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Lecture 10

BIO120H1 Lecture Notes - Lecture 10: Population Genetics, Dont, Sympatric Speciation

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June Larkin

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Chapter 7- The Origin of Species 04/06/2012
Ernst Mayr set off for the wilds of Dutch New Guinea to collect plants
and animals
Mayr brought back many specimens new to science including 26
species of birds and 38 species of orchids
This concordance between two cultural groups with very different
backgrounds convinced Mayr, as it should convince us that the
discontinuities of nature are not arbitrary but an objective fact
Nature is discontinuous. When you look at animals and plants each
individuals almost always falls into one of many discrete groups
Although there is variation among individuals within a cluster, the
clusters nevertheless remain discrete in organism space
We see cluster in all organism that reproduce sexually
These discrete clusters are known as species
Darwin apparently didn’t see the discontinuities of nature as a problem
to be solved or thought that these discontinuities would somehow be favored
b natural selection. Either way HE FAILED TO EXPLAIN NATURES CLUSTER IN
A better title for the origin of species then would have been the origin of
adaptations: while Darwin did figure out how and why a single species change
over time, he NEVER explained how one species splits in two.
All this diversity comes from a single ancient ancestor
If speciation didn’t occur there would be no biodiversity at all—only a
single, log evolved descendent of that very first species
Biologist struggled and field to explain how continuous process of
evolution produces the discrete groups known as species. The problem of
speciation was in fact not seriously addressed until the mid 1930s
How we recognize species: as a group of individuals that resemble one
another more than they resemble members of other groups this is known as
the morphological species concept (tiger as cat)
This definition has problems. In sexually dimorphic species, males
and females can look very different females of single species as
members of two different species

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There is also the problem of variation within a interbreeding groups
(ex: humans could be classified into few discrete groups based on
eye color)
What point is differences between populations large enough to make us
call them different species?
This concept makes designation of species an arbitrary exercise, yet
we know that species have an objective reality and are not simply
arbitarary human constructs
Some groups that biologist recognize as different species look either
exactly alike or nearly alike. These cryptic species are found in most
groups of organism including birds, mammals, plants and insects.
Why if these cryptic forms look so similar we think that they’re
actually different species. The answer is that they coexist in the
same location and yet never exchange genes: the members of one
species simply don’t hybridize with member of another
The groups are thus reproductively isolated from one another: they
consisted distinct gene pools that don’t intermingle
These cryptic forms are distinct
Brown eye and Blue or Iut and Kung are members of the same
species, we realize that its because they can mat with each other and
produce offspring that contain combinations of their genes
They belong to the same gene pool. When you ponder cryptic specie
and variation within humans you arrive at the notion that species are
distinct not merely because they look different but because there are
barriers them that prevent interbreeding
Mayr and Theodious Dobzhansky, proposed a definition of species that
has become the gold standard for evoloutonaty biology
Mayr defined a species as a group of interbreeding natural populations
that are reproductively isolated from other such groups this known as
biological species concept BSC
Reproductively isolated simply means that members of different species
have traits—difference in appearance, behavior or physiology that prevent
them from successfully interbreeding, while members of the same species can
interbreed readily
What keeps member of two related species from mating with each

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Species might not interbreed simply because their mating or
flowering seasons don’t overlap
Closely related species living in the same are remain distinct because
their peak spawning periods are several hours apart, preventing eggs
of one species from meeting sperm from another
Animal species often have different mating displays or pheromones
and don’t find one another sexually attractive
Species can also be isolated by preferring different habitats so they
simply don’t encounter one another,
Many insects can feed and reproduce on only one single species of
plant and different species of insects are restricted to different
species of plants this keeps them from meeting others at mating
Isolating barriers can also act after mating pollen from one plant
species might fail to germinate on the pistil of another
When they succeed at mating the sperm of one species is poor at
fertilizing the eggs of the other so that relatively few offspring’s are
The species exchange virtually no genes in nature and we have
confirmed this result by sequence their DNA, these can be considered
good biological species
Advantage of BSC
These population might not mate directly with each other but there is
potential gene flow from one population to the other through
intermediate geographical areas and little doubt that if they did mate
they’d produce fertile offspring
Males and females are members of the same species because of their
gene unite at reproduction
According to BSC the species is a reproductive community – a gene
pool and this means that a species is also an evolutionary community
If a good mutation crops up within a species, say a mutation in tigers
that boosts female output of cubs by 10% then the gene containing
that mutation will spread throughput the tiger species. But it wont go
any further
The biological species the unit of evolution – it is to a large extent
the thing that evolves
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