Evolution and Natural Selection

Evolutionary theory Originally described by Charles Darwin. On The Origin of Species, 1859 There are slight variations in traits of a species. These traits are inheritable. Some of these traits increase an organism’s chances of survival and reproduction. Those individuals who survive and reproduce pass along their genetic material (“survival of the fittest”).

Evolutionary Theory The offspring are more likely to have the variation of the trait that allowed better survival & reproduction. Natural selection—individuals with beneficial traits are more likely to survive and pass on these traits With each generation, there are slight modifications in traits. Over enough time, modifications accumulate so that eventually the population is very different from the ancestral organisms.

Evolutionary theory Darwin theorized a gradual, continual change Not supported by fossil record More recent scientists have suggested “punctuated equilibrium”: long periods of no or minor change, followed by short periods of dramatic change Time Amount of Change Time Amount of Change GradualismPunctuated Equilibrium

Evolutionary theory Origin of life 4.4 billion years ago Began with simple chemicals, gradually became more complex through bonding Eventually these formed amino acids As polypeptides formed, eventually became simple life Primitive organisms (cells) likely in ancient oceans

Evolutionary theory Natural selection resulted in more complex single-celled organisms Benefit to working together These eventually evolved into multi- celled organisms Organisms changed based on evolutionary pressures

Evolutionary theory Organisms starting moving onto land Those who had traits that increased their survival on land evolved Organisms continued to evolve and modify based on environmental pressures Most “fit” of each generation would survive Over time the changes led to completely new organisms

Evolutionary theory—animals Single-celled organsisms Simple multi- celled organisms Simple germ (tissue) layers, radial symmetry Complex germ layers, bilateral symmetry False body cavity True body cavity Body segments, exoskeleton Spinal cordEndoskeleton

Evolutionary theory—animals Single-celled organsisms Sponges Cnidarians (jellyfish) Worms, mollusks, arthropods Echinoderms (starfish) LampreysCartilaginous fishBony fishLungfishAmphibiansReptilesMammals, birds

Evidence—fossils Older fossils show fewer variety of organisms Older fossils show more primitive features Newer fossils show changes and progression among characteristics New characteristics appear in newer fossils The complexity of organisms increases when looking at newer versus older fossils

Evidence—fossils Transitional forms “Missing links” Archaeopteryx Eustheopteron Seymouria

Evidence—fossils Problems? Radiocarbon dating may not be accurate Inaccuracies have been calculated (dating volcanic rock known to be 200 years old as billions of years old) Some transitional forms questioned Archaeopteryx likely an extinct species and not a bird ancestor Fewer than expected transitional forms Living, unevolved “fossils” Coelacanth— “extinct” 80 million years ago, rediscovered 1938

Evidence—origin of life Several experiments have created simple organic molecules under “primitive Earth” conditions “Building blocks” for life Fossilized bacteria Living multicellular colonies Portuguese Man o’ War

Evidence—origin of life Problems? Dispute and debate over conditions of primitive Earth No proven mechanism for evolving from simple compounds to primitive cells Spontaneous generation? “Life” from “unlife” Many theories, often conflicting Really an unanswered question

Evidence—anatomy Homologous structures—similarities in body parts between groups. Morphological divergence—Variations in structures of different species based on a basic form in a common ancestor

Evidence--anatomy Problems? Different genes can produce homologous structures Body segments in fruit flies and wasps The same gene can produce non- homologous structures

Evidence—biogeographical Similar species in different parts of the world Rheas, emus, ostriches Common ancestor, separated because of plate tectonics (movement of sections of the earth’s crust)

Evidence—biogeographical Rhea—South America Emu—Australia Ostrich—Africa

Evidence—mutations Theory—mutations of DNA (insertion, deletion, inversion, translocation, duplication, etc.) can result in new traits or features. These are random events. If these new features give the organism a survival advantage, they are more likely to be passed along.

Evidence—mutations Mutations do happen (well established) Mutations can be beneficial Bacterial resistance to antibiotics Sickle cell anemia giving resistance to malaria Resistance to atherosclerosis in Italian village

Evidence—mutations Problems? Virtually all mutations are harmful or neutral Many new traits created in lab are not seen in the wild (fruit flies) New structures do not mean benefit Second pair of fruit fly wings lack muscles and harm flight ability Truly beneficial mutations only found in bacteria & other single-celled organisms Beneficial “mutations” often can be argued to be recessive traits that already exist Mutations really beneficial? Sickle-cell anemia

Evidence—DNA Similar DNA sequences in many species The more closely they appear to be related, the more DNA is shared Humans & bananas: 50-60% Humans & worms: 75% Humans & chimpanzees: 98% The less DNA in common, the more distant the common ancestor

Evidence—DNA & proteins Problems? Small differences in DNA can mean big differences in appearance & function Similarities could result from similar actions & functions, in the same way that sports cars share similarities with each other, but not with SUVs

Evidence—natural selection Populations evolve, not organisms Phenotypic variation Morphological—physical features Physiological—metabolic activities and products Behavioral—responses to situations and stimuli Gene pool—possible trait variations within a population

Evidence—natural selection Mutation changes or creates new alleles Other factors shuffle existing alleles Crossing over (Meiosis I) Homologous chromosome arrangement (Meiosis I) Fertilization Some alleles have greater frequencies in the population than others Red hair Albinism Allele frequencies can change over time

Evidence—natural selection Natural selection— “Survival of the fittest”. Some traits allow individuals to survive or reproduce better than others. These traits therefore increase in a population. Three types Directional Stabilizing Disruptive

Evidence—natural selection Directional selection— Natural selection “favors” a phenotype, increasing the frequency of this allele Peppered Moth Two variant phenotypes Pre-industrial, light were more common After industrial pollution, dark were more common

Evidence—natural selection Stabilizing selection—Intermediate forms of a trait are favored, extreme forms are not Human birth weight Very large or very small babies less likely to survive compared to average-sized

Evidence—natural selection Disruptive selection—Extreme forms of variation are favored, intermediate forms selected against “Darwin’s” finches Speculated to be derived from common ancestor Different beaks adapted for different foods

Evidence—natural selection Natural selection (“microevolution”) is readily accepted, even by critics “Classic” examples not unchallenged Questions of validity of peppered moth studies Galapagos finches show variations in bills, but return to “normal”

Verdict? Scientists overwhelmingly support evolution Much is still not understood about the processes involved Many intelligent, non-religious people have problems with aspects of evolutionary theory Darwin’s theories have always been challenged Much evidence for AND against evolution Decisions need to be made on facts and science

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