• Chapter 20: Genes within population

• Lamarck’s theory of evolution

i) New needs are generated by environmental changes

ii) These changes determine use or disuse of organs

iii) Use of organs determines their development and disuse determines diminish of organs

iv) Acquired characters are hereditary or inheritance of acquired characters

v) Functions create organs

• Darwin’s theory of evolution

i) Differences are displayed by different individuals

ii) Individuals with superior differences (natural selection) are able to reach adulthood, reproduce and transmit their traits to their off springs.

iii) Individuals with inferior qualities cannot survive or lose

Polymorphic variation• When loci have more than

one allele occurring at a frequency greater than 5%

• Vertebrates are less polymorphic (5-8%)

• Drosophila and invertebrates (15%)

• Plants (8%) Ex: Loosestrife – Lythrum salicaria exhibits variation in flower color

• Hardy-Weinberg principle• Proposed by Godfrey H. Hardy and Wilhelm Weinberg in

1908• Explains stability of population for reasons:

1. No mutation takes place

2. No genes are transferred to or from other sources (no immigration or emigration takes place)

3. Random mating is occurring

4. Size of population is very large

5. No selection occurs• Since no change in genotype proportions therefore also

known as Hardy-Weinberg equilibrium

• p and q designates the frequency of B and b alleles, respectively

• Sum of three genotypes BB + Bb + bb = 1

• For homozygous BB and bb – probability to receive B and b alleles is possible from both the parents (p2 or q2)

• For heterozygous Bb – probability to receive B allele from father and b allele from mother or vice versa (pxq and qxp, respectively)

• P2+2pq+q2 = 1

• Five agents of Evolutionary change

1. Mutation changes alleles- Mutation in any allele can

change proportions of particular alleles in population

- Occurrence is 1/10,000 cell division

- Not affected by natural selection

• Five agents of Evolutionary change

2. Gene flow occurs when individuals or gametes move between populations

- Drifting of gametes or immature stages of plants or marine animals

- Wind dispersal of pollen or seeds or carried by animals

- Mating of individuals of adjacent populations

• Five agents of Evolutionary change

• Nonrandom mating

- Assortative mating happens when phenotypically similar individuals mate and result in more homozygotes.

- Disassortative mating happens when phenotypically different individuals mate and result in more heterozyotes

• Five agents of Evolutionary change

• Genetic drift- Some descendants of a

generation/population survive

- Does not lead to adaptations

- Is not a result of natural selection

- Can happen because of a catastrophe

The Founder effect – One or few individuals disperse and become founders of a new isolated population away from their place of origin.

The Bottleneck effect – Organisms do not move from place to place and their population decreases due to various natural forces.

• Five agents of Evolutionary change

• Selection- Natural and Artificial selection

• Based on phenotype and behavior, an individual is able to produce more progeny.

• Natural selection can lead to evolution due to three reasons

- Variation must exist among individuals in a population

- Variation must result in differences in the number of offspring surviving in the next generation

- Variation must be genetically inherited

• Selection to avoid predators:

- Selection leading to adaptative evolutionary changes

- To avoid predation population is evolving via natural selection and adapting to genetic changes.

• Selection to match climatic conditions

• - Enzyme allele frequencies vary with latitude

- Mummichog (Fundulus heteroclitus) has enzyme lactate dehydrogenase.

- Lactate dehydrogenase is a better catalyst at low temperature for fishes of north side than south side

- Selection for pesticide resistance

- Houseflies have genes pen, kdr and dld-r which act as resistance genes that decrease uptake of insecticide

Fig. 20.8

• Selection occurs when individual of one phenotype produce more surviving offspring than its counter phenotype

• Reproducing ability can be related to fitness of an organism• Fitness is a combination of several factors like survival,

mating success and number of offspring per mating

• Gene flow can be constructive as it can spread evolutionary change carrying alleles

• Gene flow can also constrain evolution or natural selection

Example of gene flow opposing natural selection is occurring on abandoned mine sites in Great Britain

• Frequency-dependent selection: Fitness of an individual depends upon its frequency within population

• Negative frequency-dependent selection: Search image formed by predators makes latter prey upon common types

• Positive frequency-dependent selection: Search image formed by predators makes latter prey upon uncommon types

Fig. 20.12

Resistance to malaria and increased fertility in

heterozygotes

Heterozygote exhibit greater fitness than homozygote

How disruptive selection removes intermediates?

Disruptive selection eliminates finch birds with intermediate beaks which are neither able to feed on large nor small seeds

How directional selection eliminates phenotypes on one end of a range?

Individuals concentrated towards one extreme of the array of phenotypes are favored

How stabilizing selection favors individuals with intermediate phenotypes?

Both smaller and larger weight babies have a greater tendency to die than those of intermediate weight.

Fig. 20.17

Field and laboratory

experiments proved that

selection can lead to rapid evolutionary

changes

Fig. 20.19

Genetic variation can get removed by intense selection pressure

Limitations of selection

Fig. 20.20

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