The Evolution of Darwin s Theory Pt 2. Chapter 16-17

Transcription

1 The Evolution of Darwin s Theory Pt 2 { Chapter 16-17

2 Natural Selection If an individual has an allele that allows them to survive better in an environment than another allele, they will have a better chance of reproducing If they have a better chance of reproducing, the next generation will have a higher percentage of offspring with the alleles for the favorable trait Eventually, the alleles for the favorable trait will be the only alleles in the population

3 Generation 1

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5 Generation 2

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7 Generation 3

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9 Generation 4

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11 Generations 5-9

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13 Generation 10?

14 Generation 1 Generation 10

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16 Directional Selection Directional selection is when an extreme phenotype is favored and the distribution of alleles shifts toward the alleles that cause this phenotype Example, drug resistance If an antibiotic for fighting the bacteria caused by tuberculosis kills 98% of bacteria, it is successful in the moment Ten years later, however, those 2% of bacteria have become the same population size as the original population Now, the population is the same size AND they have been selected for resistance to the antibiotic.

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18 Stabilizing selection Stabilizing selection is when an intermediate phenotype is favored. Extreme phenotypes are selected against. Example: Swiss starling birds contain genes for laying between 1-9 eggs 1-3 eggs/starling result in too small of a population in the next generation 6-9 eggs/starling result in not enough resources to support all the hatchlings 4-5 eggs/starling result in optimal conditions for the population

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21 Disruptive Selection Disruptive selection is when the extreme phenotypes are favored over the intermediate phenotype Example: British land snails have genes for light colored shells, dark colored shells, or both Snails with shells of all one color are able to blend and hide in various habitats of the same colors Snails with banding are easy to spot in all habitats.

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24 Sickle-Cell and Malaria Sickle-cell is codominant trait. Individuals homozygous recessive for sickle-cell usually die at a young age if they are born at all. Geneticists in Africa have noticed that heterozygote individuals have an interesting side-affect: immunity to malaria If an individual contains the allele for both normal and sickle cells, they have mostly normal blood. Infection by the malaria parasite causes the blood to change to sickle-cell shape, which kills the parasite Thus, being a heterozygote for sickle-cell makes you more immune to death from sickle-cell disease than homozygous recessive individuals AND more immune to death from malaria than homozygous dominant individuals. Unfortunately, it also ensures the safety of the sickle-cell allele

25 Speciation Speciation is the splitting of one species into two or more species, or the transformation of one species into a new species over time. A species is a group of organisms all capable of interbreeding and producing fertile offspring Gene flow occurs between populations of similar species, not different ones. Therefore, it s important to identify and separate species from each other. It s also important to note when species separate themselves to the point where a new species emerges.

26 Speciation For two species to separate, they must be reproductively isolated. There are many ways this occurs 1) Habitat isolation When two species occupy different habitats, even within the same geographic range, they are less likely to meet and attempt reproduction. Red Maples of the East Coast vs Bigleaf Maples of the West Coast 2) Temporal Isolation Two species live in the same area but reproduce at different times of the year The leopard frog, which mates in April and the bull frog, which mates in July

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29 Speciation 3) Behavioral Isolation Two species have different courtship patterns that allow males and females to recognize one another. Fireflies of different species have different flashing patterns to attract mates Gypsy moths give off pheromones only detectable by members of their own species 4) Mechanical Isolation When two species genitalia or structures are incompatible, reproduction cannot occur Male dragonflies anatomy are so uniquely shaped they only fit with other members of their own species

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31 Speciation 5) Gamete Isolation Two species may be able to mate, but their gametes may be incompatible with each other The Gray Fox has 66 chromosomes; the Red Fox has 34 6) Zygote Mortality Fertilization occurs, but the zygote does not survive Many zygotes of coral hybrids do not survive development

32 Speciation 7) Hybrid Sterility The zygote of a hybrid is able to develop and be born, but the offspring are sterile When a male donkey and a female horse reproduce, the result is a mule. Mules live healthy lives, but they are sterile 8) F 2 fitness F 2 fitness means that the offspring can survive and reproduce, but the F 2 generation cannot Crosses between primrose and cotton.

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34 Speciation Sometimes the reproductive barrier can be removed in later generations Example: volcanic eruption causes destruction of habitat between two environments, but 200 years later the vegetation has re-grown. By this time, however, the two species have diverged so much that they still cannot mate even though the barrier is gone This type of speciation overall is called allopatric speciation Example: different tortoises on the Galapagos Islands

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36 Speciation A similar issue arises when a reproductive speciation occurs in a small section of a population This smaller population subset eventually passes on to other members in later generations The species are never separated, but issues with gametes or zygotes result in speciation This type of speciation is called sympatric speciation Example: Development of polyploid gametes and zygotes in a population of carnations.