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Evolution Genes and Variation Review of Genetics • What are genes? – Specific genetic makeup of an individual • What is an allele? – Different forms of a gene – Remember BB, Bb, bb? • What is a mutation? – A change in the DNA sequence that affects genetic information Genes & Variation • What is a population? – all the organisms of the same group or species who live in the same geographical area and are capable of interbreeding • What is a gene pool? – The total number of alleles in a population • What is variation? – The differences among members of a population Natural Selection • Natural selection acts on populations • Environmental pressures cause individuals to survive or die Modes of Selection Directional Selection Low Mortality Old average High Mortality New average • In a given population, the most fit individuals are now at one extreme or the other • Causes the entire curve to shift Stabilizing Selection • Organisms at the center of the graph (the average) are the most fit • Often occurs after a natural disaster • Nature tries to maintain the ‘status quo’ • Same average Low Mortality High Mortality Original Population # of birds Beak size • Curve stays at same position, but narrows New Population population Disruptive Selection New – 2 phenotypes! Original Population – 1 phenotype Low Mortality High Mortality # of birds Beak size # of birds Beak size • Individuals at both extremes have a higher fitness than individuals in the center • Leads to two distinct phenotypes! Genetic Drift • Over time, a series of chance occurrences can make one allele more common in the population. • A chance event wipes out members of the population and leaves only a select few à doesn’t represent original population • Small populations more easily affected • Random chance – no way to plan Genetic Drift No grey! Survivor Group #1 Survivor Group #2 Original population 3 phenotypes – black, white, grey No black! Types of Genetic Drift- Founder Effect • Members of a population migrate to a new area • New population established • May be very different gene pool than original population • Starts as smaller gene pool & fewer individuals The “Blue People” of Kentucky Types of Genetic Drift- Bottleneck Effect • Severe reduction in population size due to a random event • What could these random events be? – Predation – Disaster – Hunting • New population may look nothing like original A bottleneck effect is genetic drift in which a severe reduction in population size results from natural disaster, predation, or habitat reduction. Cheetah: A Real Life Bottleneck • Every one of today's 20,000 Cheetahs is genetically almost identical. They descend from survivors of a near-extinction catastrophe that resulted in generations of close inbreeding 10,000 years ago. Does evolution ALWAYS take place? • Nope. • Populations can sometimes be stable. • Genetic equilibrium Genetic Equilibrium • 1. Random mating & all members of a population have equal opportunity to mate • 2. Large population – Genetic drift has less of an affect on big groups • 3. No net movement in or out of a population – Migration & Emigration bring alleles to and from a population • 4. No mutations – – no new alleles/traits introduced • 5. No natural selection – No phenotype has an advantage Types of Reproduction • Asexual Reproduction – No mate needed – Quickly reproduce – Every individual can reproduce – Traits can easily be passed on – Mutation/disease affects everyone – No variation – No evolution • Sexual Reproduction – Variations (driving force behind evolution!) – New combinations of traits could be beneficial – Bad traits could disappear or be hidden – Fewer offspring – Competition for mates Sexual Dimorphism Distinction in appearance due to secondary sex characteristics Males of a species are distinctly different from Females Intrasexual Selection - Fighting for mates - Males usually fight for females Intersexual Selection - Also called Mate Choice - Usually the choosing is done by the females. - Males with the most impressive masculine features are most attractive to the ladies. What makes a species? Speciation What makes a species? Species = latin = kind à Means many types! • Morphology = similar physical form • Similarities in… • Body Function • Biochemistry • Behavior • Genetic Make-up Biological Species concept: Ernest Mayr, 1942 Defines a species as a population or group of populations whose members have the potential to produce viable, fertile offspring but who cannot produce viable, fertile offspring with members of other species. Barriers for Speciation • In order to have a new species, they must be reproductively isolated • Two ways: – Prezygotic: before sperm and egg meet – Postzygotic: after fertilization Prezygotic (Pre-Fertilization) Barriers to Speciation • Habitat isolation – Don’t live in same area • Behavioral isolation – Mating rituals : song, dance • Temporal isolation – Mate in different seasons, times of day, etc. • Mechanical isolation – Things don’t fit where they should. • Gametic isolation – Egg and sperm don’t attract each other Postzygotic (After Fertilization) Barriers to Speciation • Reduced hybrid viability – Zygote (fertilized egg) dies or offspring never reach sexual maturity • Reduced hybrid fertility – Offspring are sterile • Hybrid breakdown – Offspring can produce only 1 new generation Behavioral Isolation: Specific Courtship Displays Temporal Isolation: Different frog species mate at different times of the year. Colors indicate same habitat range When species cross… Man-made & Sterile Mule: offspring of a male donkey and a female horse Cama : offspring of a camel and a llama Produced by artificial insemination Hinny: offspring of a male horse and a female donkey A horse has 64 chromosomes resulting in 32 in the gamete. A donkey has 62, resulting in 31 the gametes. How many does a mule have in its cells? 63 + = Male and female mules are typically sterile because the horse and donkey chromosomes differ in number and they are not homologous. Therefore, the horse and donkey chromosome doublets fail to properly pair up with each other during synapsis of meiosis I. AGAIN - These Usually Do Not occur in nature! Liger : Male lion, female tiger Tigon: Male tiger, Female lion Zebroid: zebra & another equine animal Zebroid hybrids are STERILE. What is that called? Reduced Hybrid Fertility