Transcript Unit 9 Population Genetics Chp 23 Evolution of

CHAPTER 23
EVOLUTION OF
POPULATIONS
• Population Genetics/ Modern Synthesis
• Hardy-Weinberg Theory (Non-evolving Population)
• Microevolution (Evolving Populations)
– Genetic Drift
– Gene Flow
– Mutations
– Natural Selection
Population Genetics
One obstacle to understanding evolution is the common misconception that individual organisms
evolve, in the Darwinian sense, during their lifetimes. In fact, natural selection does act on
individuals; their characteristics affect their chances of survival and their reproductive success.
But the evolutionary impact of this natural selection is only apparent in tracking how a population
of organisms changes over time.
Consider, for example, representatives from a population of marine snails (Liguus fascitus).
Their different patterns of coloration represent genetic variations within that population. If predators feed
preferentially on snails having a particular coloration, then the proportion of individuals with that coloration
probably will decline from one generation to the next because such snails will produce fewer offspring.
Thus, it is the population, not its individuals, that evolves; some characteristics become more common within
the overall population, while other characteristics decline .
Evolution on the smallest scale, or microevolution, can be defined as a change in the allele
frequencies of a population
We begin our study of microevolution by tracing how biologists finally began to understand
Darwin’s theory of natural selection during the first half of the 20th century .
Example of Microevolution:
Individuals are selected, but populations evolve. The bent grass (Agrostis tenuis) in the foreground is growing on
the tailings of an abandoned mine in Wales. These plants tolerate concentrations of heavy metals that are toxic to
other plants of the same species growing just meters away, in the pasture on the other side of the fence.
Each year, many seeds land on the
mine tailings, but most are unable to
grow successfully there. The only
plants that germinate, grow, and
reproduce are those that inherited
genes enabling them to tolerate
metallic soil. Thus, this adaptation
does not evolve by individual plants
becoming more metal-tolerant during
their lifetimes. We can only see the
evolution of this population by observing
the proportions of metal-tolerant plants
in successive generations.
The Origin of Species convinced most biologists that species are products of evolution, but Darwin
was not nearly so successful in gaining acceptance for his idea that natural selection is the main
mechanism of evolution.
Natural selection requires hereditary processes that Darwin could not explain. His theory was
based on what seems like a paradox of inheritance: Like begets like--but not exactly.
What was missing in Darwin’s explanations was an understanding of inheritance that could explain
how chance variations arise in a population while also accounting for the precise transmission of
these variations from parents to offspring.
Although Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were
unappreciated at the time, and apparently no one noticed that he had elucidated the very principles
of inheritance that could have resolved Darwin’s paradox and given credibility to natural selection.
Ironically, when Mendel’s research article was rediscovered and reassessed at the beginning of the
20th century, many geneticists believed that the laws of inheritance were at odds with Darwin’s
theory of natural selection.
Darwin considered the raw material for natural selection to be quantitative characters, those
characteristics in a population that vary along a continuum, such as fur length in mammals or the
speed with which an animal can flee from a predator.
We know today that quantitative characters are influenced by multiple genetic loci.
C
C
CC
aaBbCC
But Mendel (and later the geneticists of the early 20th century) recognized only discrete "either-or"
traits, such as purple or white flowers in pea plants, as heritable. Thus, there seemed to be no
genetic basis for natural selection to work on the more subtle variations within a population that
were central to Darwin’s theory.
An important turning point for evolutionary theory was the birth of population genetics, which
emphasizes the extensive genetic variation within populations and recognizes the importance of
quantitative characters.
With progress in population genetics in the 1930s, Mendelism and Darwinism were reconciled, and
the genetic basis of variation and natural selection was worked out.
Mendelism
genetic variation
Darwinism
natural selection
A comprehensive theory of evolution that became known as the modern synthesis began to take
form in the early 1940s.
It is called a synthesis because it integrates discoveries and ideas from many different fields,
including paleontology, taxonomy, biogeography, and, of course, population genetics.
Dobzhansky
Wright
Mayr
Simpson
Stebbins
The architects of this modern synthesis included geneticists Theodosius Dobzhansky (1900-1975)
and Sewall Wright (1889-1988), biogeographer and taxonomist Ernst Mayr (1904-), paleontologist
George Gaylord Simpson (1902-1984), and botanist G. Ledyard Stebbins (1906-2000).
The modern synthesis emphasizes the importance of populations as the units of evolution, the
central role of natural selection as the most important mechanism of evolution, and the idea of
gradualism to explain how large changes can evolve as an accumulation of small changes occurring
over long periods of time.
Populations
Gradualism
Natural Selection
A population’s gene pool is defined by its allele frequencies
A population is a localized group of individuals belonging to the same species.
For now, we will define a species as a group of populations whose individuals have the potential
to interbreed and produce fertile offspring in nature
K = Animalia
P = Chordata
C = Mammalia
O = Carnivora
F = Felidae
G = Panthera
S = leo
K = Animalia
P = Chordata
C = Mammalia
O = Carnivora
F = Felidae
G = Panthera
S = tigris
Each species is distributed over a certain geographic range, but within this range individuals are
usually concentrated in several localized populations.
A population may be isolated from other populations of the same species, exchanging genetic
material only rarely.
Such isolation is particularly common for populations confined to widely separated islands,
unconnected lakes, or mountain ranges separated by lowlands.
However, populations are not always isolated, nor do they necessarily have sharp boundaries.
One dense population center may blur into another in an intermediate region where members of the species
occur but are less numerous. Although these populations are not isolated, individuals are still concentrated in
centers and are more likely to breed with members of the same population than with members of other populations.
Therefore, individuals near a population center are, on average, more closely related to one another than to
members of other populations
The total aggregate of genes in a population at any one time is called the population’s gene pool.
It consists of all alleles at all gene loci in all individuals of the population.
For a diploid species, each locus is represented twice in the genome of an individual, who may be
either homozygous or heterozygous for those homologous loci.
Recall that homozygous individuals
have two identical alleles for a given
character, whereas heterozygous
individuals have two different alleles
for that character.
If all members of a population are homozygous for the same allele, that allele is said to be fixed in
the gene pool.
Often, however, there are two or more alleles for a gene, each having a relative frequency
(proportion) in the gene pool.
Imagine a wildflower population with two varieties contrasting in flower color.
An allele for red flowers, which we will symbolize by R , is completely dominant to an allele for
white flowers, symbolized by r .
For our simplified situation, these are the only two alleles for this locus in the population.
RR Rr rr
Suppose an imaginary population has 500 plants, and 20 of these plants have white flowers
because they are homozygous for the recessive allele; their genotype is rr .
The other 480 plants have red flowers; some of them will be homozygous (RR ) and others will be
heterozygous (Rr ). Suppose that 320 plants are RR homozygotes and 160 are Rr heterozygotes.
Because these are diploid organisms, there are a total of 1,000 copies of genes for flower color in
the population of 500 individuals.
The dominant allele (R ) accounts for 800 of these genes:
320 X 2 = 640 for RR plants, plus 160 X 1 = 160 for Rr individuals.
Thus, the frequency of the R allele in the gene pool of this population is 800/1,000 = 0.8 = 80% .
And because there are only two allelic forms of the gene, the r allele must have a frequency
of 0.2, or 20% .
The Hardy-Weinberg theorem describes a nonevolving population
Before we consider the mechanisms that cause a population to evolve, it will be helpful to examine,
for comparison, the gene pool of a nonevolving population.
Such a gene pool is described by the Hardy-Weinberg theorem, named for the two scientists who
derived the principle independently in 1908.
G. H. Hardy
Wilhelm Weinberg
The theorem states that the frequencies of alleles and genotypes in a population’s gene pool
remain constant over the generations unless acted upon by agents other than Mendelian
segregation and recombination of alleles.
Put another way, the shuffling of alleles due to meiosis and random fertilization has no effect on the
overall gene pool of a population.
Crossing Over
Independent
Assortment
Random
Fertilization
To apply the Hardy-Weinberg theorem, let’s return to our imaginary wildflower population of
500 plants
Recall that 80% (0.8) of the flower-color loci in the gene pool have the R allele and 20% (0.2) have
the r allele.
How will meiosis during sexual reproduction affect the frequencies of the two alleles in the next
generation of our wildflower population?
We will assume that the union of sperm and ova in the population is completely random; that is, all
male-female mating combinations are equally likely.
The situation is analogous to mixing all gametes in a sack and then drawing them randomly, two at
a time, to determine the genotype for each zygote (fertilized egg).
Each gamete has one allele for flower color, and the allele frequencies of the gametes will be the
same as the allele frequencies in the parent population. Every time a gamete is drawn from the pool
at random, the chance that the gamete will bear an R allele is 0.8, and the chance that the gamete
will have an r allele is 0.2.
Random Mating
Using the rule of multiplication (see Chapter 14), we can calculate the frequencies of the three
possible genotypes in the next generation of the population
The probability of picking two R alleles from the pool of gametes is 0.8 X 0.8 = 0.64.
Thus, about 64% of the plants in the next generation will have the genotype RR .
The frequency of rr individuals will be about 0.04 (0.2 X 0.2 = 0.04), or 4% . And 32% , or 0.32, of
the plants will be heterozygous--that is, Rr or rR , depending on whether it is the sperm or ovum
that supplies the dominant allele (frequency of Rr = 0.8 X 0.2 = 0.16;
frequency of rR = 0.2 X 0.8 = 0.16; frequency of Rr + rR = 0.32).
Hardy-Weinberg Equilibrium
Notice in Figures below that the sexual processes of meiosis and random fertilization have
maintained the same allele and genotype frequencies that existed in the previous generation of
the wildflower population. For the flower-color locus, the population’s gene pool is in a state of
equilibrium--referred to as Hardy-Weinberg equilibrium.
____
____
Theoretically, the allele frequencies could remain constant at 0.8 for R and 0.2 for r forever
(though in reality, some other factor always intervenes).
The Hardy-Weinberg theorem describes how the Mendelian system has no tendency to alter allele
frequencies. For instance, the dominant allele (R ) has no tendency to increase in frequency from
one generation to the next relative to the recessive allele (r ).
The system operates somewhat like shuffling a deck of cards: No matter how many times the deck
is reshuffled to deal out new hands, the deck itself remains the same. Aces do not grow more
numerous than jacks.
And the repeated shuffling of a population’s gene pool over the generations cannot, in itself,
increase the frequency of one allele relative to another.
The Hardy-Weinberg Equation
We can use our imaginary wildflower population to describe the Hardy-Weinberg theorem in more
general terms.
We will restrict our analysis to the simplest case of only two alleles, one dominant over the other.
However, the Hardy-Weinberg theorem also applies to situations in which there are three or more
alleles for a particular locus and no clear-cut dominance.
R completely dominant over r
For a gene locus where only two alleles occur in a population, population geneticists use the letter
p to represent the frequency of one allele and the letter q to represent the frequency of the other
allele.
In our imaginary wildflower population, p = 0.8 and q = 0.2
Note that p + q = 1; the combined frequencies of all possible alleles must add to 100% for that locus
in the population. 0.8 + 0.2 = 1
If there are only two alleles and we know the frequency of one, the frequency of the other can
be calculated:
If p + q = 1 then:
p=1-q
p = 1 – 0.2 = 0.8
and
q=1-p
q = 1 – 0.8 = 0.2
When gametes combine their alleles to form zygotes, the probability of generating an RR genotype
is p2 (an application of the rule of multiplication).
In our wildflower population, p = 0.8, and p2 = 0.64, the probability of an R sperm fertilizing an R
ovum to produce an RR zygote.
The frequency of individuals homozygous for the other allele (rr ) is q2, or 0.2 X 0.2 = 0.04 for the
wildflower population.
There are two ways in which an Rr genotype can arise, depending on which parent contributes the
dominant allele.
Therefore, the frequency of heterozygous individuals in the population is 2pq:
(2 X 0.8 X 0.2 = 0.32 in our example).
If we have included all possible genotypes, the genotype frequencies add up to 1:
p2
+
Frequency of
RR genotype
2pq
+
Frequency of Rr
plus rR genotype
q2
= 1
Frequency of rr
genotype
For our wildflowers, this is:
0.64 + 0.32 + 0.04 = 1
Population geneticists refer to this general formula as the Hardy-Weinberg equation.
The equation enables us to calculate frequencies of alleles in a gene pool if we know frequencies
of genotypes, and vice versa.
Population Genetics and Health Science
We can use the Hardy-Weinberg equation to estimate the percentage of the human population that
carries the allele for a particular inherited disease.
For instance, one out of approximately 10,000 babies in the United States is born with
phenylketonuria (PKU), a metabolic disorder that, left untreated, results in mental retardation and
other problems.
(Newborn babies are now routinely tested for PKU, and symptoms can be prevented by following a
strict diet.)
The disease is caused by a recessive allele; thus, the frequency of individuals in the U.S. population
born with PKU corresponds to q 2 in the Hardy-Weinberg equation (q 2 = frequency of the
homozygous recessive genotype).
Given one PKU occurrence per 10,000 births, q 2 = 0.0001.
Therefore, assuming Hardy-Weinberg proportions, the frequency of the recessive allele for PKU in
the population is
and the frequency of the dominant allele is
The frequency of carriers, heterozygous people who do not have PKU but may pass the PKU allele
on to offspring, is
Thus, about 2% of the U.S. population carries the PKU allele.
The Hardy-Weinberg Theorem and Genetic Variation
The Hardy-Weinberg theorem is important conceptually and historically because it shows how
Mendel’s theory of inheritance plugs a hole in Darwin’s theory of natural selection.
Natural selection requires genetic variation; it cannot act in a genetically uniform population.
The Hardy-Weinberg theorem explains how Mendelian inheritance preserves genetic variation from
one generation to the next.
Pre-Mendelian theories of inheritance were mainly "blending" theories, in which the hereditary
factors in the offspring were thought to be a blend of the hereditary factors inherited from the two
parents.
If a red flower mates with a white one, blending theory predicts that the offspring will be a paler
red and will now have hereditary factors for this paler red color.
Genetic variation has been eliminated, since the two kinds of factors in the parents have been
reduced to only one kind in the offspring.
Such a hereditary mechanism would soon produce a uniform population.
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x
=
In Mendelian inheritance, however, the hereditary mechanism has no tendency by itself to reduce
genetic variation. The set of alleles inherited by each generation from its parents are in turn passed
on when that generation breeds.
This nonblending mechanism of inheritance preserves the genetic variation upon which natural
selection acts.
The Assumptions of the Hardy-Weinberg Theorem
For a population to be in Hardy-Weinberg equilibrium, it must satisfy five main conditions:
1. Very large population size . In a population of finite size, especially if that size is small, genetic
drift, which is chance fluctuation in the gene pool, can cause genotype frequencies to change over
time.
2.No migration . Gene flow, the transfer of alleles between populations due to the movement of
individuals or gametes, can increase the frequency of any genotype that is in high frequency among
the immigrants.
3.No net mutations . By changing one allele into another, mutations alter the gene pool.
4.Random mating . If individuals pick mates with certain genotypes, then the random mixing of
gametes required for Hardy-Weinberg equilibrium does not occur.
5.No natural selection . Differential survival and reproductive success of genotypes will alter their
frequencies and may cause a detectable deviation from frequencies predicted by the HardyWeinberg equation.
Thus, we do not really expect a natural population to be in Hardy-Weinberg equilibrium.
And a deviation from the stability of a gene pool--and from Hardy-Weinberg equilibrium--usually
results in evolution.