Chapter 24

CHAPTER 24 THE ORIGIN OF SPECIES

Lecture Outline
• What is a species?
• The biological species concept
• Prezygotic and postzygotic barriers & isolation of gene pools
• Modes of Speciation
• Allopatric and Sympatric Speciation
• Punctuated Equilibrium
• Macroevolution
• Evolutionary Novelties

Origin of new species
• unique species of Galapagos

• Can evolution explain macroevolution?
= origin of new taxonomic groups (new species, new genera, new families, new kingdoms)

• Speciation is the keystone process that creates diversity of higher taxa.

• Anagenesis:
phyletic evolution
transformation

• Cladogenesis: .
branching evolution .
=

What is a species?
• Species:
Latin for “kind” or “appearance”

• Linnaeus:
morphological differences, taxonomy

• Modern definitions:
differences in body function, biochemistry, behavior, genetic makeup

The biological species concept
• defined by Ernst Mayr 1942
• Species:
population or group of populations whose members have the potential to interbreed and produce fertile offspring
• Biological species:
largest set of populations in which genetic exchange is possible; genetically isolated from other populations
=

Reproductive isolation of species
• prevents populations belonging to different species from interbreeding, even if their ranges overlap

• Reproductive barriers: prezygotic or postzygotic
• isolate the gene pools of biological species
• multiple barriers common in many species
• intrinsic to organisms

Prezygotic barriers
• impede mating between species or inhibit fertilization
• habitat isolation: organisms that use different habitats in same geographic area
• e.g. garter snakes aquatic/terrestrial
• behavioral isolation: unique behaviors to attract mates
• e.g. fire flies -- rhythmic blinking
• e.g. courtship behavior in birds

Prezygotic barriers
• temporal isolation: two species breeding during different times of day, different seasons, different years
• e.g. Eastern/Western spotted skunk: late winter/late summer
• mechanical isolation: anatomically incompatible, transfer of sperm not possible
• e.g. copulatory organs of insects
• gametic isolation: gametes of two species do not form a zygote because of incompatibilities preventing fusion
• e.g. different chemical signals

Postzygotic barriers
-- postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult
• Reduced hybrid viability: genetic incompatibility between two species may abort hybrid development
• e.g. between different Rana species (frogs)
• Reduced hybrid fertility: hybrids may be infertile; hybrid cannot backbreed with either parental species
• e.g. mule = horse/donkey hybrid
• Hybrid breakdown: first generation hybrids are viable and fertile, but next generation is feeble or sterile
• e.g. some agricultural plants, like cotton

• Reproductive barriers
can occur before
mating, between
mating and
fertilization, or
after fertilization.

The biological species concept has limitations
• classification of fossils limited to morphology
• lack of information on interbreeding
• asexual reproduction
• e.g. bacteria

Alternative species concepts
• morphological species concept: defines a species by a unique set of anatomical features (most)
• genealogical species concept: defines a species as a set of organisms with a unique genetic makeup
• ecological species concept: defines a species in terms of its ecological niche
• pluralistic species concept: uses reproductive isolation or adaptation to an ecological niche, or both

CHAPTER 24 THE ORIGIN OF SPECIES
Lecture Outline
• What is a species?
• The biological species concept
• Prezygotic and postzygotic barriers & isolation of gene pools
• Modes of Speciation
• Allopatric and Sympatric Speciation
• Punctuated Equilibrium
• Macroevolution
• Evolutionary Novelties

Allopatric and sympatric speciation
• allopatric speciation:
geographic separation
of populations restricts
gene flow

• sympatric speciation: biological factors reduce gene flow in overlapping populations

Allopatric speciation
• Geographic barriers:
- mountains, glaciers, land bridges, canyons …
-- divide one population in isolated groups

• Colonization of new habitats:
- islands (Galapagos, Hawai, …)
-- isolation from mainland/parent population

What favors allopatric speciation?
• small and isolated population
- genetic drift & natural selection.

-- rare event
-- most small, isolated populations perish

Two species or just one?
-- are populations different enough so that they can no longer interbreed and produce fertile offspring?

• Ring species:
gradual divergence of new species from common ancestors

Adaptive radiation on islands Fig. 24.11

Figure 24.10 Long-distance dispersal of seeds

• Galapagos:
• small population from mainland -- many unique species

• Hawaii:
• fauna from older islands colonized newer islands -- many unique species

Sympatric speciation
• new species arise within the range of the parent populations:
=
common mechanisms:
-- in plants: increase in chromosome number
• autopolyploidy
• allopolyploidy
-- in animals:
• shift in habitat preference
• shift in mate preference

• autopolyploid plants:
• failure in meiosis results in a tetraploid (4n) individual
• can only reproduce with itself or with other tetraploids, but not with diploids from the original population

• allopolyploid plants:
• polyploid individuals are produced by mating between two different species (cross-pollination)
• hybrids usually sterile
• certain mechanisms can transform a sterile hybrid into a fertile polyploid:
=

• many agricultural are polyploid:
• polyploid oats, cotton, corn, potatoes, tobacco, wheat …
• artificial hybrids bred for high yield & resistance to disease

Sympatric speciation in animals
• genetic factors cause individuals to be fixed on resources not used by the parent
• genetic switches from one breeding habitat to another
• different mate preferences.

Models for the tempo of speciation

• punctuated equilibrium model:
• tempo of speciation is not constant
• most modifications occur
when new species first bud from their parent population
• separate species then remain
more or less static

From Microevolution to Macroevolution
• Speciation is at the boundary between microevolution and macroevolution.
• Microevolution: change over the generations in a population’s allele frequencies
(genetic drift, natural selection)
• Speciation: population diverges genetically from its ancestral population -- reproductive isolation
• Macroevolution: change over millions of speciation episodes
(fossil record)

Evolutionary novelties
• morphological transformations occur gradually
• ‘evolutionary novelties’ are modified versions of older structures
• e.g. mollusk or vertebrate eyes
• exaptations: structures that evolve in one context, but become useful for another function
• e.g. lightweight bones of birds

Fig. 24.18 Range of eye complexity in mollusks

Evolutionary development
“Evo-devo”
• interdisciplinary research
• examines how slight genetic differences can create major morphological differences

-- focuses on genes that control the rate and timing of development, and spatial patterning of an organism

• genes that control rate of development:
• Allometric growth = change in proportions due to different growth rates

• heterochrony
• modification of allometric growth leads to change in morphology
• caused by evolutionary change in the rate or timing of developmental events
• e.g. salamander feet

• Paedomorphosis:
= form of heterochrony affecting the relative timing of reproductive development and somatic development
• reproductive faster than somatic -- sexually mature juveniles
• e.g. axolotl salamander

Evolutionary trends
• fossil record seems to reveal trends in the evolution of many species and lineages
• e.g. evolution of the modern horse
• selected fossil horses -- trends toward increased size, reduced number of toes, teeth for grazing
• all fossil horses -- no such trend, variety of grazers and browsers

• selected fossil horses -- trends toward increased size, reduced number of toes, teeth for grazing
• all fossil horses -- no such trend, variety of grazers and browsers

Origin of Species: The Big Picture
-- Evolution is a response to interactions between organisms and current environment
-- Changes are dependent on stable or new environmental conditions
-- An evolutionary trend may cease or reverse

Key terms 24
macroevolution
speciation
species
biological species concept
prezygotic barrier
postzygotic barrier
morphological species concept
ecological species concept
genealogical species concept
pluralistic species concept
allopatric speciation
sympatric speciation
polyploidy
autopolyploid
allopolyploid
punctuated equilibrium
exaptations
allometric growth
heterochrony
paedomorphosis
species selection