Evolution of Biological Communities

1. Evolution of Biological Communities

2. Outline I. Evolution by Natural Selection A. Background B. Elements of Theory C. Examples II. Population Genetics

3. Background • Charles Darwin was not the first scientist to think about life’s origins or about how species might change over time. • Lamarck • Study of fossils suggested life evolves • Inheritance of acquired characteristics

4. Charles Darwin (1809-1882) • Even as a boy, interested in nature • Accumulated observations leading to theory • Lyell’s geology • Galapagos Islands • Artificial breeding programs • Malthus’ theories of population growth • Through these observations, Darwin began to see adaptation and the origin of new species as closely related processes • Descent with modification

6. Evolution by Natural Selection • Observation: In any species, more offspring are born into a population than actually survive to reproduce • Observation: Resources in any environment are limited • Inference: production of more individuals than the environment can support leads to a struggle for existence between individuals of a population, with only a fraction of their offspring surviving each generation

7. Evolution by Natural Selection • Observation: Members of a population vary in their characteristics • Observation: Much of this variation is heritable • Inference: Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing will leave more offspring than those without these inherited traits • Inference: leads to gradual accumulation of these favorable characteristics in the population over generations

8. Natural Selection • Adaptations: traits which give a selective advantage to some individuals in a population ie. traits which increase an organisms chances of survival and reproduction • Selective Agent/Pressure: environmental factors that influence which individuals leave more offspring (favor certain adaptations within the population) • Natural Selection is the name given to the process of differential reproduction-the mechanism by which evolution takes place

9. Important Points • Being “fit” is relative, if the environment changes, new traits may be favored and individuals suited to one environment may not be suited anymore • It is not “survival of the fittest” but survival and reproduction of the fittest! • Natural selection does not have foresight-no direction toward the “perfect” organism-blind process whereby existing variation is sorted out by fitness.

11. Population Genetics • Variation • Heredity an important and necessary part of the theory, not well understood in Darwin’s time • As study of heredity and genetics developed, the underlying control of an organisms traits has become better understood • 2 types of variation • Non-heritable: no genetic basis, can’t be passed down, often of environmental origin • Heritable: actual change in the DNA of an organism via mutation or mutagen, passed down via egg or sperm

12. Non-heritable Variation

13. Heritable Variation

15. Population Genetics • Gene: specific sequence of nucleotides in DNA which code for a particular protein • Allele: alternate forms of a gene • Examples • More precise definition of evolution: Evolution is a change in the allele frequencies of a population of organisms over time • Population: a group of interbreeding individuals, live in the same area at the same time