A. What is a species?

1. From Latin word for “kind” or “appearance”

2. There are different concepts for which species is defined. The most accepted is the “biological species concept”

a. a species is a population or group of populations whose members have the potential to interbreed (mate) in nature and produce fertile offspring.

3. Reproductive isolation – the existence of biological barriers that do not allow two species to produce viable, fertile, offspring.

Prezygotic barriers – can’t mate or fertilization can’t happen

a. Habitat isolation – rarely encounter one another

b. Temporal isolation – breed at different times

c. Behavioral isolation – courtship rituals not understood

d. Mechanical isolation – mating occurs, but structurally they don’t “fit”

e. Gametic isolation – sperm does not recognize egg or can’t survive in other species environment

Postzygotic barriers – hybrid can’t develop into viable, fertile adult

a. Reduced hybrid viability – development incomplete

b. Reduced hybrid fertility – hybrid is sterile

c. Hybrid breakdown – offspring weak or not viable

B. Mechanisms of speciation: Speciation can take place with or without geographic separation

1. Allopatric Speciation – “Allo”= other; “patra”= homeland

a. gene flow is interrupted when a population is divided into geographically isolated subpopulations.

2. Sympatric Speciation – “Same Country”

a. How can reproductive barriers between populations evolve when the members remain in contact with each other?

b. Switching habitat, food source, or other resource not used by the parent population

c. When two species interbreed and produce a hybrid that goes through asexual reproduction which can lead to mutations that make them fertile with other hybrids and not the parent.

d. Plant examples

• Polyploidy =

• Autopolyploidy =

• Allopolyploidy =

an organism that has more than 2 complete sets of chromosomes

A polyploid organism that is fertile where the sets of chromosomes come from a single species

A polyploid organsim that is fertile where the sets of chromosomes come from 2 different species

Autopolyploid

Allopolyploid

C. Reproductive barriers may evolve as populations diverge1. Fruit fly experiment in the lab:

2. Change in a single gene:

D. Hybrid Zones cause reproductive isolation

1. A Hybrid Zone is a region in which members of different species meet and mate, producing at least some

offspring of mixed ancestry.

a. Narrow band pattern there is an obstacle to gene flowobstacle is probably that hybrids have increased rates of embryonic mortality and a variety or morphological abnormalities (therefore, poor survival and reproductive rates)

b. Complicated spatial patterns

2. Several possible outcomes for hybrids a. Reinforcement = strengthening or reproductive barriers…hybrids gradually stop forming b. Fusion = weakening of reproductive barriers…the two species fuse to be one. c. Stability = continued production of hybrid individuals

E. Adaptive Radiation – the evolution of many diversely adapted species from a common ancestor.

1. Ancestor was introduced to various new environmental opportunities and challenges.

2. Natural Selection caused different traits to be selected for based on environment and challenges ancestor faced.

F. Tempo of Speciation (how fast/slow does it take?)

1. Punctuated Equilibrium model

2. Gradualism model