Chapter 24
The Origin of Species
Macroevolution
• the origin of new taxonomic groups
Speciation: the origin of new species
• Anagenesis: accumulation of changes within a
population, transforming that population into
a new species
• Cladogenesis: branching evolution, whereby a
new species arises from a parent species
Biological Species Concept
• Defines a species as a population or group of
populations that have the potential to breed
with each other in the wild and produce
viable, fertile offspring
What are some of the limitations
of the biological species concept?
Ecological Species Concept
• Defines a species in terms of its ecological
niche
Morphological Species Concept
• Characterizes each species in terms of its
unique set of structural features
Genealogical Species Concept
• Defines species in terms of its unique genetic
history
Pluralistic Species Concept
• The factors that are most important for the
cohesion of a species vary
Reproductive Isolation
• Factors that prevent interbreeding
Note: These are factors that intrinsic to the
species itself and does not include geographic
isolation
Reproductive barriers can either be
prezygotic or postzygotic
• Zygote: fertilized egg
Prezygotic barriers
• Impede mating between species or hinder
fertilization of ova if two members attempt to
mate
Prezygotic Barriers
• Habitat isolation – if two species live within
the same area but different habitats
• Behavioral Isolation – often depends upon
courtship rituals
Prezygotic Barriers, cont’d
• Temporal Isolation: if two species mate during
different times of the day, different seasons,
or different years
• Mechanical isolation: two species are not
anatomically compatible
• Gametic isolation: two gametes meet but fail
to fertilize
Postzygotic Barriers
• Reduced hybrid viability: hybrids are not very
healthy, don’t live to maturity
• Reduced hybrid fertility: hybrids are sterile
• Hybrid breakdown: first-generation offspring
are fertile and viable but when these offspring
mate with either each other or parent species,
offspring are feeble or sterile
Modes of Speciation
• Based on how gene flow is interrupted
Allopatric Speciation
• Speciation takes place in populations with two
geographically separate ranges
Sympatric Speciation
• Speciation takes place in geographically
overlapping areas
• How might these barriers arise?
Punctuated Equilibrium Model
• Argues that species diverge in spurts of
relatively rapid change instead of slowly and
gradually
Exaptations
• Structures that evolve in one context but
becomes co-opted for another function
– Ex: feathers in birds
Evo-devo
• The link between evolutionary biology and the
study of how organisms develop is called “evodevo”
• Allows us to understand how small changes in
the genome can lead to dramatic changes in
an organism
Toolkit Genes
• Subset of genes used to pattern the body
• Proteins encoded by these genes control the
formation, design, and patterning of most
major features of animal design and diversity
Homeotic Genes
• Control placement and spatial organization of
body parts
Hox genes
Ultrabithorax mutation
Eyeless
• First discovered in Drosophila. Humans have a
homologous protein called Pax6.
• Homeotic gene. Responsible for turning on
other genes (transcription factor)
• Loss of eyeless leads to a loss of eyes. (Flies
without eyes)
If you turn on eyeless in another
part of the fly -
Ectopic eyes!
Halder et al, 1995
Paedomorphosis
• When the adult species retains structures that
were juvenile structures in an ancestral
species
Ex: axolotl
Heterochrony
• Evolutionary change in the rate or timing of
developmental events
Allometric Growth
• The relative growth rates of different parts
during development
Allometric Growth Example – human
and chimpanzee skulls