lecture-evolution and its process

Darwin and wallance

The mechanism for evolution was independently conceived of and described by Charles Darwin and Alfred Wallace.

• Darwin traveled around the world from 1831-1836 on H.M.S. Beagle.

• Wallace traveled to many tropical locales from 1848- 1862.

evolution

hertiable changes in populations over time

hertiable-passed down from parents to offspring

changes-any varying feature of organisms

populations-collections of organisms in the same species

time-usually over quite long periods of time

evolution of process

-variety of mechanisms

-natural selection

Darwin’s observation

Darwin observed and collected thousands of plants and animals

• One major observation was that organisms in temperate South America resembled tropical organisms in South America rather than temperate European organisms.

• Further, the fossils he found resembled the living organisms of South America

Of particular importance to Darwin’s conclusions about evolution were his travels to the Galapagos Islands

. • Darwin observed species of organisms on different islands that were clearly similar but had distinct differences.

• Most species on the islands were not known from anywhere else in the world with Darwin hypothesizing that the islands had been colonized by South American mainland organisms that then diversified.

The finches of the Galapagos display an impressive diversity of beak morphology.

• Darwin imagined that the island species might all be species modified from one original mainland species

• In 1860 he wrote, “Seeing this gradation and diversity of structure in one small, intimately related groups of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”

Natural selection

On November 24th, 1859 Darwin published On the Origin of Species by Means of Natural Selection.

• Focused on the diversity of life, its origins and relationships, similarities and differences, distributions and adaptations.

• Presented evidence that many species are descendants of ancestral ones.

In The Origin of Species, Darwin proposed a mechanism for the evolutionary process - natural selection.

• He postulated that populations can change over time if individuals that possess certain heritable traits leave more offspring than other individuals.

Wallace and Darwin both observed similar patterns in other organisms and independently conceived of a mechanism to explain how and why such changes could take place.

• Darwin called this mechanism natural selection, an inevitable outcome of three principles that operate in nature.

The characteristics of organisms are inherited (passed from parent to offspring).

2. More offspring are produced than are able to survive as resources for survival and reproduction are limited. This leads to a competition for those resources in each generation.

3. Offspring vary among each other in regard to their characteristics and those variations are in turn inherited

Offspring with inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals that are less able to compete.

• Because characteristics are inherited, these traits will be better represented in the next generation.

• This will lead to a change in populations over generations in a process that Darwin called descent with modification.

variation and adaption

Natural selection can only take place if there is genetic variation, or inherited differences, among individuals in a population.

• A heritable trait that aids the survival and reproduction of an organism in its present environment is called an adaptation.

• Adaptation to an environment comes about when a change in the range of genetic variation occurs over time that increases or maintains the match of the population with its current environment.

evolutionary forces

For a population to evolve, the frequency of the alleles in the population much change over time.

• If there are no factors that affect the frequency of alleles in a population, those frequencies will remain the same from one generation to the next - Hardy-Weinberg Equilibrium.

• A population’s allele and genotype frequencies are inherently stable.

• Unless some kind of evolutionary force is acting on the population, it would carry the same alleles in the same proportions generation after generation.

natural selection

Depending on the environmental conditions, an individual’s phenotype confers an advantage or disadvantage to the individual relative to the other individuals in the population.

• If it is advantageous, then that individual will likely have more offspring than individuals with the disadvantageous phenotype(s).

• The allele behind the advantageous phenotype will then have greater representation in the next generation - the allele’s frequency in the population has now changed.

Mutation

Mutation is a change in the DNA sequence of a gene and is the ultimate source of all new alleles and all genetic variation in a population.

• A mutation can change one allele into another, but the net effect is a change in the frequency of one or more alleles.

• Mutations can be selected for, selected against, or selectively neutral.

• Harmful mutations get removed from the population by selection while beneficial mutations will spread through the population.