Natural Selection (22) The Evolution of Populations (23)

Natural Selection: A Summary Natural selection is the mechanism which directs the process of descent with modification Natural selection says: – Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals – Natural selection increases the adaptation of organisms to their environment over time – If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species © 2011 Pearson Education, Inc.

Darwin saw organisms adapted to their environment (a) Cactus-eater (b) Insect-eater (c) Seed-eater

Evolution is supported by an overwhelming amount of scientific evidence Evidence continues to fill the gaps identified by Darwin in The Origin of Species 1. Direct observations 2. Homology 3. Embryology 4. Vestigial Organs 5. Fossil record 6. Biogeography 7. Biochemistry (later chapters) © 2011 Pearson Education, Inc.

Evidence 1: Direct Observations of Evolutionary Change Two examples provide evidence for natural selection: natural selection in response to introduced plant species, and the evolution of drug-resistant bacteria © 2011 Pearson Education, Inc.

Can a change in a population’s food source result in evolution by natural selection? On native species, southern Florida Museum-specimen average On introduced species, central Florida Number of individuals Beak Beak length (mm) RESULTS 9

1 2,750,000 2,500,000 2,250,000 2,000,000 1,750,000 1,500,000 1,250,000 1,000, , , ,000 base pairs Chromosome map of S. aureus clone USA300 Key to adaptations Methicillin resistance Ability to colonize hosts Increased disease severity Increased gene exchange (within species) and toxin production Figure MRSA plasmid DNA

Evidence type 2: Homology Homology is similarity resulting from common ancestry Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor © 2011 Pearson Education, Inc.

Figure Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat

Figure Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo Evidence 3: Comparative embryology reveals anatomical homologies not visible in adult organisms

Comparing physical structure might bring erroneous conclusions regarding relationships Convergent evolution is the evolution of similar, or analogous, features in distantly related groups Analogous traits arise when groups independently adapt to similar environments in similar ways © 2011 Pearson Education, Inc.

Evidence 4: Vestigial structures are remnants of features that served important functions in the organism’s ancestors. – Human vestigial structures-ex. coccyx – Examples of homologies at the molecular level are genes shared among organisms inherited from a common ancestor © 2011 Pearson Education, Inc.

Evidence 5: The Fossil Record The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time Fossils can document important transitions – For example, the transition from land to sea in the ancestors of cetaceans © 2011 Pearson Education, Inc.

Evidence 6. Biogeography Biogeography, the geographic distribution of species, provides evidence of evolution Earth’s continents were formerly united in a single large continent called Pangaea, but have since separated by continental drift An understanding of continent movement and modern distribution of species allows us to predict when and where different groups evolved © 2011 Pearson Education, Inc.

(23) Evolution of Populations-Microevolution Natural selection acts on individuals, but only populations evolve. Consider, for example, a population of medium ground finches on Daphne Major Island – During a drought, large-beaked birds were more likely to crack large seeds and survive – The finch population evolved by natural selection © 2011 Pearson Education, Inc.

Figure 23.1

Figure (similar to the prior 3 years) 1978 (after drought) Average beak depth (mm)

Microevolution is a change in allele frequencies in a population over generations Three mechanisms cause allele frequency change: – Natural selection-individuals with beneficial heritable traits survive to reproduce – Genetic drift-chance events that alter gene frequencies – Gene flow-transfer of alleles between populations Only natural selection causes adaptive evolution © 2011 Pearson Education, Inc.

We have seen evidence for evolution now how do populations (not individuals) actually evolve? 1. Mutations 2. Sexual reproduction 3. Natural selection 4. Genetic drift 5. Need gene flow