Chapter 15-17Evolution

What is evolution?Evolution is change over timeTheory of evolution – process by which

modern organisms have descended from ancient organisms

Remember that a scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world

Where did the idea of evolution come from?Charles Darwin made numerous

observations and collected evidence that led him to propose his hypothesis about the way life changes over time◦Collected plant/animal specimens

where ship anchored◦Learned that an enormous number of

species exist◦Noticed how well suited organisms

are to their environment

Galapagos Tortoises

Lamarck and EvolutionIn 1809 Lamarck proposed that by

selective use or disuse of organs, organisms acquire or lose certain traits◦These traits can then be passed to

offspring◦This could lead to a change in the

species

Lamarck and Evolution

Darwin and Evolution“On the Origin of Species” was published in

1859 by Charles Darwin◦Darwin proposed natural selection as a

mechanism for evolution◦Competition for resources leads to survival of

the fittest◦Successful adaptation means the organism

has and advantage to survival and reproduction

◦Adaptation – any inherited characteristic that increases an organism’s chance of survival

◦Over time natural selection results in the inherited characteristics of a population

Darwin and EvolutionDarwin proposed that each species has

descended, with changes, from other species over time (descent with modification)

The principle of common descent states that all species (living and extinct) were derived from common ancestors (single “tree of life”)

Darwin hypothesized that living things have been evolving for millions of years◦Evidence found in fossil record,

geographical distribution of species, homologous structures of organisms, similarities in early development

Darwin and Evolution

Types of EvolutionMicro-evolution – change in a

population’s gene frequenciesMacro-evolution – when a new species

emerges

Species – group of organisms that can reproduce with fertile offspring in nature

Variation in the population provides the material for natural selection. Sexual reproduction (the shuffling of alleles and, thus, uniqueness) provides for this variation

Evidence for EvolutionThe fossil record - Strata of fossilized remains of

organisms extinct for many years. The oldest layers have the oldest fossils beginning with bacteria.

Biogeography – the presence of organisms in areas of the world correlates with the movement of land masses over time◦Australia has a unique abundance of marsupials

found no where else in the worldComparative anatomy – Organisms share many

characteristics called homologous structures◦Vestigial structures are parts reduced in size or

no longer used by a more recent organism, ex. hip bones of whales

Geographic Distribution of Species

Homologous Body Structures

Evidence for EvolutionComparative embryology – Similarities exist

between many animals as they are developing

DNA or molecular similarities – All organisms share the same genetic code, ATCG’s. Most of the codons match the same amino acids.

Population studies – We can see bacterial populations change over time ◦Bacteria exposed to antibiotics will die

except for the resistant few◦The resistant ones live and reproduce,

creating a population of bacteria resistant to the antibiotic.

Similarities in Embryology

Evolution and IsolationGeographic isolation – when populations are

split by geographic barriers, they sometimes change so that when brought back together, they can’t have offspring together

Reproductive isolation – when organisms in a population do not interbreed for some reason and become distinctly different groups. ◦In plants this can occur as a mistake in

meiosis, and the new plant will not have the correct chromosome number for successful zygote formation. ex. salamander ring species

Genetics and EvolutionGene pool- all genes, including the

different alleles, in a populationRelative frequency- number of times

that an allele occurs in a gene poolGenetically, evolution is any change in

the relative frequency of alleles in a population◦Mouse fur frequencies begin B is

40%, b is 60% (B=black, b=brown)◦If it changes to B=30%, then the

population is evolving

Sources of Genetic VariationMutations are a change to the sequence of

DNA, which may result in a phenotype change◦Can be caused by mistake in replication or

radiation/chemicalsDuring gamete productions gene shuffling

occurs (independent assortment and crossing over)◦Can produce a huge variety in phenotypes◦Does not change relative frequency◦Can result in unique phenotypes that

could be an advantage

Genetics and EvolutionEvolutionary fitness is an organism’s

success at passing genes to the next generation

However, natural selection never acts on genes only on the organism as a whole

In addition, evolution is about the frequency of alleles for an entire population, not and individual organism

Genetics and EvolutionNatural selection on single-gene traits

can lead to changes in allele frequencies and thus to evolution

The red lizards are more visible; black lizards warm faster, which allows them to be more active sooner

Genetics and EvolutionNatural selection can affect polygenic

traits (trait controlled by 2 or more genes) in 3 ways:◦Directional selection occurs when

individuals on one end of a phenotype curve have higher fitness

Low mortality, high fitness

High mortality, low fitness

Genetics and Evolution◦Stabilizing selection occurs when

higher fitness at the center of the curve than the ends

◦Weight of human infants

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f P

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Low mortality, high fitness

High mortality, low fitness

Brightness of

Feather Color

Selection against both

extremes keep curve narrow and in same

place.

Genetics and Evolution◦Disruptive selection occurs when the

ends of the phenotype curve have higher fitness than the middle Can cause it to split into 2 Medium sized seeds become less

common

Genetics and EvolutionGenetic equilibrium occurs when allele

frequencies remain constant; 5 requirements◦Random mating (not true in

organisms like elk)◦Large population◦No movement into or out of the

population◦No mutations◦No natural selection

History of LifeBiogenesis – Life comes only from life.Abiogenesis – spontaneous generation,

life can come from the nonliving

Louis Pasteur – disproved spontaneous generation by sterilizing broth solutions.

Stanley Miller and Harold Urey (1950’s) simulated early earth in the lab. By making amino acids in the lab. http://www.ucsd.tv/miller-urey/

Pasteur’s Experiment

Miller and Urey Experiment