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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
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
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
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
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
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
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
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
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 -
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
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.
Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem
◦ Describes a hypothetical population In real populations
◦ Allele and genotype frequencies do change over time
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
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
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
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.
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.
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.
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
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
The fourth assumption is that random mating occurs.
Non-random mating will distort Hardy-Weinberg frequencies.
4. Random mating
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
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.
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
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
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
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
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
Sexual Recombination
In sexually reproducing populations, sexual recombination◦ Is far more important than mutation in producing
the genetic differences that make adaptation possible
Three major factors alter allele frequencies and bring about most evolutionary change◦ Natural selection◦ Genetic drift◦ Gene flow
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
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
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
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
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
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
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
Neutral Variation Neutral variation
◦ Is genetic variation that appears to confer no selective advantage
Darwin explored the Galápagos Islands◦ And discovered plants and animals found nowhere
else on Earth
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
The biological species concept emphasizes reproductive isolation
Species◦ Is a Latin word meaning “kind” or “appearance”
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
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)
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
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
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)
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
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
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
(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
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
A. harrisi A. leucurus
Once geographic separation has occurred◦ One or both populations may undergo evolutionary
change during the period of separation
Sympatric (“Same Country”) Speciation
In sympatric speciation and parapatric speciation◦ Speciation takes place in geographically
overlapping populations
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
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.
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
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.
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
Adaptive Radiation
Adaptive radiation◦ Is the evolution of diversely adapted species from a
common ancestor upon introduction to new environmental opportunities
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