e.g. population: 500, two alleles: Red (R) and White (r). Total: 1000 genes for flower color in the population

Save this PDF as:

Size: px
Start display at page:

Download "e.g. population: 500, two alleles: Red (R) and White (r). Total: 1000 genes for flower color in the population"


1 The Evolution of Populations What is Evolution? A change over time in the genetic composition of a population Human evolution The gene pool Is the total aggregate of genes for a particular trait in a population Consists of all genes for that traits in all individuals of the population e.g. population: 500, two alleles: Red (R) and White (r). Total: 1000 genes for flower color in the population Example: alleles frequencies over time Each allele has a given frequency (proportion) in the population. Genotypes R 320* = 800 = 0.8 r 20* = 200 = 0.2 Alleles frequencies TODAY 20 rr 320 RR 160 Rr Alleles frequencies 50 YEARS FROM NOW R = 0.6 r = 0.4 Voila! (Micro) EVOLUTION The Evolution of Populations What is a Population? A group of individuals of the same species living in a given environment peacocks female male Species: Organisms that share a more or less distinctive form and are capable of breeding and having fertile offspring Charles Darwin (still evolving) Evolution through natural selection theory: 1859 What is the time scale we must consider to see evolutionary changes? Many and Many generations in general, million of years Microorganisms and viruses can have a rate of evolution visible in a few generations Why? short generation spans and sometimes also a high mutation rate 1

2 Before Darwin s ideas 1) The idea of fixed species (Aristotle) 2) Lamarck and his theory of Inheritance of Acquired Characteristics Animals develop the characteristics according to the environmental needs (neck elongates with the appearance of taller trees) These characteristics can be passed to the next generation Not very convincing, right? Evolution Is a Change in genes frequencies in a population A+a = = 1 That leads to changes in the expressed characteristics we observe. These changes happen due mainly to Darwinian Fitness Refers to reproductive success of each individual within the population. The fitness is determined by how well the characteristics of the individual match the demands of the environment in which it lives Natural Selection Is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce. The environment is the selective agent 2

3 Modes of Natural Selection Original population Evolved Original population population Phenotypes (fur color) (c) Stabilizing selection (a) Directional selection Shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection Favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. Removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Theory of Evolution by Natural selection (Darwin): 1) Genetic variation: Individuals within a species differ from each other 2) Inheritance: Offspring are similar to their parents 3) Excess of reproduction: More offspring are generally produced than those to survive to maturity. 4)Differential survival: Somme variations are more likely to survive tan others Darwin s Cultural and Scientific Context The Origin of Species challenged the notion that the Earth was: Relatively young (Bible: Earth is only be 6,000 years old) Populated by unrelated species (species were fixed, did not change) 3

4 Sexual Selection Ch 4 Was Darwin s ingenious idea that allowed explaining why we see obvious gender differences in many animal species Traits can get very exaggerated as long as they denote better fitness Peacocks In evolutionary terms, fitness is used to indicate reproductive success Elephant Seals EVIDENCE OF EVOLUTION Biological evolution leaves observable signs. Five of the many lines of evidence in support of evolution The fossil record Biogeography Comparative anatomy Comparative embryology Molecular biology Is the ordered sequence of fossils as they appear in rock layers Paleontologists have discovered many transitional forms that link past and present 4

5 Five of the many lines of evidence in support of evolution The fossil record Biogeography Comparative anatomy Comparative embryology Molecular biology Is the comparison of body structure between different species Confirms that evolution is a remodeling process Homology Is the similarity in structures due to common ancestry Homologous structures have same ancestry, but have different function at present and are morphologically (form) different as well is the study of the geographic distribution of species that first suggested to Darwin that today s organisms evolved from ancestral forms One example is the distribution of marsupial mammals in Australia Two or more organisms evolved from one original design, which leads to leads to divergent evolution Each organism adapted to different ecosystems example: forearms in different mammals Adaptive radiation: Is a case of divergent evolution characterized by a rapid increase in the number of closely related species. e.g. the colonization of a new environment (e.g. first reptiles that colonized the land) 5

6 Analogy Morphologic similarity and/or function but different origin, the opposite to homologous! Involves the development of similar characteristics in organisms of widely different origins leads to Convergent evolution Unrelated species become more and more similar in appearance as they adapt to the same kind of environment Euphorbia, African desert Cactus, American desert Absence of leaves to prevent water loss Cactus, South America desert, Andes mountains Spiny structures. Some of these are modified twigs and others modified leaves (different origin). These discourage herbivores from eating them Swallows, some bats, and dragonflies evolved from different ancestor to survive by hunting insects while flying. All of them have Wings Good eyesight or hearing Great agility and speed in flight Five of the many lines of evidence in support of evolution The fossil record Biogeography Comparative anatomy Comparative embryology Molecular biology Early stages of development in different animal species reveal additional homologous relationships The hereditary background of an organism is documented in: Its DNA The proteins encoded by the DNA Evolutionary relationships among species can be determined by comparing: Genes Proteins of different organisms For example, pharyngeal pouches appear on the side of the embryo s throat, which: Develop into gill structures in fish Form parts of the ear and throat in humans 6

7 Early 1900 s: Population genetics Reconciled Darwin s and Mendel s ideas Modern synthesis (1940 s) 1. Populations + Genetics = Darwin s + Mendel s ideas 2. Populations are the units of evolution 3. Natural Selection is the selective agent 4. Gradualism is the way populations evolve Main postulates Gradualism: Charles Darwin s view of evolution. Species slowly accumulates changes and finally become a different descendant species Other factors that contribute to Evolution Changes in the genetic composition of a population Genetic drift: The bottleneck effect Takes place more often when the population size is small, so it can produce huge fluctuations in the genes frequencies. The population is severely reduced in number due to chance a natural disaster, etc. Genetic diversity is also reduced, as mating among the remaining individuals will not replace the lost genes Parent population 50 % 50 % Bottleneck: drastic reduction caused for example by a change in the environmental conditions The population recovered after bottleneck 25 % 75 % Survivors 7

8 Gene flow = Migration Genetic exchange due to migration of fertile individuals or gametes between populations MAP AREA Non random mating (Sexual selection) Only those with the best genes get to reproduce! Which eventually leads to some traits becoming more common than others Fairbanks Fortymile herd range Whitehorse example: birds of paradise example: the two caribou populations in Alaska share the same area during part of the year, but they rarely breed with members of the other population 8