The Evolution of Populations - PowerPoint Presentation

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The Evolution of Populations

Chapter

11 Page 307Slide2

A.

Genetic Variation Within Populations

Genetic variation increases survival rate

Natural selection works on phenotypes differentlyWide range of phenotypes = greater chance of survival with changing environmentGene pool – combined alleles of all individuals in a pop.Allele frequency – how common certain alleles are in a pop.Slide3

2.

Genetic variation comes from several sources

a. Mutation 1) Can form new allele 2)

Mutations in gametes get passed to offspring

b.

Recombination

– alleles arranged in new ways during meiosisSlide4

B.

Natural Selection in Populations

Acts on distribution of traits

Normal distribution – frequency is highest near the mean value and decreases towards each end of the rangeBell shaped graphHeight of people or tails of pheasantsMore advantageous = more common

Changing environment could cause different traits to become more advantageousSlide5

Example – Height of people

Most will be average height

There will be some really tall

There will be some really shortSlide6

2.

Natural selection and distribution of traits

a. Microevolution 1) Observable change in allele frequency

2)

Small changes over time

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)

Natural selection helps this Slide7
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b.

Directional Selection – selection that favors phenotypes at the extreme end of the range

1) Causes a shift in a population’s phenotypic distribution 2) Phenotype was rare and is now common 3) Drug-resistant bacteria a)

We develop drug to kill them

b)

Bacteria become resilient

c)

We make a new drug, etc.Slide9

Directional Selection of BacteriaSlide10

c.

Stabilizing selection – intermediate phenotype is favored and becomes more common in a population

1) Nature prevents phenotypes at the extremes to develop 2) Gall fly – lay eggs in goldenrod plant a)

Downy woodpeckers feed on large galls

b)

Parasitic wasp lay eggs in small galls. Larva eat the fly larva

3)

Could cause extreme phenotypes to be lost completelySlide11

Stabilizing Selection – pg

312 Slide12

d.

Disruptive selection – both extreme phenotypes are favored, while intermediates are selected against

1) Species can start looking very different 2) Can lead to new species Slide13
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C.

Other Mechanisms of Evolution

Gene flow

Movement of alleles from one population to anotherHappens when organisms move b/w populationsIncreases genetic diversity of receiving populationLack of gene flow makes two populations become more genetically different over timeSlide15
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2.

Genetic Drift

a. Changing frequency of alleles due to chance b. Bottleneck Effect 1) Happens after population is greatly reduced

2)

Population numbers can recover

3)

Gene pool is still reduced – some genes are fixed and others may have disappeared completelySlide18
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c.

Founder Effect

1) Happens when small # of pop. colonize a new area 2) Gene pools are very different than larger pop.Slide20

d.

Effects of Genetic Drift

1) Lose genetic variation 2) Less individuals can adapt to changing environment

3)

Lethal mutations become more prevalent Slide21

3.

Sexual Selection

a. Certain traits increase mating success b. Intrasexual selection

1)

Competition among males

2)

Giraffes, deer, bighorn, etc. fighting

c.

Intersexual selection

1)

Males have traits that attract females

2)

Bright feathers/colors, strength, cunning Slide22
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d.

Not always a good thing

1) Frigate birds – air sacs too bright – easy to spot 2) Pheasant – tails too long = can’t hidee. Sexual selection and natural selection

How do they relate to each other?