Evolution

EvolutionA process of change

through time.

Theory of Evolution

1. Suggests that existing forms of life on earth have evolved from earlier forms over long periods of time.

2. Evolution accounts for the differences in structures, function, and behavior among life forms as well as changes that occur in populations over many generations.

(I) Evidence of EvolutionObservations supporting the

theory of organic evolution can be made through the study of:

1. Geologic record

2. Comparative Cytology, Biochemistry, Anatomy, and Embryology

(A) Geologic Record1. Earth is between 4.5 to 5 billion

years old (age was determined by radioactive dating of rocks).

2. Fossils- are the remains of traces of organisms that no longer exist.

Fossils have been preserved in ice, sedimentary rock, amber, and tar.

3. Other fossils have been formed from petrification, a process by which the tissues are gradually replaced by minerals that produce a stone replica of the original material.

4. Imprints, casts, and molds of organisms or parts of organisms are frequently found in sedimentary rock.

5. In undisturbed layers (strata) of sedimentary rock, the lower strata contain old fossils while the upper strata contain younger fossils

(B) Comparative Anatomy1. Evidence supports that similarities of basic

structures exist between different organisms.

2. Homologous structures are anatomical parts found in different organisms in origin and structure.

3. The presence of such homologous structures suggest that these organisms have evolved from a common ancestor.

(C) Comparative Embryology

Although certain adult organisms may be different from each other, a comparison of the early stages of their embryonic development may show similarities that suggest a common ancestor.

(D) Comparative Cytology

1. All living things are made up of cells.

2. Cell organelles including the cell membrane, ribosome's, and mitochondria are structurally and functionally similar in most organisms.

(E) Comparative Biochemistry

All living things contain similar biochemical compounds.

Examples would include: structure and function of DNA, RNA, and proteins (including enzymes) are similar in all organisms.

(II) Theories of Evolution1. Attempts to explain the

similarities and differences among species.

2. Adaptations- are a major component to these theories. Adaptations are features which make a species better suited to live and reproduce in its environment.

(A) Lamark

The evolutionary theory of Jean-Baptiste Lamark was based on the principle of:

1. Use and Disuse 2. Inheritance of acquired traits

Principle of Use and DisuseFor an organism, new structures

appeared in the course of evolution because they were needed. Structures that were present and were used became better developed and increased in size; structures that were not used decreased in size and eventually disappeared

Ex: muscles of an athlete vs. appendix

Inheritance of Acquired TraitsUseful characteristics acquired by an

individual during its lifetime can be transmitted to its offspring.

These acquired traits results in species that are better adapted to their environment.

Ex: a giraffe’s neck became longer as a result of stretching to reach higher branches. This acquired trait was then passed down to the offspring .

Lamarck's conception of evolution

(B) Weisman1. August Weisman did not agree with

Lamark’s theory of acquired traits.

2. In a series of experiments, Weisman removed the tails of mice.

3. The mating of these tailless mice produced offspring with tails of normal length.

4. Weisman removed the tails of these mice and allowed them to mate.

5. Again, offspring were produced with tails of normal length.

6. The acquired condition of “taillessness” was not inherited.

(C) Darwin 1809-1882

Charles Darwin devised a theory of evolution based on variation and natural selection.

Included in hid theory were five main ideas:

1. Overpopulation

2. Competition

3. Survival of the Fittest

4. Reproduction

5. Speciation

Natural Selection1. Natural selection is the evolutionary process which

selects the variation(s) of organisms best suited for a particular environment.

2. Natural selection and its evolutionary consequences provide a scientific explanation for the fossil record of ancient life, as well as for the molecular and structural similarities observed among the diverse species of living organisms.

3. The degree of kinship between organisms or species can be estimated from the similarity of their DNA sequences; this similarity often closely matches organisms' or species' classification based on anatomical similarities.

           DNA Sequence Similarities in Some Primates

                  

                                                                         

The graphic above shows that: 1.All of these primates had a common ancestor, the ancestral primate. 2.The human and chimpanzee have the closest evolutionary relationship as their DNA is the most similar.

OverpopulationWithin a population, there are

more offspring produced in each generation than can possibly survive.

                                                              

CompetitionNatural resources; like food,

water, and space available to a population is limited.

Because there are many organisms with similar nutritional requirements, there must be competition between them for the resources needed to survive.

Survival of the Fittest Variations among members of a population make

some of them better adapted to the environment than others.

It is generally the best-adapted individuals that will survive.

The environment is the agent of natural selection determining which species will survive.

The individuals who survive are the ones best adapted to exist in their environment due to the possession of variations that best suit them to their environment. This genetic variability within a species is chiefly due to mutation and genetic recombination. The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions.

Reproduction

Individuals that survive and then reproduce transmit these variations to their offspring.

Speciation1. As time and generations continue,

adaptations are passed on and new species may evolve from a common ancestor.

Ex: caveman present man

                  

   

• Small differences between parents and offspring can accumulate in successive generations so that descendants become very different from their ancestors.• An adaptation is a variation which assists an organism or species in its survival. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment. • Some characteristics give individuals an advantage over others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. The proportion of individuals that have advantageous characteristics will increase. • Behaviors have evolved through natural selection. The broad patterns of behavior exhibited by organisms have evolved to ensure reproductive success.

Modern Examples of Natural Selection Peppered moth:

• two varieties of peppered moth existed, a light colored and a dark colored one • as industrialization and coal burning increased, the environment in England where these moths lived became dirtier • the dark colored variety of the moth blended into the trees and increased in numbers, while the light colored moth was less adapted and decreased in numbers.

Insect resistance to insecticides:• Insecticides kill insects not resistant to the insecticide, while insects resistant to the insecticide live to reproduce. The insecticide acts as a selecting agent.

Bacterial resistance to antibiotics:• Bacteria not resistant to an antibiotic are killed by it, while resistant bacteria live to reproduce.   The antibiotic is a selecting agent for these bacteria.

(III) Evolution and Extinction

Evolution does not necessarily mean long term progress is going to go in a certain direction.

Evolutionary changes often appear to be like the growth of a bush. Some branches survive from the beginning with little or no change, many die out altogether, and others branch out repeatedly, sometimes giving rise to more complex organisms.

Direction of Evolution

Note the divergence of the various groups from a common ancestor and the fact that some branches

became extinct.

• Extinction of a species occurs when the environment changes and the adaptive characteristics of a species are insufficient to allow its survival.   • The fossil record indicates that many organisms that lived long ago are extinct.    • Extinction of a species is common; most of the species that have lived on earth no longer exist.

(IV) Modern Theories of Evolution

The modern theory of evolution includes both Darwin’s ideas of variation and natural selection and the current knowledge of the sources of variations.

(A) Sources of Variations1. Segregation and the recombination of alleles

during sexual reproduction.2. Mutations are random changes in the genes

or DNA of sex cells may result in new gene combinations creating variation in the offspring formed from these.

• Only mutations that occur in sex cells can be passed on to the offspring.  Mutations which occur in other cells can be passed on to other body cells only.

• The experiences an organism has during its lifetime can affect its offspring only if the genes in its own sex cells are changed by the experience.

1. Variation and Evolution

Evolution is the consequence of the following factors:1. The potential for a species to increase its numbers 2. The genetic variability of offspring due to mutation and

recombination of genes 3. A finite supply of the resources required for life 4. The ensuing selection by the environment of those

offspring better able to survive and leave offspring. Some characteristics give individuals an advantage over

others in surviving and reproducing, and the advantaged offspring, in turn, are more likely than others to survive and reproduce. The proportion of individuals that have advantageous characteristics will increase.

An Example of Variation Driving Natural Selection

                         

                         

Original group exhibits variation in neck length.

Natural selection favors longer necks better chance to get higher leaves. Favored character passed on to next

generation.

• The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions.

• The great diversity of organisms is the result of billions of years of selection for favorable variations that has filled available niches of our planet with life forms.

(C) Geographic Isolation Gene frequency- the percentage of organisms

in a population that carry an allele.1. Isolation of a population increases the

chances for speciation (the development of a new species) by separating a small group of organisms from the main population with its large gene pool (inheritable traits).

2. Changes in gene frequency are more likely to occur in small populations than in large ones.

3. Geographic isolation of a population is caused by natural barriers like mountains, large bodies of water, and deserts.

(D) Reproductive Isolation

If the isolated population becomes so different from the main population that members of the two cannot interbreed and produce fertile offspring, then they have become two distinct species.

(E) Time Frame for EvolutionThere are two different theories

proposed by scientists to address the rate of evolution:

1. Gradualism- proposes that evolutionary change is slow, gradual, and continuous.

2. Punctuated Equilibrium- proposes that species have long periods of stability (several million years) interrupted by geologically brief periods of significant change during which a new species may evolve.

PunctuatedEquilibrium

Gradualism