Chapter 24 Origin of Species
Mystery of Mysteries
• Speciation - origin of new species
focal point of evolution
new species is source of biological diversity
• Macroevolution – evolutionary change beyond a single
species
• Two patterns of evolutionary change:
Anagenesis – (phyletic) accumulation of changes
that gradually transform a given species into a
species with different characteristics
Cladogenesis – (branching) splitting of the gene
pool into two or more, giving rise to new species
Biological Species Concept
• Biological Species concept – defines a species as a
population whose members have the potential to
interbreed and produces viable, fertile offspring
- Cannot produce viable, fertile offspring with
members of other populations
- emphasizes separateness of species from
one another due to reproductive barriers
• Reproductive isolation – biological factors
(barriers) that impede members of two species
from producing viable fertile hybrids
Reproductive Isolation
Reproductive Isolation
• Prezygotic barriers – impede or hinder fertilization if
mating occurs
a. Habitat isolation – two species that occupy
different habitats in the same area; rarely interact
b. Temporal isolation – species that breed during
different day, season, or year cannot mix gametes
c. Behavioral isolation – courtship rituals or other
unique mating behaviors
d. Mechanical isolation – morphological
differences prevent successful mating
e. Gametic isolation – sperm of one species may
not be able to fertilize eggs of another species
West vs. East
Behavioral isolation
Reproductive Isolation
• Postzygotic barriers – prevent a hybrid from
developing into viable, fertile adult
a. Reduced hybrid viability – genes from
different parents impair hybrids development
b. Reduced hybrid fertility – hybrid may be
healthy and vigorous but sterile.
Chromosome differences from two parents;
meiosis will fail to produce normal gametes
c. Hybrid breakdown – first generation is
viable and fertile, but next generation is
impaired and possibly sterile
Other Species Distinctions
• Morphological species concept – characterizes
by body, shape, size, and structural features
• Palentological species concept –
morphological features of discrete species
known only from fossil record
• Ecological species concept – views species in
terms of ecological niche or role in community
• Phylogenetic species concept – species as a
set of organisms with unique genetic history
Geographic Isolation
• Allopatric speciation – gene flow is interrupted when a
population is divided into geographically isolated
subpopulations
Ex: rise and fall of lake water levels
- can occur when descendents are isolated from
parent population
- barrier of separation depends on organisms
mobility
Ex: hawk or windblown pollen
compared to squirrels or mice
* true allopatric speciation would mean that
populations could not interbreed successfully
Sympatric Speciation
• Sympatric speciation – speciation takes place in
geographically overlapping populations
- chromosomal changes and nonrandom
mating reduces gene flow
• Extra sets of chromosomes or polyploidy gives rise to
an autopolyploid- more than two sets of chromosomes
from a single species
- likely result of failures in cell division
- offspring are normally sterile, unless that can mate with
other autopolyploids (same # - tetraploids)
- Allopolyploid – a sterile hybrid that asexually propagates
and becomes a fertile polyploid- new species
Ex: oats, wheat, cotton, potatoes, and tobacco all
are polyploids
Sympatric Speciation
• Polyploid speciation can happen in animals
depending on habitat conditions and sexual
selection
Ex: Apple maggot fly whose habitat
changed from hawthorne to apple trees
*Also, cichlid fish in Lake Victoria of
eastern Africa, nonrandom mating or
selecting on coloration
Adaptive Radiation
• Adaptive radiation – evolution of many
diversely adapted species from a common
ancestor after being introduced into new
environmental opportunities and challenges
- organisms make way to new, distant habitats,
or when extinctions occur opens niches for
survivors
Ex: inhabitats of Hawaiian islands
Punctuated Equilibrium
• Punctuated equilibrium – episodes in fossil record
where a new species suddenly appeared,
persisted unchanged, and then disappear
- Time period of the fossil record skews the
perception of changes that occurred making
them unnoticeable
- External anatomy may not give best
interpretation of adaptive changes that
occurred in species
Accumulation of Speciation Events
• Speciation can result from seemingly small events, but
through continued divergence, differences accumulate
- forms basis of macroevolutionary change
- processes that facilitate the change are natural
selection, mutation, genetic drift, and gene flow
- accumulation of small speciation episodes
accounts for sweeping evolutionary changes
Ex: previous and present function of the eye
- exaptations – structures that evolve in one context
but become co-opted for another function
Ex: honeycombed bones and feathers in birds
Genes that Control Development
• Heterochrony –
evolutionary change in the
rate and timing of
developmental events
• Allometric growth –
proportioning that helps
give a body its specific form
*changing rates slightly
can substantially change
adult form
Ex: salamander feet
Controlling Development
• Timing of Reproductive development of
somatic organs can also be altered by
heterochrony
• Paedomorphosis – reproductive development
accelerates compared to somatic
development making the body retain juvenile
body parts during sexual maturity
*contrasts ancestral species in which
juvenile appearance is actually adult stage
Changes in Spatial Pattern
• Homeotic genes – control
the placement and spatial
organization of body parts
Ex: Hox genes that
determine positional
information in animal
embryos
- changes or mutations
can dramatically impact
morphology
Species Selection
• Branching evolution can result in an evolutionary
trend even if some new species counter the trend
• Species selection states that species that endure
the longest and generate the most offspring
determine the direction of the major trends
• Appearances of trends does not imply some
intrinsic drive toward a particular phenotype
• Evolution is the result of interactions between
organisms and their environments
• If environmental conditions change, those trends
may stop or be reversed
Hybrid Zones
Hybrid zone – region in which members of
different species meet, mate, and produce
offspring of mixed ancestry
Over time 3 outcomes possible:
1. Reinforcement of reproductive barriers
2. Fusion or weakening of barriers to a
single species
3. Stability or continued production of
hybrids even when selected against