The relationship between evolution and population requires that:

1
Evolution and Populations

• The relationship between evolution and population
  requires that
• There are differences between individuals
• Differences are passed from parents to
  offspring
• More offspring are born than live to
  reproduce
• Some variants are more successful than
  others
• The abundance of different forms
  (phenotypes) within a population changes over
  time or it evolves

Do human populations violate any Hardy-Weinberg
conditions?
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Lets Look at Hardy-Weinbergs Conditions as
Forces of Evolution
Natural Selection Mutations
Migration Genetic Drift (sampling errors)
Mating is random
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Non-Random Mating

• Mates are not chosen at random
• This does not by itself alter allele
  frequencies
• It can alter genotype frequencies, affecting
  evolution
• Example inbreeding
• - Inbreeding increases homozygosity
• - Small, isolated populations favor inbreeding

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Measures of Inbreeding
The inbreeding coefficient (F) is the probability
of picking two alleles that are identical by
descent (i.b.d.) by a random draw in a population.
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Inbreeding Problems
Loss of fitness on inbreeding -
Conservation genetics (Minimum viable
populations) - Rare species
Recessive deleterious alleles exposed
- Genetic diseases (genetic load accumulation of
deleterious mutations) -
Inbreeding depression (increased mortality in
young impacts reproduction)
Are there any positive consequences of inbreeding?
6
Lets Look at Hardy-Weinbergs Conditions as
Forces of Evolution
Natural Selection Mutations
Migration Genetic Drift (sampling errors)
Mating is random
7
Migration

• Sub-populations can experience different
  selective pressures, giving them different allele
  frequencies
• When individuals move between
  sub-populations they alter allele frequencies in
  the recipient population

Example 100 deer with frequencies of 0.8 A and
0.2 a join a population of 1000 deer with
frequencies of 0.5 A and 0.5 a
Dp m(pm-pi) so for A Dp 0.1 (0.8-0.5)
0.03 therefore the new frequency for A is pn
pi Dp 0.5 0.03 0.53
8
Lets Look at Hardy-Weinbergs Conditions as
Forces of Evolution
Natural Selection Mutations
Migration Genetic Drift (sampling errors)
Mating is random
9
Mutation

• Genotypes are shuffled by independent
  assortment and recombination
• But mutation is the only source of new
  alleles
• Mutations occur at random and are rare (e.g.
  1 x 10-5)
• Mutations alone change allele frequencies
  very slowly

So, selection and genetic drift work with
mutation to enhance the change in frequency of
mutant alleles.
10
Lets Look at Hardy-Weinbergs Conditions as
Forces of Evolution
Natural Selection Mutations
Migration Genetic Drift (sampling errors)
Mating is random
11
Natural Selection

• Assumption individuals of all genotypes
  have equal rates of survival and equal
  reproductive success.
• If this assumption is violated, populations
  will evolve.
• Violation of this assumption means some
  individuals are selected out of the gene pool.

Example for 100 individuals with A0.5, a0.5
If assumption is not violated, the genotype
frequencies are AA p2 (0.5)2 0.25 Aa
2pq 2(0.5)(0.5) 0.5 aa q2 (0.5)2
0.25 or 25 AA, 50 Aa, 25 aa individuals
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Why Selection alters allele frequencies

• Imagine some genotypes survive better than
  others
• All AAs (25) survive, 90 of Aas (0.9 x 50
  45) survive, and 80 of aas (0.8 x 25 20)
  survive
• When these survivors reproduce, each
  contributes 2 alleles to the gene pool 2(25)
  2(45) 2(20) 180 gametes total

• Frequency of alleles in the 180 gametes
• - 50 As from 25 AA, plus 45 As from 45 Aa 95
  As 95/180 0.53
• - 45 as from 45 Aa, plus 40 as from 20 aa
  85 as 85/180 0.47

Remember we started with 0.5 A and 0.5 a
13
Lets Look at Hardy-Weinbergs Conditions as
Forces of Evolution
Natural Selection Mutations
Migration Genetic Drift (sampling errors)
Mating is random
14
Genetic Drift
Genetic drift is a process of pure chance by
which gene frequencies can change with no
external stimuli.

• So why would the frequency of a particular allele
  change simply change by chance?
• In small populations, random chance can cause
  fluctuations in allele frequencies, it can even
  cause fixation of one
• The degree of fluctuation increases as the
  population size decreases
• This situation is called Genetic Drift

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Genetic Drift

• When you have a large sample size you can
  see ratios
• For example with 2000 breeding Aa
  individuals, you might find in 1000 progeny 237
  AA, 495 Aa, and 268 aa ( 25 AA, 50 Aa, 25 Aa)
• Frequencies stay close to 0.5 A and 0.5 a
• If you have 4 individuals that only produce
  2 offspring, you could get a lot of deviation
  from 0.5 of each allele in the next generation
• So, the larger the number of reproducing
  individuals, the smaller the effects of genetic
  drift.

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Genetic Drift
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Genetic Drift
There must be heterogeneity in order for genetic
drift to occur.
But heterogeneity doesnt guarantee genetic drift.
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Genetic Drift
What is needed?

• Differences between individuals
• Differences be passed from parents to
  offspring
• More offspring born than live to reproduce
• The abundance of different forms
  (phenotypes) within a population can then change
  over time or evolve