Evolution

Evolution = a change in gene frequencies in populations over time Evolution explains origin of species, diversity of organisms and their relationships, similarities and differences among species, adaptation to the environment

CHARLES DARWIN AND THE ORIGIN OF SPECIES Charles Darwin’s On the Origin of Species by Means of Natural Selection was published in 1859 co-presented with Alfred Wallace Darwin’s 2 main points: All organisms descended from ancestral species Natural selection was the mechanism for change

Darwin’s Theory of Natural Selection Darwin based his theory of natural selection on two key observations: Overproduction All species tend to produce excessive numbers This leads to a struggle for existence Individual variation Variation exists among individuals in a population Much of this variation is heritable

“Survival of the Fittest” Inference: Natural Selection leads to differential reproductive success Individuals best suited for the local environment leave more fertile offspring Evolution is the accumulation of more favorable variants over time Natural Selection operates at the level of the individual, NOT for the good of the species

Natural Selection in Action Examples of natural selection include Pesticide resistance in insects The development of antibiotic-resistant bacteria Kettlewell’s Peppered Moths Figure 13.1 Insecticide application Chromosome with gene conferring resistance to insecticide Survivors Additional applications of the same insecticide will be less effective, and the frequency of resistant insects in the population will grow

Kettlewell’s Peppered Moths Kettlewell studied the peppered moth (Biston betularia) from insect collections in England. Birds ate moths that were easiest to find. Selection for Light colored moths in non-polluted areas Selection for Black moths in polluted areas

Three General Outcomes of Natural Selection Directional Selection Selects in favor of one or the other extreme Diversifying Selection Selects in favor of two extreme types Stabilizing Selection Selects in favor of intermediate type 1) Directional Selection 2) Diversifying Selection 3) Stabilizing Selection Selects in favor of one or the other extreme Selects in favor of two extreme types Selects in favor of intermediate type

Patterns of Evolution Convergent Evolution evolutionary change in two or more unrelated organisms that results in the independent development of similar adaptations to similar environmental conditions Example: anteaters found in Australia, Africa, North and South America

Cactus-flower -eaters South American mainland Patterns of Evolution Divergent Evolution occurs when a single group of organisms splits into two or more groups and each group evolves in increasingly different directions Example: Darwin’s Galapagos Finches Medium ground finch Cactus Small tree finch Woodpecker Large Large cactus ground finch Vegetarian Mangrove Green warbler Gray Sharp-beaked Seed-eaters Cactus-flower -eaters Bud-eater Insect-eaters Ground finches Tree finches Warbler finches Common ancestor from South American mainland Figure 1.13

EVIDENCE FOR EVOLUTION Common ancestry and descent with modification are found by studying: The fossil record organisms appear in a historical sequence Biogeography the geographic distribution of species Comparative anatomy comparison of structures in different species Comparative embryology comparison of structures that appear during development Molecular biology comparison of genes and proteins (a) Chick embryo Gill pouches Post-anal tail (b) Human embryo (right) Comparative embryology of vertebrates supports evolutionary theory

Homologous structures Homologous (also called “divergent”) structures have a similar anatomy due to common ancestry Same form (function may or may not be the same) Ex: forelimbs of vertebrates Human write Cat Walk Whale Swim Bat Fly

Analogous structures analogous (also called "convergent") structures have evolved independently Serve the same function in different species Ex: wings of butterfly and bird ButterflyFly Bird Fly

The Origin of Species The biological species concept defines species as “groups of interbreeding natural populations that are reproductively isolated from other such groups” (a) Similarity between different species (b) Diversity within one species Figure 14.4

Mechanisms of Speciation The two modes of speciation are Allopatric speciation “other country” Sympatric speciation “same country” Allopatric speciation (b) Sympatric speciation Figure 14.8

Allopatric Speciation Members of a population become physically separated The separated populations diverge, through changes in mating tactics or use of their habitat They become reproductively separated such that they cannot interbreed and exchange genes

Viable, fertile offspring Sympatric Speciation a species splits into two due to reproductive isolation without any geographic separation Reproductive isolation may be Pre-zygotic Post-zygotic Figure 14.5 Individuals of different species Pre-zygotic barriers Temporal isolation: Mating or flowering occurs at different seasons or times of day Habitat isolation: Populations live in different habitats and do not meet Behavioral Isolation: Little or no sexual attraction between males and females Mating Mechanical isolation: Structural differences in genitalia or flowers prevent copulation or pollen transfer Gametic isolation: Female and male gametes fail to unite in fertilization Fertilization (zygote forms) Post-zygotic Hybrid inviability: Hybrid zygotes fail to develop or fail to reach sexual maturity Hybrid sterility: Hybrids fail to produce functional gametes Viable, fertile offspring

Pre-zygotic barriers Pre-zygotic barriers impede mating between species or hinder fertilization of eggs Temporal isolation Habitat isolation Mechanical isolation Gametic isolation Behavioral isolation

Post-zygotic barriers Post-zygotic barriers are backup mechanisms that operate should interspecies mating actually occur and produce hybrid zygotes Hybrid inviability Hybrid sterility Horse Mule (hybrid) Donkey Figure 14.7

Study Objectives Define evolution and describe Charles Darwin’s contribution to the current Theory of Evolution. Define natural selection and describe how natural selection works to change a population. Describe some examples of natural selection, including Kettlewell’s study with the black and peppered moths. Contrast directional, diversifying, and stabilizing selection. Define convergent evolution and divergent evolution and give an example of each. Describe five main lines of evidence in support of evolution. Contrast homologous and analogous structures. Define species. Define speciation and describe two mechanisms by which speciation can occur (allopatric, sympatric). Distinguish between pre-zygotic and post-zygotic barriers and describe ways in which sympatric populations can be reproductively isolated.