Ch 10- Evolution & Natural Selection PDF

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Chapter 10: Evolution and Natural Selection Intro Origin of Species

Descent with Modification

● Darwin observed adaptations of plants and animals that inhabited diverse enviro’s ● Origin of Species – Darwin, 1859 ● Darwin developed 2 main ideas: ○ 1. Descent of modification: explains life’s unity & diversity ○ 2. Natural Selection: cause of adaptive evolution ● Descent with modification: ○ Organisms related through descent from ancestor that lived in remote past ○ Current species are descendants of ancestral species

Darwin’s Focus on Adaptation

● Adaptation to enviro & origin of new species are closely related processes ○ Perfectly suited for food of that island ● Finch beaks on different Galapagos islands: ○ Diff food shape/hardness ○ Diff finch beak shapes & strengths more successful in obtaining that food ○ Better “fitness”

Natural Selection: Beetle Example

● Beetles living in dark environment ○ Mixed dark/light shells ○ Predatory birds see light beetles easier ○ light beetles get eaten more readily ○ More dark beetles survive & able to reproduce ○ nature has selected for dark shell gene to be passed on more than light shell gene

Natural Selection & Evolution

● Natural Selection: greater survival/reproduction of species with favorable heritable adaptations ● Natural selection results in evolution: change in proportions of indiv’s within population that express diff traits ● Indiv’s don’t evolve; populations evolve (KNOW)

Mechanisms of Evolution

● Evolution occurs by 3 mechanisms: ○ 1. Natural selection ○ 2. Genetic drift ■ Bottleneck & Founder Effect ○ 3. Gene flow

Natural Selection: Overproduction of Offspring Natural Selection: Individual Variation

● Populations of all species have potential to produce more offspring than enviro can support ● When population outgrows resources, members must compete for what’s available ● Indiv’s within population vary ● Variations can be: ○ Morphological: shape, color, size ○ Physiological: metabolism, growth ○ Behavioral: response to stimuli

Natural Selection: Genetic Variation

● Why is there variation? ○ variations are caused by genetic diffs bt indiv’s ○ Indiv’s express diff sets of traits based on their diff allele combos ● How does variation come about? ○ Diff alleles for same gene arise through mutation: random change in DNA sequence ○ If mutation occurs in gene, might result in change in function of protein that gene codes for ■ resulting new form of gene is new allele

Origin of Variation

● Most mutations are either lethal or neutral ● Mutation isn’t beneficial ● Mutations provide genetic variation that natural selection, gene flow & genetic drift can operate on to cause evolution

Natural Selection: Differential Survival & Reproduction

● fitness: reproduction of indiv compared to others in same pop ○ More offspring = greater fitness ● Adaptations: traits that increase indiv’s fitness in enviro ● Adaptations are genetically-based and arise by random mutations in genes (= formation of new alleles) ● Natural selection can only ↑ or ↓ heritable traits ● Adaptations not “produced” in response to environmental pressures or need

Natural Selection: Differential Survival and Reproduction

● indiv’s with adaptations reproduce more ○ have higher fitness than those who aren’t ○ pass their traits to their offspring

○ offspring will make up larger % of next generations ● Over generations those adaptive traits will increase in frequency Natural Selection

● indiv can’t “develop” adaptation; but selects for traits already present in population ○ Since adaptations are genetically based, they’re either born with those particular alleles or they aren’t ● Adaptations help indiv’s survive/reproduce better than others in population ● Increases match between organism & environment

Genetic Drift

● Genetic drift: random fluctuation in allele frequencies over time, due to chance ○ more significant in small populations ○ Genetic drift tends to reduce genetic variation through losses of alleles

Genetic Drift: Bottleneck Effect

● Bottleneck effect: catastrophic event reduces population size, leaving few indiv’s to reestablish population ○ EX- flood and/or fire ● Resulting gene pool no longer reflective of original population ● If death of indiv’s was random (not depending on their traits), results in genetic drift

Genetic Drift: Founder Effect

● Founder effect: few indiv’s from population start new pop with diff allele frequency than original population ● EX- Island Founding

Gene Flow

● Gene flow: movement of alleles in/out of populations ○ Alleles transferred by movement of fertile indiv’s /gametes (e.g., emigration/immigration, pollen) ○ Diffs bt populations are reduced ○ More likely than mutations to alter allele frequencies directly

Adaptive Evolution

Evidence of Evolution

● Enviro changes; adaptive evolution is continuous process ● Natural selection consistently leads to adaptive evolution ○ increase allele frequencies that provide reproductive advantages ● Genetic drift & gene flow don’t consistently lead to adaptive evolution ○ inconclusive match between organism & environment ● ● ● ● ● ●

Fossil record Biogeography Comparative morphology Comparative embryology Biochemistry Direct observations

Fossil Record

● Fossils: organismal remains found in sedimentary rock layers (strata) ○ EX- skeletons, shells, leaves, seeds, track/leaf imprints ● Fossil record: history of life recorded by physical evidence of organisms that lived long ago ● Provides evidence of: ○ extinction of species ○ origin of new groups ○ changes within groups over time (evolutionary transitions) ● fossil record reveals appearance of organisms in sequence: ○ Older & simple fossils in deeper layers

Biogeography

● Biogeography: study of the geographic distribution of

species ○ distribution of fossils & living populations are consistent w/ predictions based on evolutionary theory ● EX- Island vs mainland species ● Species often more closely resemble other species that live nearby in diff habitats than do species that live far away in similar habitats ● island species evolved from founders from mainland Comparative Morphology

● Morphology: diff species that have similar body parts inherited from common ancestor ● Homologous structures: anatomical structures in diff species are similar bc of common ancestry ○ EX- Vertebrate forelimbs ■ Similarities in # and position of bones indicates common ancestry ● 4 Animals, 1 Forelimb Structure ○ Suggests Common Ancestor ○ 1 upper bone, 2 intermediate bones, joined to 5 digits ● Analogous structures: when similar features evolved independently and species involved are not closely evolutionarily related ○ Situation is convergent evolution ■ Groups independently adapted to similar enviros in similar ways ■ Doesn’t provide info ab ancestry

Comparative Embryology

● Comparative Embryology: organisms may show similarities in morphology during embryonic stages that often indicate evolutionary relationships ● EX- early embryos of vertebrates strongly resemble eachother bc they have inherited same ancient plan for development

Biochemistry

● Biochemistry: species that have close evolutionary relationship have similar DNA sequences ○ EX- Human and chimpanzee DNA are 99% similar ● Since DNA sequences determine amino acid sequences in proteins, also more similar in closely related species

Direct Observations

● Researchers have observed populations in wild, in agricultural settings, & in lab change genetically over

time ● Such observations provide direct concrete evidence for evolution ● Examples: ○ Guppy coloration ○ Artificial selection ○ Antibiotic resistance ● Effects of predators on wild guppy populations (J. Endler) ○ Bright colored males more attractive to females, but also more vulnerable to predation ○ Color patterns correspond to predation intensity! Guppy Experiment

● genetic variation in population results in range of phenotypes, from drab to colorful ● In high predation: ○ Drab coloration is better adapted trait bc drab males are less easily seen/eaten ○ Drab guppies survive/reproduce more (have greater fitness) than colorful guppies ○ Over generations, proportion of drab guppies increased ● In low predation: ○ Bright coloration is better adapted tra2it bc brightly colored males attract mates ○ Colorful guppies reproduce more than drab ones ○ Over generations, proportion of colorful guppies increase

Direct Observations

● Artificial selection: can “force” natural selection by controlling reproduction of species that are commercially valuable

Direct observation: Antibiotic Resistance

● Evolution by natural selection currently in action...