Chapter 4

Genes and Their Evolution

Overview

• In this chapter we will look at what causes evolutionary (and by that we mean genetic) change.

• We will also discuss how evolutionary change is measured, and how the cause is determined.

• This chapter is concerned with the processes associated with microevolution.

• Microevolution contrasts with macroevolution.

• While microevolution describes the changes within a species over a relatively short time

(evolutionarily speaking), macroevolution is on a larger scale.

• Macroevolution is linked both to speciation and to longer periods of time evolutionarily.

• While the author does not label it by name, he refers to the Modern Synthesis

• The modern synthesis defies evolution as a two-stage process:

1. Stage 1: The production and redistribution of variation (inherited differences between

organisms)

2. Stage 2: Natural selection acting on this variation, whereby inherited differences, or variation,

among individuals differentially affect their ability to successfully reproduce.

• A paper on the history of this shift can be obtained here.

• New Synthesis?

• According to Smith and Ruppell (2011), a New Synthesis is emerging.

• New discoveries are changing the landscape of evolutionary explanations.

• These include: 1) Horizontal Gene Transfer (HGT) (genes move between species), 2) epigenetics,

3) developmental evolutionary biology (evo-devo) (shared groups of genes), 4) mutualisms and

coevolution (such as humans and dogs), 5) phenotypic plasticity and phenotypic integration and 6)

niche construction.

• In this chapter, we will be talking about groups of organisms exchanging genes. Let’s start with some

terms:

• Deme: A group of organisms that interbreeds and has offspring.

• Often used is the term breeding population (or interbreeding population).

• A breeding population is a group of organisms that tend to choose mates from within the group.

• The focus of microevolution is not on specific genotypes and phenotypes of individuals, but

rather on the total pattern of an entire biological population.

• Human populations are most often defined on the basis of geographic and political boundaries.

• Often the definition used depends on the research question being addressed..

• A potential problem is determining the difference between total census and the breeding

population. Another problem is when one locale contains many sub-populations.

• Gene pool: All of the genetic information in an interbreeding population.

• The concept of the gene pool assumes random mating.

• Nonrandom mating involves patterns of mate choice that influence the distributions of

genotype and phenotype frequencies.

• Allele frequencies change together with genotype frequencies, but genotype frequencies can

change without altering allele frequencies.

• One form of nonrandom mating is inbreeding.

• Assortative mating is another form of nonrandom mating that occurs when there is mating

based on phenotypic similarity or dissimilarity (i.e., blondes choosing blondes).

• Inbreeding and assortative mating involve no change in actual allele frequency, but a change in

genotype frequency. [More on these later.]

Demes, Gene Pool, Reproductive Isolation & Species

The Birth and Death of Species 1

• The origin of new species has been observed in historical times and in the present. For

instance: New diseases such as Ebola, many of the tropical fish, and types of fruit flies

• When it comes to the birth and death of species, we need to look at a these questions:

• How do species come into being?

• Why do some species die out?

• What is a species?

• The most common definition of a species is that described by the biological species

concept (BSC) wherein species can be defined in terms of reproductive capability.

• Organisms are classified in the same species if:

• 1) Individuals from two populations are capable of breeding naturally and

• 2) They produce fertile offspring.

• The mule is the most often cited example

• The result of breeding a female donkey (62 chromosomes) with a male horse (64

chromosomes) is a mule (63 chromosomes)

• The opposite (male donkey and female horse) is called a hinney and is harder to

produce

• When two mules or hinnies are breed there are no offspring

• All males are infertile and so are most females

• This means some female mules can be breed with donkeys or horses, but not

male mules.

The Birth and Death of Species 2

• Species change

• The BSC is useful when comparing two species living today, but how can we look at species over

time?

• Across time study requires looking at two different modes of evolutionary change of species:

• Anagenesis is linear evolution.

• It assumes one species evolved directly into a new species over time.

• But at what point is it species A or species B?

• Researchers often modify the species concept to deal with complications in naming

species.

• Chronospecies is a term used to label different stages of biological change over time, but

there is only one species at any given point in time.

• Later in the book the term paleospecies is used to describe the species identified from the

fossil record.

• Problems with this mode include that biological species concept does not apply.

• The time gap precludes the testing of breeding capacity.

• There needs to be a ‘stand-in’ for mating and this is usually the analysis of different

physical forms along a single lineage.

• Many contemporary evolutionary biologists tend not to use this method today

• Cladogenesis is the formation of one or more new species from another over time (branching

evolution).

• In cladogenesis, both species A and species B can exist at the same time

• We will see that this is an issue as we explore paleoanthropology later.

The Birth and Death of Species 3

• Speciation

• Speciation is the process wherein genetic differences between populations prevent successful

interbreeding with the parent species.

• The precondition for speciation is isolation; as long as gene flow is possible then mutations tend

to mix back into the population. Isolation can occur a number of ways: Geographic isolation,

reproductive isolation (pre-mating or post-mating incapabilities), and behavioral isolation.

• Speciation model:

• Step 1: A and B have not yet diverged →

• Step 2: A and B are just beginning to diverge.

• Genetic differences accumulate and are acted on by the forces of evolution:

• Mutations increase variation within a population

• Genetic drift may contribute to differences in small populations

• Without gene flow these accumulated differences can not be shared between populations.

• As the populations are in different environments, natural selection also contributes to

genetic divergence

• To accomplish this must occur first; this is the genetic change that can lead to an inability to

produce fertile offspring is due to reproductive isolation:• Geographic isolation is the most common means of reducing gene flow between

populations. This is the result of reduced or eliminated gene flow between populations.

• Behavioral isolation also occurs.

• Step 3: A and B have diverged to a point where they’re no longer able to reproduce; speciation

so that genetic divergence is the result. Speciation is complete.

The Birth and Death of Species 4

• Speciation (continued)

• There is a debate over the role of the various evolutionary forces in producing genetic

divergences

• At one time only natural selection was seen as the force behind speciation

• This suggests that natural selection operates on large populations gradually and is the

consequence of differential adaptation.

• Today, the other forces are also seen as contributors.

• Many think that speciation occurs in small populations and are extensively by

mutation and genetic drift. Genetic differences can come about as a result of all the

evolutionary forces.

• Adaptive radiation

• Adaptive radiation is the rapid formation of many new species following the availability

of new environments or the development of a new adaptation.

• A species, or group of species, will diverge into as many variations as two factors

allow:

• Its own adaptive potential

• The adaptive opportunities of the available niches.

• One example was the death of the dinosaurs, which allowed for the mammalian

radiation

Rate of Speciation 1

The rate at which evolutionary change occurs came into a debate in the 1970s

Prior to that time, most evolutionists were of the same mind as Darwin, that change was gradual (called phyletic gradualism).

Gradualism is the view that

macroevolution is a slow and gradual

process.

Charles Darwin saw evolution as a process of millions of years.

He saw natural selection as the primary mechanism

• Mutation and drift have little effect on a specific generation.

• Natural selection acting on initial mutation results in speciation.

With gradualism, the fossil record will be a smooth, gradual transition (no gaps).

Anagenesis is the term used to refer to this linear evolution

It assumes one species evolved directly into a new species over time

But at what point is it species A or species B?

Rate of Speciation 2

In some cases we lack transitional forms,

For some this is evidence of the fallacy of

evolution

For researchers this became a challenge

that was finally explained by the concept

of punctuated equilibrium

After the 1970s, evolutionists came to realize

that some cases of change went much faster

and punctuated equilibrium (PE) as a

mechanism was added.

• Punctuated equilibrium is the view that the pattern of macroevolution consists of long

periods of time when little change occurs (stasis) and short periods of time when rapid

evolutionary change occurs.

• This model infers that most genetic change occurs during speciation. Mutation occurs in a

small, isolated population and then spreads rapidly due to inbreeding

• and genetic drift.

• Stabilizing selection and other factors act to keep a species the same over time

• Because some new species appear so rapidly we do not see transitional forms. The fossil

record usually will not show the initial changes.

• We now know that within a species there can ALSO be punctuated gradualism (statis and

then rapid changes in the species morphology. Gould acknowledged this alteration of PE

• You can watch a myriad of videos on evolution topics here

Hardy-Weinberg Law 1

• Hardy-Weinberg Law

• The Hardy-Weinberg Law is a mathematical statement relating allele frequencies to

the expected genotype frequencies in the next generation. Basically it predicts the

values in the next generation based on the values in the present population.

• The Hardy-Weinberg Law makes certain assumptions:

• Assumes random mating with respect to the locus of the study

• Assumes that no new alleles are introduced by mutation or natural selection.

• Assumes that there are no changes caused by movement in or out of the

population (migration, or what we will call gene flow)

• Assumes that there is no variation is caused by random sampling (population is

large also)

• Observed change means that one or more assumptions are incorrect.

• Once the allelic frequencies of M and N are determined, the next question is: What

are the expected genotype frequencies in the next generation?

• In Chapter 3, we used the Punnett square to answer this question for a specific

male and female.

• But how might we calculate the values for all possible pairings in the population?

Answer: Use the formula identified by Hardy and Weinberg.

Hardy-Weinberg Law 2

• Genotype frequencies and allele frequencies

• Once a population has been defined, the next step in microevolutionary analysis is to

determine the frequencies of genotypes and alleles within the population

• Remember a genotype is NOT the same term as allele.

• The genotype is the combination of the allele from the father with that of the mother.

Example using the ABO blood group:

• The alleles are: A, B, O

• The possible genotypes are: AA, AO, BB, BO, OO, AB

• The blood type is actually a phenotype label which we will discuss later.

• The two types of frequency:

• The genotypic frequency is a measure of the relative proportions of different

genotypes within a population. Genotype frequencies are obtained by dividing the

number of individuals within each genotype by the total number of individuals in

the sample.

• The allele frequency is a measure of the relative proportions of different alleles

within a population. Allele frequencies are computed by counting the number of

each allele and dividing that number by the total number of alleles in the sample.

The number of alleles is TWICE the number of genotypes in a population.

• Review: www.youtube.com/watch?v=BuiPA8FJ_1M&feature=fvwrel

• Demo: www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=517

The Formula for 2-Allele System

• Terms:

• A and a are the labels for the two alleles. As A is capitalized and a is not this is a dominant-

recessive system.

• p represents the probability for the A allele; q represents the probability for the q allele.

• Allelic frequency formula: p = q = 1 (100% of the alleles).

• Most important to remember: q is key!

• Genotypic frequency (Look up at the Punnett Square):

• Determine the probabilities:

• The probability of Female A(p) will mate with Male A(p) is p2

• The probability of Female A(p) will mate with Male a(q) is pq

• The probability of Female a(q) will mate with Male A(p) is qp

• The probability of Female a(q) will mate with Male a(q) is q2

• Add together for genotypic frequency formula: p2 + 2pq + q2 1 (100% of the genotypes).

Hardy-Weinberg Law 3

• Genotype frequencies and allele frequencies (continued)

• The term frequency is not the same as the percentage although we often mix them

up!

• The frequency is based on actual numbers (both in the numerator and

denominator). For instance, 4 persons from a population of 20.

• The percentage is based on conversion to “per centum” (by the hundred).

• To record this value one has to convert the actual observation. (4 persons in a

population of 20 becomes 25/100 or 25%)

• The reason for this effort is so that a standard is created to more easily compare

across sets of observations.

• Let’s do some calculations here. The frequency is determined in this way:

• Example 1: Genotypic frequency in a co-dominant gene system

• A population of 200 persons have their blood tested and we learn the

following: 98 persons are MM, 84 persons are MN, and 18 persons are NN

• Calculation:

• MM: 98/200 or 0.49 (49%)

• MN: 84/200 or 0.42 (42%)

• NN: 18/200 or 0.09 (9%)

Hardy-Weinberg Law 4

• Genotype frequencies and allele frequencies (continued)

• Example 2: Allelic frequency in a co-dominant gene system

• Key concept: In a population of 200 persons, there are 400 alleles Calculation:

• Number of M alleles:

• MM had 98 persons and each has 2 M alleles = 98(2)

• MN had 84 persons and each has 1 M allele = 84(1)

• NN had 0 persons with M and each so = 0

• So to chain this together and M is = 98(2) + 84(1) + 0 =280 M alleles

• Number of N alleles:

• NN had 18 persons and each has 2 N alleles = 18(2)

• MN had 84 persons and each has 1 N allele = 84(1)

• MM had 0 persons with N and so = 0

• So to chain this together and N is = 18(2) + 84(1) + 0 = 120 N alleles

• Special note: This method of determining the number of genotypes and alleles can

only be used if the number of each genotype can be determined.

• This means when the alleles are co-dominant

• This means it is not useful when there is a dominant-recessive situation (we will

show this method by using Hardy-Weinberg equilibrium formula in the next few

slides).

Mutation: Only Source of New Alleles 1

• The evolutionary forces we will discuss are mutation, natural selection, genetic drift, and

gene flow.

• Changes in genotypic frequencies change as a result of evolutionary forces AND

random mating.

• Evolutionary forces are the ONLY mechanisms that can cause allele frequencies to

change over time.

• Mutation is a change in the DNA sequence that produces an altered gene. It is important to

keep in mind that mutation is the ultimate source of all variation in organisms.

• A miscopying of the DNA during replication is called a spontaneous mutation. There are

other known causes, such as X-ray exposure, and toxic chemicals can mutate the DNA

causing induced mutations.

• There are many different kinds of mutations.

• Point mutation is the substitution of one DNA base for another.

• Production of the very same amino acid are called synonymous point mutations.

• A nonsynonymous point mutation changes the amino acid sequence.

• A frameshift mutation is when 1+ nucleotides are inserted or deleted into the gene,

shifting all of the remaining nucleotides. This results in entirely different amino acids

being coded.

• Genes can copy themselves and be inserted into different regions of the genome;

these are called transposable elements.

Mutation: Only Source of New Alleles 2

• Example of a point mutation of an A allele to an a allele.

• Generation 1: 200 A alleles (this means this allelic frequency is 100% or 1)

• Then a mutation occurs:

• This changes the allelic frequency, but if there is not there is no additional evolutionary change,

the allelic frequency will be the same over generations.

• Generation 2: 199 A alleles and 1 a allele.

• Now A allelic frequency is 199/200 (or 0.995).

• The a allelic frequency is 1/200 (or 0.005).

• Without the other forces, only additional introductions of this same mutation can change (read

increase/decrease) the allelic frequency of a allele.

• Faster rates of evolutionary change are very likely do to one or more of the evolutionary forces.

• Many discrete genetic traits are polymorphisms (many forms).

• A genetic polymorphism is a locus of 2 or more alleles with frequencies too high to be by mutation

alone.

• The arbitrary frequency used is 0.01

• Other source of mutation are cross-overs between chromosomes or even addition of chromosomes or

deletion of them.

• Entire chromosomes can duplicate, resulting in three, rather than two, homologous chromosomes.

• This is called a trisomy.

• For example, Klinefelter’s syndrome is a trisomy that results in an extra X chromosome.

• Entire chromosomes can be lost, resulting in one, rather than two, homologous chromosomes.

• This is called a monosomy.

• For example, Turner’s syndrome is a monosomy that results from the loss of an X chromosome.

Natural Selection

• Natural selection does not create new genetic variation, but it can change the relative frequencies

of different alleles. Natural selection can be defined as differential survival and differential

reproduction.

• The environment ultimately selects individuals with the best suited genotypes to survive to

adulthood and to reproduce.

• Those who have more surviving offspring pass on more of their genes to the next

generation.

• Natural selection filters genetic variation.

• Analysis of natural selection focuses on fitness, the probability of survival and reproduction

of an organism.

• Fitness is measured on a scale from 0 (zero) to 1 (one).

• The record for the most live births by a single woman is 69 so she had a value of 1 (female).

Of these, 67 lived past infancy. Her husband married again and had 18 children.

• Watch: Why Genghis Khan has 16 million living relatives: youtube.com/watch?v=3Qcck7WylCk

• Depending on the fitness of each genotype, natural selection can have different effects.

• Natural selection is usually the most important mechanism of evolution.

• We now know that its effect on individuals depends on their phenotypes which in turn are

determined mostly by their genotypes.

• Different environments means different results.

• Thus, Darwin never said ‘survival of the fittest’, he said ‘struggle for survival’ because he was

talking about differing environments and different success rates.

Hypothetical Examples of Natural Selection 1

Hypothetical Examples of Natural Selection 2

• There are quite a few patterns of natural selection, as illustrated.

• These are directional selection (first column), stabilizing selection (second column), disruptive

selection (third column), and stasis (or no selection at all), shown in the last column.

• Imagine that we are looking at a something simple, like body size over time in a group of

organisms.

• In the first column (directional selection), those individuals who happen to be small have a

higher survival rate and reproduce more often than those individuals who happen to be large.

• In other words, small individuals have a higher fitness (as shown in the middle graph).

• The distribution of size shifts to the left, and thus the average size in a population drops (bottom

graph).

• Stabilizing selection would work against both the very small and very large individuals.

• Fitness would be highest in the average-sized individuals.

• Because of this, the average body size in the population would not change, even though natural

selection was still at work, eliminating the extremes in the population.

• Disruptive selection is essentially the opposite of this.

• The average individuals have the lowest fitness, and the smallest and largest individuals are the

ones that have the highest fitness.

• Over time, the average body size in the population may not change overall, but what will result

are two clusters—the smallest and the largest—eventually leading perhaps to two distinct

populations and maybe even two distinct species.

• Finally, there are situations where a population is in Hardy–Weinberg equilibrium and no selection

occurs. Fitness will vary according to other variables, but not size, and the average size will not

change over time..

Natural Selection Examples 1

• Directional selection is a form of selection in complex traits against one extreme

and/or for the other extreme.

• A direct relationship exists between survival and reproduction and the value of the

trait.

• Promotion of change in a population over time in one direction.

• The increase in human brain size in the last 4 million years is an example.

• Another example is the lighter skin color of the prehistoric peoples who left

Africa

• Directional selection: Selection against the homozygous recessive

• What if the genotypes do not have different fitnesses? Suppose that the genotypic

fitnesses are as follows:

• AA = 100%, Aa = 100% and aa = 50%

• The aa individuals only have a 50% chance of survival.

• Selection against an allele will lead to a reduction in the allele frequency over time.

• The heterozygote will continue to pass on the allele to the next generation.

• Is never totally eliminated as there is a slow, but present ‘back mutation’.

• Real example: Tay-Sachs disease, a fatal lipid storage disorder.

• Most common among eastern European Jewish descent (Ashkenazi Jews).

• Tay-Sachs is a deadly disease with no adaptive advantage.

• Right now there is no treatment; death is usually in the first few years of life.

Examples of Natural Selection 2

• Examples of how natural selection works (continued)

• Directional selection: Selection for the dominant homozygote

• Suppose AA = 100% fitness, Aa = 95% and aa = 90%

• Over time the A allele will increase.

• Directional selection: Selection against the dominant homozygote

• Selection against the dominant homozygote causes the frequency of the

dominant allele to go down and the frequency of the recessive allele to go up.

• Achrondoplastic dwarfism is an example.

• Achrondoplastic dwarfism (small body size and abnormal body

proportions) is caused by a dominant allele found in low frequencies in

human populations.

• The condition is often caused by a mutation occurring in the sex cells of

one parent.

• The allele is dominant, and individuals with one or two alleles will show

the disease.

• Low frequency of the disease is the result of natural selection acting to

remove the harmful allele from the population.

Malaria 1

• Natural selection has been operating throughout hominin evolution, resulting in a number of major anatomical

changes. This part of the chapter addresses several examples of evolutionary history and natural selection among

recent human evolution.

• Hemoglobin, sickle cell, and malaria

• What is malaria?

• Four species of Plasmodium infect humans and cause malaria (Plasmodium falciparum, P. malariae, P.

ovale, and P. vivax).

• A fifth species was identified in humans in 2007 ([Plasmodium knowlesi [nolls eye]). It is usually only seen

in macaques.

• At first it was seen as rare in humans

• Today can account for most of the infections in some parts of SE Asia.

• It has also been seen to transmit to Western travelers.

• All species are vector borne diseases, being spread by anopheline mosquitoes, and the disease is distributed

throughout much of the world.

• In the human host the parasite is found primarily inside of the red blood cells (RBC).

• The parasite reproduces asexually inside of the RBC, and following this the RBC breaks open releasing

many new parasites.

• These parasites then infect more RBC's, and this ultimately leads to the destruction of massive numbers

of RBC's.

• The characteristic "chill and fever" (paroxysm) associated with malaria occurs when the parasites are released

from the RBC's.

• Since the release of parasites is periodic, the paroxysms are periodic.

• For examples, the paroxysms associated with a tertian malaria (e.g., Plasmodium vivax) occur about every

48 hours, and those associated with a quartian malaria (e.g., Plasmodium malariae) occur about every 72

hours (view a diagram of the life cycle). Source: http://ryoko.biosci.ohio-state.edu/~parasite/plasmodium.html

Malaria Life Cycle

Malaria 2

• Hemoglobin, sickle cell, and malaria (continued)

• Distribution of the sickle cell allele and malaria

• The RBC protein, hemoglobin, functions to carry oxygen to body tissues.

• The normal beta chain of the hemoglobin is called HbA

• HbS is known as the sickle cell allele.

• Hemoglobin variants

• There are 1546 hemoglobin and thalessemia variants (http://globin.bx.psu.edu/cgi-

bin/hbvar/counter)

• The most widely studied mutations include the hemoglobin S , HbC and HbE groups (there

are several variants of each)

• A person who has two S alleles has sickle cell anemia.

• As homozygous for HbS is harmful it is selected against but mutation introduces even as

natural selection eliminates it.

• The balance point between A and S is between 10-20% Hb S

• Distribution of the sickle cell allele and malaria

• The distribution of the allele is related to the prevalence of certain forms of malaria and is

caused by a virus or parasite.

• There are 300-500 million cases each year and 1-3 million deaths.

• The evolution of the S allele

• In a malarial environment, heterozygous people have an advantage.

• Equilibrium is one in which the fitness of the entire population is at a maximum.

• The cost of adaptation, however, is an increased proportion of individuals with sickle cell

anemia.

Maps of HbS and P. falciparum

Hemoglobin S (left) & Plasmodium falciparum (right)

Examples of Natural Selection 3

• Stabilizing selection refers to selection against both extremes of a trait’s range in values

• Individuals with extreme values are less likely to survive; those closer to average

values are more likely to survive and reproduce.

• Stabilizing selection: Selection for the heterozygote (therefore against the homozygotes)

• What about a situation that creates a selection for the heterozygote?

• Many loci show intermediate frequencies. This reminds us that fitness is most often

NOT close to 1 or 0 in populations.

• Case of sickle-cell anemia

• The frequency of sickle-cell allele can reach 20% in some African populations and is

also high in Indian and Mediterranean populations as well.

• This hereditary disability persisting at high values then in these populations. Why?

• Plasmodium falciparum malaria is among the most deadly forms.

• Relative fitnesses in this example:

• Healthy persons are better hosts, sickly from malaria .

• The homozygous dominant (AA) = 70% fitness in our example.

• The homozygous recessive (aa) die and have a fitness of 0%.

• The heterozygotes (Aa) = 100% fitness.

• Results in stabilizing selection (selection for the heterozygote and against both

homozygotes) as the heterozygote is most fit.

Examples of Natural Selection 4

• Disruptive Selection: Selection against the heterozygote

• Selection against the heterozygote results in a decrease in the less common allele.

• An example is the Rhesus (Rh) blood group.

• Inheritance of the Rh blood group involves three linked loci.

• Those with Rh positive (DD or Dd genotypes) produce a certain chemical; those

with Rh negative (dd) produce a corresponding antibody.

• Some Rh negative mothers carry Rh positive fetuses.

• The child is at risk and may be selected against. These offspring are

heterozygous (D from father, d from mother).

• The risks are as follows (among those untreated):

• 13.7-29.8% will develop the response (either in the womb or at birth).

• During the next birth, 20-25% of the children will be exposed to the

mother’s response (occasionally this can happen in the original birth).

• In geographic areas where biomedicine is available the treatment is RhoGAM

which has greatly reduced the problem since the 1960s.

• Another reason for the decrease is smaller family sizes (less exposure).

• Human birth weight is a good example (see figure next slide).

• Both the environment and genetics play a role in birth weights.

• Underweight and overweight infants die more than those in the average.

Genetic Drift 1

• Genetic drift is defined as the fluctuations in the percentages of two or more alleles in a

population or as the random change in allele frequency from one generation to the next.

• Genetic drift is random and it occurs in every generation.

• Direction of allele frequency is random.

• Given enough time (and in the absence of other evolutionary forces), genetic drift leads to

reduction in variation within a population for humans.

• Changes are the result of the nature of probability, like a coin toss.

• If you toss a coin the expectation is 50% heads, 50% tails for each toss.

• But in the number of tosses is only 10, you are not surprised to see 7 heads and 3 tails.

• If you tossed 1 million times, much closer to 50/50 (Law of really big numbers).

• Population size and genetic drift

• The effect of genetic drift depends on the size of the breeding population.

• In larger populations:

• The larger the population size, the less change will occur from one generation to the next.

• The larger the population size, the fewer deviations in allele gene frequencies caused by

genetic drift.

• In smaller populations:

• In small populations, genetic drift more often results in a quick loss of one allele or

another.

• In a small population, just by chance, a few individuals may leave behind more offspring

than others in the population.

• Genetic drift has the greatest evolutionary effect in relatively small breeding populations.

Genetic Drift 2

• Genetic drift is evolutionary changes in allele frequencies

produced by random factors

• Based on the randomness of the percentages in a small

population. In fact, it is due to the population being small.

• Probably more important to human evolution many

thousands of years ago. Today very big populations

• Genetic drift is studied at the population level

• There are different types of genetic drift

1. A type of genetic drift in which allele frequencies are

altered in small populations that are taken from, or are

remnants of, larger populations is called founder effect.

• A few members of population break off from the larger

group.

• Due to randomness, the genetic composition is generally

not the same.

The two types

2. A second type of genetic drift is called bottleneck

• A bottleneck is the reduction of the size of a population within a single generation

• The genetic variation within this smaller population is lessened by randomness alone.

• Often the result of a disaster or other catastrophe

• Examples of genetic drift:

• Founder effect: The Dunker population and those in Tristan de Cunha

• Bottleneck: Toba eruption may be one.

Gene Flow

• Gene flow is the movement of alleles from one population to another.

• Migration and gene flow are not exactly the same thing

• Gene flow may or may not occur when there is migration

• Gene flow can also occur after the migrants have gone home.

• Genes may occasionally also flow between species.

• When gene flow occurs, the two populations mix genetically and tend to become more similar.

• Gene flow also introduces new variation into a population

• One of the principal reasons humans are so similar is because of gene flow

• The amount of gene flow between human populations depends on a variety of environmental and

cultural factors.

• Geographic distance is a major determinant of migration and gene flow.

• Greater distance means less likelihood of exchange of mates.

• Most marriages are between those local to each other.

• Cultural factors such as ethnicity, social status and so forth are factors (think assortative

mating).

• Genetics and farming in Europe

• Essentially all the domesticated plants are of Middle Eastern origin.

• They first appeared around 9 kya, but how were they introduced? Two competing hypotheses:

• Cavalli-Sforza’s hypothesis is that migrants from the Middle Each brought agriculture with

them (demic expansion) This is most supported.

• The second hypothesis states that the ideas of spread by diffusion, not the peoples (cultural

expansion).

Interaction of the Evolutionary Forces

• Sometimes the four evolutionary forces–mutation, natural selection, genetic drift, and

gene flow – act together and sometimes they act in opposition.

• Different evolutionary forces can produce the same, or opposite, effects

• Mostly we look at three of the forces: gene flow, genetic drift and natural selection

• When gene flow and genetic drive are both operating they counter-balance each

other

• When mutation and genetic drift are present, genetic drift can either increase or

decrease the mutation within a population

• Genetic drift can increase a harmful gene, even if it is being selected against

• Population genetics is all about the mathematics of these forces.