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Lecture 18

Lecture 18 - The Origin of Evolution and Species

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Esteban Parra

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ANT203Y5 – BiologicalAnthropology Lecture 1 – January 7, 2014 The Origin and Evolution of Species Microevolution vs. Macroevoltuion - In the previous semester, we focused on micoevolutionary processes: o Mutation, genetic drift, gene flow and natural selection, and how these evolutionary factors drive short term changes in gene frequencies. o The focus was on variation in populations (in particular human populations). - During this semester, we will focus on evolutionary change over long time periods: Macroevolution. Macroevolution - The word Macroevolution was coined by T. Dobzhansky, referring to evolution at levels higher than the population (species or higher). o Change of species through time - Macroevolution refers to evolutionary change over long periods of time. - Two key questions to understand macroevolution are: o What is a species? o How do new species arise? The Biological Concept of Species - There are numerous definitions of “species”. We will use here the biological species concept, which states that: - “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups” - Ernst Mayr o Individuals from other populations that cannot breed, they are considered to be from different species (not part of slides) o Species that can breed and produce fertile offspring, are considered to be the same species (not part of slides) - Note that for different populations to belong to the same species, they must o Be capable of interbreeding under natural conditions, and o Be capable of producing fertile offspring - Problem: This concept of species can be applied (with some difficulties) to organisms with sexual reproduction but not to organisms with asexual reproduction. - Example: Horse and Donkey o If they produce a mule; they cannot form viable offspring o By definition, donkey and horses cannot be classified as the same species Patterns of Macroevolution - Macroevolution deals with evolution of species over time. This can happen in two different ways o Anagenesis: Over time, a species can change in a linear fashion, from one form to a different form.  Change of species over time; more or less in a linear form o Cladogenesis: One or more new species branch off an original species. Cladogenesis involves the formation of new species.  One or more species are going to branch from an original species The Tempo and Mode of Macrovolution ANT203Y5 – BiologicalAnthropology Lecture 1 – January 7, 2014 - Gradualism o Macroevolution as a slow and gradual process (Darwin) o Natural selection primarily responsible for speciation o Fossil record expected to show smooth species transitions o Problem: fossil record isn’t complete, and this happens to any organisms (including human lineage). - Punctuated equilibrium (Elredge and Gould) o Long periods of stasis, punctuated by rapid change  Period of where nothing is changes, and then a burst of rapid change (speciation)  This has more to do with the idea of cladogenesis o Rapid speciation, happening at the edges of species range. o Larger role for mutation and drift in small populations o Predicts long periods of little evolutionary change, with rapid bursts of species diversity o Fewer “transitional” forms in the fossil record (which is a dilemma) - Both models are not mutually exclusive! o Some organisms can fit well with the gradualism model (can see the transition through time) o You can see some example of stratigraphic records, where you can see slow evolution Adaptive Radiation - Adaptive radiation: formation of many new species following the availability of new environments or the development of a new adaptation. o Many adaptive radiations are evident in the fossil record, including the primate fossil record. Speciation - How do new species form? - For speciation to occur, a number of factors need to be in place. o Reduction or elimination of gene flow between populations. Commonly, this happens through geographic isolation, although other factors may potentially be involved as well (behavioral isolation).  You need to have enough differentiation so that eventually members of the different populations do more mate. And if they do, their zygote will not be viable. o Genetic divergence. Speciation only happens when, in addition to restricted gene flow, other evolutionary forces (mutation, genetic drift, natural selection) act to increase the genetic differences between populations.  Populations can evolve in different, not only because of natural selection. Genetic drift, in a small population, can cause dramatic changes which can increase genetic differentiation o Reproductive isolation. The end result of restricted gene flow and increased genetic differences will be that populations will become new species. The populations will no longer
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