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 celldivision afterchromosomeduplication givesrise to tetraploidtissue.

2n

Gametesproducedare diploid..

4n

Offspring withtetraploidkaryotypes maybe viable andfertile.

Sympatric Speciation - Polyploidy --> Allopolyploid

Species A2n = 6

Normalgameten = 3

Meioticerror

Species B2n = 4

Unreducedgametewith 4chromosomes

Hybridwith 7chromosomes

Unreducedgametewith 7chromosomes

Normalgameten = 3

Viable fertilehybrid(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 ExtinctionsTo

tal e

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tion

rate

(fam

ilies

per

mill

ion

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Time (millions of years ago)

Num

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f fam

ilies

:

CenozoicMesozoicPaleozoicE O S D C P Tr J

542

0

488 444 416 359 299 251 200 145

EraPeriod

5

C P N

65.5

0

0

200

100

300

400

500

600

700

800

15

10

20

• 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 Equilibriumpattern

Gradualism pattern

Change / Time

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.