Transcript Species
“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. Why 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 • Because individuals differ from each other, some survive & reproduce better • Favorable traits (adaptations) get passed on to offspring • Cummulative effect over long periods of time results in dramatic changes in a species (evolution) Types of 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 Variation: Fuel for Evolution • Genetic Variation = differences between individuals in a population • 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. 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, traits become more prevalent in each generation 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 (add to notes) • 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 (add to notes) • 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? Allopatric or Sympatric Speciation? Allopatric or Sympatric Speciation? The European mosquito Anopheles group consists of six morphologically indistinguishable species. They are isolated reproductively as they breed in different habitats. 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. 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 Divergent Evolution Species evolve from same ancestor by differentiating from each other. ___________? Homologous structures result from divergent evolution. Phylogenetic Tree of Vertebrate Animals Cladograms are a type of Phylogenetic Tree A clade is a group that shares a common ancestor. Convergent Evolution Unrelated or distantly related species evolve similar adaptations due to similar environment Convergent Evolution for sustained rapid swimming in a marine habitat Analogous ___________? structures result from convergent evolution. Divergent Evolution 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 (add to notes) Stabilizing Selection Directional vs. Stabilizing Selection Disruptive Selection