16.2 Evolution as Genetic change Biology Mr. Hines

A large part of genetics is understanding that reproduction and passing on “fit” traits is vital. If an organism lives to reproduce, it was obviously successful at surviving. The traits that were necessary to survive will be passed on to another generation.

Natural selection works on survival and reproduction. If an organism does not possess the traits for survival, it will die, and therefore not pass on its genes. This process enables perfection over time.

What is evolution? Evolution is any change over time in the relative frequencies of alleles in a population. Individual animals do not evolve, entire populations do.

Natural selection on single gene traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Take for example a hypothetical population of lizards. 80% of the lizards are brown 10% are red 10% are black

If red lizards are more visible to predators, they would be less likely to survive. Black lizards might absorb more sunlight and warm up faster which could allow them to escape a predator more effectively. The red lizard can not reproduce if it is dead and will not affect the gene frequency. The black lizard will live to pass on its trait. This could change the gene frequency. When a gene frequency of an allele changes, we have evolution

Change of gene frequency = Evolution

Natural Selection on Polygenic traits When traits are controlled by more than one gene, the affects of natural selection are more complex. Human height is an example of a polygenic trait.

Natural selection can affect the distributions of phenotypes in any of 3 ways. 1.Directional selection 2.Stabilizing selection 3.Disruptive selection

Directional selection We will use beak size to demonstrate this idea. The dotted bell curve represents a wide range of beak sizes. The bell curve peaks where beak size is most common.

If the food supply of small seeds diminishes, birds with larger beaks would have an advantage. This would cause beak size in a population to shift towards large beaks. The bell curve would move to the right in this case. This is called directional selection

Stabilizing Selection We will use human birth weight to demonstrate this idea. Human babies can be born in a range of birth weights.

If a baby is born too small, its survival chances are lower. If the baby is born too large, complications in birth arise, and its survival chances are lower. A range of birth weights will narrow based on survival rates. The bell curve will become more narrow and steeper This is called stabilizing selection

Disruptive selection We will use beak size to demonstrate this idea. If a population of birds has a wide range of beak sizes, it will have a normal bell curve

This would mean that the food supply is very diverse. If the food supply changed so that medium sized seeds disappeared, and small and large seeds became more common, small beaks and large beaks would be the strongest traits This would cause the bell curve to be divided. The population would split into 2 subgroups.

This can also lead to new species of birds – birds with large beaks are one species, and birds with small beaks are a different species. This is called disruptive selection

Genetic drift Natural selection is not the only force in evolution. Genetics are controlled by the laws of probability – or chance. Alleles can grow in number in a gene pool simply by chance. This is called genetic drift.

Genetic drift happens more quickly in a small population than a large population. We will use beetles to demonstrate this idea. Lets say that this sample of beetles lives on the continent of South America.

A Typhoon blows through the area and carries a different sample of beetles to the Galapagos.

Notice that sample of beetles is different on each island If these beetles reproduce, it would produce the following outcome.

Therefore, the population of beetles on each island is different based and random chance. This is genetic drift.

Evolution vs Genetic equilibrium Another look at evolution is to determine what happens when no change takes place. Are there conditions when evolution will not occur? The Hardy-Weinberg principle states that if evolution does not occur, the allele frequency will remain constant.

That can be stated in reverse. If the allele frequency remains constant, evolution is not occurring. This condition is called genetic equilibrium.

What conditions will cause genetic equilibrium? 1.Random mating 2.The population must be very large 3.There can be no movement in and out of the population 4.No mutations 5.No natural selection

Random mating All individuals have an equal chance of producing offspring. This is uncommon in a population – usually the female will decide who she will mate with. This is called sexual selection**sexual selection is a major force in evolution – it is barely mentioned in the book.

Large populations Some populations are so large that genetic drift never occurs.

No movement in or out of a population Any organism moving in or out of a population will certainly change the gene frequency

No mutations No mutations can occur if an allele frequency is to remain unchanged.

No natural selection All genotypes must have an equal chance of survival and reproduction

If all 5 of these conditions are not met, a population will evolve. There is no known population that meets all 5 conditions, therefore all populations are evolving. This can be restated that non-evolution is impossible!