Chapt12_lecture-Final Sem 1 2013-14 Lecture 1

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Chapter 12

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Diversity within species and population genetics


Recognize how the concepts of species, gene pool, and population are related. •Explain the difference between the biological species concept and the morphological species concept. •State why all organisms of a species are not the same. •Distinguish between gene pool and genetic diversity. •List three methods used to distinguish species from one another. Know the factors that can cause differences in genetic diversity of different populations of the same species. •Explain how each of the following affects the genetic diversity within populations: mutation, sexual reproduction, population size, and migration. •Describe three processes that could result in different populations of the same species having different gene combinations. .

What we will learn at the end of this chapter


Learn the processes used to produce specific varieties of domesticated plants and animals. •Relate cloning and hybridization to asexual and sexual reproduction. •Explain how hybrid plants and animals are produced. •Describe how genetic engineering differs from the development of intraspecific hybrids and clones. •Describe the value and potential danger of the practice of monoculture. Recognize that population genetics principles apply to human populations. •Describe why certain diseases are more common in some groups of people than in others. •Describe how a lack of understanding about population genetics contributed to the eugenics movements. •Discuss the ethics matters in relation to human population genetics


Why study Population genetics ?

• To map populations genetic profiles• To identify key genetic markers and help prevent diseases• To apply this knowledge for solving societal issues –like food, preservation of plant and animal species•To be aware of the processes that underlie variation leading to natural selection-why populations change over time leading to evolution of species


Populations vs. Species

A species is all the organisms potentially capable of naturally breeding among themselves and having offspring that could successfully interbreed.

A population is a group of organisms in the same species in the same geographical area.

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Species –examples-human,murine,bacterial Population –eg same- Human population in BITS GOA,New

Delhi,Arctic Bacterial population in soil of different places

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Population Genetics and Gene Pools

Population genetics is the study of the kinds of genes (alleles) within a population.– Also accounts for the numbers of alleles in a

population– Predicts and observes how those numbers will

change over time– This data is used to classify organisms and study

evolutionary change.


Population Genetics and Gene Pools

In a population– Each individual has a set of alleles.

Diploid organisms have 2 alleles at most.

– The population may contain more different alleles than any one individual.

The human population has 3 alleles for blood type.

All of the alleles in a population make up the gene pool.


Example -blood groups diversity of alleles

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Genes, Populations, and Gene Pools


Another example-sub populations of mice

3 alleles Allele1 C+- gray colour C – brown colour c- white colour

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Allele 2 Tail T-long t- short Allele 3 Size- S –large , s- small

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The proportion of alleles differs based on subpopulations

In I all are brown while the other 3 they are mixed

In II none are brown Allele 2 In Population III all are long tailed while in

population IV 50% are short tailed Allele 3-population III 50% are small while in I

and II all are large

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Analogous examples

Dark and light moths allele frequency Difference in types of bacteria found in

extreme conditions and tropics-what could be difference?

In cell walls ,heat sensitivity of enzymes Human race -skin colour differences in

populations –example of what kind of inheritance??-

Polygenic 12-14


Biological Species Concept

According to the biological species concept, species is a group of organisms – That share a common gene pool– That are reproductively isolated from other

populations They do not exchange genetic information.

Local populations of a single species may have slightly different allele combinations.


Gene and Allele Frequencies

Differences in gene frequencies reflect genetic differences between populations.

Allele frequency is a measure of how often an allele is found in a population.– Expressed as a decimal or percentage– # of times an allele appears in a population/the

total number of alleles in the population


Allele Frequencies Differ in the Human Population


Allele Frequencies, Dominance, and Recessiveness

Allele frequencies are unrelated to whether the allele is dominant or recessive.

There are many instances where a recessive allele is more frequent in a population.

Blue eyes and light hair are recessive traits that are more frequent in European regions.


Example of ABO

What Determines Blood Type? ABO blood types are determined by a cell surface marker that

identifies the cell as belonging to "self" or to that individual. These cell surface markers are characterized by a protein or lipid that has an extension of a particular arrangement of sugars.

Figure 1 shows the arrangement of sugars that determines each of the A, B, and O blood types.

Note that each is identical, except that types A and B have an additional sugar: N-acetylgalactosamine for A, and galactose for B.

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Blood group types

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Relevance

These sugar arrangements are part of an antigen capable of stimulating an immune response that produces antibodies to identify and destroy foreign antigens.

People with blood type A produce antibody B when exposed to antigen B, and those with blood type B produce antibody A when exposed to antigen A.

Blood type AB, however, produces no antibodies because both antigens present on the cells are recognized as "self."

Blood type O produces antibodies A and B, because neither antigen A nor B is present on the cells of type O individuals

Though the O allele is recessive it is high in terms of frequency

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Table of frequency of O allele

The O blood type (usually resulting from the absence of both A and B alleles) is very common around the world.

About 63% of humans share it. Type O is particularly high in frequency among the indigenous populations of Central and South America, where it approaches 100%. It also is relatively high among Australian Aborigines and in Western Europe (especially in populations with Celtic ancestors).

The lowest frequency of O is found in Eastern Europe and Central Asia, where B is common

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O Allele prevalence in the world

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Implications

The ancestor O allele lacked the gene for the enzyme but A and B developed late in the course of evolution to confer an advantage over the earlier allele-studies are ongoing to prove this-or 0 allele could be a mutant of early human populations whose numbers were small

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Advantages of O allele

The O allele can provide a selective advantage since it also produces both anti-A and anti-B antibodies

In particular, it has been suggested that the O allele protects against severe malaria .

At the same time, it can be more sensitive to Helicobacter pylori infections and to severe forms of cholera

The complex pattern of putative selective agents favouring or acting against different alleles could explain the maintenance of the high ABO polymorphism as evidenced by the signal of balancing selection detected on the gene

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Research

Genetic characterization of the ABO blood group in Neandertals

Carles Lalueza-Fox1*, Elena Gigli1, Marco de la Rasilla2, Javier Fortea2, Antonio Rosas3, Jaume Bertranpetit1 and Johannes Krause4

PAPER:BMC Evolutionary Biology 2008, 8:342

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Results

This study tried to analyse samples of a Neathanderal man to confirm which blood group alleles were dominant and a O group allele was found in the same.

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Subspecies, Breeds, Varieties, Strains, and Races

These all describe different forms of organisms that are all members of the same species.– Dogs have different breeds.– Plants have different varieties.– Bacteria have different strains.– Humans have different races.

All of these are types of subspecies.


Subspecies of American Robins

(b) Turdus migratorius confirmis(a) Turdus migratorius migratorius


How Genetic Diversity Comes About

Genetic diversity describes genetic differences among members of a population.

– High genetic diversity implies that many different alleles exist in a population.

– Low genetic diversity implies that all of the individuals in the population have the same alleles.

A gene pool with greater diversity is likely to contain combinations of alleles that will allow the individuals to adapt to a changing environment.


1 Mutations

Mutations are changes in the base sequence of DNA.

Mutations are the source of new alleles.– All alleles originated with mutations.– Most mutations are harmful.– Occasionally a mutation will change a gene so that

the protein works differently or better. Example: Insecticide resistance in mosquitoes O allele formation


2 Sexual Reproduction

Sexual reproduction generates new genetic combinations.

– New combinations of alleles in individuals May not necessarily change the frequency of alleles

in a population– However, the new combination of alleles in an individual

may create a combination of traits that allows the individual to survive and reproduce more successfully than other individuals.

Example: Corn plants that inherit resistance to corn blight and resistance to insects


Another example related to wheat – doubling of chromosomes –lead to taller wheat –

A “genetic accident of nature” that lead to better crops

Today with genetic engineering we are triggering the changes

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Source

http://www.wheatbp.net/WheatBP/Documents/DOC_Evolution.php

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3 Migration

The migration is the movement of individuals into and out of populations.

– Results in alleles being added or subtracted from a population

– May change allele frequencies in the population– Examples:

Artificial migration is used in zoos to generate genetic diversity.

– Inbreeding has reduced genetic diversity in small zoo populations.

– Zoos are exchanging animals for breeding to introduce new alleles into their populations.


Example ferret,California condors,Asiatic Lion

Problems with this approach Low genetic diversity due to poor mimicry of

natural migration patterns

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100 lion cubs raise Gir's cute quotient

Link http://economictimes.indiatimes.com/environment/flora-fauna/100-lion-cubs-raise-girs-cute-quotient/articleshow/23842796.cms

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Preservation of Asiatic Lion

Gir, the world's only remaining home of wild Asiatic lions, sees about 80 to 85 new cubs every year

In 2008, International Union for Conservation of Nature (IUCN) had removed Gir lions from the critically endangered list and put them in the comparatively healthier endangered list

The Supreme Court has ordered Gujarat — much against its wishes — to part with a few of its lions for their relocation to Kuno-Palpur sanctuary in Madhya Pradesh, in the long-term interests of the lions' survival.

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4 The Importance of Population Size

Population size is directly related to genetic diversity.– The smaller the population, the less genetic

diversity a population can contain.Mutations, migrations, and death can have

dramatic effects on the genetic make-up of a population.

Frequently, random events will significantly change the gene pool.

– This is called genetic drift.


Example of Genetic Drift

Notice that in the original population, the red frogs were eliminated and failed to breed. Therefore, their genes were not passed on to the next generation. As a result, the frequencies of the genes change in the gene pool.


Why Genetically Distinct Populations Exist

Many species have wide geographic distribution with reasonable distinct subspecies.

This occurs for several reasons.


1Adaptation to Local Environmental Conditions

Genetic diversity allows populations to adapt to their specific environments.– Some individuals will have combinations of alleles

that allow them to survive and successfully reproduce in hostile conditions.

Death and migration remove or reduce the alleles that do not contribute to survival.

– Example: Lizards in the desert have lighter coloration than those that live in other environments.



2 Founder Effect

The founder effect is a type of genetic drift that occurs when a new population is established by a few colonizing individuals.– The small colonizing group may have different

allele frequencies than the original population.– When the colonizing individuals mate and

multiply, their allele frequencies will tend to persist, making the new population different from the parent population.


3 Genetic Bottleneck

Genetic bottleneck is another form of genetic drift.

Occurs when there is a dramatic reduction in population size

– Usually due to some chance event like a natural disaster examples eruption of Mt Vesuvius,end of Dinosaurs

– Could be due to over-hunting by humans The remaining members of the population

will mate and pass on their alleles, limiting their genetic diversity.

Many endangered species are undergoing genetic bottlenecks.


4 Barriers to Movement

When migration is limited, populations become geographically and reproductively isolated.– Perpetuates the effects of genetic drift caused by

founder effect and bottleneck– Limits genetic diversity and generates subspecies