Evolution and Natural Selection

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Pre – Darwin “Origin of species”

Earth relatively young (thousands of years) ; this hypothesis was being replaced

In the 1700’s and early 1800’s, geological evidence suggested that the earth was not young, but quite old, and that it had undergone considerable change over its history. Massive geologic formations, such as the Grand Canyon, were seen as the result of slow geologic processes

Pre – Darwin “Origin of species”

There were a limited number of fossils found and most did not appear dramatically different from current species; species were thought not to change between generations and the number of species on the earth was constant.

Organisms were thought to be perfectly adapted to their environment.

Post- Darwin “Origin of species”

Species are related by descent

Adaptation to the environment is the result of the interplay of random variation and natural selection

The number of species was not constant and species changed over time. Descent with modification

Conditions necessary for decent with modification

Variation within a population

The variation is heritable

The reproductive success depends on the available variation.

Evolution theory

Variation exists among individuals within a species

Organisms produce more offspring than the environment can support

Competition exists among individuals

The organisms whose variation best fit them to the environment are the ones who are most likely to survive, reproduce, and pass those desirable variations to the next generation

Gene Variation

Macroevolution - Evolutionary change on a grand scale, encompassing novel designs, evolutionary trends and episodic mass extinction.

Microevolution - Differential survival and reproduction due to natural selection. Gradually alters population to include more individuals with advantageous characteristics.

Gene Variation

Darwin’s explanation of evolution: Adaptation by natural selection is responsible

for evolutionary changes within a species(microevolution), and accumulation of these changes leads to development of new species (macroevolution).

Evidences for evolution: fossil record

Fossils are preserved traces of once-living organisms created when organisms become buried in sediment and calcium in hard surfaces mineralizes. Often provide evidence of successive

evolution.

Take out a blank sheet of paper On the paper write :

1. your name (first and last)

2. The date today (6/2)

3. The letter of the correct answer to the following questionWhich of the following Conditions necessary for decent with modification do you know are conditions of the ecosystem of the predator-prey (Daphnia and Hydra) project ?

(i.e. exclude those conditions that would require experimentation with to know that the condition was being met)?

A.) Variation in traits exists within a population.

B.) The variation is heritable.

C.) The reproductive success depends on the available

variation.

D.) All of the above

E.) Only condition B is known.

Fossilization process limits available fossils

Requires burial, sedimentary rock, deposition of minerals replacing hard parts of an organism. Soft parts preserved in

impressions or casts in soft sediment

Dating the fossil record – isotopic dating

History of Earth

0.00E+005.00E+081.00E+091.50E+092.00E+092.50E+093.00E+093.50E+094.00E+094.50E+095.00E+09

4.5 billion 500 million

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Molecular Record

Evolutionary theory allows evolutionary change involves substitution of new versions of old genes. New alleles arise by mutation and come to

predominate through favorable selection. Molecular Clocks

The longer the time since divergence, the greater the number of differences in nucleotide sequence of cytochrome C.

Changes accumulate at constant rate.

Molecular Record

Phylogenetic Trees Evolutionary history of a gene can be mapped

as a phylogenetic tree.

Mechanisms of evolution

Genetic drift Random fluctuations in the allele frequencies

within a population

Mutation A random change in the coding of an allele or

gene

Natural selection (including sexual selection) the differential reproduction of genotypes

caused by factors in the environment.

Anatomical Record – products of evolutionary process Homologous Structures - Structure and

function have diverged between body parts of different animals, but are derived from same part present in a common ancestor. Forelimbs of Vertebrates

Analogous Structures - Features resemble each other as a result of parallel evolution in separate lineages. Flippers of penguins and dolphins

Vestigal Organs - Organs no longer of use.

Products of evolution

Adaptation – organisms are not perfectly adapted ( have to make due with your genetics)

Examples: panda thumb

Hardy-Weinberg Rule

From 1920’s onward, scientists began formulating theory of how alternative gene forms (alleles) behave in a population, and how changes in gene frequencies lead to evolutionary change. 1908 Hardy and Weinberg pointed out in the absence

of forces, in a large population with random mating, allelic frequencies remain constant.

Hardy Weinberg equilibrium

Hardy-Weinberg - individuals do not evolve

Assumptions: Large population size Random mating No mutation No immigration Absence of natural selection

Change in Allelic Frequencies

Mutation Error in replication of a nucleotide sequence

in DNA (Very slow rate). Migration

Movement of individuals from one population to another (Dependent on strength of selective forces).

Change in Allelic Frequencies Genetic Drift

Change in allelic frequencies due to random events.

Founder Effect - Population started by few individuals and thus a restricted gene pool (Rare genes may become common).

Bottleneck Effect - Gene pool becomes very small, usually due to small population size.

Change in Allelic Frequencies

Nonrandom Mating Individuals with certain genotypes mate with

more or less commonly than expected on a random basis.

Inbreeding - Mating with relatives. Increases homozygosity

Change in Allelic Frequencies

Selection Some individuals leave behind more progeny

than others. Artificial - Breeder selects desired characteristics. Natural - Environment determines adapted

characteristics.

Forms of Selection

Disruptive Selection acts to eliminate intermediate phenotypes.

Stabilizing Selection acts to eliminate both extremes from an

array of phenotypes. Directional

Selection acts to eliminate one extreme from an array of phenotypes.

Three Forms of Selection

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Balancing selection

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Sickle-Cell Anemnia

Hereditary disease affecting hemoglobin molecules. Sickle-Cell homozygosity frequently leads to a

reduced life span. Heterozygosity causes enough hemoglobin to be

produced to keep red blood cells healthy. Very common in Africa.

Stabilizing selection as heterozygosity infers less susceptibility to malaria.

One of leading causes of death in Africa.

Stabilizing Selection in Sickle-Cell

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Peppered Moths and Industrial Melanism

Until the mid nineteenth century, Peppered Moths ,Biston betularia, had predominately light-colored wings. Subsequently, dark individuals became

predominant. Industrial smog helped turn lichens on tree trunks

dark. Contrasting colors between trunk color and moth

color led to differential predation by birds.

Peppered Moths and Industrial Melanism

Second half of the twentieth century saw widespread implementation of pollution controls, thus trends reversed and light colored moths again dominated.

But, caution must be taken, as the selective agent could be some factor other than wing coloration.

Industrial Melanism

Example of directional selection

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Species Concept

A species is generally defined as a group of organisms unlike other such groups and does not integrate extensively with other groups in nature.

Species Formation

Local populations adapt to the specific circumstances each faces.

When they become different enough, the populations become ecological races.

Natural selection reinforces differences through isolating mechanisms.

Two races become incapable of interbreeding and are considered two separate species.

Prezygotic Isolating Mechanisms

Prevent formation of Zygote: Geographic Isolation Ecological Isolation Behavioral Isolation Temporal Isolation Mechanical Isolation Prevention of Gamete Fusion