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 populations actually or potentially : are or could be if in close proximity natural : not in captivity or under coercion reproductively 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 - Cladogenesis
Figure 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 organisms
Figure 24.5

distance may be a barrier to mobile organisms
Figure
24.6

Species Formation
• Three modes of cladogenesis
1. 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
– auto polyploidy
»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 cladogenesis
2.
sympatric speciation occurs without physical separation
• most often through polyploidy
– allo polyploidy
»formation of a diploid hybrid
»asexual reproduction
»formation of diploid gametes
»self or infraspecific fertilization

diploid & tetraploid populatiion
Distributions
Figure 24.8
Allopolyploidy

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 versicolor a 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

Figure 24.10
spatial isolation reinforced by behavioral differences

third-party behavioral isolation
Figure 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 variation
Figure
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 i o a t n d i r a
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 micro evolution
• 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.