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Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
Review From Last Time
Darwin based his theory of natural selection on two key observations.
Overproduction
of offspring
Individual
variation
Conclusion
Natural selection:
unequal reproductive success
How does e volution happen? All of these require genetic diversity – so mutation is a requirement for all evolution
1. Natural S election
2. Sexual S election
3. Genetic Drift
4. Gene F low
Summary from Textbook
11.4 Speciation • Speciation occurs along two main pathways: geographic separation ( allopatric s peciation) and through mechanisms that occur within a shared h abitat ( sympatric speciation). Both p athways force reproductive isolation between populations. Sympatric s peciation can occur through errors in meiosis that form gametes with extra chromosomes, called polyploidy. Autopolyploidy occurs within a single species, whereas allopolyploidy occurs b ecause of a mating b etween closely related s pecies. Once the p opulations are isolated, evolutionary d ivergence can take p lace leading to the evolution of reproductive isolating traits that p revent interbreeding s hould the two populations come together again. The reduced viability of hybrid offspring after a p eriod of isolation is expected to s elect for stronger inherent isolating mechanisms. What is a Species?
• A group of organisms that c an interbreed and produce fertile offspring. Zannie Dallara 1
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
What does it mean to Speciate? What does it mean to Speciate? New species may form over long periods of time • In the graduated model, a species acquires small adaptations to its environment over millions of years.
New species may form r elatively r apidly
• In the punctuated equilibrium model, there are periods of stasis interrupted by occasional bursts of speciation.
530 million years ago during a period called the Cambrian explosion, the rate of evolution was an order of magnitude higher than the normal rate. (It still required millions of years.)
2 types o f S peciation
• Allopatric • Sympatric Zannie Dallara 2
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
Figure 14.7
Ammospermophilus
leucurus
Ammospermophilus
harrisii
New species may form after geographic isolation • Allopatric speciation may occur when a physical barrier isolates populations.
The formation of the Grand Canyon produced two isolated habitats. One species of squirrel is now found exclusively on each side of the canyon.
Allopatric Speciation
• Geologic processes can
– fragment a population into two or more isolated populations and – contribute to allopatric s peciation.
Video: G rand Canyon © 2 0 1 3 P earso n Ed u catio n , In c.
Zannie Dallara 3
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
New species may form within a parent species • Sympatric speciation may occur quite suddenly due to large-­‐scale genetic changes. (There is no physical barrier.)
http://evolution.berkeley.edu/evosite/evo101/VC1eSympatric.shtml
Sympatric Speciation
• Sympatric speciation occurs in populations that live in the same geographic area.
• An accident during cell division that results in an extra set of chromosomes is a common route to sympatric speciation in plants.
• Many polyploid species arise from the hybridization of two parent species.
© 2 0 1 3 P earso n Ed u catio n , In c.
Three General Outcomes of Natural Selection
– If we graph the coat color of a p opulation of mice, we get a bell-­‐shaped curve.
– If n atural s election favors certain fur-­‐color phenotypes, • the populations of mice w ill change over the generations and
• three general outcomes are possible.
© 2 0 1 3 P earso n Ed u catio n , In c.
Zannie Dallara 4
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
Three General Outcomes of Natural Selection
1. Directional s election s hifts the overall makeup of a population b y s electing in favor of one extreme phenotype.
2. Disruptive s election can lead to a b alance between two or more contrasting p henotypic forms in a p opulation.
3. Stabilizing selection favors intermediate phenotypes, occurs in relatively s table environments, and is the most common.
© 2 0 1 3 P earso n Ed u catio n , In c.
Frequency of
individuals
Fi g u re 1 3 .2 9
Original
population
Evolved
population
Phenotypes (fur c olor)
(a) D irectional selection
(b) D isruptive selection
Original
population
(c) Stabilizing selection
Fi g u re 1 3 .2 9 a
Three General Outcomes of Natural Selection
1. Directional s election s hifts the overall makeup of a population b y s electing in favor of one extreme phenotype.
(a) D irectional selection
Zannie Dallara 5
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
Fi g u re 1 3 .2 9 b
Three General Outcomes of Natural Selection
2. Disruptive s election can lead to a b alance between two or more contrasting p henotypic forms in a p opulation.
(b) D isruptive selection
Figure 1 1.6
•
As the Industrial Revolution caused trees to darken from soot, darker colored peppered moths were better camouflaged than the lighter colored ones, which caused there to be more of the darker colored moths in the population.
Fi g u re 1 3 .2 9 c
Three General Outcomes of Natural Selection
3. Stabilizing selection favors intermediate phenotypes, occurs in relatively s table environments, and is the most common.
(c) Stabilizing selection
Zannie Dallara 6
Bio 1 1 -­‐ EVOLUTION -­‐ Speciation
LEC #18
Frequency of
individuals
Fi g u re 1 3 .2 9
Original
population
Evolved
population
Phenotypes (fur c olor)
(a) D irectional selection
(b) D isruptive selection
Original
population
(c) Stabilizing selection
Biology and Society:
The Sixth Mass Extinction
• Our current rate of extinction, over the past 400 years, indicates that we may be living in, and contributing to, the sixth mass extinction period.
• Mass extinctions pave the way for the evolution of new and diverse forms, but it takes millions of years for Earth to recover.
© 2 0 1 3 P earso n Ed u catio n , In c.
Zannie Dallara 7