Mechanisms of Evolution

Transcription

1 Mechanisms of Evolution Mutation Gene Flow (migration) Non-random mating Genetic Drift Natural Selection...individuals don t evolve, populations do

2 1. Mutation The ultimate source of genetic variation. A change in a DNA sequence, usually occurring because of errors in cell replication. The mistakes are RANDOM. Mutations that don't occur in sex cells are not passed on to future generations. These are very rare which is good because most mutations are not beneficial to survival.

3 A mutation could cause parents with genes for bright green colouration to have offspring with a gene for brown coloration. That would make the genes for brown beetles more frequent in the population. This is evolution in itself.

4 2. Gene Flow (migration) Individuals from one population of brown beetles enter and breed with a different population of green beetles. Then brown beetle genes more frequent.

5 Another example, the grey wolf Grey wolves have large territories A lone grey wolf may travel 800 km for a new territory or breeding partner Very often, a grey wolf from one population will mate with a member of a nearby population, bringing new alleles into the gene pool As a result, the genetic diversity in the nearby population may increase

6 3. Non-random Mating Mating among individuals on the basis of mate selection for a particular phenotype or due to inbreeding (if mating was random we d have no way to predict which females will choose with male or vice versa)

7 .preferred phenotypes Individuals may choose mates based on their physical and behavioural traits

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9 .inbreeding An extreme example of inbreeding is the selffertilization of some flowers Because close relatives share similar genotypes, inbreeding increases the frequency of homozygous genotypes..and harmful recessive alleles are more likely to be expressed Purebred animals have a higher incidence of deformities and health problems

10 Originated in China 2000 years ago An inherited gene mutation enhanced through inbreeding increases the production of hyaluronic acid The overproduction of hyaluronic acid contributes to a skin disorder called mucinosis The consequence? An excessive build-up of mucin accumulates under the skin.and produces wrinkles Example, Shar-pei

11 4. Genetic Drift Imagine that some green beetles were killed when someone stepped on them and had no offspring. Other green beetles by chance were washed away by a storm. The next generation would have more brown beetles than the previous generation This happens by chance it s a random change in genes.

12 Genetic Drift: The Founder Effect One pregnant individual or a small group manages to colonize a new isolated region. This is the most probably explanation for the habitation of volcanic islands such as the Galapagos (occurs mostly on islands)

13 Founder Effect: Human Population The lack of genetic diversity can be a medical concern The Amish population of Philadelphia, Pennsylvania, for example, was founded in the 1700s by just a few families The current population of Amish in the region has an unusually high frequency of polydactylism (presence of a sixth finger or toe)

14 Genetic Drift: Bottleneck Effect An environmental disaster reduces a population s numbers drastically (starvation, disease, natural disasters, severe weather) The survivors are a poor representation of the original genetic make-up of the population. (only a fraction of the alleles are now present lack of diversity)

15 Genetic Drift: Human Population The bottleneck effect can be seen on the small island of Pingelap in the Pacific Ocean (part of Micronesia) In 1775, a typhoon devastated the island and there were fewer than 30 survivors Once of the survivors carried the genetic mutation that causes colour vision deficiency The current population on the island traces back to this person about 10% of the population have colour vision deficiency

16 Individuals in the population are nearly genetically identical High infertility Inbreeding Extremely low sperm count High cub mortality Poor immune systems Susceptibility to disease African Cheetah

17 The bottleneck effect is often seen in species driven to the edge of extinction Northern Elephant Seal In 1890 there were approximately 30 individuals left due to overhunting Today there are tens of thousands Genetic diversity is very low

18 5. Natural Selection survival of the fittest Several types.. Stabilizing selection Directional selection Disruptive selection

19 Natural Selection: Stabilizing Selection Favours an intermediate phenotype Acts against the extremes

20 Natural Selection: Directional Selection Favours the phenotypes at one extreme over another Common during times of environmental change or when a population migrates to a new habitat (peppered moths) The increase in antibiotic resistance in infectioncausing bacteria is another example

21 Natural Selection: Disruptive Selection Takes place when the extremes of a range of phenotypes are favoured over intermediate phenotypes Example: the size differences in the male coho salmon

22 The smaller phenotype averages 500 g while the larger phenotype averages 4500 g The difference in size reflects the means by which each phenotype gains access to females Smaller salmon sneak opportunities to fertilize the eggs of females The larger salmon are better equipped for fighting for the same access Coho Salmon

23 copy Factors That Cause Evolutionary Change Factor Mutation Gene Flow (migration) Description and Effect Description: Mutation randomly introduces new alleles into a population Effect: Mutation changes allele frequency Description: Gene flow occurs between two different interbreeding populations that have different allele frequencies Effect: Gene flow may change allele frequencies in either or both populations through a movement of genes

24 Non-random Mating Genetic Drift Description: During non-random mating, individuals in a population select mates, often on the basis of their phenotypes Effect: Non-random mating increases the proportion of homozygous individuals in a population Description: Genetic drift refers to random change in genetic variation form generation to generation due to chance. - Founder effect: a few individuals start a new population. - Bottleneck: results from a rapid decrease in population size/loss of diversity Effect: Genetic drift changes frequencies of alleles

25 Natural Selection Description: Natural selection is the result of the environment selecting for individuals in a population with certain traits that make them better suited to survive and reproduce than others in the population 3 types: -Stabilizing selection (favours intermediate phenotype) -Direction selection (favours one extreme over the other) -- Disruptive selection ( extremes are favoured over the intermediate phenotype)

26 Effect: Over many generations, frequencies of alleles of many different genes may change, resulting in significant changes in the characteristics of a population

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