natural selection and mechanisms of evolution (ii)

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Natural Selection and Mechanisms of Evolution (II) Descent with Modification: A Darwinian View of Life The Origin of Species The Evolution of Populations Prepared by Raajeswari Rajendran

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Page 1: Natural Selection and Mechanisms of Evolution (II)

Natural Selection and Mechanisms of Evolution

(II)Descent with Modification: A Darwinian View of Life

The Origin of SpeciesThe Evolution of Populations

Prepared by Raajeswari Rajendran

Page 2: Natural Selection and Mechanisms of Evolution (II)

The Smallest Unit of Evolution One common misconception about evolution is that

individual organisms evolve, in the Darwinian sense, during their lifetimes

Natural selection acts on individuals, but populations evolve.

Genetic variations in populations◦ Contribute to evolution

Page 3: Natural Selection and Mechanisms of Evolution (II)

The Modern Synthesis Population genetics provides a foundation for studying

evolution Microevolution

◦ Is change in the genetic makeup of a population from generation to generation

Population genetics◦ Is the study of how populations change genetically over

time◦ Reconciled Darwin’s and Mendel’s ideas

The modern synthesis◦ Integrates Mendelian genetics with the Darwinian theory of

evolution by natural selection◦ Focuses on populations as units of evolution

Page 4: Natural Selection and Mechanisms of Evolution (II)

Gene Pools and Allele Frequencies A population

◦Is a localized group of naturally occurring individuals that are capable of interbreeding and producing fertile offspring.

The gene pool◦ Is the total aggregate of genes in a population of

species at any one time◦ Consists of all gene loci in all individuals of the

population

Page 5: Natural Selection and Mechanisms of Evolution (II)

The Hardy-Weinberg Theorem The Hardy-Weinberg theorem

◦ Describes a population that is not evolving◦ States that 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.

Mendelian inheritance◦ Preserves genetic variation in a population

Page 6: Natural Selection and Mechanisms of Evolution (II)

Preservation of Allele Frequencies - Hardy-Weinberg Equilibrium In a given population where gametes contribute to

the next generation randomly, allele frequencies will not change.

Hardy-Weinberg equilibrium◦ Describes a population in which random mating

occurs◦ Describes a population where allele frequencies do

not change

Page 7: Natural Selection and Mechanisms of Evolution (II)

A population in Hardy-Weinberg equilibrium

Gametes for each generation are drawn at random fromthe gene pool of the previous generation:

80% CR (p = 0.8) 20% CW (q = 0.2)

SpermCR

(80%)CW

(20%)

p2

64%CRCR

16%CRCW

16%CRCW

4%CWCW

qp

CR

(80

%)

Egg

s

CW

(20

%)

pq

If the gametes come together at random, the genotypefrequencies of this generation are in Hardy-Weinberg equilibrium:

q2

64% CRCR, 32% CRCW, and 4% CWCW

Gametes of the next generation:

64% CR fromCRCR homozygotes

16% CR fromCRCW homozygotes+ = 80% CR = 0.8 = p

16% CW fromCRCW heterozygotes+ = 20% CW = 0.2 = q

With random mating, these gametes will result in the samemix of plants in the next generation:

64% CRCR, 32% CRCW and 4% CWCW plants

p2

4% CW fromCWCW homozygotes

Page 8: Natural Selection and Mechanisms of Evolution (II)

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then◦ p2 + 2pq + q2 = 1 for the next generation◦ And p2 and q2 represent the frequencies of the

homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Page 9: Natural Selection and Mechanisms of Evolution (II)

Frequency of different alleles of a gene in a population can be estimated by examining the proportion of individuals that have particular phenotypes and genotypes.

E.g. human MN blood group – controlled by 2 codominant alleles, LM and LN. The heterozygote is blood type MN and the homozygotes are blood types M and N.

A population of Aboriginal people from Elcho Island in Australia’s Northern Territory has 28 individuals with blood type M, 129 individuals with blood type MN and 195 individuals with blood type N. (Refer to handout (Knox) – pg 838-839).

A population in Hardy-Weinberg equilibrium -

Page 10: Natural Selection and Mechanisms of Evolution (II)

Current generation: Frequency of LM allele in the population –

185/704 = 0.26 = p. Frequency of LN allele in the population –

519/704 = 0.74 = q. p + q = 1 Next generation: p2 + 2pq + q2 = 1 Blood group M, (p2) = (0.26)2 = 0.07 Blood group N, (q2) = (0.74)2 = 0.54 Blood group MN, (2pq) = (2 x 0.26 x 0.74) = 0.39

Page 11: Natural Selection and Mechanisms of Evolution (II)

If the genotype frequencies for the next generation is similar to the frequency predicted by the Hardy-Weinberg theorem, then evolution has not taken place.

Page 12: Natural Selection and Mechanisms of Evolution (II)

Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem

◦ Describes a hypothetical population In real populations

◦ Allele and genotype frequencies do change over time

Page 13: Natural Selection and Mechanisms of Evolution (II)

Five assumptions/conditions to apply the Hardy-Weinberg theorem The five conditions for non-evolving

populations are rarely met in nature◦ Extremely large population size◦ No gene flow◦ No mutations◦ Random mating◦ No natural selection

Page 14: Natural Selection and Mechanisms of Evolution (II)

The first assumption is population size is large.

If the population is very small, or if it goes through periodic ‘bottlenecks’ of low numbers, alleles in the population may drift to high or low frequencies purely by chance.

These random changes in allele frequencies in small isolated populations is termed genetic drift and can lead to rapid fluctuations in allele frequencies.

1.Population size

Page 15: Natural Selection and Mechanisms of Evolution (II)

Genetic Drift Statistically, the smaller a sample

◦ The greater the chance of deviation from a predicted result Genetic drift

◦ Describes how allele frequencies can fluctuate unpredictably from one generation to the next

◦ Tends to reduce genetic variation

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

Page 16: Natural Selection and Mechanisms of Evolution (II)

The Bottleneck Effect In the bottleneck effect

◦ A sudden change in the environment may drastically reduce the size of a population

◦ The gene pool may no longer be reflective of the original population’s gene pool

Originalpopulation

Bottleneckingevent

Survivingpopulation

(a)

Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

Page 17: Natural Selection and Mechanisms of Evolution (II)

Bottleneck effect Understanding the bottleneck effect

◦ Can increase understanding of how human activity affects other species bringing about a fall in the population of species making it vulnerable to a genetic drift.

(b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

Page 18: Natural Selection and Mechanisms of Evolution (II)

The Founder Effect The founder effect

◦ Occurs when a few individuals become isolated from a larger population

◦ Can affect allele frequencies in a population◦ This too leads to a genetic drift.

Page 19: Natural Selection and Mechanisms of Evolution (II)

The second assumption is that individuals with particular alleles do not migrate at different rates.

Migration of individuals with particular alleles at different rates can lead to changes in allele frequencies.

2. No gene flow / migration

Page 20: Natural Selection and Mechanisms of Evolution (II)

The third assumption is that different alleles mutate at the same rate and therefore unlikely to lead to rapid changes in the population.

If alleles mutate at different rates, they may lead to changes in the allele frequencies of the next generation.

3. Mutation rate

Page 21: Natural Selection and Mechanisms of Evolution (II)

The fourth assumption is that random mating occurs.

Non-random mating will distort Hardy-Weinberg frequencies.

4. Random mating

Page 22: Natural Selection and Mechanisms of Evolution (II)

Constant Hardy-Weinberg frequencies will occur only if there are no differences in the Darwinian fitness of individual genotypes.

The assumption made here is that the environment is constant and has no changes.

Fitness is measured as the relative contribution of offspring to subsequent generations due to differential survival and or reproduction.

If environment changes and natural selection is in progress, then the allele frequencies will change.

5. Darwinian fitness/no natural selection

Page 23: Natural Selection and Mechanisms of Evolution (II)

Given the above assumptions, the genotype frequencies are expected to remain constant from generation to generation.

If one or more of these assumptions is not met, allele frequencies in a population may change and the population may evolve.

Page 24: Natural Selection and Mechanisms of Evolution (II)

Population Genetics and Human Health

We can use the Hardy-Weinberg equation◦ To estimate the percentage of the human

population carrying the allele for an inherited disease

Page 25: Natural Selection and Mechanisms of Evolution (II)

When Darwin proposed natural selection as the primary mechanism for evolution, he emphasized the importance of heritable differences among individuals.

He knew that natural selection could not cause evolutionary change unless individuals differed in their inherited characteristics.

What are the sources of genetic variation?

Genetic variation

Page 26: Natural Selection and Mechanisms of Evolution (II)

1. Mutation – source of new alleles in a population

2. Crossing-over and recombination 3. Gene flow – immigration and emigration 4. Random segregation and independent

assortment 5. Random fertilization 6. Random mating

Sources of genetic variation

Page 27: Natural Selection and Mechanisms of Evolution (II)

Genetic variation◦ Occurs in individuals in populations of all species◦ Is not always heritable

Mutation and sexual recombination produce the variation that makes evolution possible

The two processes, mutation and sexual recombination◦ Produce the variation in gene pools that

contributes to differences among individuals

Page 28: Natural Selection and Mechanisms of Evolution (II)

Mutation Rates

Mutation rates◦ Tend to be low in animals and plants◦ Average about one mutation in every 100,000

genes per generation◦ Are more rapid in microorganisms

Page 29: Natural Selection and Mechanisms of Evolution (II)

Sexual Recombination

In sexually reproducing populations, sexual recombination◦ Is far more important than mutation in producing

the genetic differences that make adaptation possible

Page 30: Natural Selection and Mechanisms of Evolution (II)

Three major factors alter allele frequencies and bring about most evolutionary change◦ Natural selection◦ Genetic drift◦ Gene flow

Page 31: Natural Selection and Mechanisms of Evolution (II)

Natural Selection Differential success in reproduction

◦ Results in certain alleles being passed to the next generation in greater proportions

Natural selection is the primary mechanism of adaptive evolution

Natural selection◦ Accumulates and maintains favorable genotypes

in a population

Page 32: Natural Selection and Mechanisms of Evolution (II)

Gene Flow Gene flow

◦ Causes a population to gain or lose alleles◦ Results from the movement of fertile individuals or

gametes◦ Tends to reduce differences between populations

over time

Page 33: Natural Selection and Mechanisms of Evolution (II)

Variation Within a Population Both discrete/quantitative and quantitative

characters◦ Contribute to variation within a population

Discrete characters◦ Can be classified on an either-or basis

Quantitative characters◦ Vary along a continuum within a population

Page 34: Natural Selection and Mechanisms of Evolution (II)

Variation Between Populations Most species exhibit geographic variation

◦ Differences between gene pools of separate populations or population subgroups

1 2.4 3.14 5.18 6 7.15

XX1913.1710.169.128.11

1 2.19 3.8 4.16 5.14 6.7

XX15.1813.1711.129.10

Page 35: Natural Selection and Mechanisms of Evolution (II)

Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

EXPERIMENT Researchers observed that the average size

of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants.

RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments.

CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates therole of genetic as well as environmental differences.

Mean

heig

ht

(cm

)A

titu

de (

m)

Heights of yarrow plants grown in common garden

Seed collection sites

Sierra NevadaRange

Great BasinPlateau

Page 36: Natural Selection and Mechanisms of Evolution (II)

A Closer Look at Natural Selection From the range of variations available in a

population◦ Natural selection increases the frequencies of

certain genotypes, fitting organisms to their environment over generations

Page 37: Natural Selection and Mechanisms of Evolution (II)

Evolutionary Fitness The phrases “struggle for existence” and “survival of

the fittest”◦ Are commonly used to describe natural selection◦ Can be misleading

Reproductive success◦ Is generally more subtle and depends on many factors

Fitness◦ Is the contribution an individual makes to the gene pool of

the next generation, relative to the contributions of other individuals

Relative fitness◦ Is the contribution of a genotype to the next generation as

compared to the contributions of alternative genotypes for the same locus

Page 38: Natural Selection and Mechanisms of Evolution (II)

Neutral Variation Neutral variation

◦ Is genetic variation that appears to confer no selective advantage

Page 39: Natural Selection and Mechanisms of Evolution (II)

Darwin explored the Galápagos Islands◦ And discovered plants and animals found nowhere

else on Earth

Page 40: Natural Selection and Mechanisms of Evolution (II)

The origin of new species, or speciation◦ Is at the focal point of evolutionary theory, because

the appearance of new species is the source of biological diversity

Evolutionary theory◦ Must explain how new species originate in addition

to how populations evolve Macroevolution

◦ Refers to evolutionary change above the species level

Page 41: Natural Selection and Mechanisms of Evolution (II)

The biological species concept emphasizes reproductive isolation

Species◦ Is a Latin word meaning “kind” or “appearance”

Page 42: Natural Selection and Mechanisms of Evolution (II)

The Biological Species Concept

The biological species concept◦ Defines a species as a population or group of

populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations

Page 43: Natural Selection and Mechanisms of Evolution (II)

Similarity between different species. The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) have similar body shapes and colorations. Nevertheless, they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild.

(a)

Diversity within a species. As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed.

(b)

Page 44: Natural Selection and Mechanisms of Evolution (II)

Reproductive Isolation

Reproductive isolation◦ Is the existence of biological factors that impede

members of two species from producing viable, fertile hybrids

◦ Is a combination of various reproductive barriers

Page 45: Natural Selection and Mechanisms of Evolution (II)

Prezygotic barriers◦ Impede mating between species or hinder the

fertilization of ova if members of different species attempt to mate

Postzygotic barriers◦ Often prevent the hybrid zygote from developing

into a viable, fertile adult

Page 46: Natural Selection and Mechanisms of Evolution (II)

Prezygotic and postzygotic barriersPrezygotic barriers impede mating or hinder fertilization if mating does occur

Individualsof different

species

Matingattempt

Habitat isolation

Temporal isolation

Behavioral isolation

Mechanical isolation

HABITAT / ECOLOGICAL ISOLATION TEMPORAL ISOLATIONBEHAVIORAL / ETHOLOGICAL ISOLATIONMECHANICAL ISOLATION

(b)

(a)(c)

(d)

(e)

(f)

(g)

Page 47: Natural Selection and Mechanisms of Evolution (II)

Viablefertile

offspring

Reducehybrid

viability

Reducehybridfertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRID VIABILITY

REDUCED HYBRID FERTILITYHYBRID BREAKDOWN

(h) (i)

(j)

(k)

(l)

(m)

Post-zygotic barriers

Page 48: Natural Selection and Mechanisms of Evolution (II)

Limitations of the Biological Species Concept

The biological species concept cannot be applied to◦ Asexual organisms◦ Fossils◦ Organisms about which little is known regarding

their reproduction

Page 49: Natural Selection and Mechanisms of Evolution (II)

Other Definitions of Species

The morphological species concept◦ Characterizes a species in terms of its body shape,

size, and other structural features The paleontological species concept

◦ Focuses on morphologically discrete species known only from the fossil record

The ecological species concept◦ Views a species in terms of its ecological niche

The phylogenetic species concept◦ Defines a species as a set of organisms with a

unique genetic history

Page 50: Natural Selection and Mechanisms of Evolution (II)

(a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population.

(b) Sympatric speciation. A smallpopulation becomes a new specieswithout geographic separation.

Speciation can take place with or without geographic separation

Speciation can occur in two ways◦ Allopatric speciation◦ Sympatric speciation and

parapatric speciation

Speciation – Formation of New Species

Page 51: Natural Selection and Mechanisms of Evolution (II)

Allopatric (“Other Country”) Speciation

In allopatric speciation◦ Gene flow is interrupted or reduced when a

population is divided into two or more geographically isolated subpopulations

Page 52: Natural Selection and Mechanisms of Evolution (II)

A. harrisi A. leucurus

Once geographic separation has occurred◦ One or both populations may undergo evolutionary

change during the period of separation

Page 53: Natural Selection and Mechanisms of Evolution (II)

Sympatric (“Same Country”) Speciation

In sympatric speciation and parapatric speciation◦ Speciation takes place in geographically

overlapping populations

Page 54: Natural Selection and Mechanisms of Evolution (II)

Causes of sympatric speciation -Polyploidy

Polyploidy◦ Is the presence of extra sets of chromosomes in

cells due to accidents during cell division◦ Has caused the evolution of some plant species –

grass and wheat◦ There are two types of polyploidy – autopolyploidy

and allopolyploidy

Page 55: Natural Selection and Mechanisms of Evolution (II)

An autopolyploid◦ Is an individual that has more than two

chromosome sets, all derived from a single species

2n = 64n = 12

2n

4n

Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.

Gametes produced by flowers on this branch will be diploid.

Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

Page 56: Natural Selection and Mechanisms of Evolution (II)

An allopolyploid◦ Is a species with multiple sets of chromosomes derived from

different species

Meiotic error;chromosomenumber notreduced from2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes Viable fertile hybrid

(allopolyploid)

Normal gameten = 3

Normal gameten = 3

Species A 2n = 4

Species B 2n = 6

2n = 10

Page 57: Natural Selection and Mechanisms of Evolution (II)

Habitat Differentiation – Parapatric speciation

Parapatric speciation is a mode of speciation in which divergence occurs among populations that have contiguous distributions and hence are incompletely geographically separated.◦ As a result of divergent selection, gene flow

between the populations becomes prgressively reduced and eventually the differentiated populations become reproductively isolated.

◦ Sympatric and parapatric modes of speciation assume that speciation takes place without complete geographical isolation.

Page 58: Natural Selection and Mechanisms of Evolution (II)

Allopatric and Sympatric Speciation: A Summary

In allopatric speciation◦ A new species forms while geographically isolated

from its parent population In sympatric speciation

◦ The emergence of a reproductive barrier isolates a subset of a population without geographic separation from the parent species

Page 59: Natural Selection and Mechanisms of Evolution (II)

Adaptive Radiation

Adaptive radiation◦ Is the evolution of diversely adapted species from a

common ancestor upon introduction to new environmental opportunities

Page 60: Natural Selection and Mechanisms of Evolution (II)

The Hawaiian archipelago◦ Is one of the world’s great showcases of adaptive

radiation

Dubautia laxa

Dubautia waialealae

KAUA'I5.1

millionyears O'AHU

3.7millionyears

LANAI

MOLOKA'I

1.3 million years

MAUI

HAWAI'I0.4

millionyears

Argyroxiphium sandwicense

Dubautia scabra Dubautia linearis

N