Chapter 14: Migration

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• Chapter 14 Migration
• Portion of a species divide into separate
  populations
• Different evolutionary forces on populations
  different allele frequencies
• Migration subsequent movement of individuals
  between populations

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• 14.1 Mating systems
• Random mating
• (choice of mates independent of genotype and
  phenotype)
• Inbreeding
• (Mating between relatives)
• Preferential mating
• (non-random, based on phenotype and genotype)
• - Assortative mating
• (Same phenotype attracts e.g. human choice
  based on intelligence) (Mm)
• - Disassortative mating
• (Self-sterility systems in plants)

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• 14.2 Estimating migration
• Migration is the movement of individuals from
  one breeding population to another.
• Assume that random individuals are involved.
• After migration Pop 1 increase in size by
  proportion m, which comes from Pop 2.
• n2 number of migrants, n1 native pop size
• If the frequency of a given gene is q2 among
  immigrants and q1 among the natives, the new
  frequency will be
• qm m(q2 - q1) q1 and m n2 / (n1 n2)

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Example For given Pop 1, n 8,000 2,000
animals from Pop 2 emigrates to Pop 1 q1 was
0.2 q2 0.6 m n2 / (n1 n2) 2,000 /
(8,000 2,000) 0.2 The new frequency of q
after migration (qm) is qm m(q2 - q1)
q1 0.2 (0.6 0.2) 0.2 0.28 Change in
allelic frequency ?q 0.28 - 0.2 0.08
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• 14.3 The island model of migration
• In this model, the overall population is divided
  into many sub-populations, like islands in an
  archipelago.

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• 14.3 The island model of migration
• In this model, the overall population is divided
  into many sub-populations, like islands in an
  archipelago.
• Each sub-population is so large that random
  genetic drift is not possible.
• Consider two alleles, A and a ( p and q)
• The amount of migration is m
• The generation is t
• P in each population is constant
• (because of infinite size)
• pt p (po p)(1 m)t

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Suppose there are 2 populations, with initial
frequencies of A 0.2 and A 0.8 respectively
and m 0.1 What is the frequency of A in the 2
populations after 10 generations? With movement
of migrants in both direction, the frequency of A
in migrants (0.2 0.8) / 2 0.5 For Pop
that started with A 0.2 P10 0.5 (0.2
0.5)(1 - 0.1)10 0.395 For Pop that started
with A 0.8 P10 0.5 (0.8 0.5)(1 - 0.1)10
0.605
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The change in allele frequency with time, in five
sub-populations exchanging migrants at a rate of
m 0.1 per generation
Because migration rates are much greater than
mutation, changes in allele frequencies occur
much faster with migration.
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14.4 One-way migration When migration occurs
from one population onto another, without an
equal migration in the reverse direction.
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• 14.5 Isolate breaking and Wahlunds principle
• Isolate breaking fusion of formerly isolated
  sub-populations, by migration.
• Fusion reduces the frequency of homozygotes
• Wahlunds principle

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• 14.6 How migration limits genetic divergence
• A relatively low level of migration between
  populations can prevent significant divergence
  between the populations.
• Migration can be expressed as Nm, which denotes
  the number of migrants per generation.
• Nm is derived from F
• When F 1, Nm 0

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• 14.7 Patterns of migration (actual patterns)
• Migration in real populations is more complex
  than is assumed in the island model of migration.
• In nature, migrants come mostly from nearby
  populations (as opposed to several random
  populations)
• Humans migration rates depend on age, sex,
  status, population density.
• Managed wildlife populations ?

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• 14.8 Genetic distance
• A coefficient to express genetic differentiation
  among populations.
• Several varieties exist, with the most common
  measure
• Neis genetic distance or D
• D -ln(I)
• Where I genetic identity the correlation of
  allele frequencies between 2 populations.

• Possible values for D can vary from 0 (no
  difference) to 1 (no similarity).

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Example For 3 populations and 1 locus with 3
alleles
Genetic distance between populations 1 and
2 For Pop1 ?pix2 (0.023)2 (0.977)2
(0.0)2 0.955 For Pop2 ?piy2 (0.019)2
(0.885)2 (0.096)2 0.793 And ?(pixpiy)
(0.023x0.019) (0.977x0.885) (0.0x0.096)
0.865
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Genetic distance between populations 1 and
2 For Pop1 ?pix2 (0.023)2 (0.977)2
(0.0)2 0.955 For Pop2 ?piy2 (0.019)2
(0.885)2 (0.096)2 0.793 And ?(pixpiy)
(0.023x0.019) (0.977x0.885) (0.0x0.096)
0.865
IN ?(pixpiy) / v (?pix2 )(?piy2) 0.865
/ v (0.955 x 0.793) 0.994 And genetic
distance -ln(IN) -ln(0.994) 0.006
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Chapter 15. Mutation A sudden (and heritable)
change in the genetic material
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Mutation

• With each generation gene pool is shuffled
  produce new genotypes (combinations)

• Large number of possible combinations only a
  fraction in living members of the population

(for example 5 loci 3 alleles give 7,776
possible genotypes)

• Enormous genetic reserve - produce new
  genotypic combinations continuously

• Assortment and recombination do not produce new
  alleles

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Mutation create new alleles
Random process, without regard for any benefits
or disadvantages to the organism
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• 15.1 Number of alleles maintained in populations
• An average protein contains 300 amino acids
  900 nucleotides
• The number of possible alleles 10542
• We can therefore assume that every time a
  mutation occurs, it is a new allele that does not
  already exist in the population.
• the Infinite allele model of mutation.

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• 15.2 The neutrality hypothesis
• The neutrality hypothesis predicts that many
  mutations have such mild effects, that their
  influence on survival and reproduction are
  negligible.
• The frequencies of such alleles are determined
  by forces such as migration and random genetic
  drift, instead of selection.
• (The evidence for this hypothesis comes from
  early work on allozymes, i.e. protein phenotypes
  used as indicators of underlying genotypes).

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• 15.3 Mutations and behaviour
• Mutations that effect processes in the brain
  result in different alleles that cause conditions
  such as Huntingtons disease, PKU and
  Schizophrenia.

• The mechanism of mutations
• Mutations in egg and sperm cells will be
  transmitted unless natural selection intervenes.
• Most mutations are not translated into proteins,
  because they occur in DNA regions that are not
  transcribed (introns). They thus have no visible
  effect.

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A single-base mutation results in the insertion
of a different amino acid into a protein. (
altered function) e.g. ATT TAC
CGC becomes ATT TCC CGC TAC codes for
methionine TCC codes for arginine The effect
of replacing a single amino acid can be anything
from small to lethal.
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A deletion is usually more damaging than a
substitution, because the entire reading frame of
the DNA triplet code is shifted e.g. TAC AAC
CAT After the loss of C becomes TAA ACC AT-
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• 15.4 Targeted mutations
• Techniques used during genetic modification of
  organisms, by which genes are changed in a
  specific way to alter their function.
• Knock out deleting key areas of DNA sequence,
  preventing the gene from being transcribed.
• Newer techniques alter gene expression in more
  subtle ways, by inducing underexpression or
  overexpression.
• In the field of neurogenetics, targeted
  mutations are used to elucidate processes such as
  learning and memory (in suitable laboratory
  animals).

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Mutation rate

• A gene undergoes mutation to a dominant allele
• 2 out of 100,000 births exhibit mutant phenotype
• Parents are phenotypically normal
• Zygote carries two copies of the gene
• Study 200,000 copies of the gene
• Assume affected births are heterozygous
• Uncovered 2 mutant alleles out of 200,000

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The mutation rate is 2 / 200,000 1 /
100,000 1 x 10
-5
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Example
Achondroplasia dominant form of
dwarfism Individuals enlarged skull, short arms
and legs Diagnosed by X-ray at birth Mutation
rate 0.5 x10 Estimate the extent to which
mutation can cause allele frequencies to change
from one generation to the next
- 5
d ? normal alleles D ? allele for achondroplasia
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• Population 500,000 individuals
• 100 dd
• Initial frequencies d 1.0
• D 0.0
• Each individual contributes 2 gametes
• Gene pool contains 1,000,000 gametes
• Assume 1.4 of every 100,000 d alleles mutates
  into a D allele

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Frequency of allele d (1,000,000 -14)
1,000,000
0.999986
Frequency of allele D ___14___

1,000,000
0.000014
It will take 70,000 generations to reduce the
frequency of allele d from 1.0 to 0.5