University:
Felician University
Course:
BIO-209 | Evolution
Academic year:

2020
Views:

105

Pages:

1
Author:

Clint W.

EvOLuTIOn Evolution Evolution is the cumulative change in the heritable characteristic of a population (i.e. biological change over time) • These characteristics are encoded by genes and transferred between generations as alleles Hence, evolution is a change in the allele frequency of a population’s gene pool over successive generations Theories of Evolution Lamarck Proposed that species change via habitual use and disuse • A giraffe stretches it neck to reach leaves in tall trees • The giraffe’s neck becomes extended from constant use • The giraffe’s offspring inherit its long neck Darwin (and Wallace) Proposed that species change via natural selection • A giraffe with a longer neck can reach leaves in tall trees • The giraffe will get enough food to survive and reproduce • The giraffe has more offspring (that inherit a long neck) This theory has been rejected because these acquired traits do not have a genetic basis (and thus cannot be inherited) Darwin’s theory has been reinforced by our understanding of modern genetics (incorporated as neo-Darwinism) Mechanisms of Change Fundamental to the process of evolution is the presence of variation within populations upon which selective forces act There are three main mechanisms by which genetic variation within a population is maintained: • Mutations – changes to the gene sequence • Sexual reproduction – new gene combinations • Gene flow – immigration and emigration Mutation Sex There are two mechanisms by which population variety can be altered (⬇ biodiversity): • Random chance (genetic drift) • Directed intervention (natural or artificial selection) The impact of a change is greater if the population is small (this may occur via population bottlenecks or founder effect) Gene Flow Speciation If populations become isolated, the level of genetic divergence gradually increases the longer the populations remain separated • Continuous variation across a geographical range of related populations matches this concept of gradual divergence Speciation will occur when populations diverge to the extent that they can no longer interbreed and produce fertile, viable offspring chance event Genetic Drift selection pressure Natural Selection