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BI 102 Lecture 15:Macroevolution

Galapagos Islands

• Darwin visited the Galapagos Islands on the voyage of the Beagle• Recognized that many species exist there that exist nowhere else

Galapagos Islands

• These species are distinct from other similar species found elsewhere• Darwin hypothesized that long-term isolation led to the emergence of

distinct traits• Provided an advantage in this unique environment

• Could these traits accumulate to such an extent that a new species could emerge?

• This concept is called speciation• Speciation lies at the boundary between microevolution and macroevolution

Galapagos Islands

• Example: Galapagos marine iguana• Iguanas exist elsewhere, but this is the only one that swims and

sneezes out salt!• Is this natural variation within a species, or is it a distinct species?

What is a Species?

• What makes a species?• Traditional view has been that individuals that look similar are of the

same species• Massive problems with this view…

What is a Species?

• Are two individuals of the same species if they can’t breed, even if they look similar?

Eastern Meadowlark Western Meadowlark

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What is a Species?

• What if two individuals look nothing alike, but have the same mother?

Worker Soldier

What is a Species?

• What if two individuals look nothing alike, but reproduce successfully with each other?

What is a Species?

• Currently the most accepted definition of a species is called the biological species concept

• Individuals are of the same species if…• They are able to mate and produce viable, fertile offspring• They are unable to reproduce with other species

What is a Species?

• There are problems with the biological species concept too• How much of a reproductive barrier is necessary?

What is a Species?

• There are problems with the biological species concept too• What about ring species?• All of these salamanders in California mate

successfully with their immediate neighbors except in the very south

• Where to draw the speciation line?

What is a Species?

• There are problems with the biological species concept too• What about asexual reproduction – how do we classify organisms that don’t

mate?• Morphological – how they look• Ecological – how they live

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What is a Species?

• There are problems with the biological species concept too• What about extinct species?

• If we have two similar fossils from the same time period, how can we tell if they were the same species?

• How similar are the fossils?• Are their differences due to natural variation?• Would they mate with modern organisms?

What is a Species?

• There are problems with the biological species concept too• What about chronospecies?• Here the reproductive barrier is time• If they weren’t alive at the same time, they couldn’t mate• But if they could have, would they have?• Where do we draw the speciation line?

Reproductive Isolation

• How do two populations become two species?• A reproductive barrier must form

• Prevents the exchange of alleles

• The two populations become reproductively isolated• Evolve separately

Reproductive Isolation

• Reproductive isolation relies on the formation of a reproductive barrier

• Pre-mating barriers• Geographic isolation• Habitat isolation• Temporal isolation• Behavioral isolation• Mechanical incompatibility

• Post-mating barriers• Gametic incompatibility• Reduced Hybrid viability

Reproductive Isolation

• Pre-mating barriers• Geographic isolation

• A physical barrier forms between two populations

Reproductive Isolation

• Pre-mating barriers• Habitat isolation

• Two populations live and mate in different environments

Water Land

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Reproductive Isolation

• Pre-mating barriers• Temporal isolation

• Mate at different times of the day or year

Spring Fall

Reproductive Isolation

• Pre-mating barriers• Behavioral isolation

• Mates become attracted by different characteristics and behaviors

Reproductive Isolation

• Pre-mating barriers• Mechanical incompatibility

• Structural differences prevent mating

Reproductive Isolation

• Post-mating barriers• Gametic incompatibility

• Sperm and egg no longer recognize each other

Reproductive Isolation

• Post-mating barriers• Reduced hybrid viability

• Mating produces hybrids that do not develop completely, or are too frail to survive and reproduce

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1. Rana pipiens2. Rana blairi3. Rana sphenocephala4. Rana berlandieri

Reproductive Isolation

• Post-mating barriers• Reduced hybrid viability

• Alternatively, the hybrid may be strong but unable to reproduce

• Especially true when hybrid has an oddnumber of chromosomes

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Reproductive Isolation

• What happens if two populations become reproductively isolated for a time, but then are reconnected?

• Many possibilities• Short-term separation

• Population fusion• Gene flow erases differences• Populations freely interbreed

• One population goes extinct• Founding of a stable hybrid zone• Creation of a new species by hybridization

Reproductive Isolation

• What happens if two populations become reproductively isolated for a time, but then are reconnected?

• Many possibilities• Long-term separation

• Genomes diverge and are no longer compatible• Populations co-exist but no longer reproduce• Hybrids are less viable

Patterns of Speciation

• Once reproductive barriers form, speciation can occur in one of several patterns

• Allopatric speciation• Sympatric speciation• Adaptive radiation

Patterns of Speciation

• Allopatric speciation• Geographic separation restricts gene flow• Populations diverge in different areas

Part of a mainlandpopulation reachesto an isolated island

Divergence may eventuallybecome sufficient to causereproductive isolation

The isolated populations beginto diverge due to genetic driftand natural selection

Patterns of Speciation

• Sympatric speciation• Reproductive barriers evolve within a population

in the same geographic area• “Habitat isolation”

• Reduces competition for resources and benefits both populations

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Part of a fly population that lives only on hawthorne trees moves to an apple tree

The flies living on the apple tree do not encounter the flies living on the hawthorne tree, so the populations diverge

Patterns of Speciation

• Adaptive radiation• Rapid speciation from one common ancestor to fill ecological

niches• Minimizes competition for resources and benefits everyone• Also called divergent evolution• Examples

• Silversword alliance• Darwin’s finches

Patterns of Speciation

• Adaptive radiation• Silversword alliance

• Family of plants on the Hawaiian Islands

• Isolated, diverse• Single ancestor on one island

diverged into more than 50 species on several islands

Patterns of Speciation

• Adaptive radiation• Darwin’s finches

• Darwin observed 14 different beak shapes in finches• Beak shape corresponded with habitat and diet• Some were more suited to cracking nuts, some to reaching

insects in trees• All came from a common ancestor

Ground and Cactus FinchesVegetarianTree Finch

WarblerFinches

Certhideafusca

Platyspizacrassirostris

Certhideaolivacea

Tree Finches

Cactospizapallida

Camarhynchuspauper

Camarhynchusparvulus

Geospizaconirostris

Geospizamagnirostris

Geospizafuliginosa

Geospizafortis

Geospizascandens

Geospizadifficilis

Camarhynchuspsittacula

Cactospizaheliobates

How Fast is Evolution?

• Evolution is slow• Most mutations are repaired• Most mutations that aren’t repaired are in junk DNA• Mutations not in junk DNA are usually harmful

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How Fast is Evolution?

• Two proposed temporal patterns of evolution• Gradualism

• Proposed by Darwin• Slow, gradual changes over time

• Punctuated equilibrium• Proposed by Steven Jay Gould• Rapid evolution over short periods of time, followed by short

periods of no change

How Fast is Evolution?

• Which model is correct?• Depends• Fossil records have a few examples of gradualism

• Example: The horse• Most records tend to favor punctuated equilibrium

• Example: The Cambrian explosion• 541 million years ago• Rapid development of nearly all modern phyla in a short

period of time• Why?

• Satisfactory explanation lacking• One of the admitted problems with Neo-Darwinism

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