The Evolution of Populations

1
The Evolution of Populations

• Chapter 23

2
Modern Synthesis

• What difficulty did Darwin have with his
  mechanism of natural selection?
• Genetic basis
• Even after Mendel inheritance was between
  discrete characteristics
• By 1940s modern synthesis combined ideas from
  different fields
• Population is the unit of evolution
• Natural selection is the main mechanism
• Gradualism explained accumulation of small
  changes led to large ones over time

3
Population Genetics

• What is a population?
• Members of the same species living in the same
  area
• Gene pool is the total collection of alleles in
  the population
• What is microevolution?
• The change in the frequency of alleles in a
  population over time

4

• Population of wildflowers where pink allele is A
  and white allele is a
• 500 total plants in the population, 20 are white
  and 480 are pink (320 AA and 160 Aa)
• How many total alleles for flower color are
  there in this population of 500 flowers?
• 1000
• What is the frequency of A allele and a allele?
• A 0.8 a 0.2

5
Hardy Weinberg Theorem

• Conditions for non-evolving population
• Very large population size
• No migration
• No mutations
• Random mating
• No natural selection
• How would our wildflower population work
  according to these criteria?
• A 0.8 and a 0.2

6

• p frequency of dominant alleles
• q frequency of recessive alleles
• What are the three possible genotypes that result
  from random mating of these wildflowers?
• AA, Aa and aa
• Using the rule of multiplication we can calculate
  the frequencies of each genotype in the next
  generation (assuming no evolution)
• Notice that p and q (allele frequencies) have
  remained the same

7

• 2 equations are generated from the probabilities
  involved
• p q 1
• This means that there are only 2 possible alleles
  p and q
• The equation that corresponds to the frequency of
  individuals regarding these two alleles
• p2 2pq q2 1
• p2 frequency of homozygous dominant individuals

8

• q2 frequency of homozygous recessive
  individuals
• 2pq frequency of heterozygous individuals
• Round head is dominant to cone heads, 51 of the
  individuals in the population have round heads.
  What portion of this 51 are homozygous?
• 0.49 q2 therefore q 0.7, so p 0.3
• p2 is the frequency of homozygous dominant
  individuals 0.09 or 9

9
Causes of Microevolution

• We can use the 5 criteria for non-evolution to
  determine causes of microevolution
• The Hardy Weinberg equations are used to
  determine the degree of microevolution that is
  occurring for a given allele

10
Genetic Drift

• Changes in the gene pool of a small population
  due to chance
• Examples
• Bottleneck
• Founder Effect where few individuals colonize an
  isolated area
• Is genetic drift an adaptive or non-adaptive
  mechanism?
• Non-adaptive

11
Gene Flow

• Negating the HW rule of no migration is the
  mechanism of gene flow
• Migration of fertile individuals will alter the
  frequency of alleles
• Gene flow can combine populations that were at
  one time isolated
• Human populations have increased gene flow
  through world travel
• Is gene flow adaptive or non-adaptive?

12
Mutations

• Mutations can immediately alter p and q in a
  population
• Individual mutations are rare in a population,
  but there may be cumulative mutations that have
  an effect.
• Mutations are the original source of variation in
  a population
• Are mutations adaptive or non-adaptive?

13
Non-random Mating

• Inbreeding
• Selfing is most extreme inbreeding
• Leads to increase in homozygous individuals at
  expense of heterozygous individuals
• When Aa individuals self only 1/2 their
  offspring are Aas, every generation sees a
  decrease in the heterozygote
• Assortative mating - choosing individuals more
  like self
• Adaptive or non-adaptive?

14
Natural Selection

• Differential success among offspring leads to
  shift in frequency of alleles in the population
• Adaptive or non-adaptive mechanism?
• ADAPTIVE!

15
Types of Variations

• Variations may be due to additive effects such as
  with polygenic inheritance
• Variations of discrete traits such as
  polymorphisms where 2 or more discrete morphs can
  be seen within the population (e.g. A,B O blood
  groups)
• Clines are types of geographical variations

16
Sources of Variations

• Mutations
• At first the mutation may not be beneficial to
  the organism (resistance to antibiotics)
• Once antibiotics are introduced the mutation is
  beneficial
• Sexual recombination increases variation

17
How are variations maintained?

• What prevents natural selection from eliminating
  all variations?
• Diploidy where the recessive allele is
  unfavorable and has a very low frequency in the
  population the heterozygous individual allows it
  to persist
• Balancing selection

18

• Neutral variations
• fingerprints
• variations in non-coding portions of genome
• Can we be certain that these variations are truly
  neutral?

19
Fitness

• Fitness is determined by reproductive success
• Variations and natural selection affect fitness
• Fitness is not determined by one trait but by the
  totality of traits in the organism

20
Modes of Selection

• Stabilizing selection
• Directional Selection
• Diversifying Selection

21
Sexual Dimorphism

• Males are usually more colorful and larger
• Doesnt help cope with environment
• Does lead to reproductive success
• Can be termed Female Selection

22
Natural Selection Does Not Design Perfect
Organisms

• Organisms are locked into historical constraints
• Adaptations are often compromises as with seals
  that need to swim and walk
• Not all evolution is adaptive
• Selection can only select what is already there.
  Organisms cant get a trait because they need
  it.

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