Speciation

How are new species created?

Warmup:

• Get out your learning targets paper• In the warmup section of your lab book:

Create a concept map using at least 12 of the terms listed in part b) on the front side

• Share current events article• Notes:

– Directional selection, stabilizing, disruptive– Genetic Drift– Founder Effect– Speciation

• Ensatina eschscholtzii salamanders• Create a scenario…• Homework

Natural Selection on Polygenic Traits

• Individuals at one end of curve have higher fitness

• Range of phenotypes shifts

Food becomes scarce

Low mortality, high fitness

High mortality, low fitness

Directional Selection

Natural Selection on Polygenic Traits

• Individuals near center of curve have highest fitness

• Keeps center of curve at same position and narrows graph

Stabilizing Selection

Low mortality, high fitness

High mortality, low fitness

Per

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age

of P

opul

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n

Birth Weight

Selection against both extremes keeps curve narrow and in same place

Natural Selection on Polygenic Traits

• Individuals at upper and lower ends of the curve have higher fitness than individuals in the middle

• Selection acts strongly against individuals of the intermediate type

Disruptive SelectionLargest and smallest seeds become more common

Low mortality, high fitness

High mortality,Low fitness

Num

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f Bird

s in

Pop

ulat

ion

Num

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s in

Pop

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Beak Size Beak Size

Population splits into two subgroups specializing in differentseeds.

Speciation• Speciation = formation of new

species

• Species = group of organisms that breed with one another and produce fertile offspring

• As new species evolve, populations become reproductively isolated from each other

Isolating Mechanisms (lacewing interactive)

• Behavioral Isolation– Differences in courtship rituals or other

reproductive strategies

• Geographic Isolation– Two populations separated by geographic barrier

such as rivers, mountains or bodies of water

• Temporal Isolation– Two or more species reproduce at different times

of the day or year

results from

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produced by produced byproduced by

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which result in

Isolating mechanisms

Behavioral isolation Temporal isolationGeographic isolation

Behavioral differences Different mating timesPhysical separation

Independentlyevolving populations

Formation ofnew species

Reproductive Isolation

Speciation

• In a phylogenetic tree, it is represented by a branching point

• Example: Drosophila

A new species of fruit fly

• Example: Fruit flies

http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_42

A population of wild fruit flies on several bunches of rotting bananas, laying their eggs in the mushy fruit...

A new species of fruit fly

• Disaster strikes: A hurricane washes the bananas and the fruit flies out to sea. The banana bunch washes up on an island off the coast of the mainland. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them.

A new species of fruit fly

• The populations diverge: Conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does.

• Food preferences, and courtship displays change over the course of many generations of natural selection.

A new species of fruit fly

• So we meet again: When another storm brings the island flies back to the mainland, they will not mate with the mainland flies since they've evolved different courtship behaviors.

• The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations.

• Two separate species now exist since genes cannot flow between the populations.

Geographic Isolation• Populations are separated by geographic

change or dispersal to geographically isolated places– Rivers change course– Mountains rise– Continents drift– Organisms migrate– Roads are built

• Note: a barrier for one species may not be a barrier for another species

Small populations face risks

• Founder effect: when only a few individuals colonize a new place, genetic variation is low

• Genetic drift: changes in gene pool due to chance (which individuals reproduce)

• Bottleneck effect: disasters that eliminate a large number of individuals and greatly reduce the gene pool

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic DriftSection 16-2

A new species:

• If a group splits off from the main population– evolves to adapt to its environment– the changes accumulated make it unable

to breed with the larger population • Then a new species has been formed

Other reasons for reproductive isolation:

• Timing (Temporal Isolation): Different breeding seasons– Example: Spotted skunks

• Western skunks breed in the fall, Eastern skunks breed in the late winter

http://s190.photobucket.com/albums/z257/americanwildlife/Mammal/Z-western-spotted-skunk1.jpg

http://www.redorbit.com/modules/reflib/article_images/42_7de29e7e859b48896129c3d380d3cfbb.jpg

Other reasons for reproductive isolation:

• Behavior (Behavioral Isolation): Different courtship or mating behaviors– Example: Eastern and Western

Meadowlarks• Different songs

Other reasons for reproductive isolation:

• Habitat: Adapted to different habitats in the same general location– Example: Stickleback fish in British

Columbia• Live in different levels of water, have different

diets

http://ecoreb.org/imgs/o_gasacu2.jpg

Other reasons for reproductive isolation:

• Others: different reproductive structures, insects only transfer pollen to certain plants, hybrid offspring is sterile

http://www.birkenholz.com/IMAGES/MuleColt05Right.jpg

Speciation of Darwin’s Finches

• Founders Arrive• Separation of

Populations• Changes in the Gene

Pool• Reproductive Isolation• Ecological Competition• Continued Evolution

Speciation in the Andes (Ecuador)

• Hummingbird video• Explain the hypothesis presented by the scientists

profiled in this segment to explain the process of speciation in hummingbirds and possibly other species.

• How does this hypothesis differ from the traditional view that speciation often requires geographic separation of populations?

• Why were the researchers collecting blood from the populations they studied? Discuss at least two possible analyses that could be performed on those samples and, identify at least two different questions that might be answered with sufficient data.

Ensatina Salamanders• California salamanders

– Live and lay eggs on land– Studied by R.C. Stebbins in the 1940s

• You used his data to map the locations of various subspecies

• Video of mating behavior:– http://evolution.berkeley.edu/evolibrary/article/0_0_0/devitt_07

• Pictures of each subspecies:– http://www.indiana.edu/~ensiweb/lessons/stepsal4.html

http://evolution.berkeley.edu/evolibrary/article/0_0_0/devitt_02

Ring Species

• All subspecies interbreed with their immediate neighbors EXCEPT at southern end– E. klauberi and E.

eschscholtzii do not interbreed

• Where should speciation be marked?

http://evolution.berkeley.edu/evolibrary/article/side_0_0/biospecies_01#ring

Did you get it? Watch this!

• Link to video about Ensatina salamanders in California (3 minutes) http://www.youtube.com/watch?v=PjcFSy1KCTI

Do Now!

• Devise a scenario in which a particular type of selection leads to speciation.– Use the 5 tenants of natural selection– What type of selection is at work?

Homework

• Read section 17-4 in your textbook (pg. 435-440), Patterns in Evolution

• Answer the three learning targets for this section (#3, 4 and 5 under chapter 17) from your LEARNING TARGETS paper.