BIO120H1 Lecture Notes - Lecture 9: Sympatric Speciation, Ecological Speciation, Allopatric Speciation

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12 Jul 2017
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BIO120: Species, Speciation, and Hybridization (Lecture 9)
Flightless Birds
-high frequency of flightlessness on islands
-hypothesis: no selection against mutations that impair flight
important as antipredator mechanism, but not many predators on islands
flight is a neutral character
may be disadvantageous (could fly out to sea and never come back)
-not flying can be an advantage when there are no predators
-example: cormorants
loss of wings increases diving ability
can be impediment when going down deep into water
allows them to dive in a much more physically efficient manner
Intro: Species
-Hawaiian honeycreepers have all descended from a single species of finch in the last 10
million years
-Darwin: “I look at the term species, as one arbitrarily given for the sake of convenience
to a set of individuals closely resembling each other…” (morphological characterization
of species)
-Coyne: “When we understand the origin of reproductive isolating factors, we understand
the origin of species” (to understand species, have to understand how reproductive
isolation occurs)
Speciation: Key Questions
1. What ecological and genetic conditions are required for speciation to occur?
2. How does reproductive isolation evolve?
3. How many genes are involved?
4. Is the evolution of adaptation required for speciation?
Two Main Species Concepts
1. taxonomic/morphological: based primarily on distinct morphological differences
2. biological: based on inter-fertility (crossability) among individuals
-concepts vary among different groups of organisms (no universal species concept)
neither of these concepts are right or wrong
-no universal species concept:
asexual organisms: reproduction important for biological concept, but asexual
organisms do not reproduce sexually
very old trees: hard to do breeding experiments to determine inter-fertility
use morphology in these cases
-more satisfied with biological: some judgement value with morphological, subjective
Ernst Mayr, 1942
-introduced the biological species concept: a group of interbreeding natural populations
that are reproductively isolated from other such groups
-although Mayr first formally defined it, Dobzhansky first suggested the idea
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Two Main Modes of Speciation
-allopatric: often known as the theory of geographic speciation since geographical
isolation is involved
there is a population with a barrier to gene flow, diverge over time
may diverge in sufficient number of genes that if they come back into same area, they
can’t interbreed
common, main form of speciation on Earth today
-sympatric: within species, some individuals become genetically differentiated, leads to
new species within sympatry
controversial (how is divergence occurring when gene flow could be occurring?)
happens often in plants (polyploidy), rare in animals
What Causes Reproductive Isolation?
-stages when reproductive isolation can occur:
-all stages have to be investigated
Reproductive Isolating Mechanisms
-speciation involves the evolution of reproductive isolation between populations
1. premating isolation
2. postmating isolation
Premating
-preventing zygote formation
1. geographical, ecological
species are in different environments
adapted to different ecological environments (e.g. growing in wet vs. dry
environments)
2. temporal, behavioural
time of season that insect emerges might be different from others, plants flower at
different times in season
courtship differences
3. mechanical, prevention of gamete fusion
too big or too small to mate with some other animals
Example: Premating Isolation in Apple Maggot Flies
-habitat and temporal isolation
-before apples introduced, lived and laid eggs on hawthorne (same family as apples)
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