Ch 24  Speciation -As individuals are no longer able to mate with each other, they are classified as different species.  This is caused by reproductive isolation. This occurs when members of 2 populations cannot interbreed with each other to produce fertile offspring.

Types of Reproductive Isolation (Count them: There’s 5!)  Behavioral Isolation – populations do not breed because they have different mating rituals or behavioral reproductive strategies  Ex: A fruit fly will only breed with another fruit fly after recognizing a particular mating ritual “dance” performed by the male. (PICTURE FROM LAB BENCH)

 Geographic Isolation – populations do not breed because they are physically separated from each other (by a river, mountain range, etc) Temporal Isolation – populations do not breed because they reproduce at different times Ex: flowers opening up at different times of day

Mechanical Isolation  Anatomical incompatibility may prevent sperm transfer when closely related species attempt to mate. EX: male dragonflies use a pair of special appendages to clasp females during copulation. When a male tries to mount a female of a different species, he is unsuccessful. (male is on left)

Gamete Isolation Gametes of different species that meet rarely fuse to form a zygote. EX: For animals that use internal fertilization, the sperm of one species may not be able to survive the internal environment of the female reproductive tract of a different species.

 All of these previous isolations were prezygotic (not able to make a fertilized egg)  What if an egg is made?  Postzygotic barriers= When prezygotic barriers are crossed and a hybrid zygote forms, hybrids may be (1) frail or unable to survive to develop or (2) have different chromosome numbers so meiosis cannot happen normally so hybrid has no fitness

Two Types of Speciation…  Allopatric Speciation= Speciation forms due to geographic barrier.

 Sympatric Speciation- Formation of new species within the range of parent populations (no geographic barrier)

Early Earth---Ch 26  Life on Earth began between 3.5 and 4.0 billion years ago.  Earliest organisms were prokaryotes at least 2 billion years ago before eukaryotes arrived  Stromatolites= banded domes of sediment constructed by colonies of bacteria and cyanobacteria currently living in salty marshes (3.5 billion years ago!!, west Austrailia and southern Africa)

The first cells…  Little to no oxygen  Lightning, volcanic activity, meteorite bombardment, and intense UV light  Earth’s crust had just solidified

First living organisms may have been products of chemical evolution in 4 stages…  1. Abiotic synthesis and accumulation of monomers  2. Joining of monomers into polymers  3. Formation of protobionts- droplets which formed from aggregates of abiotically produced molecules and were different from chemical surroundings  4. Origin of heredity during or before protobiont appearance

Polymers  …are made from dehydration synthesis  Must have occurred without enzymes in early Earth  Clay must have been important because: -monomers bind to charged sites in clay, concentrating amino acids + monomers -metal ions (iron, zinc) could catalyze dehydration reactions

More on Protobionts  They are molecules that can maintain an internal environment different from surroundings and have some life properties such as metabolism + excitability  Experimental evidence for spontaneous formation of these? Yes. Example: Microspheres. Undergo shrinking, swelling. Selectively permeable membrane and have a potential energy in the form of membrane potential.

Microspheres

It’s an “RNA WORLD” *  Today’s cells transcribe DNA into RNA…then to proteins BUT  We hypothesize in primitive times, RNA became before DNA. Evidence:  RNA can self-replicate (in small amounts)  RNA is autocatalytic as ribozymes have the ability to act as catalysts  RNA unfolds uniquely depending on sequence (unlike DNA)

Debate about life continues…  Panspermia- organic compounds may have reached Earth by way of meteorites and comets. Organic compounds were recovered.  Most researchers believe life appeared first in shallow water or moist sediments. Some believed life started on sea floor due to harsh conditions on surface. This was supported by discovery of sea vents.

Sea Vents

Tube Worms…have adapted to immense pressure, total darkness, and proximity to super-heated water that reaches 662 degrees F.