Evolution and PopulationsChapter 17

Genetics and Molecular Biology

Darwin had no idea how heredity worked, and he was worried that this lack of knowledge might prove fatal to his theory.

As it happens, some of the strongest evidence supporting evolutionary theory comes from genetics. A long series of discoveries, from Mendel to Watson and Crick to genomics, helps explain how evolution works.

Also, we now understand how mutation and the reshuffling of genes during sexual reproduction produce the heritable variation on which natural selection operates.

Life’s Common Genetic Code All living cells use information coded in DNA and

RNA to carry information from one generation to the next and to direct protein synthesis.

This genetic code is nearly identical in almost all organisms, including bacteria, yeasts, plants, fungi, and animals.

A Testable Hypothesis Darwin hypothesized that the Galápagos finches he

observed had descended from a common ancestor.

He noted that several finch species have beaks of very different sizes and shapes. Each species uses its beak like a specialized tool to pick up and handle its food. Different types of foods are most easily handled with beaks of different sizes and shapes.

Darwin proposed that natural selection had shaped the beaks of different bird populations as they became adapted to eat different foods.

Genetics Joins Evolutionary Theory

In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time.

Researchers discovered that heritable traits are controlled by genes.

Changes in genes and chromosomes generate variation.

Genotype and Phenotype in Evolution An organism’s genotype is the particular

combination of alleles it carries.

An individual’s genotype, together with environmental conditions, produces its phenotype.

Phenotype includes all physical, physiological, and behavioral characteristics of an organism.

Genotype and Phenotype in Evolution

Natural selection acts directly on phenotype, not genotype.

Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

Populations and Gene Pools

A population is a group of individuals of the same species that share a geographical area, mate and produce offspring. Smallest unit in which evolution occurs

A gene pool consists of all the genes, including all the different alleles for each gene that are present in a population. Combined genetic information of all the members of a

particular population

B b b b B b b b B bb b b b b B b b b b

Populations and Gene Pools

Researchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

Allele Frequency = # of a certain allele total # of alleles of all types

in the populationRed Allele - 9/36 = ¼ = 0.25

White Allele - 27/36 = ¾ = 0.75

Populations and Gene Pools For example, this diagram shows the gene

pool for fur color in a population of mice.

Populations and Gene Pools• Evolution, in genetic terms, involves a

change in the frequency of alleles in a population over time.

• Note: Although natural selection acts on individuals it is the population that evolves, not individuals

Allele Frequency Example

Calculate the frequency of the dominant and recessive alleles in the gene pool below.

A a a A a A a a A aA =a =

4/10 = 0.46/10 = 0.6

Alleles Frequencies & the Gene Pool:

Phenotype Frequency = # of individuals with a particular

phenotypetotal #of individuals in population

Determining Phenotype and Allele Frequencies using Japanese four o’clock

flowers: 1st Generation:

RR RR RW RW RR RW RW RR

Phenotype Frequency: Allele Frequency:White - R =Pink - W =Red -

04/8 = 0.504/8 = 0.50

12/16 = 0.754/16 = 0.25

Predicting Genotypes & Phenotypes of Second Generation: According to the laws of probability, the chance of an R gamete meeting with

another R gamete is the product of the allele frequencies in the gene pool. Red (RR) = R x R = RR= 0.75 x 0.75 = 0.5625

White (WW) = W x W = WW = 0.25 x 0.25 = 0.0625

The frequency of all types expected in the second generation

must add up to 1.01.0 - RR - WW = RW

1.0 - 0.5625 - 0.0625 = 0.375

Single-Gene Traits A single-gene trait is a trait controlled by

only one gene. Single-gene traits may have just two or three distinct phenotypes.

Dominance of an allele for a single-gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population.

No widow’s peak is a recessive trait

Natural selection on single gene traits• Evolution does not act on genes. Instead it acts on

phenotype frequencies by changing allele frequencies!

• Evolution = any change in the relative frequencies of alleles in a population’s gene pool

Initial Population Generation 10 Generation 20 Generation 30

90% 80% 70% 40%

10% 20% 30% 60%

Polygenic Traits Polygenic traits are traits controlled by two

or more genes. Each gene of a polygenic trait often has two or

more alleles. A single polygenic trait often has many

possible genotypes and even more different phenotypes.

How Natural Selection Works Evolutionary fitness is the success in passing

genes to the next generation. Evolutionary adaptation is any genetically

controlled trait that increases an individual’s ability to pass along its alleles.

 

Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can

lead to changes in allele frequencies and then to evolution. 

For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

 

Natural Selection on Single-Gene Traits If red lizards are more visible to predators, they

might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.

Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

Natural Selection on Polygenic Traits

Polygenic traits have a range of phenotypes that often form a bell curve.

The fitness of individuals may vary from one end of the curve to the other.

Natural selection can affect the range of phenotypes and hence the shape of the bell curve.

3 Types of Selection

Natural selection on polygenic traitsNatural selection can affect the relative fitness of phenotypes involving polygenic traits in any of 3 ways:1. Stabilizing Selection -average form of a trait is

favored.2. Directional Selection -one extreme form of a

trait is favored.3. Disruptive Selection - either/both extremes of a

trait are favored over an average form of a trait.

Types of Natural Selection (2:21)

Directional selection

Stabilizing selection

Disruptive selection

Directional Selection Directional selection occurs when individuals

at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.

Directional Selection:Anteaters feed by breaking open termite nests (extend their sticky tongues into the nests).

New species of termites that build very deep nests.

Anteaters with long tongues more effective than those with average or short tongues

Directional Selection

Directional Selection A drought on the Galápagos island of Daphne

Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size between 1976 and 1978.

Stabilizing Selection Stabilizing selection occurs when individuals

near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.

Stabilizing Selection:o Most common kind of selectionA – small lizards may not be able to run fast

enough to escapeB – large lizards may be more easily spotted,

captured, & eaten by predators

Stabilizing Selection

Disruptive Selection Disruptive selection occurs when individuals

at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.

Disruptive Selection:Limpets are marine organisms that have shells that vary in

color from white to dark brown.  Limpets live their adult life attached to rocks.  On light colored rock, white shelled limpets are at an advantage

because the birds that prey upon them have a difficult time locating them. 

On dark-colored rock, dark-shelled limpets are well camouflaged. 

On the other hand, tan-colored limpets are easily spotted on either light or dark rocks.

Genetic Drift Genetic drift occurs in small populations

when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.

The smaller the population, the more susceptible it is to such random changes.

Evolution vs. Genetic Equilibrium The Hardy-Weinberg principle states that allele

frequencies in a population should remain constant unless one or more factors cause those frequencies to change.

The Hardy-Weinberg principle makes predictions about certain genotype frequencies.

According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large (2) There can be no mutations(3) There must be random mating (4) There can be no movement into or out of the population(5) No natural selection

Evolution vs. Genetic Equilibrium

A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve.

Real populations rarely exist under the rigid conditions of the Hardy-Weinberg Equilibrium.

The Hardy-Weinberg principle predicts that 5 conditions can disturb genetic equilibrium and cause evolution to occur:1. Nonrandom mating – individuals select mates based on

heritable traits2. Small population size – evolutionary change due to

genetic drift happens more easily in small populations3. Immigration or emigration – movement of individuals

into (immigration) or out of (emigration) may introduce new alleles or remove alleles from the gene pool

4. Mutations – introduce new alleles changing allele frequencies

5. Natural Selection – different genotypes have different fitness

One or more of these conditions usually holds for real populations = evolution happens most of the time

Nonrandom mating example – female peacocks choose mates on the basis of physical characteristics such as brightly patterned tail feathers

Isolating Mechanisms When populations become reproductively

isolated, they can evolve into two separate species. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation.

Speciation is the formation of a new species. A species is a population whose members can interbreed and produce fertile offspring.

Isolating Mechanisms Reproductive isolation occurs when a

population splits into two groups and the two populations no longer interbreed.

When populations become reproductively isolated, they can evolve into two separate species.

Behavioral Isolation Behavioral isolation occurs when two

populations that are capable of interbreeding develop differences in courtship rituals or other behaviors.

Blue footed boobies perform an elaborate courtship display that involves the male ‘skypointing’ (pointing the head and beak upwards and spreading the wings), alternately lifting each blue foot, with the tail held cocked, and often

emitting a whistling call. Both members of the pair may then skypoint, touch beaks, lift the feet, or pick

up twigs or stones and place them on the ground

Blue Footed Booby Courtship

watch to 1:30 min

Behavioral Isolation Eastern and western meadowlarks are difficult

to distinguish between based on size, shape, and color, however, their calls are quite distinct. Presumably this difference serves to distinguish

mates from the different species. Behavioral isolation may also prevent different

firefly populations from mating because different species have their own pattern of light pulses. Other examples include mating dances and various courtship rituals.

Western Meadowlark Call

Eastern Meadowlark Call

Synchronous Firefly Display

Geographic Isolation Geographic isolation occurs when two

populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

Once completely separated, the two populations possess variations of some genes, resulting in two "species" that differ in appearance (color, size, etc.) and behavior.

For example, the Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. Separate gene pools formed, and genetic changes in one group were not passed on to the other.

Geographical Isolation The Kaibab squirrel

(Sciurus aberti kaibabensis, left) became geographically isolated from the common ancestor with its closest relative, the Abert squirrel (Sciurus aberti aberti, right) in the North Rim of the Grand Canyon about 10,000 years ago.

Since then, several distinguishing features, such as the black belly and forelimbs have gradually evolved.

Temporal Isolation Temporal isolation happens when two or

more species reproduce at different times. For example, three species of orchid live in the

same rain forest. Each species has flowers that last only one day and must be pollinated on that day to produce seeds. Because the species bloom on different days, they cannot pollinate each other.

Temporal Isolation The Red-legged Frog (Rana aurora, left)

breeding season lasts from January to March. The closely related Yellow-legged Frog (Rana boylii, right) breeds from late March through May.

Drosophila persimilis breeds in early morning, while closely related Drosophila pseudoobscura breeds in the afternoon

Temporal Isolation

Founders Arrive Many years ago, a few finches from South America—species M—arrived on one of the Galápagos islands, as shown in the figure. 

Geographic Isolation Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population. 

Changes in Gene Pools Over time, populations on each island adapted to local environments.  

Behavioral Isolation Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype. 

Competition and Continued Evolution

Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E). 

Speciation in Darwin’s Finches

Speciation in Galápagos finches occurred by:

1. founding of a new population

2. geographic isolation3. changes in the new 

population’s gene pool4. behavioral isolation 5. ecological competition

Gradualism Gradualism involves a

slow, steady change in a particular line of descent.

The fossil record shows that many organisms have indeed changed gradually over time.

The pattern of slow, steady change does not always hold.

Horseshoe crabs, for example, have changed little in structure from the time they first appeared in the fossil record.

This species is said to be in a state of equilibrium, which means that the crab’s structure has not changed much over a very long stretch of time.

Punctuated Equilibrium

Punctuated equilibrium is the term used to describe equilibrium that is interrupted by brief periods of more rapid change.

This is a proposed theory to explain the gaps in the fossil record.

Rapid Evolution After Equilibrium

Rapid evolution may occur after a small population becomes isolated from the main population. This small population can evolve faster than the larger one because genetic changes spread more quickly among fewer individuals.

Rapid evolution may also occur when a small group of organisms migrates to a new environment. That’s what happened with the Galápagos finches.

Genetic Drift A change in allele frequencies that

occurs due to chance events rather than differences in fitness

Genetic drift can cause big losses of genetic variation for small populations. Population Bottlenecks Founder Effect

http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml

Population Bottlenecks Population bottlenecks occur when a

population’s size is reduced to just a few individuals, often as a result of some form of disaster such as disease The smaller population has less genetic variation

than the original, larger one. Even when population increases, genetic variation

remains low.Cheetahs have extremely low genetic variation due to their

population having gone through a population bottleneck about

10,000 years ago - at the end of the last ice age when a

remarkable extinction of large vertebrates occurred on several

continents.

Population Bottleneck Because genetic drift acts more quickly to reduce

genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations. This is illustrated by the bags of marbles shown

below, where, in generation 2, an unusually small draw creates a bottleneck.

Bottleneck Example Northern elephant seals have reduced genetic variation probably

because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml

Reduced genetic variation means that the population may not be able to adapt to new selection pressures,

such as climatic change or a shift in available resources, because the genetic variation that

selection would act on may have already drifted out of the population.

Founder Effect A founder effect occurs when a new colony is

started by a few members of the original population.

This small population size means that the colony may have: Decreased genetic variation from the original

population by reducing the number of alleles. a non-random sample of the genes in the original

population. Alleles are regularly lost from populations.

However, the process of mutation produces new alleles.

http://evolution.berkeley.edu/evosite/evo101/IIID3Bottlenecks.shtml

Founder Effect

Founder Effect Example The Afrikaner population of Dutch settlers in

South Africa is descended mainly from a few colonists.

Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease (nerve cells in the brain degenerate) those original Dutch colonists just happened

to carry that gene with unusually high frequency.

Founder Effect ExamplePennsylvania Amish In the 1700s, a small group of Europeans settled in eastern Pennsylvania. Among this small group was an individual who carried an allele for Ellis-van

Creveld syndrome. Ellis-van Creveld syndrome is a very rare form of dwarfism causing short stature,

extra fingers (polydactyly), abnormal teeth and nails, and heart defects. The allele for Ellis-van Creveld syndrome is found at a frequency of 7% in the

Pennsylvania Amish in comparison to only 0.1% in the general population. The low allelic frequency of 0.1% was also the allelic frequency of the original

European population from which the Amish migrated. While the Amish live in close proximity to large, diverse human populations that

would be capable of breeding, the culture of the Amish restricts marriage outside of the group.

This results in genetic isolation and group interbreeding that allows the frequency of the allele for Ellis-van Creveld syndrome to not only persist but increase over time.