Tuesday, February 24, 2009

Chapter 24 Notes!

Unit 8: Mechanisms of Evolution
Chapter 24 The Origin of Species

What is a Species?
Macroevolution refers to the origin of new taxonomic groups.

Speciation is the process by which a new species arise.

There are many barriers that prevent members of different species from interbreeding these barriers can be broken down into two types – prezygotic barriers (those that prevent mating between species or hinder fertilization) and postzygotic barriers (those that prevent a fertilized egg from developing into a fertile adult.)

Examples of prezygotic barriers include:
1. Habitat isolation: – two species can live in the same geographic area, but not in the same habitat; this will prevent them form mating.
2. Behavioral Isolation: Some species use certain signals or types of behavior to attract mates, and these signals are unique to their species. Members of other species will not recognize them, which prevents them from mating
3. Temporal Isolation: Species may breed at different times of the day, different seasons, or different years, and this can prevent them from mating.
4. Mechanical Isolation: Species may be anatomically incompatible
5. Genetic Isolation: Even if the gamete of two species do meet, they might be unable to fuse to form a zygote.

Example of postzygotic barriers include:
1. Reduced Hybrid Vitality: When a zygote is formed, the fact that two species are genetically incompatible may cause development to cease.
2. Reduced Hybrid Fertility: Even if the two different species produce a viable offspring. Reproductive isolation is still occurring if the offspring are sterile and can’t reproduce.
3. Hybrid Breakdown: If the two different species produce offspring that are viable and fertile, these hybrids may mate to produce weak or sterile offspring.
Modes of Speciation
There are two main types of speciation-allopatric speciation, in which a population forms a new species because it is geographically isolated form the parent population, and sympatric speciation, in which a small part if a population becomes a new population without being geographically separated from the parent population.

Same geologic events or processes that can fragment a population include the emergence of a mountain range, the formation of a land bridge, or the evaporation of a large lake to produce several small lakes.

Small, newly isolated populations undergo allopatric speciation more frequently because they are more likely to have their gene pools significantly altered. Allopatric speciation has occurred when an individual from the new population us unable to mate successfully with an individual from the parent population.

One mechanism that can lead to the formation of a small new population within the parent population (sympatric speciation) in plant is the formation of autopolyploid plants through nondisjunction in meiosis. These plants have 4n chromosomes, instead of the normal 2n number, and they are unable to breed with members of the parent population—though they are still able to breed with other tetraploids.

Polyploid speciation occurs in animals, but it is not common. Instead, in animals, sympatric speciation can result from part of the population switching to a new habitat, food source, or other resource.

The punctutated equilibrium model states that species diverge in relatively quick spurts rather than slowly and gradually.

Adaptive radiation occurs when many new species arise from a single common ancestor.
Extra Notes:
While the biological species concept emphasizes the separateness of species from one another due to reproductive barriers, several other definitions emphasizes the unity within a species.
  • Morphological Species Concept: characterizes a species by body shape and other structural features
  • Ecological Species Concept: views a species in terms of it ecological niche, the sum of how members of the species interact with the nonliving and living parts of their environment.
  • Phylogenetic Species Concept: defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life

Allopolypoid: When two different species interbreed and produce hybrid offspring.

1. The largest unit within which gene flow can readily occur is
a. population
b. species
c. genus

2. Which of the following factors would not contribute to allopatric speciation?
a. Gene flow between the two populations is extensive.
b. The isolation populated is exposed to different selection pressures than the ancestral population.
c. A population becomes geographically isolated from the parents population.

3. Does polyploidy result in
a. a complete set of chromosomes
b. more than two complete sets of chromosomes
c. only two complete sets of chromosomes

Anwers: 1) B 2) A 3) B

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