Species and Their Formation

Patterns of Speciation

Biological Species

• What are biological species?

– fundamental units of classification

Biological Species

• Species definitions

– several exist

• Morphological species

–species are discrete morphological units

»members and non-members are distinguishable

but

Biological Species

• Morphological species

– some problems exist

• some reproductively distinct species are not easily distinguishable

and

• some morphospecies freely interbreed

therefore

• animal species are generally defined by their reproductive patterns

Biological Species• The Biological Species Concept

– Ernst Mayr (1940)

“Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”

Biological Species• “Species are groups of actually or potentially

interbreeding natural populations which are reproductively isolated from other such groups.”groups: collections of local populationsactually or potentially: are or could be if in

close proximitynatural: not in captivity or under coercionreproductively isolated: prevented from

genetic exchange

Biological Species• A “Biological Species” is a group of

individuals that shares a gene pool.

Biological Species

• The BSC fits some organisms better than others.

– most plant species are morphologically distinguishable

– many plants freely interbreed with clearly distinct morphospecies

– many plants reproduce asexually, almost exclusively

Species Formation

• Evolutionary change occurs according to one of two patterns

1. anagenesis

• change over time in a single lineage

• may produce a new species

Species Formation

• Evolutionary change occurs according to one of two patterns

2. cladogenesis

• interruption of gene flow between two segments of a population

–the two groups evolve independently

–the two gene pools cannot exchange information

Species Formation - CladogenesisFigure 24.3

Species Formation• Evolutionary change occurs according to one

of two patterns– cladogenesis

• changes that occur may prevent interbreeding when (if) the two groups are reintroduced

Species Formation• Three modes of cladogenesis

1. allopatric speciation (a.k.a. geographic speciation)• speciation due to physical barriers

–land barrier to aquatic species–water barrier to land species–habitat barrier to fastidious species–distance barrier to mobile species

water may be a barrier

to land organsims

Figure 24.4

distance may be a barrier to mobile organismsFigure 24.5

distance may be a

barrier to mobile

organismsFigure 24.6

Species Formation

• Three modes of cladogenesis1. allopatric speciation

• a population may be divided• divided groups may experience different evolutionary agents

• once reintroduced, they may be reproductively incompatible

• if gene flow does not resume, they are distinct species

Species Formation

• Three modes of cladogenesis

2. sympatric speciation occurs without physical separation

• most often through polyploidy

–autopolyploidy

»formation of diploid gametes

»self or infraspecies fertilization

»tetraploid offspring

• new population can’t interbreed effectively with diploid parent population

Figure 24.7

Species Formation

• Three modes of cladogenesis2. sympatric speciation occurs without

physical separation• most often through polyploidy

–allopolyploidy»formation of a diploid hybrid»asexual reproduction»formation of diploid gametes»self or infraspecific fertilization

Allopolyploidydiploid &tetraploid populatiion DistributionsFigure 24.8

Species Formation

• polyploidy is common among plants

– ~70% of flowering plants, 95% ferns are polyploid

– polyploid species can spread rapidly

– polyploid species can be more successful than their diploid parents

Species Formation

• sympatric speciation among animals is less common, but not unknown

– more common is strict habitat selection and mating behavior

• sympatric picture-winged fruit flies reproduce on different fruits

Hyla versicolora tetraploid

frog

Species Formation• Three modes of cladogenesis

3. parapatric speciation occurs between two adjacent populations without a physical barrier

– e.g. plant populations differing in tolerance to heavy metal toxicity

Reproductive Isolation

• conditions or mechanisms that prevent gene flow between two populations

• geographically separated populations may still be “potentially interbreeding”

Figure 24.9

Reproductive Isolation

• conditions or mechanisms that prevent gene flow between two populations

• geographically separated populations may still be “potentially interbreeding”

• reintroduced populations may be unable to reproduce for many different reasons

– reproductive isolating mechanisms may operate before fertilization (prezygotic) or after fertilization (postzygotic)

Prezygotic Reproductive Isolation

• Spatial: two new species have come to prefer different habitats

• Temporal: two species have adopted reproductive periods that do not overlap

• Mechanical: sizes/shapes of reproductive organs have become incompatible

• Gametic: gametes cannot fuse because of chemical incompatibility

• Behavioral: the other is not seen as a mate

spatial isolation reinforced by

behavioral differences Figure 24.10

third-party behavioral isolationFigure 24.11

Postzygotic Reproductive Isolation

• Hybrid zygote abnormality: developing embryos die or produce abnormal adults

• Hybrid infertility: normal adult hybrids are sterile

• Low hybrid viability: low survival rates of hybrids

Incomplete Reproductive Isolation• separated populations may be reintroduced

before complete isolation has occurred– hybrids may be vigorous; the populations

may merge– hybrids may be weak; isolation may

continue to strengthen

Figure 24.13

Reproductive Isolation

• reproductive isolation does not require extensive differentiation

speciation without

extensive genetic variationFigure 24.14

Speciation Rates

• Factors affecting speciation rates

1. species richness

• the more species are part of a lineage

–more opportunity for polyploidy

–more opportunity to be separated by a barrier

Speciation Rates

• Factors affecting speciation rates

2. species range

• the larger the range of a species

–more likely to be fragmented by a barrier

–more likely that isolated subpopulations will experience different conditions

Speciation Rates

• Factors affecting speciation rates

3. Dispersal rates

• species that do not disperse well will be separated by relatively small barriers

Speciation Rates

• Factors affecting speciation rates

4. sexual selection

• species that discriminate among potential mates - engage in non-random mating

Speciation Rates

• Factors affecting speciation rates

5. environmental change

• environmental change may increase or decrease available habitat

Speciation Rates

• Factors affecting speciation rates

6. ecological specialization

• discontinuous populations more easily become isolated

Speciation Rates

• Factors affecting speciation rates

7. generation times

• short generation times: more generations per unit time

• more generations: more rapid response to evolutionary agents

Evolutionary Radiations

• evolutionary radiations: rapid speciation with low extinction

– following mass extinctions

– following colonization of a new habitat

• low competition rates

• low predation

• abundant resources

Evolutionary Radiations

• common on the Hawaiian Islands

– descendants of U.S. west coast tarweeds

• silverswords occupy all major habitats and exhibit growth forms not represented in the ancestral populations

H a w a i i a n

radiation

Figure 24.16

Species and Their Formation

• we know the results of macroevolution from the study of fossils and relics

– the mechanisms of macroevolution are inferred to be those of speciation extrapolated over many millions of years

Species and Their Formation• Likewise -

– multiple factors influence microevolution• assortative mating, small population size, migration, high mutation rate, directional natural selection, sexual selection

– macroevolution may depend on additional processes that are• invisible to short term studies

or• processes that are rare or one-time events.