Transcript PPT

CHANGE IN POPULATIONS
AND COMMUNITIES
Important Terms

Communities are made up of populations
of different species of organisms that live
and potentially interact with one another in
a particular area at a particular time
Important Terms
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a Population is a group of individuals of
the same species that live in a localized
area
A Species is a group of populations that
have the potential to interbreed in nature
Populations
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Individuals within a species tend to cluster
into groups or separate populations and
tend to breed with other members of their
population
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Because of this, members of populations tend
to be more closely related to members of their
own population
Populations
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A population’s gene pool is the total aggregate of
genes in a population at any one time
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Even though members of a population are similar
there is still lots of variation within that population
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Diploid species
Alleles ‘P’ and ‘p’
Genotypes PP, Pp, pp
Frequencies of alleles and genotypes in a population remain
fairly constant over time
Populations
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Frequencies of alleles and genotypes in a
population remain fairly constant over time
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Red haired Scotts
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Tounge rollers (dominant gene)
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65% carry dominant gene
Blood types A, B, AB, O
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IA, IB, I
In N.A. 45% of Whites
and 49% of blacks are
Type O (recessive allele
has greater frequency
Studying Populations
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Population sampling is used to study large
populations
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A representative group of the population is
randomly selected and the frequency of traits
of this group are used as indicators for the
entire population
Hardy-Weinberg Theorem
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Frequencies of alleles and genotypes in a
populations gene pool remain constant
from one generation to the next
(Equilibrium)
***For non-evolving populations***
Hardy-Weinberg Equilibrium
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In order to maintain the Hardy-Weinberg
Equilibrium the following 5 conditions must be
met
1.
2.
3.
4.
5.
Population sizes are large
Populations are isolated from other populations
No net mutations
Random mating
No Natural selection
Hardy-Weinberg Equilibrium
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The Hardy-Weinberg ratio is the ratio of genotype
frequencies that evolve when mating is random
and neither selection nor drift are operating
For two alleles (A and a) with frequencies p and
q, the Hardy-Weinberg frequencies are
Genotype AA : Aa : aa
Frequency p2 : 2pq : q2
Hardy-Weinberg Equilibrium
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The Hardy-Weinberg principle states
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p2 + 2pq + q2 = 1
Hardy-Weinberg Equilibrium
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The Hardy-Weinberg principle states that allele
and genotype frequencies do not change over
time given the 5 conditions:
1.
2.
3.
4.
5.
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Population sizes are large
Populations are isolated from other populations
No net mutations
Random mating
No Natural selection
What is the use of the H-W principle then?
Evolutionary Change
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Factors that change a population’s gene
pool contradict the 5 conditions of the H-W
Principle and lead to evolution
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Mutation
Genetic Drift
Migration
Mutation
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Very rare events (1/half million genes per
generation)
Usually occurs during meiosis and present
in either egg or sperm
The mutation will be present for life of the
individual and will possibly be passed on to
future generations
Genetic Drift
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Disruption of genetic equilibrium due to
small population sizes and chance events
Larger population sizes have less sampling
error (probabilities)
Small population – gene pool may not be
accurately represented in following
generations
Causes of Genetic Drift
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Bottleneck effect
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Drastic reduction in population size due to a
catastrophic event (fire, flood, earthquake,
disease)
Small surviving population is unlikely to be
representative of original population
Causes of Genetic Drift
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Founder effect (Darwin’s Finches)
A few individuals from an original
population move to an isolated location
(island)
Most radical example would be a pregnant
female moving to an island with no other
members of her species
Migration (gene flow)
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Movement of members of a species in or
out of a population
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Immigration
Emigration