ch. 23 the evolution of populations · 2011. 12. 19. · pink flowers incomplete dominance red...

40
1 Ch. 23 The Evolution of Populations

Upload: others

Post on 02-Nov-2020

1 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

1

Ch. 23The Evolution of Populations

Page 2: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

2

Essential question:

Do populations evolve?

Page 3: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

3

Mutation and Sexual reproduction produce genetic variation that makes evolution possible

What is the smallest unit of evolution?

genetic variations come from microevolution = change in allele frequencies in a population over generations

Three main mechanisms that cause allele frequency:1. natural selection*2. genetic drift (chance events that alter allele frequencies)3. gene flow (transfer of alleles between populations)

* natural selection is the only mechanism that improves the match between organisms and their environment

Page 4: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

4

When looking at genetic variation, need to look at:

­discrete characters­ classified as either/or Ex. black or white­ many determined by a single gene locus with different alleles at that locus (think dominant/recessive)

­Quantitative characters ­ characteristics that vary along a continuum within a population; 2 or more genes involved

Examples?? Calico cats

Page 5: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

5

Scientists measure variation by looking at:1. whole gene level (gene variability) 2. molecular level of DNA (nucleotide variation)

Gene variability can be quantified by Average heterozygosity = the average % of loci that are heterozygous

Heterozygous = two different alleles for a given locusEx. Fruit flies ­ 13,700 genes, heterozygous for 1,920

loci (14%)

Nucleotide variability ­ measured by comparing DNA sequences of 2 individuals in a population and then average data of many comparisons.

What is the difference between you and me?

Page 6: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

6

Variation between populations

geographic variation­ differences between gene pools of separate populations or population subgroups

Ex. mice in Madeira ­patterns of fusedchromosomes differ from one population to the other.

Page 7: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

7

clines­ a graded change in a trait along a geographic axis

Page 8: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

8

What factors do you think affect clines?

clines suggest natural selection because otherwise "there would be no reason to expect a close relationship between the environmental variable and the frequency of the allele"

Page 9: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

9

Ultimate source of new alleles is Mutations

mutation = any change in the nucleotide sequence of an organism's DNA

Types of mutations:1. point mutation ­ change of one base in a gene

­as long as the amino acid that a section codes for is not changed, then it will have no effect.­ but can be detrimental

2. translocation ­ movement of one part of chromosome to another chromosome or within itself.­beneficial if it links DNA segments to have a beneficial effect

Page 10: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

10

Gene duplication ­ when a gene segment is duplicated due to errors in meiosis, slippage during DNA replication or pieces moving

if are not harmful, can lead to mutations and natural selection

Sexual reproduction leads to genetic variation and makes evolution possible

Mechanisms that make this possible:1. crossing over during meiosis2. independent assortment of chromosomes3. fertilization

Page 11: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

11

Darwin couldn't explain how inherited variations were maintained in populations over time

Gregor Mendel had a model, but his manuscripts weren't really looked at until early 20th century

geneticists put Darwin's views and Mendel's view together = population genetics­ the study of how populations change

over time

mid 20th century ­ modern synthesis­ a comprehensive theory of evolution that integrated ideas from many fields

Fisher (1890­1962) ­ demonstrated rules of Mendelian inheritance

Haldane (1892­1964) ­ studied rules of natural selection

Page 12: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

12

Population = a localized group of individuals that are capable of breeding and producing fertile offspring

­some populations of species are isolated so rarely exchange genetic materialEx. Blue people of Kentucky

Gene pool = all of the alleles at all gene loci in all individuals of the populationfixed allele= one allele that exists at a particular locus in a

population (all individuals are homozygous for that allele)

Each allele has a frequency in the population

Page 13: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

13

Ex. 500 wildflower plants with 2 alleles for color C = red, C = white ­absence of red pigment (total = 1000 alleles)

R

W

C C x C C R R W W

C CR W

pink flowers ­ incomplete dominance

red flowers white flowers

In the population of 500 plants:320 plants = red 160 plants = pink 20 plants = white

C allele = 800 of the genes (320 x2 = 640 for C C plants, + 160 x1 = 160 for C C plants)

R

R

RR

W

Page 14: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

14

C allele = 200 of the genes (160 x 1 = 160 for C C plants , +,20 x 2 for C C plants

if have two alleles for a particular locus use p to represent frequency of one allele and q to represent frequency of other allele

p = dominant allele frequencyq = recessive allele frequency

W R W

W W

p + q = 1

Page 15: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

15

Page 16: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

16

p = frequency of C allele in the gene pool of this population

C accounts for 800 of the genes (640 + 160)800C alleles/ 1000 total alleles of population = 0.8 or 80%

frequency

R

R

R

C accounts for 200 of the genes (160 + 40)200 C alleles/ 1000 total alleles of population = 0.2 or 20 % frequency

W

W

q = frequency of the C allele in the gene pool of this population

W

Page 17: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

17

the sum of all frequencies must equal 1therefore p + q = 1

Even if you have more than two alleles, the sum of the frequencies must still = 1

Page 18: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

18

Hardy­Weinberg Principle (Theorem)

1908 by Hardy and Weinberg ­ worked independently

= the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

­works if gene pools not evolving

Hardy Weinberg Equilibrium = The condition describing a nonevolving population (one that is in genetic equilibrium)

Page 19: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

19

Conditions for Hardy­Weinberg equilibrium*

1. works for extremely large population size­smaller the population, more chance for fluctuation in allele frequencies

2. No gene flow ­ no transfer of alleles between populations

3. no mutations ­ mutations could modify the gene pool

4. Random mating ­ allele frequencies would be different if choose mates with specific traits

5. No natural selection ­ allele frequencies could be changed if have different survival and reproductive success

*changes in these conditions result in evolution

Page 20: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

20

Hardy­Weinberg Theorem

Page 21: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

21

Hardy­Weinberg equilibrium

­using previous example and rule of multiplication­can calculate the frequencies of the three possible genotypes

assuming random union of sperm and egg

probability of two C alleles will come together0.8 x 0.8 = p x p = p = 0.64

R

2

64% of the next generation will have genotype C CRR

The frequency of C C individuals 0.2 x 0.2 = q =0.04 or 4%

W W

2

Page 22: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

22

The frequency of heterozygotes C C can be from a sperm with C and egg with C or a sperm with C and an egg with C

R W R

W W R

0.8 x 0.2 = 0.16 or 16%

Can summarize unions of gametes using algebraic equation

p + 2pq + q = 12 2

Page 23: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

23

Hardy­Weinberg can be used to estimate percentage of population carrying an allele for an inherited disease

Ex. PKU ­ phenylketonuria = metabolic disorder1 in 10,000 people have this in U.S.

because allele is recessive corresponds to q2

frequency of the recessive allele:q = 0.0001 = 0.01

frequency of dominant allele:p = 1 ­ q = 1­ 0.01 = 0.99frequency of carriers (heterozygous)2pq = 2 x 0.99 x 0.01 = 0.0198 (approximately 2% of U.S. population

Page 24: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

24

Other ways to alter a population's genetic comparison

any change in the conditions that Hardy­Weinberg is based on

1. Natural Selectionindividuals with variations in their heritable traits that are better suited to environmental conditions can produce more offspring

Page 25: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

25

Genetic Drift­ unpredictable fluctuations in allele frequencies from one generation to the next because of a population's small size

Page 26: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

26

Two situations of genetic drifta. bottleneck effect­ happens when a sudden change in environment (like a fire or flood) drastically reduces the population size, so only survivors can pass genes on

­certain alleles may be more present than others, some may be eliminated­humans ­ can cause other species to go through this

­endangered species

Page 27: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

27

b. The Founder Effect­ when a few individuals become isolated from main population and establish a new gene pool not reflective of original population.

accounts for some human genetic disordersEx.

1814 colonist brought recessive alleles for retinitis pigmentosa (form of blindness) to Tristan da Cunhaislands1960's 4 people had disease (10x higher than normal)

Page 28: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

28

Gene Flow ­ genetic additions or subtractions from a population resulting from the movement of fertile individuals or gametes

­tends to reduce differences between populations­could eventually form a single large gene pool

Page 29: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

29

Natural Selection is the primary mechanism of adaptive evolution

­accumulates and maintains favorable genotypes in a population

Page 30: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

30

Evolutionary fitness

fitness = adaptive advantage, the contribution that an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

relative fitness = contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus

relative fitness = "0" if an animal or plant is sterile

Natural Selection

Page 31: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

31

(Disruptive)

Page 32: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

32

Directional selection­ favors individuals that deviate from the average­caused by environmental changes or migration of members to new environment with different environmental conditions than original habitat

Disruptive selection­ both extremes of phenotypic range are favored

Stabilizing selection ­ against extreme phenotypes and favors intermediate variants

selection favors heritable traits

Page 33: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

33

Directional Selection for beak size in Galapagos population of medium ground finch

Page 34: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

34

Sexual Selection ­ a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.

Page 35: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

35

Sexual selection­ natural selection for mating success

sexual dimorphism =marked differences between sexes in secondary sexual characteristics, not associated with reproduction

differences can be size, color, ornamentation

Page 36: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

36

intrasexual selection ­selection "within same sex" like a direct competition among individuals of same sex­usually in male birds

Bower bird ­ builds a colorful nest to attract femalepeacocks ­ fanning their feathers

Page 37: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

37

intersexual selection = mate choicemates are choosy about mates of other sexfemale's choice depends on male's appearance or behavior

Page 38: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

38

Preservation of genetic variation in a population ­

1. Diploidy­ most eukaryotic organisms are diploid (2 of each chromosome ­ one from each parent)

­organisms have recessive alleles­only shown when inherit both recessive genes­ frequency is very low­ the rarer the recessive allele, more protection from natural

selection

2. Balancing selection ­ when two or more forms exist in a population.

Page 39: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

39

a. heterozygote advantage heterozygote individuals have a greater fitness than homozygotes.

Ex. sickle ­cell allele ­ most common in Africa; also provides protection against malaria

Page 40: Ch. 23 The Evolution of Populations · 2011. 12. 19. · pink flowers incomplete dominance red flowers white flowers In the population of 500 plants: 320 plants = red 160 plants =

40

Why natural selection cannot make perfect organisms

1. Selection can edit only existing variationsfavors only fittest phenotypes among those currently in the population

2. evolution is limited by historical constraintsEx. ancestral anatomy can't be changed

3. Adaptations are often compromisesEx. humans have prehensile hands and flexible limbs, but prone to injury

4. Chance, natural selection and the environment interact­storms blowing insects or plant seeds to new areas­ they might not be the most fit organisms