• One or more populations of individuals that can inter breed under natural conditions and produce fertile offspring. Different species may arise from genetic divergence between populations

• Genetic divergence is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between the groups
– If the environments are different between isolated units then natural selection, mutations, and genetic drift will work independently on each. This will speed up genetic divergence and speciation.

• Mechanisms that lead to, or cause genetic divergence by preventing interbreeding between two groups.
– As long as two groups do not interbreed their gene pools will continue to drift further and further apart.
– The longer two species are genetically isolated from each other the more different they become from each other.

• Isolating mechanisms can be divide into two different types.
– Isolating mechanisms that occur before fertilization are called Prezygotic mechanisms
– Isolating mechanisms that occur after fertilization are called postzygotic mechanisms

• Behavioral isolation : Potential mates meet but cannot figure out what to do about it because patterns of courtship may be altered to the extent that sexual union is not achieved
• Temporal isolation : (Time) Different groups overlap in range but may not be reproductively mature in the same season.
•
Mechanical isolation : Potential mates attempt engagement but sperm cannot be successfully transferred . This may be due to differences in reproductive organs .

•
Gametic isolation : Sperm is transferred but sperm and egg are incompatible.
•
Ecological isolation : potential mates never meet because they live in different habitats

• Zygotic mortality : Egg is fertilized but zygote does not develop properly dies before birth because parents are genetically incompatible.
•
Hybrid inviability
: Hybrid very weak and can’t live outside the uterus.
• Hybrid offspring : Hybrid is sterile.

• Speciation occurs when a species gives rise to one or more different species.
• There are three main speciation patterns
– Allopatric speciation
– Sympatric speciation
– Parapatric speciation

• Allopatric (Allo= different, Patric=homeland) Population are separated due to geographical barriers
• Rivers, earthquakes, continental drift, glaciation, archipelagos cause allopatric speciation.
• Examples:
– Cave fish,
– Darwin’s finches
– Antelope squirrels of the Grand Canyon
– Isthmus of Panama

• Sympatric (Sym = same, Patric=homeland) speciation occurs in the same geographical region without physical isolation.
• A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population.
• Example: Accidents during cell division that result in extra sets of chromosomes (Polyploidy). Self-fertilization can give rise to new individuals that are unable to mate and form fertile off-spring with the parent species.

• Parapatric (Para = near, Patric=homeland) neighboring populations become distinct species while maintaining contact through Hybrid zones
• If hybrid zone is removed through increased natural selection, natural disasters or some other means the extremes of a population fail to mate.
• Examples
1 Toad population along the north rim of the Grand
Canyon.
2 Dogs

• Cladogenesis : Branched evolution. Occurs as populations split and become reproductively isolated from each other.
A
C
• Anagenesis : Unbranched evolution. Occurs as changes in allele frequency and morphology accumulate over long periods of time. New species do not live within the same time period
A B
C
Related species are only seen in the fossil record

• Species which are intermediate in body form between two groups of organisms.
• Species that are intermediate in time in the fossil record.
Archaeopteryx

Intermediate between birds and dinosaurse
• Archaeopteryx lived after the development of dinosaurs but before that advent of birds.
• Archaeopteryx contains features that are both characteristic of reptilian dinosaurs as well as modern birds. Some features are half way inbetween.

Feature Birds Archaeopteryx Coelurosaurs
Body covering Feathers Feathers Scales (?)
Metatarsals Fused
Bones
Clavicles
Sternum
Partly fused Not fused
Hollow and pneumatic
Hollow, not pneumatic Not hollow or pneumatic
Not fused Fused
(wishbone)
Fused (wishbone)
Large, keel Small, no keel Small, no keel
Absent Present present Abdominal ribs

• Phyletic gradualism : New species develop slowly and gradually as an entire species changes over time. This idea was asserted by Darwin.
• Punctuated equilibrium
: This theory states that new species arise suddenly and rapidly as small subpopulations of a species split from the populations of which they were a part.