Population Genetics and Evolution

1
Population Geneticsand Evolution

• Ch 15. 2

2

• Darwin didnt know anything about genes, when he
  developed his theory.
• Now we are able to study the complex behavior of
  genes in a population.
• This is called Population Genetics

3

• Population Genetics is based on the idea that
  populations evolve NOT individuals.
• The phenotype of an individual organism cannot
  evolve in its lifetime.
• However, if an organisms phenotype is poorly
  suited for the environment, the organism may be
  unable to survive and reproduce. This is the idea
  of Natural Selection.

4

• We know that traits (i.e. hair color) develop
  based on our genes (i.e. a section of DNA) and
  genes exist in pairs and different versions of
  genes are called alleles (red hair, black hair,
  brown hair, blonde hair, etc).
• All the alleles of a populations genes can be
  grouped together into what is called a gene
  pool.

5

• You can look at a specific allele (B for black
  hair) and calculate its percentage in the gene
  pool
• This is called
• allelic frequency.

6
Phenotype frequency Black Mixed White
Allele frequency B W
12/16 ¾ .75
5/8 .625
2/8 ¼ .25
4/16 ¼ .25
1/8 .125
7
Phenotype frequency Black Mixed
Allele frequency B W
.50
12/16 ¾ .75
.50
4/16 ¼ .25
8

• If the frequency of alleles remains the same over
  generations it is said that the population is at
• genetic equilibrium

9
Changes in Genetic Equilibrium

• A population that is in genetic equilibrium is
  NOT evolving
• However, there are factors that can affect the
  genes in a gene pool and change the allelic
  frequency which ultimately leads to the process
  of evolution.

10
Changes in Genetic Equilibrium

• A change in the populations genetic equilibrium
  can be caused by
• Mutations naturally occurring and ones that
  resulted from environmental factors (radiation,
  UV rays)
• Genetic drift the alteration of allelic
  frequency by chance events
• Migration the movement of individuals in and out
  of a population. New genes could be added or
  lost. (aka gene flow)

11
Genetic Drift

• Genetic Drift can greatly affect small
  populations.
• For example, the Amish people in PA became
  isolated originally for religious practices.
• One of the original 30 settlers carried a
  recessive allele that resulted in short arms and
  legs and extra fingers and toes.
• Because of the small gene pool, many individuals
  inherited the recessive allele over time.

12
Amish

• An Amish person has a 1 in 14 chance of getting
  the recessive allele.
• An American in the USA has a 1 in 1000 chance of
  getting this recessive allele.

• This is the effect seen on small populations
  known as genetic drift

13
(No Transcript)
14
Quick Review
15
3 types of natural selection that act on variation

• Stabilizing selection
• Directional selection
• Disruptive selection

16
Stabilizing Selection

• Stabilizing selection is natural selection that
  favors average individuals in a population.

17
Stabilizing Selection

• A classic example of this is human birth
  weight. Babies of low weight lose heat more
  quickly and get ill from infectious disease more
  easily whereas babies of large body weight are
  more difficult to deliver. The mean is about
  6lbs.

18
(No Transcript)
19
Directional Selection

• Directional selection occurs when natural
  selection favors one of the extreme variations of
  a trait.

20
Directional Selection

• For example, woodpeckers feed on insects under
  the bark. Suppose a species of insect invades
  that burrows deeper in the bark. Only woodpeckers
  with long beaks could feed on that insect.
  Therefore, long-beaked woodpeckers would have a
  selective advantage.

21
(No Transcript)
22
Disruptive Selection

• Disruptive selection occurs when natural
  selection favors individuals with either extreme
  of a traits variation.

23
Disruptive Selection

• Consider a population of limpets. There is a
  wide variation of shell color (white, tan, dark
  brown). On light colored rocks, white shelled
  limpets have an advantage because birds cannot
  see them so easily. On dark colored rocks,
  dark-colored limpets are camouflaged. However, if
  a tan colored limpet is on a light or dark rock,
  they are spotted immediately. Disruptive
  selection tends to eliminate the intermediate
  phenotype.

24
Limpets
25
(No Transcript)
26

• The Evolution of Species

27

• Species a group of organisms that can interbreed
  and produce fertile offspring
• Speciation the process by which a new species
  evolves. This means that two similar populations
  can no longer interbreed and produce fertile
  offspring.

28
How can new species evolve?

• Geographic isolation when a physical barrier
  divides a population. i.e. lava, water,
  earthquakes
• Reproductive Isolation populations become
  increasingly distinct and formerly interbreeding
  organisms can no longer mate and produce fertile
  offspring

29
Geographic isolation

• Over time, each small population might adapt to
  its environment through natural selection and
  develop its own gene pool. The gene pools might
  become so different that they could no longer
  interbreed.

30
Reproductive isolation

• Method 1 When the genetic material of the
  populations becomes so different that
  fertilization cannot occur.
• Method 2 Also another type is behavioral. One
  population could mate in the fall while the other
  population mates in the summer.

31
How else can new species evolve?

• Also, chromosomes could affect the evolution of a
  new species from an existing population.
• An individual organism with multiple sets of
  chromosomes polyploid
• This is commonly seen in plants because they can
  self-fertilize. If a plant has 4 sets of
  chromosomes and reproduces with itself and it
  survives and is successful ? this could arise
  into a new species

32
Scientists argue how evolution occurs

• Some believe it occurs at a slow steady rate with
  small, adaptive changes gradually accumulating
  over time in populations. (Gradualism)
• Some believe that speciation occurs relatively
  quickly, in rapid bursts, with long periods of
  genetic equilibrium in between. (Punctuated
  equilibrium)
• Both methods feel that fossils provide evidence.

33
Punctuated Equilibrium
Gradualism
Gradual change
Rapid bursts
34
(No Transcript)
35
Patterns of Evolution

• Adaptive radiation an ancestral species evolves
  into an array of species to fit a number of
  diverse habitats
• Example Hawaiian honeycreepers

36
Hawaiian Honeycreepers
37
Divergent Evolution

• Divergent evolution the pattern of evolution
  in which species that once were similar to an
  ancestral species diverge.

Divergent evolution is type of adaptive
radiation.
38
Convergent evolution

• A pattern of evolution in which distantly
  related organisms evolve similar traits

This occurs when unrelated species occupy similar
habitats in different parts of the world because
they share similar environmental pressures.