EVOLUTIONARY THEORY EVOLUTION: • Change through time • Processes that transformed early life forms into the current, vastly

diverse forms • Processes at work today

LAMARCK •

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Life forms have changed over time Supported evolution

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Evolution responded to needs of organism Use & disuse o Increase use = parts become

larger & stronger o Decrease use = parts become

smaller & weaker

• Inheritance of acquired traits (modifications acquired during lifetime passed onto offspring)

DARWIN Descent with Modifications • Species not created in

present forms • Species evolved from

ancestral species • All organisms are related • Organisms descended from a

common ancestor • Descendants have

accumulated mutations over vast periods of time.

Natural Selection • Struggle for survival • Survival of the fittest (best adapted

will survive to reproduce) • Unequal success in reproduction

leads to gradual change • Differential success in reproduction

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DESCENT WITH MODIFICATION (cont.)• Evolution is a process of descent with modification.

• Evolution is a remodeling process.• Related species can have characteristics that have an underlying

similarity yet function differently. • Similarity resulting from common ancestry is known as

homology.• Structural and molecular homologies reveal evolutionary relationships.

• Today, biologists represent patterns of descent with an evolutionary tree, often turned sideways.

• Homologous structures can be used to determine the branching sequence of an evolutionary tree.

• These homologies can include • anatomical structure and/or• molecular structure.

Darwin’s Greatest Contribution Page 2 of 7

• Darwin’s greatest contribution to biology was his explanation of how life evolves.

• Insights into how incremental change occurs could be seen in examples of artificial selection.

• Darwin reasoned that if artificial selection can bring about so much change in a relatively short period of time, then natural selection could modify species considerably over hundreds or thousands of generations.

Natural Selection Key PointsIt is important to emphasize three key points about evolution by natural selection.

1. Although natural selection occurs through interactions between individual organisms and the environment, individuals do not evolve. Rather, it is the population, the group of organisms, that evolves over time.

2. Natural selection can amplify or diminish only heritable traits.3. Evolution is not goal directed; it does not lead to perfectly adapted

organisms.

These examples of evolutionary adaptation reveal two important points about natural selection.

1. Natural selection is more of an editing process than a creative mechanism.

2. Natural selection is contingent on time and place, favoring those heritable traits in a varying population that fit the current, local environment

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Variations in Populations• Organisms typically show individual variation.• Mutations are the ultimate source of the genetic variation that serves

as raw material for evolution.• In organisms that reproduce sexually, most of the genetic variation in

a population results from the unique combination of alleles that each individual inherits.

• Fresh assortments of existing alleles arise every generation from three random components of sexual reproduction: 1. crossing over2. independent orientation of homologous chromosomes at

metaphase I of meiosis3. and random fertilization.

HARDY-WEINBERG THEOREM

POPULATION and SPECIESPopulation=Localized group of individuals belonging to same species

Species = group of populations whose individual have potential to interbreed

GENE POOL • Total aggregate of genes in a population at any one time • All alleles at all loci in all individuals • Example – flower population with white and pink flowers

Pop. of 500 individuals 20 white 320 heterozygous pink160 homozygous pink Allele frequency: a = 2(20) + 160 = 200 =0.2 1000 1000

A = =0.8

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HARDY-WEINBERG THEOREM Frequency of alleles and genotypes in a population’s gene pool remain constant over generations unless acted upon by agents other than sexual recombination

HARDY-WEINBERG EQUILIBRIUM Allele frequency is constant from generation to generation

REQUIRED CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM

Very large population Random mating

Isolation from other populations No natural selection

No net mutations

HARDY-WEINBERG EQUATION p + q = 1 p = frequency of dominant allele q = frequency of recessive allele p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant genotype (AA)2pq = frequency of heterozygous genotype (Aa) q2 = frequency of homozygous recessive genotype (aa)

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HARDY-WEINBERG SAMPLE PROBLEM

In a population of mice, 245 are black, 210 are brown, and 45 are white.

1. KEY BB= Black = p2 B = p

Bb = Brown = 2pq b = q bb = White = q2 2. Calculate allele frequencies

B = = = 0.7 b = 1−B = 1−.7 = 0.3

3. Calculate genotype frequencies BB = (.7)2 = .49 = 49% Bb = 2(.7)(.3) = .42 = 42% bb = (.3)2 = .09 = 9%

4. Question – Is the population in H-W equilibrium?

Genotypes Expected Actual BB.49(500) = 245 245 Bb .42(500) = 210 210

bb .09(500) = 45 45 Answer = Yes, the population is in H-W equilibrium

because the expected genotype frequencies match the actual frequencies

MICROEVOLUTION• Change in gene pool of a population • Change in relative frequency. of alleles in population • Evolution on smallest scale

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CAUSES OF MICROEVOLUTIONGenetic Drift • Changes in gene pool of small population due to

random chance

• Bottleneck effect • Founder Effect

Gene Flow • Change in gene pool due to migration of fertile individuals or due to the transfer of gametes between populations

Mutation • Introduction of a new allele

Nonrandom mating

• Mates not randomly chosen Sexual selection

Natural Selection

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Differential reproductive success Some phenotypes selected against

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