Mechanisms of Evolution Changes in genes mutations produce

Mechanisms of Evolution

Changes in genes (mutations) produce variations that can be passed on to offspring. Natural selection works on these variations. • Genetic variation is studied in populations. • Because members of populations reproduce together (interbreed), they share a common group of genes called a gene pool.

Relative frequency of alleles: The number of times a particular allele occurs in a gene pool • In a population of 100 slugs, there are 50 yellow slugs (YY), 35 yellow-brown (YB), and 15 brown slugs (BB).

Remember: in a population of 100 slugs, there are: 50 yellows (YY), 35 yellowish-browns (YB), and 15 browns (BB). • To figure out relative frequencies, we must see how many times the alleles for Y and B occur in our population. • There are 100 slugs. Each has two alleles for color. That means there are 200 alleles total. • For Y, there are 50+50+35= 135 Y alleles. • Divide 135/200=. 675 x 100 = 67. 5%! • Do the same thing for B. You should get…

35+15+15 = 65 B alleles 65/200=. 325 x 100 = 32. 5%! • So, we have a relative frequency of 67. 5% for the Y allele and 32. 5% for the B allele. • Let’s say that a species of frog started to eat these slugs. The yellow ones were much easier for the frogs to see! What might happen? • If the relative frequencies change, EVOLUTION has occurred!

What are some sources of genetic variation? • A mutation is any change in the sequence of DNA. Mutations may or may not have an effect on the organism’s phenotype. If there is a change in phenotype, it could have a negative or positive effect on that organism’s fitness. • Gene shuffling describes how the 23 chromosomes from your mother and 23 chromosomes from your father came together in a unique way during meiosis.

Single-gene and Polygenic Traits • Single-gene traits are controlled by a single gene that has two alleles. • Traits controlled by two or more genes are called polygenic traits. In humans, height is a polygenic trait. The presence or absence of a widow’s peak hairline is a single-gene trait.

Review: • What is the difference between a phenotype and a genotype? • What is an allele? • What are dominant and recessive alleles? • What is the difference between homozygous and heterozygous?

Speciation: The formation of new species How does it happen? As new species experience genetic changes over time (evolution), populations become reproductively isolated from each other.

Speciation:

What are some types of reproductive isolation? • Behavioral isolation (Species have different reproductive behaviors. ) • Geographic isolation (Populations are physically separated by geographical barriers like water, canyons, or mountains. ) • Temporal isolation (Species reproduce at different times. )

Consequence of Isolation • Over time the two species experience different selection pressures • After many years the species will become more and more different • After thousands of years these species may not even appear similar Speciation!

Three types of natural selection: • Directional selection • Stabilizing selection • Disruptive selection

Stabilizing Selection Average traits are favored Example: Human Birth Weight Babies that are too small tend to lose too much heat and have more illnesses. Average weight babies (around 8 lbs. ) tend to be the healthiest Babies that are too large have a hard time fitting through the birth canal.

Disruptive Selection Favors traits at both extremes Ex: Darwin’s finches in the Galapagos Because there were different seed sizes available on the different islands, birds with bigger beaks ate the big seeds and birds with smaller beaks ate the small seeds. Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive.

Directional Selection Allele frequency shifts in one direction Example: Peppered moths, found in England, were originally light colored and hid on light colored trees. Then came the industrial revolution, covering England with soot… So, the darker colored moths were selected for, since now they blended in with the soot-covered trees!

Once more…

Mechanisms of Evolution part deux

The Hardy-Weinberg Principle • This principle states that allele frequencies in populations will remain constant unless something causes those frequencies to change. • When allele frequencies remain constant, a state of genetic equilibrium is reached. But what could cause allele frequencies to change…?

Lots of things could cause allele frequencies to change! • • • Mutations Individuals moving into the population Individuals moving out of the population Disease Predation What else? ? ?

Genetic Drift Random changes in allele frequency – What could cause this? What might happen if a small group of individuals colonizes a new habitat?

Founder effect in Amish populations. . .

So, how does natural selection affect ecosystems? ? ? Natural selection is the unique survival and reproduction that results from the presence or absence of physical traits. If you are a deer with thicker than normal fur and you happen to live in a cold climate, you’re going to have a better chance of surviving and reproducing. Your offspring may have thicker fur too. This inherited trait is an ADAPTATION.

Adaptive Radiation The process by which a species evolves into a different species that lives in a different way What does adaptive mean? What does radiation mean?

Adaptive Radiation The process by which a species evolves into a different species that lives in a different way

Convergent Evolution Occurs when organisms that are not closely related independently evolve similar traits Wings evolved separately in: insects (arthropods) pterosaurs (extinct flying reptiles) birds (birds) bats (mammals) So, flight has evolved at least four separate times!

What is an analogous structure, and how is it different from a homologous structure? Analogous structures = convergent evolution!

Analogous structures: similarities due to convergent evolution, not common ancestry Homologous structures: similar structures due to a common ancestor

Sometimes, two or more species can evolve in response to each other. This is called coevolution. This often happens between predators and prey. Predators that are good at catching prey are naturally selected. This means they survive because they can catch food. Prey animals might evolve to have better camouflage or better escaping or fighting skills. They evolve in response to each other.

What is the difference between punctuated equilibrium and gradualism?

Gradualism: small, gradual changes over time Punctuated Equilibrium: Long periods of equilibrium that are interrupted by episodes of speciation

Animals that are in danger of extinction are called endangered species. Here a few of the most endangered species on Earth.

These are already gone… Tasmanian Wolf Dodo