Introductory Questions #1 (fifth overall)

1) How would you define a species? What are two key factors you must consider?

2) What is the key as to how a new species forms?

3) Explain the difference between a prezygotic barrier and a postzygotic barrier.

4) How is allopatric speciation different from sympatric speciation?

5) Name the two models that have been proposed to explain evolution observed in the fossil record.

Introductory Questions #21) Give two examples of a prezygotic and postzygotic

barrier. 2) Why is sympatric speciation more common in plants

vs. animals?3) Which model (gradualism or puntuated equilibrium)

is more reflective of the fossil record? Briefly explain why?

4) How do new body designs and evolutionary novelties form?

5) What is allometric growth and paedomorphosis? 6) When was the last mass extinction event? How

many have occurred in the last 600 million years?

Macroevolution & Speciation

Chapter 24• Define a Species• Isolation• Extinction Events• Geological Timetable• Phylogenetics

Overview of the Existence of Species

• Estimated number of 13-14 million different species• Only 1.75 million have been scientifically named• The breakdown:

-250,000 Plants

-42,000 Vertebrates

-750,000 Insects

How would you define a species?

What is a species?

• Biological species concept (Mayr): a population or group of populations whose members have the

• potential to interbreed• produce viable, fertile

offspring

(genetic exchange is possible and that is genetically isolated from other populations)

• Considered separate species Considered separate species if they if they cannot interbreedcannot interbreed (or (or are reproductively isolated)are reproductively isolated)

What is a Species?What is a Species?

How Does a new Species Emerge?

• There has to be some ISOLATION event that separates a population of individuals

• Separation has to be maintained with barriers

• Applies to sexually reproducing organisms

• Asexual reproducers: species concept is difficult to apply

-classified by structural & biochemical differences

Modes of Reproductive Isolation

Prezygotic Barriers

• Prezygotic barriers: impede mating between species or hinder the fertilization of the ova

• Habitat (snakes; water/terrestrial)

• Behavioral (fireflies; mate signaling & courtship)

• Temporal (salmon; seasonal mating)

• Mechanical (flowers; pollination anatomy)

• Gametic (frogs; egg coat receptors)

Postzygotic BarriersPostzygotic Barriers: fertilization occurs, but the

hybrid zygote does not develop into a viable, fertile adult

Reduced hybrid viability frogs; zygotes fail to develop or reach sexual maturity

Reduced hybrid fertility mule; horse x donkey; cannot backbreed

Hybrid breakdowncotton; 2nd generation hybrids are sterile

What if they breed, but don’t What if they breed, but don’t produce viable offspring?produce viable offspring? (mules)(mules)

Problem With “Species” Problem With “Species” Definition:Definition:

If they never have the If they never have the opportunity to interbreed, opportunity to interbreed, how do you know if they how do you know if they can?can?

We Can Separate Species We Can Separate Species Based On % Of Shared DnaBased On % Of Shared Dna

How Much of a difference is How Much of a difference is needed to call 2 organisms needed to call 2 organisms

separate species?separate species?

Modes of speciation (based on how gene flow is interrupted)

Allopatric: populations segregated by a geographical barrier; can result in adaptive radiation (island species)

Sympatric: reproductively isolated subpopulation in the midst of its parent population (change in genome);

-polyploidy in plants

-cichlid fishes

Punctuated Equilibrium

• Tempo of speciation: gradual vs. divergence in rapid bursts; Niles Eldredge and Stephen Jay Gould (1972); helped explain the non-gradual appearance of species in the fossil record

• The fossil of the earliest known bird, Archeaopteryx, was discovered in 1861

• Fossils of dinosaurs with feathers may support the bird-dinosaur theory

MicroevolutionMicroevolution• SmallSmall genetic changes in a genetic changes in a

populationpopulation• Change in frequency of a Change in frequency of a single single

alleleallele due to due to selectionselection

MacroevolutionMacroevolution• Large-scale changes in Large-scale changes in

organismsorganisms• Involves Involves new new generagenera

• Macroevolution consists of the major changes in the history of life– The fossil record chronicles these changes,

which have helped to devise the geologic time scale

Macroevolution

Macroevolution: the origin of new taxonomic groups

• Speciation: the origin of new species

• 1- Anagenesis (phyletic evolution): accumulation of heritable changes

• 2- Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity)

Adaptive RadiationAdaptive Radiation

• By forming new islands, volcanoes can create opportunities for organisms

– Example: Galápagos

• But volcanic activity can also destroy life– Example: Krakatau

Figure 15.4B, C

• The elimination a species from the The elimination a species from the earthearth

• Background Extinction RateBackground Extinction Rate - - relatively constant rate of relatively constant rate of extinction in the fossil recordextinction in the fossil record

• Mass ExtinctionMass Extinction - major loss of - major loss of species: climate change, humans, species: climate change, humans, catastrophiescatastrophies

ExtinctionExtinction

Figure 15.5

90 million years ago 80 70 65

Cretaceousextinctions

60

?

- These mass extinctions may have been a result of an asteroid impact or volcanic activity

– Every mass extinction reduced the diversity of life

– But each was followed by a rebound in diversity

Ex. Mammals filled the void left by the dinosaurs

Six Mass Extinction Events in the last 600 million years

(2) of the major extinctions are:

-Permian (90% of all marine species went extinct)

-Cretaceous (Killed the dinosaurs)

Mass Extinctions

Figure 15.1

• Every mass extinction reduced the diversity of life

CRITICAL QUESTION:CRITICAL QUESTION:

How Do Humans Affect How Do Humans Affect Extinction Rates?Extinction Rates?

• SimplifySimplify ecosystems ecosystems –(monocultures/disturbed (monocultures/disturbed

habitats) habitats)

• StrengthenStrengthen pest populations pest populations

• EliminateEliminate predators (can predators (can create new pests)create new pests)

How Do Humans Affect How Do Humans Affect Extinction Rates?Extinction Rates?

• IntroduceIntroduce new species new species (starlings)(starlings)

• OverharvestOverharvest

• InterferInterfer with chemical cycling with chemical cycling and energy flow (UV/ozone, and energy flow (UV/ozone, heat pollution)heat pollution)

How Do Humans Affect How Do Humans Affect Extinction Rates?Extinction Rates?

• Did birds evolve from dinosaurs?

• Evolutionary biologists investigate this question by looking at the fossil record

Are Birds Really Dinosaurs with Feathers?

• The sequence of fossils in rock strata indicates the relative ages of different species

• Radiometric dating can gauge the actual ages of fossils

The actual ages of rocks and fossils mark geologic time

• Continental drift is the slow, incessant movement of Earth’s crustal plates on the hot mantle

Continental drift has played a major role in macroevolution

Figure 15.3A

PacificPlate

NorthAmerican

Plate

NazcaPlate

SouthAmerican

Plate

AfricanPlate

EurasianPlate

Splitdeveloping

Indo-AustralianPlate

Edge of one plate being pushed over edge of neighboring plate (zones of violent geologic events)

Antarctic Plate

• This movement has influenced the distribution of organisms and greatly affected the history of life

– Continental mergers triggered extinctions

– Separation of continents caused the isolation and diversification of organisms

– Rate : 1-2 cm/yearFigure 15.3B

Mil

lio

ns

of

ye

ars

ag

o

EurasiaCE

NO

ZO

ICM

ES

OZ

OIC

PA

LE

OZ

OIC

North America

AfricaIndiaSouth

America

AntarcticaAustra

lia

Laurasia

Gondwana

Pangaea

Continental Drift/Plate Tectonics

Pangea (Paleozoic)

Laurasia Gondwana (Mesozoic)

• Europe -S. America• Greeland -Australia• N. America -Africa (Cenozoic)

**First Proposed by Alfred Wegner (1912)**Later Reproposed in the 1960’s after WWII and sonar mapping of the ocean

floor

• Continental drift explains the distribution of lungfishes

– Lungfishes evolved when Pangaea was intact

Figure 15.3C

Figure 15.3D

NORTHAMERICA

SOUTHAMERICA

EUROPE

AFRICA

ASIA

AUSTRALIA

= Living lungfishes

= Fossilized lungfishes

• Plate tectonics, the movements of Earth’s crustal plates, are also associated with volcanoes and earthquakes– California’s

San Andreas fault is a boundarybetween two crustal plates

Connection: Tectonic trauma imperils local life

Figure 15.4A

San Andreas fault

San Francisco

Santa Cruz

Los Angeles