lecture 3: ch. 24, 25 - origin of species
TRANSCRIPT
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Lecture 3: The Origin of Species
Campbell chapters:
Chapter 24Chapter 25
Speciation - the origin of new species from pre-existing species.
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What is a species? (Latin for kind, type)
Biological Species:
= A set of naturally interbreeding populations that aregenetically reproductively isolated from other sets of populations.
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Other species “concepts” exist
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Interbreeding within species= lineage
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A BEvolutionary
change
Speciation:Divergence, followed byevolutionary change.
Evolutionarychange
Divergence
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Types of Speciation1) Allopatric
2) Sympatric
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Allopatric speciation= evolutionary change occurring
in different geographic ranges.
Ancestral population divides; each can undergo independent evolutionary change.
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Allopatric speciation
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Sympatric speciation= evolutionary divergence
occurring in same (overlapping) geographic ranges.
Rare in nature, but may occur by:- Initial disruptive selection (e.g., different food sources).- Local ecological niche specialization (e.g., races/ecotypes)
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Reproductive Isolating Mechanisms
•Geographic – Continental Drift– Mountain uplifting– Changes in sea level– Changes in climate– Island formation
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Reproductive Isolating Mechanisms (Genetic)
Polyploidy = evolution of chromosome no. that is multiple of an ancestral set.
Hybridization of 2 species followed by polyploidy ----> instant speciation. Polyploid hybrid reproductively isolated from both parents.
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Polyploid Speciatio
n:
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Reproductive Isolating Mechanisms (Genetic)
PRE-ZYGOTIC (pre-mating)i) Habitat isolation - differences in
habitat preference
ii) Temporal isolation - differences in timing of reproduction
garter snakes: aquatic vs. terrestrial species
spotted skunk species: mate in different seasons
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Reproductive Isolating Mechanisms (Genetic)
PRE-ZYGOTIC (pre-mating)iii) Behavioral (sexual) isolation -
differences in behavioral responses with respect to mating
mating “dances” of birds differ among species
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Reproductive Isolating Mechanisms (Genetic)
PRE-ZYGOTIC (post-mating)iv) Mechanical isolation
- differences in sex organs, don’t “fit”
v) Gametic isolation - sperm / egg incompatibility
left- vs. right-handed snail species can’t mate
sperm & egg of different sea urchin species incompatible
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Reproductive Isolating Mechanisms (Genetic)
POST-ZYGOTICvi) Reduced hybrid viability
- embryo doesn’t live.
vii) Reduced hybrid fertility - hybrids develop but sterile.
salamander hybrids frail or don’t mature
horse + donkey mule: sterile
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Reproductive Isolating Mechanisms (Genetic)
POST-ZYGOTICviii) Hybrid (F2) breakdown
- F1 fertile, but future generations
sterile or reduced fitness
hybrid rice plants small, reduced fitness
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Time for Speciation to occur?
Varies, dependent on group. E.g.,
Spartina angelica hybrid polyploidCa. 20 years
Hawaiian Drosophila spp. (Fruit flies)Average speciation time = 20,000 yrs
Platanus spp. (Sycamores)P. orientalis & P. occidentalis separated ca. 50,000,000 years, still not genetically reproductively isolated
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Adaptive Radiation
- spreading of populations or species into new environments,with adaptive evolutionary divergence.
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Adaptive Radiation
• Promoted by:• 1) New and varied niches
- provide new selective pressures
• 2) Absence of interspecific competition- enables species to invade niches previously occupied by others
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Examples of Adaptive Radiation:
GalapagosTortoises
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Examples of Adaptive Radiation:
“Darwin’s” Finches
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Close North American relative,the tarweed Carlquistia muirii
Argyroxiphium sandwicense
Dubautia linearisDubautia scabra
Dubautia waialealae
Dubautia laxa
HAWAII0.4
millionyears
OAHU3.7
millionyears
KAUAI5.1
millionyears
1.3millionyears
MOLOKAIMAUI
LANAI
Examples of Adaptive Radiation: “Tarweeds” of Hawaiian Islands
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Macroevolution
• = large scale evolution at & above species level
• [Microevolution = small scale evolution at the population level.]
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Tempo of Speciation
• 1) Gradualism (gradualistic speciation)
= gradual, step-by-step evolutionary change
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Evolution of horses
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Species showing very little evolutionary change:
• E.g.:– Coelacanth (Latimeria) - 250 myr,
rediscovered 1938
– Horseshoe crab
– Dawn-Redwood Tree (Metasequoia)
– Maidenhair Tree (Ginkgo)
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Tempo of Speciation
• 2) Punctuated Equilibrium
= rapid evolutionary change during speciationfollowed by relatively long periods of stasis (no change).
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Punctuated Equilibrium:
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Punctuated Equilibrium:
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How can rapid speciation (resulting in punctuated
equilibrium) occur?1) Founder principle or population bottleneck
2) Major environmental change, new niches open up.
- both can accelerate evolutionary change
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How can rapid speciation occur?
3) Major genetic change:
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E.g., Change in a gene that regulates development (homeotic / regulatory gene)
Hox gene 6 Hox gene 7 Hox gene 8
About 400 mya
Drosophila Artemia
Ubx
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Heterochrony
• = change in the rate or timing of development
• Neotony = type of heterochrony: decrease in rate of development
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å ß
Chimp
Human
NEOTONY
Feature
Developmental Time
• Many features of humans evolved by NEOTONY!
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Heterochrony - NEOTONY
Mature human adult resembles fetus of both.
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult
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Extinction• “Opposite” of Speciation
• Over 99% of all species on earth are now extinct.
• E.g., – ammonites– seed ferns– dinosaurs– Irish Elk– dodo bird
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Extinction is a major driving force of evolution
• How?
• Opens up new niches, by removing interspecific competition.
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