evolution [part 3]

EVOLUTION

TOPIC OUTLINE

A)The pattern of evolutionB) Evolutionary agents

C) Artificial selection

D)Polymorphism

Biological Evolution:

is a change over time in the genetic composition of a population

Evolution creates two patterns across:

1. TIME:

vertical evolution – a change in a single lineage through time

Evolution creates two patterns across:

2. SPACE:

speciation (divergent evolution) - the process by which one evolutionary unit splits into two units, which thereafter evolve as distinct lineages

Darwin in his book ‘On the Origin of Species’:

was concerned with vertical evolution

http://en.wikipedia.org/wiki/File:Origin_of_Species_title_page.jpg

Darwin in his book ‘On the Origin of Species’:

did not discuss how a single species splits into two or more daughter species

Darwin in 1840

Darwin:

proposed a mechanism (natural selection) to explain adaptation

Adaptation has two meanings in evolutionary biology:

a characteristic (anatomy, physiology, behaviour) of an organism that is well suited to its environment

the process by which a population becomes better suited to its environment (better adapted)

adaptation is not perfect, just an improvement on what came before

Or

Microevolution vs Macroevolution

Microevolution or adaptive evolution:

refers to changes in allele frequencies

plant and animal breeding are examples of microevolution

Macroevolution or speciation :

is an evolutionary process by which species originate

the fossil record provides the best evidence

Question: [MAY, 2010]

Natural selection is the only evolutionary process that leads to adaptation.

What is an adaptation? (1)

Adaptations are structures or behaviours that allow efficient use of the environment. For example, the webbed foot of a duck enables it to swim better than a foot that is not webbed. Adaptations are due to genes, that is, they are inherited.

Two opposing views of evolution models:

1. Gradualism 2. Punctuated equilibrium

Time

What does ‘Punctuated equilibrium’ mean?

Long periods of stasis (equilibrium) are broken up (punctuated) by bursts of

speciation.

Both rely on the same evidence: fossils

They differ in how they:

interpret gaps in the fossil record

Extreme views of gradualism vs. punctuated equilibrium

Gradualism Punctuated equilibrium

Mutations & phenotypical changes are gradual and explain the fossil record gaps as simply missing because fossils are hard to find.

In long periods of time, mutations simply accumulate but do not cause any drastic phenotypical changes, followed by short periods where these mutations are suddenly expressed. This would account for the lack of transitional fossils.

Interpreting an evolutionary tree

a graph of time against change in form

Question: [SEP, 2001]

How may the following observation be interpreted in the light of evolutionary theory?

Viral DNA has the same basic structure as the DNA of prokaryotes and eukaryotes.(2)

All organisms originated from a common ancestor. Thus DNA of all organisms has the same basic structure.

Queston: [MAY, 2009]

The graph in Figure 1 shows the change in the length of a vertebrate limb (y-axis) over geological time (x-axis). The labels A and B on the graph represent time intervals. During the period being considered, the limb changes from length β1 to length β2, as shown in the graph. A palaeontologist studying this species concludes that the change from β1 to β2 is sufficient to make this a speciation event.

Figure 1: Change in the length of a vertebrate limb over geological time

1. What is speciation? (2)

Speciation is the evolutionary process by which new biological species arise.

2.During which time interval, A or B, is speciation occurring? Give a reason for your answer. (2)

B - the change in the length of a vertebrate limb is taking place.


3. What model of evolutionary change does this speciation event seem to be following? (1)

Punctuated equilibrium.


4 Briefly describe the principal characteristics of this model of evolutionary change. (3)

Characteristics of a species remain unchanged for a long period of time, then in a relatively short span, changes occur which lead to new species in such a way that no intermediate forms exist. This accounts for gaps in the fossil record.

5.Name ONE other model of evolutionary change and briefly describe its basic principles. (3)

Gradualism model. Evolutionary change occurs very slowly and at a constant rate. Intermediate forms thus are expected.

TOPIC OUTLINE

A) The pattern of evolution

B)Evolutionary agentsC) Artificial selection

D)Polymorphism

What are ‘Evolutionary agents’?

1. mutation

2. gene flow

3. random genetic drift

4. non-random mating

5. natural selection

Forces that change allele and genotypic frequencies in populations

Evolutionary agents cause deviations from the

Hardy-Weinberg equilibrium

Because such changes in the gene pool constitute small-scale evolutionary changes, they are referred to as microevolution m

acro

evo

luti

on

1. Mutations

mutation rates are generally so low that they have little effect on the Hardy-Weinberg proportions of common alleles

typical mutation rate is 1 in a million per generation

Exposure to chemicals or

radiation

DNA fails to copy accurately

1. Mutations

mutations can restore to populations alleles that other evolutionary agents remove

mutations both: create and help maintain variation within

populations

2. Gene flow

gene flow happens when migrating individuals breed in their new location

Immigrants may:

add new alleles to the gene pool of a population

change the frequencies of alleles already present if they come from a population with a different allele frequency

3. Genetic drift

is a change in the pool of genes of a small population that takes place strictly by chance

Explain why although genetic drift is a mechanism of evolution, it does not work to

produce adaptations.

It is an entirely RANDOM process.

Genetic drift can result in genetic traits :

being lost from a population

becoming widespread in a population without respect to the survival or reproductive value of the gene pairs (alleles) involved

OR

Effects of genetic drift on a small population are large:

the marble-drawing scenario illustrates why drift affects small populations more

bag has only 20 marbles (a tiny bag!) & you can only draw four marbles to represent gene frequencies in the next generation

something like this might happen:

Notice how quickly and drastically the marble ratio changed: 1:1, 1:3, 0:1.

What is the ‘bottleneck effect’?

When the size of a population is drastically reduced due to an event/disaster

E.g. during the 1890’s hunting reduced the number of elephant seals to about 20 in a single population

During population bottlenecks, genetic variation can be reduced by

genetic drift

The ‘founder effect’:

is a form of bottlenecking

a small group in a population splinters off from the original population and forms a new one

Island

The founder effect is a special case of genetic drift

by chance alone, a few individuals that establish a new population have allele frequencies that differ from those of the original population

Original population

4. Non random mating occurs when individuals choose mates

with particular phenotypes

The Zoo’s Panda breeding program was still getting nowhere.

TOO FAT, TOO WHITE, TOO SMALL….

Mating patterns may alter genotype frequencies

if individuals in a population do not choose mates at random

E.g., if they mate preferentially with individuals of the

same genotype, then:

AA x AAaa x aaAa x Aa

100% AA 100% aa25% AA : 25% aa: 50% Aa

Homozygous genotypes will be overrepresented

Heterozygous genotypes will be underrepresented relative to

the Hardy-Weinberg expectations

100% AA 100% aa25% AA : 25% aa: 50% Aa

AA x AAaa x aaAa x Aa

Alternatively, individuals may mate

primarily or exclusively with individuals of different genotypes

nonrandom mating is seen in some plant species, such as primroses where pin-eyed flowers mate with thrum-eyed flowers

Self fertilisation (selfing): another form of nonrandom mating

is common in many organisms, especially plants

Selfing:

1. reduces the frequencies of heterozygous individuals from Hardy-Weinberg equilibrium

2. increases the frequencies of homozygotes

3. does not change the allele frequencies

5. Natural Selection

What is ‘Selection’?

The process by which those organisms which appear physically, physiologically and behaviourally better adapted to the environment survive and reproduce; those organisms not so well adapted either fail to reproduce or die

Two types of selection:

1. Artificial selection:

a breeder selects for the desired characteristics

2. Natural selection:environmental conditions determine which individuals in a population produce the most offspring

Generation 1: 1.00 not resistant0.00 resistant

Resistance to antibacterial soap

How natural selection works



mutation!





Selection determines which alleles are passed to the next generation

natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can result in evolutionary change

NATURAL SELECTION

Mutations cause variation in traits

Bad

Die

No offspring

Good = adaptation

Survive

Reproduce and pass adaptation to

offspringEVOLUTION

predators

shelterenvironmental

factors

amount of food

moreover, natural selection can occur without producing evolutionary change;

only if variation is genetically based will natural selection lead to evolution

Selection pressure is a means of:

increasing

or decreasing

the spread of an allele within the gene pool

What can these changes in allele frequency lead to?

Evolutionary change

Three conditions for natural selection to occur and to result in evolutionary change:

1. Variation must exist among individuals in a population.

3. Variation must be genetically inherited.

2. Variation among individuals must result in differences in the number of offspring surviving in the next generation.

Selection processes

competition sets in as certain factors become

limiting e.g. food

What happens when a population increases in size?

Which organisms survive and reproduce?

those showing characteristics which give them a competitive advantage to obtain the resources

organisms without, die before reproducing

Environmental limiting factors

&

Population size

What is a ‘selection pressure’?

A selection pressure is an abstract force that shapes organisms as

they evolve

operate together to produce a selection

pressure

Potential sources of selection pressure:

Availability of preyPresence of predatorsObtaining foodCompetition with other speciesParasitesCompetition for a mateClimate


Availability of prey variants with adaptations allowing them

to obtain food have more offspring

- e.g. Speed, senses for finding prey/food, weapons for killing prey

or obtaining food, camouflage for stealth

Sole


Presence of predators

variants with adaptations allowing them to escape predators have more offspring

- e.g. speed, defensive weapons, camouflage, mimicry

A noxious mixture shoots into the face of an attacker with a loud pop at boiling temperatures.

- camouflage

Can you spot the frog?

- mimicry


The leaf bug

The selection pressure is predators

Its strategy is to mimic a leaf

Question: [SEP, 2004]

Comment on the biological significance of the following observation: Removal of predators may reduce

diversity of prey organisms. (2)

Predators create a selection pressure, favouring certain phenotypes instead of others. Removal of this pressure, no longer drives populations to diversify.


Competition for a mate variants with adaptations allowing them

to attract a mate to have offspring

- e.g. strong, attractive, good provider

Zebra finches.

Brightness of male’s bill is an indicator of health.

Color is due to carotenoids, which are also important in the immune system. A brighter bill indicates more carotenoids and greater overall health.

Original population

(c) Stabilising selection(b) Disruptive selection(a) Directional selection

Phenotypes (fur color)

Fre

qu

ency

of

ind

ivid

ual

s

Originalpopulation Evolved

population

Three types of selection processes:

1. Stabilising Selection

Natural selection favors the average for population selected

Happens when the extremes of the trait are not as well suited

E.g. tail length of birds

Example: Stabilising Selection

Human birth weight underweight or overweight babies die,

therefore a certain weight is favoured

a study on 13, 730 babies [1935 -1946] showed that there is an optimum birth weight of about 3.6 kg

2. Directional Selection

Natural selection favors one extreme of the population for that trait

often happens when environment changes in a consistent way- e.g. climate gets colder

Examples: Directional Selection

Peppered moths before and after the industrial revolution

Up to 1848:

All moths were creamy white

In 1848:

A black form was reported

By 1895:

19% were black


Insecticide resistance

Random mutation

Resistant insects survive

Resistant insects multiply


Selective breeding

3. Disruptive Selection

Natural selection favors both extremes selected

Causes species to diverge

[polymorphism]

Disruptive Selection Results in a Bimodal Distribution

bill size in black-bellied seed crackers

Tim

eStabilising selection

Disruptive selection

Directional selection

What is ‘fitness’?

Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to how good

other genotypes are at it

Lower fitness Higher fitness

F1

F2

I’m proud to

be so fit !

A genotype’s fitness includes:its ability to survivefind a mateproduce offspring—and

ultimately leave its genes in the next generation

These strategies help the parents get more of their offspring into the next generation.

Strategies which increase fitness:

Parental care

Producing thousands of young

Sporting fancy feathers that attract females

TOPIC OUTLINE


B) Evolutionary agents

C)Artificial selectionD)Polymorphism

Artificial Selection :

is the gradual improvement of animal and plant characteristics over time, for human benefit

the term was utilised by Darwin in contrast to natural selection

Over thousands of years man has selected characteristics in dogs that are useful…..

•Very small dogs•Used for pets

•Large hunting dogs•Strong & powerful

•Large working dogs•Strong and athletic

Look at the variety of plants that have been artificially selected from mustard

What is ‘inbreeding’?

Mating of closely related individuals

Twins – result of inbreeding

Advantages of inbreeding:

For example:

Parental genotypes: Ffgg x Ffgg

Gametes: Fg Fg

F1 genotypes: FFgg

(pure breeding)

X

Create pure line over time

Better adapted to steady environment

Disadvantages of Inbreeding:

1. Increasing the chance for recessive genes to be homozygous

2. Most recessive genes are undesirable traits

3. Leads to a high frequency of defects present at birth

4. Reduce the genetic variability vigour and fertility of a population.

How do breeders overcome the problems of inbreeding?

Breeders resort to outbreeding after

several generations of inbreeding

What is ‘Outbreeding’?

crossing individuals from genetically distinct populations

outbreeding usually takes place between: members of different varieties

or strains

in certain plants between closely related species

Advantages of outbreeding:

1. Progeny are heterozygous and the bad recessive genes are masked by normal dominant alleles.

2. Hybrid vigour – Progeny are tougher, more fertile and have a greater chance of survival.

3. Produces variation/heterozygosity; on which natural selection can act.

No more pure line exists

Disadvantage:

A simple genetic explanation of increased vigour in F1 hybrids

Parental genotypes (2n) :

FFgghhIIjj x FFGGHHiiJJ

Gametes (n) :

F1 genotype (2n) : FFGgHhIiJj

FghIj

FGHiJ

X

Hybrids have characteristics superior to either of the parental stocks: the

phenomenon is called

hybrid vigour

TOPIC OUTLINE


B) Evolutionary agents

C) Artificial selection

D)Polymorphism

Polymorphism is the:

existence of two or more forms of the same species within the same population, and can apply to:

biochemical morphological behavioural

characteristics

http://www.sciencephoto.com/image/374534/large/Z4850033-Genetic_polymorphism_in_banded_snails_Cepaea_-SPL.jpg

Polymorphism plays a significant role in the process of natural selection. Why?

As different phenotypes are produced

Shell color polymorphism in the snail, Liguus fascitus

Two forms of Polymorphism:

1)transient polymorphism

2) balanced or stable polymorphism


1) transient polymorphism arises when different forms (morphs),

exist in a population undergoing strong selection pressure

Transient Polymorphism:

usually applies in situations where one form is gradually being replaced by another

e.g. the peppered moth

Normal situation:

peppered (grey) form more numerous than melanic (black) form

Industrial Revolution:

peppered (grey) form less numerous than melanic (black) form


2) balanced or stable polymorphism occurs when different forms coexist

in the same population in a stable environment

Examples of Balanced Polymorphism:

1. the existence of the two sexes in animals and plants

The sex ratio for the entire world population – 101 males :100 females

2. in humans, the existence of the A, B, AB and O blood groups

3. red-green colour blindness in humans

Examples of Balanced Polymorphism:

4. the existence of workers, drones and queens in social insects 5. pin-eyed and thrum-

eyed forms in primroses

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%

7.5–10.0%

5.0–7.5%

>12.5%

10.0–12.5%

6. sickle-cell anaemia

Question: [MAY, 2007]

Use your knowledge of biology to explain the following statement.

Islands generally comprise several endemic species.

(5 marks)An endemic species is found only on one island. This is the result of allopatric speciation. No gene flow between populations on different islands – sea is a geographical barrier. Different mutations may occur on the different islands. Islands could have different selection pressures in the form of climate, predators and food sources. Chance events like the founder effect contribute to a change in allele frequency.

Summary

One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes

Natural selection acts on individuals, but only populations evolve

Genetic variations in populations contribute to evolution

Microevolution is a change in allele frequencies in a population over generations

The End

evolution [part 3]

Documents

graph of time

longperiod of time

time intervals

evolutionary tree

geological time xaxis

long periods of time

single lineagethrough

light of evolutionary