16 - Evolutionary Mechanisms.docx

7/27/2019 16 - Evolutionary Mechanisms.docx

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Chapter 15 Evolutionary Mechanisms 1

Gene pools

• Population: a group of organisms of the same species living together in a particular

place at a particular time.• Geneticists: scientists who specialise in the study of inheritance, (consider

characteristics of the population rather than individuals)

• Gene pool is the sum of all the alleles in a given population.

• Allele frequencies: how often each allele of a gene occurs in the gene pool for that

population.

• e.g an allele for cystic fibrosis is found on chromosome 7. If the frequency of the cystic

fibrosis allele in a given population is 5%, then among population members, 5/100 of

chromosome 7 will carry that allele.

• Pplns with differing characteristics different gene pools.

• e.g Scandinavians blue eyes (high blue eye allele frequency) whereas black Africans

brown eyes. (high brown eye allele frequency)

• Over time, the frequency of particular alleles or composition in a population may

change.

• Due to chance (mutation that alters the expression of a gene), or by natural means

(changes in the environment)

Changes to allele frequencies

Caused by:

1. Mutation

2. Natural Selection

3. Random Genetic Drift

4. Founders Effect

5. Migration

1. Mutations• New variations in offspring, that show no resemblance to either parent, occurring

suddenly and randomly.

• Mutant: An organism with a characteristic resulting from a mutation.

• Two main types of mutations:

gene mutations: single gene, traits produced by that gene are changed or destroyed.

chromosome mutations: in which all or part of a chromosome is affected.

• Somatic mutation: (body cells) Reproductive cells are not affected so that when the

individual dies, the mutation is lost. Somatic mutations are often the cause of many

cancerous growths.




• Germline or germinal mutations. (reproductive cells). Person not usually affected but

can be passed on to the next and subsequent generations.

• May improve survival chances likely to be passed on to future generations change

in allele frequencies of the gene pool.

2. Natural selection• The selection of favourable alleles at the expense of others. (Not random)

• The alleles enhance survival and reproduction of the individual and therefore species.

• Major cause of changes to allele frequencies.

• Generally allele frequencies in a large population is same from gen to gen.

• If natural selection is operating on some characteristics at the expense of others, then,

given time, the allele frequencies for that characteristic will change.

3. Random genetic drift (Sewall-Wright effect)• In small populations, there is often a random, non-directional variation in allele

frequencies which has a direct effect on the allele frequencies of future generations.

e.g Dunkers

• In Pennsylvania but originally from Hesse (Germany).

• Religion does not allow them to marry outside group isolated breeding population

within U.S.

• The study investigated frequency of ABO, Rh and MN (LM

and LN) blood groups, mid-

digital hair, left- or right-handedness, and attached or free ear lobes.

• Generally allele frequency varied from present-day ppln of Hesse and from American

ppln even though environment is very similar. (in US)

e.g Aborigines

• isolated populations of the islands of Bentinck and Mornington in the Gulf of

Carpentaria. (Queensland)

• The occupants of Bentinck Island show allele frequency (very high proportion of the IB

allele and a complete absence of the IA), unlike the mainland population, (low

proportion of the IB allele and a high proportion of the IA)

4. Founder effect.• a small group moves away from homeland and establishes a new community

expands, but group (small sample of the original ppln), is not genetically representative

of them.

• This new community, shows features that are not typical of original population.

e.g Inhabitants of Pitcairn Island (in Pacific)

• 1790 by the descendants of nine mutineers from the Bounty , together with eighteen

Polynesians from Tahiti.

• Due to isolation few alleles have been introduced. The descendants of the original

Pitcairn islanders show less genetic diversity than the original parent populations

because they all descended from the small number of original settlers.




e.g Inhabitants of Tristan da Cunha (South Atlantic Ocean between South Africa and South

America.)

• 15 British immigrants formed the first permanent settlement (1816) 250 present day.

(only seven surnames) Very few migrations. Different gene pool from the British

population today.

e.g Finland

• Settled approx 2000 years ago.

• Ppln relatively isolated, with little immigration. Gene pool different from the gene pools

of neighbouring European populations.

e.g Inhabitants of Pingelap (in Micronesia)

• 1775 a typhoon reduced the ppln to 20.

• Among the survivors was a person heterozygous for achromatopsia. (total colour

blindness - recessive).

• Today incidence of achromatopsia on Pingelap is 5% while in other parts of the world itis 0.0033%.

• Furthermore 30% of the Pingelap population are carriers.

5. Migration• gene flow from one population to another.

• Therefore, if immigrants to a certain country bring alleles that are not already in the

population, the frequencies for the alleles of that gene will be altered.

E.g China

• Originally completely Rh +. European immigrants and sailors introduced Rh – allele in the

16th century (due to trading). Frequency of Rh- allele is still comparatively low.

E.g. Mongolians

• IB allele across Europe and Asia. The inhabitants of East Asia (Mongols), have a high

frequency of IB

compared with Europe. Most Western Europeans lacked IB

completely. In

the 12th and 13th

centuries, the Mongols invaded Europe several times spreading their

culture and genes resulting in a steady decrease in the IB allele from Central Asia to

Western Europe.

• lowest concentrations of the IB are in Pyrenees and Caucasus mountains (where the

original Europeans fled to escape the invading Mongols)

e.g. Europeans in Australia

• Before British colonisation (1788), indigenous Australians had no contact with European

disease and therefore poor immunity.

• Diseases such as chickenpox, smallpox, influenza and measles spread rapidly throughout

the Aboriginal population. Approx 90% of decline in Aboriginal population due to

disease.

Barriers to gene flow




• Something that inhibits the amount of interbreeding between populations.

• Results in slightly different characteristics being favoured in one population compared to

the other.

• These changes in each population, over many generations, result in the populations

becoming less alike as they develop characteristics better suited to their respective

environments.

• Isolation results in the development of separate gene pools.

• Geographical barriers include oceans, mountain ranges, large lake systems, deserts and

expansive ice sheets.

• Eg. Indigenous Australians were isolated for thousands of years by ocean barriers

• Sociocultural barriers include religion, language, economic status, educational

background and social position

• E.g Australians tend to marry people of similar educational background, and members of

particular religious groups favour partners who share the same faith.• E.g The Basque people of the Pyrenees (France and Spain) have a unique language.

Helps to unite them and to preserve their cultural identity, even though they have the

same religion and occupations as neighbours.

• They are characterised by broad foreheads, narrow jaws. 35% are Rh-whereas in

the surrounding European population only 16% of people are Rh –.

Evolution through Natural Selection

• Charles Darwin (& Russel Wallace) – 1858

• Darwin’s major work was on the HMS Beagle to Galapagos Islands, New Zealand &

Australia.

• Published ‘On the Origin of the Species’

• Influenced by Carolus Linnaeus, Charles Lyell & Thomas Malthus.

Darwin’s Three key observations: • 1.Variation: There is variation within a species

• 2.Birth rate: More offspring of a species are produced than the environment can sustain

• 3.Natures Balance: Each species appeared to maintain their numbers at a constantlevel.

Darwin’s Two key interpretations of these observations: • 1.Struggle for existence: Due to excessive birth rate, and limited resources, there is a

competition for survival

• 2.Survival of the fittest: The individuals with characteristics best suited to the

environment have more chance of surviving and reproducing.

• Favourable characteristics (those with survival value) are passed on to the next

generation




• In the gene pool, the proportion of alleles that produce favourable characteristics

gradually increases.

• Examples of Natural Selection in Humans

• e.g. Cold Environment & short stature.

• Individuals with long bodies and short limbs (e.g. Inuit) have a smaller surface area in

relation to body volume than those with short bodies and long limbs (e.g. African). Such

individuals lose less heat in very cold environments.

Genetic diseases An allele causing an inherited, fatal disease would be expected to be gradually

eliminated from a population because people with the allele would die and would not

pass it on to the next generation.

This has probably happened many times in the past but sometimes alleles that cause

serious disease do persist in certain populations.

e.g. Tay-Sachs disease (TSD)

• A hereditary disorder of lipid metabolism that occurs most frequently in individuals of

Jewish descent from Eastern Europe (Ashkenazi Jewish population).

• Caused by a missing enzyme that results in the accumulation of a fatty substance in the

nervous system. Death usually occurs by the age of 4 or 5.

• Frequency worldwide approx 1/500 000 births. Ashkenazi Jews approx1/2500.

• High frequency possibly due to genetic drift. Jewish populations have tended to be small

and isolated, factors that increase the chances of genetic drift.

• Possibly due to observation that individuals who are heterozygous for Tay-Sachs, appear

to have increased resistance to tuberculosis (TB). (Advantageous in situations where TB

is prevalent)

• individuals with two normal alleles would be more susceptible to TB, and would possibly

die, while individuals with two Tay-Sachs alleles would die early in life. Heterozygotes,

on the other hand, would have a survival advantage and would be more likely to

reproduce and pass their alleles on to the next generation.

• Due to discrimination, the Ashkenazi Jews often found themselves isolated in

overcrowded ghettos under conditions that would increase the threat of TB high

frequency of Tay-Sachs.

• The same mutation that causes Tay-Sachs in Ashkenazi Jews also occurs in the Cajun

population of southern Louisiana in the United States. (Possibly when a Jewish family

assimilated into Cajun society)

E.g Sickle-cell anaemia.

• mutation of the gene for haemoglobin production causes the substitution of one amino

acid (valine) for another (glutamic acid).

• The affected haemoglobin is often referred to as haemoglobin S

• Mainly in black Africans, or in people of black African ancestry.

• In the tropical zone of Africa, up to 40% of some populations carry the allele for sickle-

cell anaemia.




• The disease is usually fatal, as the sickle-shaped cells do not carry as much oxygen as

normal red blood cells and also stick together and block small blood vessels.

• Sickle-cell trait: Individuals with only one allele for sickle-shaped cells show no ill effects

unless oxygen is in short supply. When this occurs their red blood cells show mild

sickling.

• Also provides a degree of immunity to malaria, a disease prevalent in parts of the world

where the sickle-cell gene is found.

• For this reason, the allele is maintained in areas where malaria is present.

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