Evolution

Sumeet AgarwalELL796

17 Jan 2018

Heredity

● Long before DNA, Mendel and others had hypothesised genes as the units of discrete heredity

● Alleles: Different variants of a gene. Mendel's experiements suggested that only one kind was expressed in a given individual's traits: the notion of dominant and recessive alleles

● Genotype to Phenotype: A central question in biology

[Parts taken from NCERT Science Class X]

Variation in Heredity

● Heredity is not entirely exact; else, there would be no variation

● Mutation and crossover (in sexual reproduction: meiosis) create random variation

Meiosis

[Wikipedia]

Evolution by natural selection

● Darwin's key idea: Variants with high fitness (ability to reproduce) would naturally outcompete less fit variants over time

● Notion of fitness can be fraught: how to quantify? At the level of genotype or phenotype?

● Wasn't the first evolutionary theory: Lamarck had suggested inheritance of acquired characteristics. Mendel's work played a key role in supplanting this.

Evolution of complexity

● Frequent creationist argument: How did selection from random variants lead to complex organs such as the eye?

● Darwin's response: progressive build-up of somewhat useful/better functionality, over long periods of time

● Many other additional hypotheses about progressive evolution; we will discuss some of these in due course

Endosymbiosis● Suggests that eukaryotic organelles like

mitochondria and chloroplasts were originally free-living bacteria that entered into endosymbiotic relationships with other cells

[Wikipedia]

Origin of Eukaryotes

● Eukaryotes closer to archaea (nuclear DNA, genetic machinery); but membrane resembles bacteria more

● Autogenous models: Proto-eukaryotic cell with nucleus acquired mitochondria/chloroplasts via endosymbiosis

● Chimeric models: Fusion or endosymbiosis of archaeon and bacterium

Origin of Multicellularity● Theories:

– Symbiosis of different species of single-cell organisms

– Cellularisation of unicellular organism with multiple nuclei, e.g., slime molds

– Colonies of cells of a single organism

[Wikipedia]