Chapter-9
Heredity and Evolution
Accumulation of Variation during Reproduction
(i) Depending upon the nature of variation different individual would have different kinds of advantage.
Example, Thermostatic Bacteria that can withstand heat will survive better in a heat wave.
(ii) Main advantage of variation to species is that is increase the chances of its survival in a changing enviroment.
Free ear lobes and attached ear lobes are two variants found in human populations.
Mendel and His Work on Inheritance
• Gregor Johann Mendel (1822 & 1884) : Started his experiments on plant breeding and hybridisation.He proposed the laws of inheritance in living organisms.
Mendel was known as Father of Genetics.
• Plant selected by Mendel : Pisum sativum (garden pea). Mendel used a number of contrasting characters for garden pea.
| CHARACTER | DOMINANT TRAIT | RECESSIVE TRAIT |
|---|---|---|
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Pod shape | Inflated/full | Constricted |
(i) Availability of detectable contrasting traits of several characters.
(ii) Short life span of the plant.
(iii) Normally allows self-fertilisation but cross-fertilisation can also be carried out.
(iv) Large no. of seeds produced.
Monohybrid Cross
Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross.
Example : Cross between a tall and a dwarf plant (short).
| Pod colour | Green | Yellow |
| Flower position | Axial | Terminal |
| Stem height | Tall | Dwarf |
Observations of Monohybrid Cross
(i) All F1 progeny were tall, no medium height plant. (Half way characteristic)
(ii) F2 progency 3/4 were short. 3/4 were tall.
(iii) Phenotypic ratio F2 -3 : 1 (3 tall : 1 short)
| Genotypic ratio F2 -1 : 2 : 1 | (TT : Tt : tt)
(1 : 2 : 1) |
Conculusions
1. TT and Tt both are tall plants while tt is a short plant.
2. A single copy of T is enough to make the plant tall, while tall copies have to be 't' for the plant to be short.
3. Characters/traits like 'T' are called dominant trait (becuase it express itself) and 't' are recessive trait (becuase it remains suppressed).
Dihybrid Cross
A cross between two plants having two pairs of contrasting characters is called dihybrid cross.
Phenotypic Ratio :
Round, yellow : 9
Round, green : 3
Wrinkled, yellow : 3
Wrinkled, green : 1
Observations :
(i) When RRyy was crossed with rrYY in F1 generation all were Rr Yy round and yellow seeds.
(ii) Self pollination of F1 plants gave parental phenotype and two mixtures (recombinants round yellow and wrinkled green) seeds plants in the ratio of 9 : 3 : 3 : 1.
| 9 | : | 3 | : | 3 | : | 1 |
| (Round yellow) |
(Round green) |
(Wrinkled yellow) |
(Wrinkled green) |
Conculusions
1. Round and yellow seeds are Dominant characters.
2. Occurrence of new phenotype combinations show that genes for round and yellow seeds are inherited independently of each other.
How do these traits get expressed
| Cellular DNA (Information source) |
|---|
| ↓ For synthetic of |
| Proteins (Enzyme) |
| ↓ Works efficiently |
| More Hormone |
| ↓ Produced |
| Tallness of plant |
Therefore, genes control characteristic/traits.
| Enviromental ↓ |
Genetic ↓ |
| In some animals, the temperature at which the fertilized eggs are kept decides the gender. E.g., in turtle |
In some animals like humans gender of individual is determined by a pair of chromosomes called sex chromosomes. XX - Female Xy - Male |
Sex Chromosomes : In human beings, there are 23 pairs of chromosome. Out of these 22 chromosome pairs are called autosomes and the last pair of chromosome that help in deciding gender of that individual is called sex chromosome.
XX - Female SEX
XY - Male CHROMOSOMES
This shows that half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless whether they are boys or girls. Thus, sex of children will be determined by what they inherit from their father, and not from their mother.
EVOLUTION
* Evolution is the sequence of gradual changes which takes place in the primitive organisms, over millions of years, in which new species are produced.
Situation I
|
Group of red beetles ↓ Colour variation arises during reproduction ↓ |
|---|
|
All beetles red except one that is green ↓ Crows feed on red beetle ↓ No of red beetle reduces |
One beetle green ↓ Reproduction Progency beetles green ↓ Crows could not feed on green beetles as they got Camouflaged in green bushes ↓ Number of green beetles increase |
Conclusion
* Green beetles got the survival advantage or they were naturally selected as they were not visible in green bushes. This natural selection is exerted by crows resulting in adaptations in the beetles to fit better in their enviroment.
Situation II
Group of red beetles
↓ Reproduction
All beetles are red except one that is blue One blue beetle
↓ Reproduces
All beetles are red except one that is blue No. of blue beetles increases
|
↓ |
|
Crows can see both blue and red beetles and can eat them |
Conclusion
* Blue beetles did not get survival advantage. Elephant suddenly caused major havoc in beetles populations otherwise their number would have been considerably large.
* From this we can conclude that accidents can change the frequency of some genes even if they do not get survival advantage. This is called genetic drift and it leads to variation.
Situation III
|
Group of red beetles ↓ Habitat of beetles (bushes) suffer from plant disease ↓ Average weight of beetles decreases due to poor nourishment ↓ Number of beetles kept on reducing ↓ Later plant disease gets eliminated ↓ Number and average weight of beetles increase again |
Conclusion
No genetic change has occurred in the populations of beetle. The population gets affected for a short duration only due to enviromental changes.
| Acquired Traits | Inherited Traits |
|---|---|
|
1. These are the traits which are developed in an individual due to special conditions.
2. They cannot be transferred to the progency. 3. They cannot direct evolution. E.g., Low weight of starving beetles. |
1. These are the traits which are passed from one generation to the next.
2. They get transferred to the progency. 3. They are helpful in evolution. E.g.,Colour of eyes and hair. |
WAYS BY WHICH SPECIATION TAKES PLACE
Speciation takes place when variation is combined with geographical isolation.
1. Gene flow : Occurs between population that are partly but not completely separated.
2. Genetic drift : It is the random change in the frequency of allele (genepair) in a population over successive generations.
3. Natural selection : The process by which nature selects and consolidate those organisms which are more suitable adapted and possesses favourable variations.
4. Geographical isolation : It is caused by mountain ranges, rivers etc. Geographical isolation leads to reproductive isolation dur to which there is no flow of genes between separated groups of population.
Evolution and Classification
Both evolution and classification ar interlinked.
1. Classification of species is reflection of their evolutionary relationship.
2. The more characteristic two species have in common the more closely they are related.
3. The more closely they are related, the more recently they have a common ancestor.
4. Similarities among organisms allow us to group them together and to study their characteristic.
I. Homologous Organs : (Morphoological and antomical evidences). These are the organs that have same basic structural plan and origin but different functions.
Homologous organs provides evidence for evolution by telling us that they are derived from the same ancestor.
|
Example : |
|||
|
Forelimb of horse Wings of bat Paw of a cat |
(Running) (Flying) (Walk/scratch/attack) |
Same basic structural plan, but different functions perform. |
II. Analogous Organs : These are the organs that have different origin and structural plan but same function.
Example : Analogous organs provide mechanism for evolution.
| Wings of bat → Elongated fingers with skin folds
Wings of bird → Feathery covering along the arm |
Different basic structure, but perform similar function i.e., flight. |
III. Fossils : (Paleontological evidences)
The remains and relics of dead organisms of the past.
FOSSILS ARE PRESERVED TRACES OF LIVING ORGANISMA
Fossil Archaeopteryx possess features of reptiles as well as birds. This suggests that birds have evolved from reptiles.
Examples of Fossils :|
AMMONITE TRILOBITE KNIGHTIA RAJASAURUS |
- - - - |
Fossil-invertebrate Fossil-invertebrate Fossil-fish Fossil-dinosaur skull |
AGE OF THE FOSSILS
| I. | Deeper the fossil, older it is. |
1. (Top layer ofthe earth Recent → • surface) |
|
| II. |
Detecting the ratios of difference of the same element in the fossil material i.e., Radio-carbon dating [C-(14) dating] |
2. ................... 3. ................... 4. ................... 5. ................... |
Layer of Earth surface |
|
• ← Older 6. ................... |
Evolution takes place in stages i.e., bit by bit over generations.
I. Fitness Advantage
Evolution of Eyes : Evolution of complex organs is mot sudden. It occurs due to minor changes in DNA, however takes place bit by bit over generations.
• Flat worm has rudimentary eyes. (Enough to give fitness advantage)
• Insects have compounds eyes.
• Humans have binocular eyes.
II. Functional Advantage
Evolution of Features : Feathers provide insulation in cold weather but later they might become useful for flight.
Example, Dinosaurs had feathers, but could not fly using feathers. Birds seem to have later adapted teh feathers to flight.
Evolution by Artificial Selection
Human have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artifical selection. E.g.,
(i) From wild cabbage many varieties like broccoli, cauliflower, red cabbage, kale, cabbage and kohlirabi were obtained by artifical selection).
(ii) Wheat (many varieties botained due to artifical selection).
Molecular Phylogeny
• It is based on the idea that cahnges in DNA during reproduction are the basid events in evolution.
• Organisms which are most distantly related will Accumulate greater difference in their DNA.
| Tools to study Human evolutionary relationship |
|---|
| ↓ |
| Excavating | Time dating | Fossils | Determinign DNA sequences |
* Although there is great diversity of human forms all over the world, yet all humans are a single species.
| Hundreds/thousand of years ago |
|---|
| Earlier members arose in Africa
↓ |
| ↓ | ↓ | ↓ | ↓ | ↓ | ↓ | ↓ | ↓ | ↓ | ↓ |
| Philippines | Africa | South Africa |
Island of Indonesia |
West Asia |
East Asia |
Central Asia |
Eurasia | Australia | Eurasia |
• They did not go in a single line.
• They went forward and backward.
• Moved in and out of Africa.
• Sometimes came back to mix with each other.
Genetic Terminology
1. Gene : Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the inheritance of character.
2. Allele : A pair of genes that control the two alternatives of the same phenotypic characteristic e.g., TT/tt.
3. Heterozygous : The organism in which both the genes of a character are unlike e.g., Tt.
4. Homozygous : The organism in which both the able of genes of a character are similar e.g., TT, tt.
5. Dominant : The gene which express itself in F1 generation is known as dominant gene.
6. Recessive : The gene which is unable to express itself in presence of the dominant gene.
7. Genotype : It is the genetic constitution of an organism which determines the characters.
8. Phenotype : It is the appearance of an individual.
9. Micro-evolution : It is the evolution which is on a small scale.
10. Species : A group of similar individuals within a population that can interbreed and produce fertile offspring.
11. Chromosome : Thread like structures present in the nucleus of a cell, containing hereditary information of the cell.
12. DNA : Deoxyribose Nucleic Acid.
It is present in chromosome which carries traits in a coded form, one generation to the next.
