Speciation and Extinction
Microevolution vs Macroevolution
• Microevolution consists of changes that
evolve within a population; we are talking
about the changes in 1 gene pool/allele
frequencies; think Hardy-Weinberg
• Macroevolution refers to evolutionary
changes above the species level; think new
species evolving as a result of so many
microevolution changes
Speciation
• A parent organisms evolving into one or more
new species
– Can be slow and gradual or can occur in bursts
followed by relatively quiet periods
– Called adaptive radiation in the extreme form
(many new species arising around the same time)
Four different ways to define a species
• Biological Species Concept – states that a species is a
group of organisms who are able to interbreed in
nature and produce fertile offspring; they do not breed
successfully with other species.
– This concept cannot be applied to fossils or asexual
organisms and is therefore a limited definition
• The following concepts focus on the unity within a
species rather than the separateness; can be applied to
sexual and asexual organisms
– Morphological species concept
– Ecological Species Concept
– Phylogenetic Species Concept
Reproductive Isolation
• For the purpose of this class we will accept the
biological definition of a species
• Takes place as a result of barriers that cause
species to become unique
• Reproduction Isolation: barriers that prevent
two species from producing fertile offspring
Pre-Zygotic Barriers (Before
Fertilization)
• Does not allow fertilization to happen:
– Temporal: separate by time
– Habitat: different habitats (land vs. water)
– Behavior: different courtship rituals
– Mechanical: structures do not allow for
fertilization to occur
– Gametes: sperm of one species may not be able to
fertilize egg of another species
Post-Zygotic Barriers (After
Fertilization)
• Sometimes the species are similar enough for
sperm to fertilize egg; however, the offspring
either dies or is unable to produce fertile
offspring (genes are not passed to future
generations)
• Hybrids: the offspring organisms of two
different species; hybrids either die or are
unable to reproduce (can’t pass along genes
to future generations)
Speciation can occur in two ways:
• Allopatric speciation: geographic barrier
separates the population (river, mountain range,
great wall of China)
– Gene flow is interrupted
– Separated populations evolve independently through
mutations, natural selection, and genetic drift
• Sympatric speciation: no geographic barrier
– Species overlap and could share a gene pool
– Polyploidy, appearance of new ecological niches, and
sexual selection can all drive sympatric speciation
Polyploidy
• Presence of extra sets of chromosomes due to
accidents during cell division
– Autopolyploid – more than two chromosome sets
derived from one species
– Allopolyploid – multiple chromosome sets derived
from different species
• Common in plants (oats, cotton, potatoes,
tobacco, and wheat)
• Not common in animals
Sympatric Speciation via Polyploidy is Common in Plants
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
4n
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Sympatric Speciation - Polyploidy --> Allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Sexual Selection
• Natural selection driven by mate selection
• Can result in sexual dimorphism (different looking
males vs females
• Two types:
– Intrasexual: selection within the same sex (males
preventing other males from mating)
– Intersexual (aka mate choice): female choose male based
on behavior and looks
• Many of these traits can be disadvantageous; however,
because the sexual selective pressure is so high it wins
out over environmental selection
– Peacock tail: Male showiness due to mate choice can
increase a male’s chances of attracting a female, while
decreasing his chances of survival
Sexual Selection
Extinction
• Mass extinction: dramatic increase in the rate of
extinction around the same time period
• Evidence shows the Earth has experienced five
mass extinction events – more than 50% of
Earth’s species became extinct
• Causes for mass extinction: continental drift,
volcanoes, comet, carbon dioxide level changes,
gamma rays, sea level changes, climate change,
methane hydrate, ocean anoxia, ocean
circulations
Five Big Mass Extinctions
800
20
600
15
500
400
10
Era
Period
300
5
200
100
0
E
542
O
Paleozoic
S
D
488 444 416
359
C
Tr
P
299
251
Mesozoic
C
J
200
Time (millions of years ago)
145
0
Cenozoic
P
65.5
N
0
Number of families:
Total extinction rate
(families per million years):
700
• The break-up of Pangaea lead to allopatric speciation.
• The current distribution of fossils reflects the movement of
continental drift. Similarity of fossils in parts of South
America and Africa supports the idea that these continents
were formerly attached.
• The fossil record shows that most species that have ever
lived are now extinct.
• The presence of iridium in sedimentary rocks suggests
a meteorite impact about 65 million years ago.
• The Chicxulub crater off the coast of Mexico is
evidence of a meteorite that dates to the same time.
Is a Sixth Mass Extinction Under
Way? Consequences …
• Scientists estimate that the current rate of extinction is 100
to 1,000 times the typical background rate.
• Data suggest that a sixth human-caused mass extinction is
likely to occur unless dramatic action is taken.
• Mass extinction can alter ecological communities and the
niches available to organisms.
• It can take from 5 to 100 million years for diversity to
recover following a mass extinction.
• Mass extinction can pave the way for adaptive radiations.
Adaptive Radiations - New
Environmental Opportunities …
• Adaptive radiation is the evolution of diversely adapted
species from a common ancestor upon introduction to new
environmental opportunities.
• Mammals underwent an adaptive radiation after the
extinction of terrestrial dinosaurs.
• The disappearance of dinosaurs (except birds) allowed for
the expansion of mammals in diversity and size.
• Other notable radiations include photosynthetic
prokaryotes, large predators in the Cambrian, land plants,
insects, and tetrapods.
Regional Adaptive Radiations
• Adaptive radiations can occur when organisms
colonize new environments with little competition.
• The Hawaiian Islands are one of the world’s great
showcases of adaptive radiation.
The Time Course of Speciation
• Broad patterns in speciation can be studied using the fossil
record, morphological data, or molecular data.
• The fossil record includes examples of species that appear
suddenly, persist essentially unchanged for some time, and
then apparently disappear
• Niles Eldredge and Stephen Jay Gould coined the term
punctuated equilibrium to describe periods of apparent
stasis (no change) punctuated by brief periods of rapid
change.
• The punctuated equilibrium model contrasts with a
Darwinian model of gradualism: slow continuous change
over time in a species’ existence.
Patterns in Speciation
Punctuated Equilibrium
pattern
Change / Time
Gradualism pattern
You should now be able to:
1. Define and discuss the limitations of the four species
concepts.
2. Describe and provide examples of prezygotic and
postzygotic reproductive barriers.
3. Distinguish between and provide examples of
allopatric and sympatric speciation.
4. Explain how polyploidy can cause reproductive
isolation.
5. Distinguish among the following sets of terms:
intrasexual selection, intersexual selection, and sexual
dimorphism.
4. Briefly describe the Cambrian explosion.
5. Describe the mass extinctions that ended the Permian
and Cretaceous periods.