9. Biodiversity& Species 生物多样性

• Reproductive barriers
– Serve to isolate a species’ gene pool and prevent interbreeding
– Are categorized as prezygotic or postzygotic
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• The phylogenetic species concept
– Defines a species as a set of organisms representing a specific evolutionary lineage
Speciation requires:
1. Genetic variation
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In allopatric speciation a population is geographically divided.
• Geographic isolation on east and west slopes of the Rockies, eastern and western Great lakes, and on the eastern and western slopes of the Appalachians has resulted in new species of organisms. • Isolated ponds in deserts has led to new species of pupfish and plants
• In sympatric speciation
– New species may arise without geographic isolation – Microhabitat isolation as in red (moist) and black (dry) oaks – Gametic isolation where species can chemically distinguish their own pollen and destroy that of other species
• In microhabitat differences
– Two species occupy different microhabitats Ex. Quercus coccinea (scarlet oak) and Q. velutina (black oak) of eastern USA are isolated in different microhabitats. Scarlet oak grows in wet habitats adjacent to
In mechanical isolation of plants:
Morphological differences in flowers allow them to be specifically adapted to certain pollinators.
Pollen is transferred only between plants of the same species with the same floral anatomy. Length of the stigma and style and growth of the pollen tube Is often important in mechanical isolation of plants.
• Gene flow
–Is the movement of individuals or gametes
between populations. Occurs in plants by ____
and _____
–Can alter allele frequencies in a population
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New species can also arise within the same geographic area as the parent species
• In mechanical isolation – Female and male sex organs or gametes are not compatible
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Three possible effects of natural selection
Frequency of individuals
Original population
Original population
Evolved population
Phenotypes (fur color)
Stabilizing selection
• Many plant species have evolved sympatrically by polyploidy
Biodiversity
Studied in a relatively new field called Conservation Biology
Consists of three components: 1. Species 2. Genes 3. Ecosystems
The Biological Species Concept
Parents A1 A1
X
A2
A3
Meiosis
Gametes
A1
A2
A3
Fertilization
Figure 13.12
Offspring, with new combinations of alleles
A1
A2 and
A1
A3
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• In behavioral isolation – There is little or no sexual attraction between species, due to specific behaviors
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Other Species Concepts
• The _____concept – Classifies organisms based on observable phenotypic traits
• The _____concept
– Defines a species by its ecological role
2. Natural Selection
3. Reproductive Barriers
Mutation, sexual recombination, gene flow, and genetic drift can generate variation needed for speciation
Prezygotic Barriers
• Prezygotic barriers – Prevent mating or fertilization between species
Seasonal Flowering Differences
-In temporal isolation
- Three orchid species of the genus Dendrobium live in the same tropical rain forest and all flower after thunderstorms. One flowers eight days after the storm, a second species nine days after, and a third species ten days after the storm
Postzygotic Barriers
• Postzygotic barriers – Operate after hybrid zygotes are formed
• One postzygotic barrier is hybrid sterility – Where hybrid offspring between two species are sterile and therefore cannot mate
population
– Can alter allele frequencies in a population
Natural selection can alter variation in a population in three ways
• Stabilizing selection
– Favors intermediate phenotypes • Directional selection – Acts on individuals at one of the phenotypic extremes • Disruptive selection – Favors individuals at extremes of the phenotypic range