BiodiversityStudied in a relatively new field called Conservation Biology

Consists of three components:1. Species2. Genes3. Ecosystems

The Biological Species Concept

• The biological species concept defines a species as

– A population or group of populations whose members can interbreed and produce fertile offspring

Other Species Concepts

• The _____concept

– Classifies organisms based on observable phenotypic traits

• The _____concept

– Defines a species by its ecological role

• The phylogenetic species concept

– Defines a species as a set of organisms representing a specific evolutionary lineage

Speciation requires:

1. Genetic variation

2. Natural Selection

3. Reproductive Barriers

Mutation, sexual recombination, gene flow, and

genetic drift can generate variation needed for

speciation

• Mutations, or changes in the nucleotide sequence of

DNA

–Can create new alleles

• Sexual recombination

– Generates variation by shuffling alleles during meiosis (independent assortment), and cross overs

A1 A2A1 A3

A1

A1

A2 A3

A2A1 A3

and

XParents

Meiosis

Gametes

Fertilization

Offspring,with newcombinationsof allelesFigure 13.12

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• Gene flow

–Is the movement of individuals or gametes

between populations. Occurs in plants by ____

and _____

–Can alter allele frequencies in a population

•Genetic drift

–Is a change in the gene pool of a population

due to chance

–Can alter allele frequencies in a population

• Natural selection

– Leads to differential reproductive success in a

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

• Three possible effects of natural selection

Originalpopulation

Stabilizing selection

Originalpopulation

Evolvedpopulation

Fre

qu

en

cy o

f in

divi

du

als

Phenotypes (fur color)

Directional selection Disruptive selection

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Reproductive barriers keep species separate

• Reproductive barriers

– Serve to isolate a species’ gene pool and prevent interbreeding

– Are categorized as prezygotic or postzygotic

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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

• In behavioral isolation

– There is little or no sexual attraction between species, due to specific behaviors

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• In mechanical isolation

– Female and male sex organs or gametes are not compatible

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Morphological differences in flowers allow them tobe specifically adapted to certain pollinators.

Pollen is transferred only between plants of the same specieswith the same floral anatomy.

Length of the stigma and style and growth of the pollen tubeIs often important in mechanical isolation of plants.

In mechanical isolation of plants:

• 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

black oak in dry upland habitats

• In gametic isolation

– Two species can chemically distinguish between

pollen of the same species and that of different

species. Germination of pollen from foreign

species is retarded by chemical components in

the stigma and style

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

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• 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

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In allopatric speciation a population is geographically divided.

New species can also arise within the same geographic area as the parent species

• 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

• Many plant species have evolved sympatrically by polyploidy (DE VRIES and BOEDIJN 1923)

– Multiplication of the chromosome number due to errors in cell division

Parent species

Meioticerror

Self-fertilization

Offspring may be viable and self-fertile

Zygote

Unreduced diploid gametes

2n = 6Diploid

4n = 12Tetraploid

O. gigas

Oenothera lamarckiana

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AA BB

AB

AA BB DD

ABD

AA BB DD

Triticum monococcum(14 chromosomes)

Wild Triticum(14 chromosomes)

Sterile hybrid(14 chromosomes)

Meiotic error andself-fertilization

T.turgidumEmmer wheat(28 chromosomes)

T.tauschii(wild)(14 chromosomes)

Sterile hybrid(21 chromosomes)

Meiotic error andself-fertilization

T.aestivumBread wheat(42 chromosomes)

Polyploid plants clothe and feed us

• Many plants, including food plants such as bread wheat

– Are the result of hybridization and polyploidy

Polyploidy is a common mechanism for sympatric speciation:

Hybrid polyploids (allopolyploids) have characteristics of both parents. They have more genes and enzymes to deal with changes in the environment and thus are often better adapted than the parent species. Since their chromosome numbers are doubled chromosomes can no longer pair with parent chromosomes during meiosis and thus are reproductively isolated from their parents.

Many plant species have been shown to undergo this typeof rapid sympatric speciation. Invertebrates and lower chordates also often speciate in this manner.

Adaptive radiation can also be rapid and may occur in new or newly vacated habitats

• In adaptive radiation, the new species

– Occur when mass extinctions or colonization provide organisms with new environments

– Ex. Over the course of millions of years, the descendants of the pioneer plant in the Hawaiian silversword `ohana (family) evolved into 28 distinct species in three genera, occupying many different habitats. 

Flowering rosette of the extremely rare Mauna Kea silversword (Argyroxiphium sandwicense subsp. sandwicense) Copyright © The Botanical Society of America