egg  1. Reduced hybrid viability – genetic incompatibility may abort development of hybrid during embryonic development

Post-zygotic Barriers  2. Reduced hybrid fertility – if two different species do mate and produce a hybrid, many hybrids are sterile and cannot produce offspring (mule)  3. Hybrid Breakdown – occasionally, first generation hybrids are viable and fertile, but when hybrids mate the next generation is often feeble or sterile sterile

Other concepts of species  Biological species concept doesn’t work in all situations. Here are some alternative concepts (like for fossils/prokaryotes)  *Ecological species concept – species is defined in terms of their niche (role in environment)  *Phylogenetic Species concept- species is the smallest group of individuals that share a common ancestor, forming one branch in the tree of life

 *Morphological species concept – relies on structural features to distinguish species –i.e. body shape and other structural features (problem – can be subjective)

Modes of Speciation  1. Allopatric Speciation – speciation takes place in populations with geographically separate ranges. As the isolated population’s gene pool changes, reproductive isolation can result.

Allopatric Speciation  *geologic processes can fragment a population (i.e. mountain emerges, land bridge forms separating marine life etc)  *amount that geography separates organisms depends on their ability to move (fly, swim etc)  *allopatric speciation increases when population becomes small and isolated from others. These populations are more likely to have gene pool changed dramatically.

Allopatric Speciation  *Adaptive radiation – evolution of many diversely adapted species from a common ancestor (i.e. finches on Galapagos islands, many indigenous species on Hawaiian islands )

How do reproductive barriers evolve?  *must be intrinsic to organism and prevent interbreeding even when species are brought back together  *most likely coincidental – not that organism is trying to make a barrier to reproduction  *Fruit fly experiment shows formation of pre-zygotic barrier due to different mating ritual

How do reproductive barriers evolve?  *When trying to mate same plant from different areas of U.S., those that were close together (Wisconsin/Michigan) could interbreed while farther apart (Wisconsin/Texas) were unable to interbreed (post-zygotic barrier due to hybrid breakdown)

2. Sympatric Speciation  speciation takes place in geographically overlapping populations. Biological factors such as chromosomal changes and non- random mating reduce gene flow.

2. Sympatric Speciation  *In Plants – polyploidy – cell division that results in extra sets of chromosomes  -autopolyploid – individual that has more than two sets of chromosomes derived from a single species  *could be tetraploid instead of diploid if meiosis does not occur. Tetraploid can self fertilize or mate with other tetraploids, but can not mate with diploid plants (new species formed)

Sympatric Speciation  -allopolyploid – two different species interbreed, combine chromosomes and produce a hybrid. Hybrid can only mate with other hybrids  Ex. – oats, cotton, potatoes, tobacco, wheat

Sympatric Speciation  *In animals -can become reproductively isolated within the same geographic region (wasps – dif species pollinate dif fig plant)

3. Punctuated Equilibrium  Species diverge in spurts of relatively rapid change (thousands of years) followed by long periods of stasis (unchanging) – based on fossil evidence

Speciation Rates  In general, once speciation begins, it can be a relatively rapid process (in geologic time)  It varies from species to species – can take anywhere from 4000 years to 40 million years – avg about 6.5 million years

From Speciation to Macroevolution  *Most evolutionary novelties (like human eye) are modified versions of older structures that have evolved in many stages.  *genes that control rate of development play a major role in evolution. Mutations in these genes can cause macro-evolutionary changes.  *Long-term evolutionary trends may be a result of adaptations to surrounding environment or may results when species with certain characteristics endure longer and speciate more often than those with other characteristics.