Evolutionary forces

HARDY WEINBERG 'S LAW

1. DESTABILISING FORCES OF EVOLUTIONARY EQUILIBRIUM BY ANILA VARGHESE I MSc ZOOLOGY

2. What is evolutionary equilibrium??? Evolutionary equilibrium = Hardy Weinberg equilibrium Hardy Weinberg Law states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary forces. Evolutionary forces = Destabilising forces

3. What is allele frequency? Allele is the alternative form of a gene. Allele frequency = no. of copies of a particular allele in a population total no. of all alleles for that gene in a population No of times a particular allele is present in that particular locus in a population.

4. p + q = 1 allelic perspective (p + q) (p +q) = genotype perspective = + 2pq + = 1 = pp = homozygous dominant 2pq = heterozygous = qq = homozygous recessive.  

5. Evolutionary forces GENETIC DRIFT Change in allele frequency due to chance. Acts in a small population. Reduces genetic variability. Examples : Bottleneck effect and Founder effect

6. Bottle neck effect It occurs when there is a sudden sharp decline in a population’s size typically due to environmental factors (natural disasters such as earthquakes or tsunamis, epidemics that can decrease the number etc.). S ome genes (there is not any distinction) are extinguished from the population . R eduction of the total genetic diversity of the original gene pool.  S urviving population is be farther from the original one in its genetic makeup.

7. Example : Northern elephant seals Coastal areas of U.S and Mexico Bottleneck event Humans hunting (1890s) Population size 20 individuals (at the end of the 19th century) Now Population 30,000 B ut their genes still carry the marks of this bottleneck Much less genetic variation

8. Founder effect Loss of genetic variation that occurs when a new population is established by a small number of individuals that are cleaved from a larger population.  N ew population does not have the genetic diversity of the previous one . Some genetic traits are becoming more prevalent in the population . L eads to the presence of certain genetic diseases in the next generations

9. Example :- Dutch settlers in south Africa Afrikaner population High frequency of Huntington's disease allele ( nerve cell in brain degenerate)

10. 2. MUTATION Sudden heritable changes Beneficial H armful lead to reduce fitness reproductive success increase susceptibility to disorders & illness better adaptability to environment spread and get fixed in population

11. Example :- Influenza A virus, causes human influenza pandemic An RNA virus, made of 8 ssRNA Sub-divided into hemagglutinin (HA) and neruaminidase (NA), based on the antigenic protein present . RNA polymerase High error rate Mutations Antigenic drift   Viral evolution Antibody provides only partial immunity. Therefore flu vaccine needs to be modified very often

12. 3. MIGRATION OR GENE FLOW In an evolutionary sense, migration is the movement of alleles among populations. A unifying force, prevent population from diverging Homogenisation Is effect of migration on alleles same or opposite of the effect which of genetic drift and natural selection have on alleles???

13. No selection, Only Migration Homogenisation Selection stronger than migration More diversity Migration stronger than selection Less diversity

14. One Island Model Scenario: a small island close to the mainland. If migration occurs from the small island to the mainland, the population on the mainland will not be affected much. If migration occurs from the mainland to the small island, the population can change drastically

15. Example One Island Model, Lake Erie Water Snake, Nerodia sipedon . Two varieties – banded and unbanded, genetically These snakes can be found on both the islands and on the mainland. On the islands’ limestone rocks the young, small, banded snakes are much more vulnerable to predation than the unbanded snakes. Why do the banded snakes exist on the islands if they are selected against?

16. Migration works against selection E very generation several banded snakes move from the mainland to the islands. The migrants bring with them copies of the allele for banded coloration. Interbreeding contributes these copies to the population. Migration is working against selection preventing fixation of any one allele.

17. Conclusion Migration can cause allele frequencies to change from one generation to another which violates the first Hardy-Weinberg conclusion . Migration can be a powerful mechanism for evolution . Migration tends to homogenize allele frequencies across populations.

18. 4. MEIOTIC DRIVE Any process which causes some alleles to be over represented in the gametes. Mendel’s law is disobeyed. One of the alleles is consistently found in more than half of the offspring. Example: T-locus in mice. T-locus affect the tail length and viability. TT - H omozygotes : normal tail length Tt - H eterozygote : Short tail and transmit ~90% of the alleles to the gamete t t - H omozygote: sterile Meiotic drive increases the frequency of t allele

19. 5. NATURAL SELECTION A directional process that leads to the decrease or increase in allele frequency. Disrupts Hardy Weinberg’s equilibrium. O ne allele gives individual a reproductive advantage over other individuals. T hese leave more offspring, produces adaptation; “relative fitness” “Survival of the fittest” - what does “fittest” mean? Alleles that allow the greatest reproductive success increase in frequency, other alleles decrease.

20. Is Hardy Weinberg's law followed in nature?? NO APPLICATIONS To calculate gene frequency and allele frequency To develop hypothesis about populations To predict how a population will behave with modifications of certain environmental factors.

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