LU 4 Genetic Diversity I

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Population Genetics

Study of genetic variation in a population

Darwinian selection + Mendelian inheritance

Modern synthesis 1940's, many won't accept Darwin's theory of natural selection.

Modern Synthesis

Combined discoveries from different fields = Paleontology, Taxonomy, Biogeography & Population genetics.

Emphasize importance of populations as units of evolution

Natural selection as the most important mechanism of evolution

Backs up idea of gradualism

Population = group of individuals of the same species living together in a place at the same time.

Species = group of populations that are able to interbreed with each other to produce fertile offspring.

Population's gene pool = total of all genes in the population at any one time.

If all have homozygous for 1 allele, the allele is fixed in the gene pool.

Individuals that are near the population center, on average are more closely related to each other compared to members of other populations

The Hardy - Weinberg Theorem

Describing a non-evolving population where shuffling of allele by meiosis & random fertilisation won't effect the overall gene pool.

Gene pool of this non-evolving population will remain the same over multiple generation where the frequency of allele does not change.

Assumptions :

Large population size (small ones can fluctuate in allele frequencies from fire storm dll)

No migration = Changes of frequency of alleles will happen because the immigrant will bring in new alleles to a population.

No net mutations = Allele frequency can change if the alleles change from one to another.

Random mating = More desirable traits will be chosen & this won't allow random mixing of alleles.

No natural selection = If some survive & reproduce, the offspring will carry the genes, and eventually the frequency will change in the next generation. No NO NO !

H-W Equation

1.0 = p2 +2pq + q2

p2 = frequency of RR genotype

2pq = frequency of Rr + rR genotype

q2 = frequency of rr genotype

Causes of evolution = microevolution = Changes in allele frequency in a gene pool from one generation to the other. This represents a gradual change in a population.

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Causes of Microevolution

Genetic drift = alteration of the gene pool (small population) due to chance.

Factors causing GD :

Bottleneck Effect = When a large disturbance occurs, it will remove a large portion of the population. So this leads to reduced genetic variability where the population that survives doesn't represent the allele frequency in the original population.

Founder Effect = A few individuals become founder at a new isolated populations = involves migration. It has lesser genetic variation compared to the original population. The allele frequency also may be a lil different compared to the original population.

2. Gene flow = Genetic exchange because of the migration of fertile individuals / gametes between populations.

3. Mutation = Change in an organism's DNA & is shown by changing alleles. These mutation can be transmitted in gametes of offsprings. So it immediately effect the composition of the gene pool.

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Natural Selection

Genetic Variation exists within & between populations.

Environment has the power to alter an individual's phenotype. Such as the changing of colours of a butterfly in different seasons.

Variation within population

Most variations = quantitative characters - vary along a continuum/set of numbers = heights. It indicates polygenic inheritance = occurs when one characteristic is controlled by two or more genes.

Few are discrete = colours

Polymorphism = existence of two or more forms of a character in high frequencies, within a population. (Discrete char only) - red vs white.

Variation between populations

Geographic variations = Differences in gene pools because of environmental factors.

Natural selection may contribute to GV. Usually occurs when populations are located in diff areas / isolated individuals.

Preservation of Genetic Variation

Hiding of recessive alleles in heterozygotes = Diploidy hides genetic variation from selection in the form of recessive alleles. Dominant = expressed. Recessive = silent.

Balanced polymorphism = ability of natural selection to maintain stable frequencies of at least 2 phenotypes. = Heterozygote advantage = Heterozygote has greater survival & reproductive success compared to homozygote.

Neutral variation = genetic variation, where in the end there wouldn't be any competitive advantage to any individual = human fingerprints.

Natural selection = Mechanism of adaptive evolution. NS increases frequencies of certain alleles. The way NS works is =

a) Evolutionary fitness contribution of an individual to the gene pool just like the contributions of other individuals where the no of offsprings could be more or less than the no of offspring produced by others.

b) Relative fitness = Contribution of a genotype to the next generation, compared to the contributions of alternative genotypes for the same locus. Just because an individual survives, it doesn't necessarily increase relative fitness. RF = 0 for sterile animal/plant.

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Natural selection maintains sexual reproduction

Sex

Creates genetic variation through meiosis & fertilisation

Need to be continued because it is important for the generation-to-generation variation.

Asexual produces more offspring but sexual reproduction produces more variation. Sexual wins ! woohoo !

Sexual selection leads to differences between sexes

Intrasexual selection = direct competition between males/felames for mates of opposite sex. So this will give rise to a secondary sexual equipment jeng jeng = antlers, used to compete for females (gaduh dush !)

Intersexual selection = mate choice, where they're choosy of their mates. This give rise to a secondary sexual characteristics = peacock feathers.