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Chapter 3

ANTB14 CH3.odt

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Michael Schillaci

CHAPTER 3: MACROEVOLUTIONAND EVOLUTIONARYANTHROOLOGY -typological: the study or systematic classification of types that have characteristics or traits in common -however, individuals that vary morphologically can be members of the same species so in response Darwin's idea of common descent, the definition of typological species was modified as follows: individuals are the same species if they can successfully produce fertile offspring >however, modified concept no longer favoured for for 3 reasons: 1) it fails to account for geographic variations w/in species 2) strict application of concept can lead to the erroneous conclusion that males of the same species, or conspecific (members of the same species) males, represent separate species. B/c two male mammals cannot sexually reproduce, then a typological definition of species would imply that they represent different species 3) this concept cannot account for instances in which distinct species mate and produce a fertile hybrid (offspring from mating plants or animals from different species, varieties, or genotypes There are many definitions for species however for evolutionary purposes, scientists stick to 2 relevant modern definitions: biological & phylogenetic Biological Species Concept (BSC): -researchers of this field note that any one feature rarely defines a species: diff species share features bc of common descent -a species represents a population of inds that varies from other species in a variety of ways & they exchange genes by interbreeding -biological features prevent one species from breeding w another species Ernst Mayr: biological species are defined as actually or potentially interbreeding populations, which are reproductively isolated from other such groups -a strict application of the BSC concept could lead a researcher to spend considerable time and effort trying to get two potentially different species to interbreed which can be dangerous depending on animals size so instead they opt to collecting and analyzing as much morphological, behavioural, and ecological data as they can on a potential new species >assumption: conservative application of these data provides deductive proof of reproductive isolation -this concept cannot be applied to fossilized species therefore an alternative definition is needed -concept is applied particularly to paleoanthropologists -phylogenetic species concept: a species is the smallest aggregation of (sexual) populatoins diagnosable by a unique combination of character states: “alternative expressions of a character. For example, each character is described in terms of its states, such as 'hair present' or 'hair absent', where hair is the character and present/absent are its states >these data are then analyzed using cladistics, which is the study of evolutionary relationships w/in and b/w organisms on our planet. The book's definition is “a system of biological taxonomy based on the quantitative analysis of comparative data that is used to reconstructive the (assumed) phylogenetic relationships and evolutionary history of groups of organisms >cladistics came abt in response to weakness of phenetics: the classification of organisms based on their overall phenotypic similarities, regardless of their evolutionary relationships -PSC can be applied to both fossilized organisms such as extinct humans and to rare, extant primates The most fundamental issue in speciation research is determining how diff populations can be formed w/o gradually evolving forms that are intermediate in shape and appearance from the original species -a speciation form -allopatric speciation: some form of physical barrier (i.e., body of water, mountain range, or an expansive unsuitable habitat) causes geographic isolation and reduced gene flow b/w populations. The barrier only has to limit gene flow; complete isolation is not necessary *remember, gene flow is the movement of genes between populations which increases w/in population genetic variation and eventually reduced b/w population genetic variation >isolation refers to reduced movement of individuals b/w populations rather than geographic distance between populations -there are two models of allopatric speciation: 1) vicariant speciation: vicariant speciation occurs when a physical barrier creates large, geographically separated populations which then (with time) diverge from each other and can no longer interbreed 2) peripatric speciation: occurs when a small, peripherally isolated colony of the main population diverges to become a new species >the key difference bw the two are the population size after the split (large [vicariant] and small [peripheral]) -all life forms on Earth are united by evolutionary history, which means that each organism is related in some way to all other organisms creating a vast tree of life -cladogram (also known as phylogeny OR evolutionary tree) is a branching diagram used to illustrate phylogenetic relationships -cladistics comprises three main assumptions about the relationships among organisms 1) there are changes in characteristics, which are the genetic or physical features of the organisms, w/in lineages over time –changes w/in lineages provide info abt evolutionary relationships 2) all organisms are descended from a common ancestor (at some level, which creates a vast tree of life) 3) when a lineage splits, it divides into exactly two groups >last assumption is a real issue b/c there are cases where one species evolved rapidly in terms of geologic time frames into multiple species. For example, the original finch that arrived on the Galapagos Islands evolved into multiple lineages Data use to create cladistic trees -morphological and molecular data needed—called total evidence -most phylogenetic analyses of fossils are based primarily on teeth and a few cranial features -biological evolution can be tricky in it that it can produce a homoplastic character, a trait shared b/w differing taxa but not because pf inheritance from a common ancestor or as prof explains, individual adaption to similar environments such as wings in birds and bats this is because they are adapted to environments that requires flying >due to this, phylogenetic analyses needs to be viewed as hypothetical and not factual Reading and Interpreting a Cladistic Tree -two commonly used cladograms are diagonal and rectangular cladograms >each describes hypothetical relationships -the above illustration depicts relationships among 6 hypothetical taxa (ABCDEF) and each letter refers to a species -each upper case letter is referred to as a terminal node: the start point (internal) or end point (terminal) of a line segment in a cladogram -each terminal node is connected to another by a series of branches or lines that join at internal nodes -each internal node represents a speciation event (lineage splitting) that resulted in the evoluation of a descendent sister group—in essence, it represents a common ancestor -researchers specifically interested in tree topology which is the branching patterns of line
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