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Chapter 23: Species and their Formation (Part I & II)

Biological Sciences
Course Code
Kamini Persaud

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Chapter 23: Species and their Formation
Mar 10/2011
What are Species?
Speciation: the process by which one species splits into two or more daughter
Occurs gradually, and through a time line, you can tell where the distinction
Species = kinds
o1. Morphological Species Concept:
Recognize species by their appearance (Darwin did this)
Share alleles
Binomial system of nomenclature (genus names ex: homosapiens)
Did not recognize offspring differences and male/female differences
o2. Biological Species Concept:
Species are groups of actually and potentially interbreeding natural
populations which are reproductively isolated from other such
Individuals in a population can mate with each other but not
with those of another population
They are distinct evolutionary unit within which genes
How do new species arise?
Species sometimes change by adapting to the changes in their environment
If a species becomes isolated into 2 populations, within each isolated gene pool,
allele and genotype frequencies change and result of evolutionary mechanisms
They no longer can interbreed once the barrier is removed
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oAllopatric speciation: physical barrier between species
Dominant mode of speciation
Allopatric species can occur through vicariant event and dispersal
(recall chapter 22)finches that Darwin evaluated dispersed and did
not interbreed
oSympatric Speciation: does not require physical isolation
a.Ecological Isolation: different resources or different niches.
Through disruptive selection, exploiting a new niche 
reduces gene flow
Reproduction is isolated because the species are ecologically
Duplication of the whole set of chromosomes
Most arise through hybridization by self-fertilization
(flowering plants and ferns)
Auto-polyploidy: chromosome duplication in a single
Occurs if a cell accidentally duplicated the chromosomes,
resulting in a tetraploid individual.
i.Do not synapse properly with diploids because it has
odd numbers of chromosomes
Allo-polyploidy: combining of chromosomes from two
species result of two related species that mate
What happens when newly formed species come together?
Reproductive isolation evolves as an incidental by-product of genetic changes in
allopatric populations
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