Warm-up What is evolution? What is heredity?

Mechanisms of Evolution

What do we know? Evolution is: A change in species over a very long period of time Heredity is: Passing of alleles through generations of organisms

How are Evolution and Alleles Related? 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).

For Y, there are 50+50+35= 135 Y alleles. 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 x100 = 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. Independent Assortment describes how the 23 chromosomes from your mother and 23 chromosomes from your father came together in a unique way during meiosis. The biological selection of a particular allele for one trait has nothing to do with the selection of an allele for another trait

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

Stabilizing Selection Average traits are favored; Genetic Diversity decreases as population stabilizes Example: Human Birth Weight Babies that are too large have a hard time fitting through the birth canal. 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

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 So, the darker colored moths were selected for, since now they blended in with the soot-covered trees! Peppered moths, found in England, were originally light colored and hid on light colored trees. Then came the industrial revolution, covering England with soot…

Once more…

Random changes in allele frequency Genetic Drift Random changes in allele frequency What could cause genetic drift? Population Bottleneck: When a population’s size decreases dramatically in a short period of time (due to environmental one-time disaster—hard frost, forest fire, flood, etc.)

What could cause genetic drift? Founder Effect: when a small group in a population splinters off from the original population and forms a new one.

Founder effect in Amish populations... Ellis-van Creveld Syndrome

Warm-up Get out your guided notes from Friday. On the very last line change pq to 2pq EVOLUTION TEST THIS FRIDAY 3/8

What did G. H. Hardy say? Allele frequencies will stay the same from generation to generation IF: There are no mutations Mating is random Populations are infinitely large There is no selection *This is the Hardy-Weinberg Principle!

Hardy-Weinberg Equation A = dominant allele; a = recessive allele p = (freq)A q = (freq)a So p + q = 1 If the population is in EQUILIBRIUM: p2=(freq)AA (homozygous dominant) q2=(freq)aa (homozygous recessive) 2pq=(freq) Aa (heterozygous) So p2 + 2pq + q2 = 1

p + q = 1 AND p2 + 2pq + q2 = 1 You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that knowledge, calculate the following: Frequency of “AA” genotype Frequency of “a” allele Frequency of “A” allele Frequencies of genotype “Aa”

p + q = 1 AND p2 + 2pq + q2 = 1 You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that knowledge, calculate the following: Frequency of “AA” genotype = 0.16 (16%) Frequency of “a” allele = 0.6 (60%) Frequency of “A” allele = 0.4 (40%) Frequencies of genotype “Aa” = 0.48 (48%)

Mechanisms of Evolution part deux

Individuals moving into the population Lots of things could cause allele frequencies to change! These are things that Hardy-Weinberg does not take into account. Mutations Individuals moving into the population Individuals moving out of the population Disease Predation What else???

Speciation: The formation of new species How does it happen? As species experience genetic changes over time (evolution), populations become reproductively isolated from each other. (Populations’ gene pools don’t mix!)

What are some types of reproductive isolation? Behavioral isolation (Species have different reproductive behaviors) Courting individual (usually male) does not display to members of other species Signal and response are species-specific

What are some types of reproductive isolation? Geographic isolation (Populations are physically separated by geographical barriers like water, canyons, or mountains; continental drift) Differences in populations are the consequences of geographic isolation; Separate populations evolve independently (from adaptation and genetic drift)

What are some types of reproductive isolation? Temporal isolation (Species reproduce at different times) Can be because of timing of breeding cycles (seasons) or the time of day/night that species search for mates

Once populations are separated…

Adaptive Radiation Diversification of a single ancestral species into several forms that are each specialized to a particular environmental niche What does adaptive mean? What does radiation mean?

Convergent Evolution Occurs when organisms that are not closely related independently evolve (acquire)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! How was the adaptation for flight useful?

What is an analogous structure, and how is it different from a homologous structure? Analogous structures have a SIMILAR function but DIFFERENT evolutionary origin. Homologous structures are similar in structure AND show common ancestry!

Sometimes, two or more species can evolve in response to each other 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. Predator and prey evolve in response to each other.

Punctuated equilibrium vs. gradualism 2 ways in which evolution of a species can occur Punctuated Equilibrium= speciation is rapid; long periods of stasis Gradualism= constant, gradual evolution (smooth and continuous)

Gradualism Punctuated Equilibrium