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Evolution (the Cliff Notes version) Millions of years Natural selection in 5 easy steps 1. Individuals in a population vary in traits, such as shape or size Natural selection in 5 easy steps 2. Some of these trait differences are heritable – meaning that they can be passed on to offspring Natural selection in 5 easy steps 3. In each generation, more offspring are produced than can survive. • only some individuals will survive long enough to produce offspring • some of these individuals will produce more offspring than others The struggle for existence Natural selection in 5 easy steps 4. Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment are likely to leave more offspring. Survival of the fittest!! Natural selection in 5 easy steps 5. The unequal ability of individuals to survive and reproduce will lead to gradual changes in a population, with favorable characters accumulating over generations. Evolution! Evolution (the Cliff Notes version) Millions of years Important points • According to Darwin: – At what level does natural selection occur? – At what level does evolution occur? Important points • According to Darwin: – At what level does natural selection occur? – At what level does evolution occur? – Natural selection acts on individuals • Individuals vary in their ability to survive and produce offspring – Evolution can only be measured as changes in relative proportions of heritable variation in a population over many generations. Natural selection • Individuals do not evolve!! • Natural selection DOES NOT involve organisms “trying” to adapt. • Natural selection DOES NOT give organisms what they need to survive. • Natural selection simply favors traits that enhance survival and reproduction in a new environment. Survival of the fittest • We often hear the process of natural selection called “the survival of the fittest”: – Define fitness – Based on your knowledge of the process of evolution by natural selection, why is this statement an incomplete description? Fitness Sparrowhawks • Of 693 female offspring hatched, only 194 survive to maturity, 499 do not • Of 194 females surviving to breeding age, 33 fail to produce any offspring – Thus: 532 of 693 or 85% females leave no descendents • Australian red-bellied black snakes adapt to the invasion of toxic cane toads Red-bellied black snake, Pseudechis porphyriacus Morphology • Larger snakes should be better able to withstand toxins • Hypothesis: Snakes will evolve larger bodies in areas with toads • Analysis: Compared mean size of snakes (SVL: snout to vent length) in areas that have had toads for different lengths of time (statistically controlled for other variables that might influence snake size like climate) Physiology • Hypothesis: Snake populations with longterm exposure to toads will show increased tolerance of toad’s cardiotoxin • Measured swimming speed of snakes before and after non-lethal oral dose of toxin calibrated to body size (80 µg of toxin per g body mass). Behavior • Hypothesis: Snakes in populations with longterm avoidance of toads will show an evolved, innate avoidance of toads. • Methods: Offered cane toads and non-toxic frogs to snakes from toad infected (40-60 yrs) and toad free areas. Gave 24 h to consume. For Red-bellied snakes • How does the environment change for the population? Is this a biotic or abiotic change? • What are the different phenotypes or the variation seen in the population? • What is the specific selection agent (environmental factor) acting on the population? • As a result of this selective pressure, which phenotypes had the highest fitness? Why? • What is the result of the selective pressure? Natural selection terminology • With the coming of the Industrial Revolution and the blackening of the environment, there was selection for darker bodies in moths and other species. • There is selection on body size in snakes. • Because white flowers are visited by relatively few pollinators and hence produce relatively few seeds, white flowers are selected against. Sexual Selection Intrasexual competition: males fight other males for mates How should males differ from females? Giraffes • Long neck in giraffes – attributed to an evolutionary advantage of being able to reach more food when competing with other browsers. • May actually be a result of sexual selection. Sexual Selection Intersexual competition: females choose mates with specific traits (phenotypes) How should males differ from females? Peafowl • Do males with showier ornaments have “better” genes? – The larger a male’s peafowl’s tail, the more females he is likely to mate. – If the “good genes” hypothesis is correct, what relationship should there be between fitness and “showiness”? Peafowl Growth (measured by offspring weight at 84 days of age) Conclusion? Survival (percent of released offspring that survived for ≥2 years ) • Widowbirds – Both sexes normally brown – During breeding season, males molt and produce black feathers • Red and yellow wing patches • Long tail (up to 0.5 m) • • • • • Tail Length Manipulations Group 1—tails shortened Group 2—tails uncut Group 3—tails cut and re-glued onto the male, length unchanged Group 4—tails elongated by gluing on the length cut from Group 1 males Number of active nests before treatment • • • • • Tail Length Manipulations Group 1—tails shortened Group 2—tails uncut Group 3—tails cut and re-glued onto the male, length unchanged Group 4—tails elongated by gluing on the length cut from Group 1 males Number of active nests AFTER treatment Sexual selection can generate different species Wow, look at that blue fin! Sexy! No thanks, I prefer mine orange. You can have him! Let’s back up a bit • Define evolution… Let’s back up a bit • Define evolution… • Evolution is measured as changes in relative proportions of heritable variation (morphology, physiology and/or behavior) in a population over many generations. – Population shifts from a predominance of grey snakes to a predominance of blue snakes. – Proportion of long-billed hummingbirds in the population increases Another definition • Geneticist’s definition of evolution: a change in the relative frequencies of different alleles in a population's gene pool over the course of generations – Gene pool: all the alleles that exist in the population for a particular gene or set of genes. Population genetics Why is variation important? How do you get variation? • Mutations – Imperfect copies during mitosis – Environmental factors like UV and chemicals How do you get variation? • Mutation – Source of new alleles – Rare and random – Harmful, beneficial or neutral • However, since almost all conceivable beneficial mutations of a population in a stable environment have already been selected in the recent past, the occurrence of new beneficial mutations are rare. (Ernst Mayr, 2001. What Evolution Is). How do you get variation? Meiosis Independent Assortment Crossing over Meiosis Importance of variation • In a population or species, if every organism was genetically identical, what might happen? How do you change the gene pool? How do you change the gene pool? • Through natural selection – the individuals with alleles that increase fitness have more offspring that carry those alleles, increasing the proportion of those alleles in the gene pool. How do you change the gene pool? • Through natural selection – the individuals with alleles that increase fitness have more offspring that carry those alleles, increasing the proportion of those alleles in the gene pool. • Through sexual selection – the individuals with alleles that allow them to attract more mates have more offspring that carry those alleles, increasing the proportion of those alleles in the gene pool. Changes in allele frequencies • Natural selection – Differential survival/fecundity of certain genotypes • Non-random mating – Many types of non-random mating. One type: sexual selection • Mutation – Mutation can cause (slight) changes in allele frequencies by: • changing one allele to its alternative (e.g. A-->a or a -->A) • creating new alleles for a gene locus (e.g., A ---> b ) • Gene “flow” – Alleles entering/leaving population as a result of individuals immigrating into, or emigrating out, of population. • Genetic Drift – More serious in small populations: bottleneck and founder effects Mechanisms of Evolution • Gene flow – Individuals move between populations and bring new alleles into the area. Genetic drift Founder effects Bottleneck event Proportion of the gene loci that are polymorphic Mean no. of gene loci at which individuals are heterozygous Elephant seals 0% 0% Values for >1000 other mammal species w/ no bottleneck event 15-50% 3-4% Blue crabs – possible bottleneck? Blue crabs – possible bottleneck? • If you examined the genetics of blue crabs, what would you expect to see if the population had gone through a bottleneck? Blue crabs – possible bottleneck? • If you examined the genetics of blue crabs, what would you expect to see if the population had gone through a bottleneck? – Low polymorphism and low heterozygosity = low genetic diversity and evolutionary potential Blue crabs – possible bottleneck? • If you examined the genetics of blue crabs, what would you expect to see if the population had gone through a bottleneck? – Low polymorphism and low heterozygosity = low genetic diversity and evolutionary potential – Baltimore crab populations: high haplotype diversity and high microsatellite heterozygosity = high diversity • maintained by not overusing wild individuals in hatchery Rats! Coming to America! Rats in Baltimore • Rat populations in Baltimore were likely introduced from Europe. What do you call this phenomenon? How should this effect diversity of the rat population in Baltimore? Rats in Baltimore • Rats in Baltimore = Founder event – Low genetic diversity expected in Baltimore Rats in Baltimore • What aspects of the rat lifestyle may counteract low diversity due to isolation and founder events? Rats in Baltimore • Rats breed frequently and have large litters – more opportunities for variation to enter population. • Rats can move long distances = gene flow What about Baltimore rats? Baltimore rats • Most rats stick close to home – Baltimore rat populations are geographically isolated and genetically structured. – The Jones Falls is a big barrier to rats – Some rats to go much farther than others – Neighborhood restoration causes large-scale dispersal If you were a rat… • In terms of fitness – – Why might it be advantageous to stay in the same area as your relations? – Why might it be advantageous to head out on your own? Speciation • How do you get new species? – Prezygotic mechanisms- prevent mating or fertilization • • • • • Mechanical isolation Gametic isolation Habitat isolation - Allopatry Temporal isolation - Sympatry Behavioral isolation - Sympatry – Postzygotic mechanisms- prevent development of viable, fertile young • Hybrid inviability • Hybrid sterility • Hybrid breakdown Lice/Human Evolution Chimpanzee lice = Pediculus schaeffi Head lice Pediculus humanus capitis “cooties” Lice/Human Evolution Human Chimp Gorilla Orangutan Gibbon Old world monkey 30 25 20 15 10 million years ago 5 0 The human and chimpanzee lines diverged approximately 6 million years ago Lice/Human Evolution The human and chimpanzee lines diverged approximately 6 million years ago Lice literally came along for the ride. What type of speciation is this? But there’s more… Head lice Pediculus humanus capitis “cooties” Body lice Pediculus humanus corporis Pubic lice Phthirus pubis “crabs” Lice/Human Evolution • How did we go from one louse species to three?? Phthirus – pubic lice Closely related to gorilla lice Pediculus – head and body – closely related to chimp lice Lice/Human Evolution • DNA comparison suggests the most recent common ancestor of human head lice and human pubic lice lived ~12 million years ago. • The most recent common ancestor of gorillas and humans lived ~ 12 million years ago • Humans lose body hair 3-4 million years ago • Head lice trapped in heads. • DNA comparisons show that human pubic lice and gorilla lice last shared a common ancestor 3-4 million years ago. Lice/Human Evolution • Conclusion: – Common ancestor of humans, chimps and gorillas had both Pediculus and Phthirus lice. – When the human/chimp line split from gorillas, Pediculus went extinct in gorillas and Phthirus went extinct in the chimp/human ancestor. – Early humans “caught” pubic lice from gorillas 3-4 million years ago. • Sleeping in nests recently used by early gorillas. • Preying on early gorillas. – Lice are known to jump from prey to predator. Lice/Human Evolution • A DNA comparison suggests the most recent common ancestor of human body lice lived less than 72,000 years ago. • Humans starting wearing tight fitting clothes ~ 107,000 years ago. • Very hotly contested (chicken and egg) • Uncertain as to different species – (intro to species concepts) Cool websites • http://www.pbs.org/wgbh/evolution/educato rs/index.html • http://evolution.berkeley.edu/ • http://www.sciencecases.org/three_lice/note s.asp