Mechanisms of Evolution Darwinian Evolution. Theory of Natural Selection. Descent with modification by means of natural selection

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1 Mechanisms of Evolution Darwinian Evolution Descent with modification by means of natural selection All life has descended from a common ancestor The mechanism of modification is natural selection Theory of Natural Selection Darwin s theory of natural selection was formed by the early 1840 s He was reclusive and in poor health, but he was very well known as a naturalist from the specimens, fossils, notes and letters he sent back to Britain during his Beagle voyage He continued to correspond and meet with Lyell, Henslow and other scientists In 1844, he wrote a long essay on the origin of species and natural selection He was reluctant to introduce his theory publicly, anticipating the uproar it would cause and wishing to gather more evidence Evolutionary thought was emerging at the time and Lyell encouraged him to publish 1

2 In June 1858, he received a manuscript from Alfred Russell Wallace Wallace had developed a theory of natural selection similar to Darwin s Alfred Russel Wallace ( ) Wallace asked Darwin to evaluate the manuscript and forward it to Lyell for publication Lyell presented Wallace s manuscript and excerpts of Darwin s 1844 essay to the Linnean Society of London on July 1, 1858 Darwin quickly finished The Origin of Species and published it the following year Darwin is considered the main author because he developed and supported natural selection much more extensively. The book and its supporters quickly convinced the majority of biologists that biodiversity is a product of evolution. The Origin of Species Darwin developed two main ideas Evolution explains life s unity and diversity Natural selection is a cause of adaptive evolution 2

3 Millions of years ago Moeritherium Barytherium Platybelodon Deinotherium Mammut Stegodon Mammuthus Years ago Millions of years ago Moeritherium Barytherium Platybelodon Deinotherium Mammut Stegodon Mammuthus Years ago Descent with Modification The phrase descent with modification Summarized Darwin s perception of the unity of life States that all organisms are related through descent from an ancestral population that lived in the remote past Adaptations (modifications) accumulated over millions of years as descendants moved into various habitats Darwin viewed, the history of life as a tree With multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms Sirenia Hyracoidea (Manatees (Hyraxes) and relatives) Elephas Loxodonta Loxodonta maximus africana cyclotis (Asia) (Africa) (Africa) Figure 22.7 At each fork is an ancestral population common to all lines of descent branching from that fork Species that are very similar share a more recent common ancestor Less closely related species share a more ancient common ancestor Sirenia Hyracoidea (Manatees (Hyraxes) and relatives) Elephas Loxodonta Loxodonta maximus africana cyclotis (Asia) (Africa) (Africa) Figure

4 Darwin s Tree of Life Linneaus Taxonomic Scheme To Darwin, Linneaus taxonomic scheme reflected the branching genealogy of the tree of life He recognized that the diversity of species can be ordered into groups subordinate to groups, with organisms at the various taxonomic levels related through descent from common ancestors Classification hierarchy Phylogenetic tree Species Panthera pardus Family Order Panthera Genus Felidae Carnivora Mammalia Class Chordata Phylum Animalia Kingdom Eukarya Domain Order Family Genus Species Panthera pardus (leopard) Mephitis mephitis (striped skunk) Lutra lutra Canis (European familiaris otter) (domestic dog) Panthera Mephitis Lutra Canis Felidae Mustelidae Canidae Carnivora Canis lupus (wolf) Lines of Evidence Often Agree Classification alone does not confirm common descent, but relationships are often validated by other lines of evidence Example: Genetic analyses of species thought to be closely related based on anatomical features or other criteria often reveal a common hereditary background 4

5 Natural Selection Darwin focused on the role of natural selection in adaptation Natural Selection and Adaptation Evolutionary biologist Ernst Mayr Has dissected the logic of Darwin s theory into three inferences based on five observations Ernst Mayr ( ) a leading evolutionary biologist of the 20th century, renowned taxonomist, tropical explorer, ornithologist, science historian, naturalist, professor emeritus (Harvard). He contributed to the conceptual revolution that led to the modern synthesis and the development of the biological species concept. Observation #1: For any species, population sizes would increase exponentially If all individuals that are born reproduced successfully Figure 22.8 maple 5

6 Observation #2: Nonetheless, populations tend to be stable in size Except for seasonal fluctuations Observation #3: Resources are limited Inference #1: Production of more individuals than the environment can support Leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving each generation Observation #4: Members of a population vary extensively in their characteristics No two individuals are exactly alike Figure 22.9 ladybugs Observation #5: Much of this variation is heritable Inference #2: Survival depends in part on inherited traits Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment have higher fitness and are likely to leave more offspring than other individuals 6

7 Inference #3: This unequal ability of individuals to survive and reproduce Will lead to a gradual change in a population, with favorable characteristics accumulating over generations Stephen J. Gould s Summary Fact 1: Overproduction and struggle for existence Fact 2: Individual variation The inescapable conclusion: Unequal reproductive success It is this unequal reproductive success that Darwin called natural selection The product of natural selection is adaptation Natural Selection Natural selection involves a struggle for existence Darwin was already aware that overproduction causes a struggle for existence. He read an essay on human suffering by Reverend Thomas Malthus (1798) Malthus held that much human suffering was a consequence of the population growing faster than the food supply The capacity for overproduction is common to all species and only a fraction of each generation survives and reproduces 7

8 Natural Selection Occurs from the interaction between the environment and the inherent variability in a population Is NOT chance. Variations arise by chance, but the environment sets definite criteria for reproductive success. Some variations are favored over others. Artificial Selection Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits Terminal bud Lateral buds Cabbage Brussels sprouts Flower cluster Leaves Cauliflower Kale Flower Stem and stems Broccoli Wild mustard Kohlrabi Figure Artificial Selection 8

9 Darwin reasoned that if such changes could be achieved by artificial selection in relatively short time periods, then natural selection should be capable of considerable modifications over hundreds of thousands of generations Even if the advantage of some heritable trait is slight, it will accumulate in the population after many generations of natural selection Natural Selection Gradualism is Fundamental to Darwinism According to Darwin, life did not evolve by sudden quantum leaps, but rather by a gradual accumulation of small changes Natural selection operating in differing contexts over vast spans of time account for the diversity of life 9

10 Some Subtleties of Natural Selection A Population The Unit of Evolution A population is the smallest unit that can evolve. Population = a group of interbreeding individuals of one particular species living together in space and time. Natural selection is a consequence of the interactions between individual organisms and their environment, but individuals do NOT evolve Evolution can only be measured as a change in relative proportions of heritable variations in a population over several generations Some Subtleties of Natural Selection Natural Selection does not create variation Natural selection can only amplify or diminish existing heritable variations. Organisms may adapt to changes in the environment and may be modified by life s experiences, but these acquired characteristics are not heritable We must distinguish between adaptations that are acquired during the lifetime and those that are the result of inheritance Some Subtleties of Natural Selection Natural Selection is Situational The environment varies from area to area and time to time An adaptation that is advantageous under one set of conditions may be useless or even detrimental under other conditions 10

11 Summarizing Natural Selection Natural selection is differential success in reproduction It results from the interaction between individuals that vary in heritable traits and their environment Over time, natural selection can produce an increase in the adaptation of organisms to their environment (a) A flower mantid in Malaysia (b) A stick mantid in Africa Figure Summarizing Darwinian Evolution The diverse forms of life have arisen by descent with modification from ancestral species The mechanism of modification has been natural selection working gradually over long periods of time 11