Download Population Genetics and Speciation

Populations,
Genes and
Evolution
Ch. 16.1 and Ch.17
Population Genetics


Study of diversity in a population at the
genetic level.
Alleles

1 individual will provide 2 alleles for a given
phenotype
DD- homozygous Dominant
 Dd- heterozygous
 dd- homozygous recessive



In a population, we would expect a mixture of D
and d alleles in the gene pool
The allele frequency is the proportion of each
allele in the gene pool.
Hardy-Weinberg Equilibrium
 When
there is no change in allele
frequencies over time, the population is
said to be in Hardy-Weinberg Equilibrium
 Allele
frequencies can be determined by
p2 + 2pq + q2 = 1
p2 = DD frequency 2pq = Dd frequency
q2 = dd frequency
HW only applies to populations
under 5 conditions…
1.
No mutations-No new alleles arise by
mutation


Mutations are changes in DNA sequence
and provide genetic variation.
Most mutations come from errors during
DNA replication, but some can occur
through exposure to physical or chemical
agents called mutagents.
HW only applies to populations
under 5 conditions…


Mutations only affect the gene pool if they
can and are passed on to next generation.
Though mutations are rare, they are
essential to evolution.
HW only applies to populations
under 5 conditions…
2.
No Migration- no new members moving
in and no existing members moving out


When plants and animals migrate between
populations, we get gene flow- movement
of alleles between populations.
Gene flow is determined by:
1.
2.
3.
The distance between populations
The ability of individuals to move
The behavior that determines if they will
move
HW only applies to populations
under 5 conditions…

If migration does not occur, two
populations become genetically different
and therefore reproductively isolated. This is
the first step towards new species.
HW only applies to populations
under 5 conditions…
3.
Large Gene Pool- Large population size

A small population size is affected more by
genetic drift (changes in allele frequency
by chance)
 Bottleneck
affect- when large populations
suddenly become very small due to natural
disaster.
 Founder affect- when a few members of a
population split off to form a new population.
HW only applies to populations
under 5 conditions…

In small populations, the chance of
inbreeding increases greatly after a few
generations. While this does not lead to a
change in allele frequency, it can increase
the appearance of rare recessive disorders.
HW only applies to populations
under 5 conditions…
4.
Random Mating- Individuals mate at
random with no bias for genotype or
phenotype


Random mating will not change the allele
frequencies
Non-random mating will change
frequencies because mates are chosen by
specific phenotypes and therefore alleles
assort according to mating behavior
(Assortive Mating)
HW only applies to populations
under 5 conditions…

Non-random mating, in itself, does not
cause population evolution, it does play a
role towards it.
HW only applies to populations
under 5 conditions…
5.
No selection- natural selection does not
favor one genotype over another


In HW equilibrium, every genotype has an
equal chance of survival and reproduction.
In nature, one genotype may provide
advantages to the individual leading to
natural selection.
Speciation
 The
splitting of one species into 2 or more
different species.
 What is a species?

There are 3 species concepts
 Morphological
species concept
 Evolutionary Species Concept
 Phylogenetic Species Concept
Morphological Species
Concept
 Each
species is defined by one or more
distinct physical characteristics
(diagnostic traits)
 This is the concept Linnaeus used.
 Drawbacks:


bacteria and other microorganisms doesn’t
have many measurable traits
Cryptic species- look almost identical but
have different behaviors (habitat or
courtship)
Evolutionary Species Concept
 Proposed
to explain speciation in the fossil
records.
 Again, uses diagnostic traits but also
requires the members to share the same
evolutionary pathway.
 This means, abrupt changes in traits
classifies a new species.
Phylogenetic Species
Concept
A
phylogeny or “family tree” is used to
identify species based on a common
ancestor
 Species is considered the smallest set of
interbreeding organisms
Reproductive Isolating
Mechanisms
 For
two species to remain, there cannot
be any gene flow (mating) between the
two.
 Reproductive barriers that prevent
reproduction are called isolating
mechanisms


Prezygotic- before fertilization
Postzygotic- after fertilization
Modes of Speciation
 Principal



modes of speciation
Allopatric Speciation
Sympatric Speciation
Adaptive Radiation
Allopatric Speciation
 Speciation
mode in which populations
are segregated by geographical barriers.
Sympatric Speciation
 Daughter
species arises from a group of
individuals within an existing population.
The geographical distributions of these
species overlap.
Adaptive Radiation
 Speciation
that occurs when a single
ancestor rapidly gives rise to new species
that adapt to specific environments