1. Speciation and Extinction

2. 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

3. 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)

4. 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

5. 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
Reduced hybrid viability -- weak offspring
Reduced hybrid fertility -- sterile offspring
Hybrid breakdown.

6. 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

7. 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)

8. 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
Sympatric speciation can also result from the appearance of new ecological niches.
For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees.
Sexual selection can drive sympatric speciation.
Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria.

9. 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
An autopolyploid is an individual with more than two chromosome sets, derived from one species.
An allopolyploid is a species with multiple sets of chromosomes derived from different species.
Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.

10. Sympatric Speciation via Polyploidy is Common in Plants
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Fig Sympatric speciation by autopolyploidy in plants

11. Sympatric Speciation - Polyploidy --> Allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Figure One mechanism for allopolyploid speciation in plants
Normal
gamete
n = 3
Species A
2n = 6

12. 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
How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for.

13. Sexual Selection
Figure Sexual dimorphism and sexual selection

14. 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

15. (families per million years):
Five Big Mass Extinctions 20
800
700 15
600
500
Number of families:
(families per million years):
Total extinction rate 10
400
300 5
200
100
Figure Mass extinction and the diversity of life
Era
Period
Paleozoic
Mesozoic
Cenozoic E O S D C P Tr J C P N
542
488
444
416
359
299
251
200
145
65.5
Time (millions of years ago)

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. Patterns in Speciation
Punctuated Equilibrium
pattern
Change / Time
Gradualism pattern
Figure Two models for the tempo of speciation

22. You should now be able to:
Define and discuss the limitations of the four species concepts.
Describe and provide examples of prezygotic and postzygotic reproductive barriers.
Distinguish between and provide examples of allopatric and sympatric speciation.
Explain how polyploidy can cause reproductive isolation.
Distinguish among the following sets of terms: intrasexual selection, intersexual selection, and sexual dimorphism.
Briefly describe the Cambrian explosion.
Describe the mass extinctions that ended the Permian and Cretaceous periods.