17C+-+Evolution+Mechanism

Charles Darwin (1809-1882)

 * Englishman who spent two years in medical school at Edinburgh and then trained as a clergyman at Cambridge for three years.
 * He was mostly bored by his studies but while at college he became friends with some of the most respected scientists of the time
 * Through a college contact he accepted an unpaid post as naturalist aboard the HMS Beagle which sailed around the world for 5 years (1831-1836) when he was 22.
 * When Darwin left England in 1831 he accepted the religious dogma of special creation. On his return to England in 1836 he was convinced of the idea of "descent with modification", i.e. all organisms, including humans, are modified descendents of previously existing forms of life
 * Darwin thoughts developed in two stages:
 * 1) the realization that organisms are not fixed and unchangeable
 * 2) to provide an explanation of the process of evolutionary change (mechanism)
 * Darwin noticed 13 different species of finches within the different islands of the Galapagos
 * The finches appeared to be related to the only finch found on the mainland of South America
 * The finches all had different beaks that appeared to have different functions
 * An ancestral stock had migrated to the islands where they underwent profound changes under the different conditions of the individual islands. A single ancestral group could give rise to several different varieties or species.

OBSERVATION 1 Populations have the potential to increase exponentially OBSERVATION 2 Populations are fairly constant in size OBSERVATION 3 Natural resources are limited DEDUCTION 1 Only some organisms survive. There is a struggle for existence among individuals in a population.

OBSERVATION 4 There is variation within a species, and variation is inherited DEDUCTION 2 Individuals with favourable variations are more likely to survive and reproduce. DEDUCTION 3 Accumulation of variation over many generations is evolution.

__**Reference:**__
 * [|Natural Selection: Charles Darwin & Alfred Russel Wallace]
 * Mechanism of Evolution - Natural Selection
 *  [|The Origin of Species by Means of Natural Selection by Charles Darwin]
 * Misconceptions about natural selection and adaptation
 * [|Finches & Speciation]

===Evolution Mechanism: Theory of Natural Selection=== In other words, there is:
 * 1) Variations in a population  – caused by meiosis, mutation,  cross fertilization, environmental  influences etc.
 * 2) Over production of off springs  – organisms will produce more young than needed
 * 3) Struggle for survival  – competing for resources, space, mate etc.
 * 4) Differential survival and reproduction - will determine the composition of the gene pool


 * In other words, natural selection that occurs over a long period of time results in evolution
 * As long as the environment changes, a population will continue to evolve
 * E.g. changes in predators, changes in climate, changes in natural resources for life
 * Evolution may result in the organisms changing to a new species

===Summary of Theory of  Natural Selection===
 * Overproduction: More offspring produced than will ultimately survive and reproduce
 * Variation: Inheritable features vary from individual to individual
 * Change in environment: Changes in climate, topography, food supply, predators, etc
 * “Struggle for existence”: Mainly competition within the species, for food, habitat, survival from being eaten
 * “Survival of the fit” (not necessarily the strongest): Those with more adaptive traits tend to survive longer and/or produce the most offspring; these are the “naturally selected”
 * Inheritance of “selected” features: Traits involved are already inheritable, but may involve new combinations
 * New species, better adapted to the new environment: when the collective traits of the population differ significantly from the earlier population, and can no longer reproduce with the earlier population.

Assumptions Made

 * In order to have natural selection, some basic assumptions were made
 * The traits seen must be found in the genes and hence be able to be passed on to the next generation
 * Traits arose from random events like meiosis and mutation and not by intent

Lamarck’s Hypothesis

 * The hypothesis of The Inheritance of Acquired Characteristics
 * A changing environment creates a need for certain features to be developed in order to survive.
 * “Acquired Characteristics”: Through use and /or non-use, those features needed for survival are developed in each individual
 * Inheritance: Those characteristics developed (“acquired”) by individuals are somehow passed on to their offspring, who can continue that development.
 * New species: Eventually, over many generations, enough differences have developed that we can say we have a new species.

Difference between Lamarck’s & Darwin’s theory

 * Lamarckian theory was incorrect as the traits acquired by overuse of an organ can be transferred to the next generation
 * Lamarck also believed that traits can arise due to intent, with organisms seemingly able to “will” their organs to grow or develop
 * Darwin’s natural selection was based on passive processes like random variation in a population while Lamarck’s theory was based on active response to change

The reference from Evolution 101 on the mechanism will throw more light. We will only be focusing on the natural selection process and speciation but please DO read up on all of them to have a more indepth understanding. How does evolution work? ||
 * || [|Mechanisms: the processes of evolution]
 * **[|Descent with modification]**
 * **[|Mechanisms of change]**
 * **[|Genetic variation]**
 * **[|Mutations]**
 * **[|The causes of mutations]**
 * **[|Gene flow]**
 * **[|Sex and genetic shuffling]**
 * **[|Development]**
 * **[|Genetic drift]**
 * **[|Natural selection]**
 * **[|What about fitness?]**
 * **[|Sexual selection]**
 * **[|Artificial selection]**
 * **[|Adaptation]**
 * **[|Misconceptions about natural selection]**
 * **[|Coevolution]**