EVOLUTION “Nothing in Biology Makes Sense Except in the Light of Evolution.” Theodosius Dobzhansky (Geneticist, 1900-1975) “Lamarckism” • Before Darwin published his theory of evolution, Jean-Baptiste Lamarck’s idea of evolution was widely accepted. • Lamarck proposed that acquired traits could be passed on to offspring. Ex.: “Giraffes got their long necks by striving to reach the leaves on the tops of trees, so their babies were born with long necks.” (NOT!) • After Darwin published his theory, and after inheritance was explained through genetics, Lamarckism was abandoned. If you grow big muscles by lifting weights, will you pass this acquired trait on to your offspring? Charles Darwin • Charles Darwin was an English naturalist (1800’s). • During the voyage of the Beagle, Darwin observed many species in their natural environments, especially in South America / the Galapagos Islands. • While studying the Galapagos finches, he developed the theory of natural selection. • Darwin published “The Origin of Species” in 1859, containing his theory of evolution by natural selection to explain how the variety of living things existing today developed through time. Darwin was not the only one! • Alfred Russel Wallace also inferred evolution by natural selection. • Darwin had the idea first, but only published his book once he knew Wallace had also arrived at the same conclusion – Darwin had been working on it for over 20 years!! • Darwin explained his theory more completely and with more supporting evidence, and established his priority. Evolution of Species • Evolution = change or development through time • Species = interbreeding population of organisms that can produce healthy, fertile offspring • Adaptation = inherited trait that allows organisms to survive in a particular environment • Species are adapted to their niches. • Speciation = evolution of a NOT evolved from each other: new species share a common ancestor 5-6 m.y.a. Adaptations are… Shape, size, color, hormones, ability to make eggs, and other genetic traits. Leaves that capture sunlight, flowers containing gametes for reproduction Poison to deter predators, enzymes that digest organic matter, coloration Small size, wings, feathers, beak size Behavioral traits, including mating behavior that optimizes reproduction and parental care Artificial Selection / Selective Breeding of Cattle (Videos at pbs.org – 5-6 min.) Artificial Selection of Crop Plants Artificial Selection / Selective Breeding Domestication and selective breeding by people have generated many breeds/varieties of animals and plants. Why and How do Species Evolve? Current biodiversity ~ 5-30 million species 99% species that ever lived on the earth are now extinct! Environments change advantage for some, disadvantage for others Natural Selection • Mechanism of evolution suggested by evidence • Four factors: – – – – Variation within populations some favorable Struggle for survival limited resources, competition Overproduction of offspring not all young survive Differential survival & reproduction individuals w/ favorable variations survive & reproduce more, traits become more prevalent in each generation Cummulative effect over time results in changes in a species (evolution) Variation • Genetic Variation = differences between individuals in a population fuel for evolution • Environmental changes variation increases the chances of survival for a species Ex.: new disease some individuals may survive • Sources of genetic variation: – Recombination of existing alleles (forms of a gene) by sexual reproduction; – Mutation – change in the sequence of a gene; most are neutral or harmful. Selective Pressure • Selective pressure = environmental factor that creates struggle to survive/compete • Individuals have to compete with other individuals of the same species and other species for limited resources: – Space/territory – Food, water, sunlight, soil nutrients – Mates Sexual selection – selection by member of the opposite sex (mate) Evolution is a Compromise Between Different Selective Pressures Illustration: Guppy studies by John Endler at pbs.org Peppered Moths & The Industrial Revolution – A Natural Selection Tale Before factories… After factories… “industrial melanism” Tell me the story using the words variation, reproduction, genes, and natural selection. After environmental standards improved / pollution control… Species Evolve in Interaction with Other Species • Coevolution = two species evolving interdependently Ex.: predator and prey • Each species is under selective pressure (environmental limitations) for adaptations that are favored in its habitat • A species niche includes interactions with other species • When two species evolve in symbiosis, they become adapted to each other Coevolution Examples Symbiosis: Parasitism Symbiosis: Mutualism Predator & Prey Reproductive Isolation Results in Speciation • Two populations of the same species may become reproductively isolated no more gene exchange, evolve separately • Geographic isolation causes reproductive isolation: living in separate places breeding in separate groups Ex.: islands colonized by mainland species • Different environments next to each other may favor different variations of the same trait ex: forest hummingbirds, grassland hummingbirds Reproductive Isolation • Pre-Zygotic before fertilization/mating; no zygote/embryo forms • Post-Zygotic after fertilization/mating: zygote/embryo not viable or not fertile Ex.: Drosophilla melanogaster x D. pseudoobscura hybrids have atrophied testes (infertile) – Hybrid is sterile, inviable, or has low fitness Ex.: horse x donkey mule (infertile) Allopatric vs. Sympatric Speciation • Allopatric speciation – populations diverge due to geographic isolation (islands, areas separated by a mountain, river, etc.) • Sympatric speciation - populations diverge without geographic isolation (usually due to reproductive isolation in the same environment) Allopatric or Sympatric Speciation? Galapagos Finches Allopatric or Sympatric Speciation? Allopatric or Sympatric Speciation? Allopatric or Sympatric Speciation? The European mosquito Anopheles consists of six morphologically indistinguishable species. They are isolated reproductively as they breed in different areas of the same habitat. Some breed in brackish water, some in running fresh water and some in stagnant fresh water. Therefore, they never meet to breed. If this happens for subpopulations of a species, speciation may follow. How Fast Does Evolution Happen? • Darwin originally proposed evolution happens at a slow, gradual rate Gradualism • Gould & Eldredge (1972) proposed that populations remain stable through long periods of time, interrupted by rapid genetic changes Punctuated Equilibrium Fossil record lacks many intermediate forms. Common ancestor Relatedness Between Species • Depends on how long ago species shared a common ancestor (diverged from ea. other) • Scientists compare fossils, structures & DNA sequences to establish relatedness • A phylogenetic tree is a diagram showing relationships between different species Phylogenetic Tree of Vertebrate Animals Analogous or homologous structures? Divergent Evolution Species evolve from same ancestor by differentiating from each other. Common ancestor of crocs & birds fishapod-like ancestor of land vertebrates Common ancestor of all vertebrates (first fish with a backbone) Cladograms are a type of Phylogenetic Tree A clade is a group that shares a common ancestor shared characters Shared characters: quadruped body plan, amniotic eggs, skin that preserves moisture, etc. Convergent Evolution Unrelated or distantly related species evolve similar adaptations due to similar environment Selective pressure for sustained rapid swimming in a marine habitat Analogous or homologous structures? Divergent Evolution CONVERGENT EVOLUTION CONVERGENT EVOLUTION Analogous or homologous structures? No blind spot – nerves behind retina Blind spot – nerves & cell layers in front of retina Convergent Evolution East African cichlid fishes Lake Tanganyika cichlids Lake Malawi cichlids Similar diets in different lakes have favored similar jaw/mouth shapes. Can Convergent Evolution happen in homologous structures? Bones are homologous – inherited from a common quadruped ancestor Evidence suggests that wings evolved separately in mammals and dinosaurs convergent evolution of flying structures Convergent Evolution: Distantly related species, similar environments Adaptive Radiation Divergent evolution in which several or many species evolve from one ancestor, filling available niches. Ex.: Marsupials in Australia Adaptive Radiation Ex.: Archaeoptherix, a transitional form between dinosaur and bird, thought to be the ancestor of modern birds Directional Selection Trait value Trait value Stabilizing Selection Trait value Trait value Directional vs. Stabilizing Selection Disruptive Selection What type of selection has happened? Disruptive Selection Divergent Evolution Common ancestor Evolution Reminder: Keep your sense of humor…