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Chapter 23

Chapter 23

Biological Sciences
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Chapter 23 Species and Their Formation
Sex stimulates speciation (among other things)
-Charles Darwin said sexual selection was the cause for evolution conspicuous trait in species
-ex. larger antler of male deer
-enlarged tail feathers of male peacock
-traits are exaggerated in species in which individuals of one sex (usually male) compete to mate
with opposite sex
-exaggerated trait evolves in competition among males for access to female/ attracting
discriminating females
-sexual selection can also increase rate of new species form
-comparison of the number of species found in sister clades (same ancestor) suggests
development of new species from sexual selection
-in promiscuous mating system, male bird evolved bright plumage and ornament (long tail
feathers), female raises its child with no help from male
-in monogamous species, members form pair bond and share responsibilities of raising the young
-individuals of both sexes have dull plumage and look alike
-sexual selection stimulates divergence of a lineage into many species
-random mutation results in different physical characteristics
23.1 What Are Species?
We can recognize and identify many species by their appearance
-Linnaeus developed morphological species concept
-classification of species on the basis of their appearance
Species form over time
-speciation is process by which one species splits into two or more daughter species, which
evolve as distinct lineages
-process is gradual
-when the two populations at various stages become new species, it is impossible to decide
whether individual belongs species 1 or 2
-reproductive isolation is when individuals of a population mate with one another, but not
members of the other population
-constitute distinct group in which genes recombine
-become independent evolutionary unit, separate branches on the tree of life
-Mary’s definition of species, biological species concept is:
Species are groups of actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups.”
-actually means individuals live in the same area and interbreed with one another
-potentially means although individuals do not live in the same area (cannot interbreed), but they
would interbreed if they get together
-does not apply to organisms that reproduce asexually
-reproductive isolation is one element to identify species
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23.2 How Do New Species Arise?
-not all evolutionary changes result in new species
-lineage may change over time without giving rise to new species
-speciation takes place when one species splits into 2 or more daughter species
-in isolated gene pool, allele and genotype frequencies change due to action of evolutionary
-if 2 populations are isolated from each other and there are sufficient differences in their genetic
structure accumulate in isolation, members of 2 populations cannot exchange genes when they
come together
-speciation requires gene flow in the population whose members formerly exchanged genes be
Allopatric speciation requires almost complete genetic isolation
-allopatric speciation is when population is divided by a physical barrier
-also known as geographic speciation
-dominant mode of speciation in most groups of organism
-physical barrier includes water body, mountain, and dry land for aquatic organism
-is formed when continents drift, sea levels rise, glaciers advance and retreat, and climates
-population that are separated are often initially large
-evolve differences due to gene drift and change in environment
-allopatric speciation also happens when some members of population cross existing barrier and
found a new isolated population
-species differentiate that when immigrants arrive to their place, they do not survive because they
often do not breed with resident species or their offspring is less competitive than that of resident
-genetic distinctness and cohesiveness of finch species are maintained
-closest relative of a species on one island is often a species on a neighbouring island rather than
species on the same island
-physical barrier’s effectiveness at preventing gene flow depends on size and mobility of species
-gene flow can sometimes be interrupted even in the absence of physical barriers
Sympatric speciation occurs without physical barriers
-sympatric speciation is partition of a gene pool without physical isolation
-disruptive selection in which certain genotypes have high fitness on one or the other of 2
different resources
-2 species become partly reproductively isolated because the mate primarily with individual
raised on the same type of location in the environment
-one species become more adapted to a certain type of location that they would not mate with
individual of another location
-were born at different time of the year so they do not interact
-2 incipient species are partly reproductively isolated
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