BIO120H1 Chapter Notes - Chapter 7: Reproductive Isolation, Speciation, Chromosome

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27 Nov 2018
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BIO120: Why Evolution is True, Jerry A. Coyne (Ch.7 The Origin of
Species)
In association with Reading Quiz 5 and Lecture 9
The Origin of Species
- The concordance between two cultural groups with very different backgrounds should convince
us that the discontinuities of nature are not arbitrary, but an objective fact
- Each individual almost always falls into one of many discrete group
- Although there’s variation among individuals within a cluster, the clusters nevertheless remain
discrete in “organism space.”; where the clusters are evident in all organisms that reproduce
sexually discrete clusters are known as species
- Evolution is a continuous process, so how can it produce groups of animals and plants that are
discrete and discontinuous, separated from others by gaps in appearance and behavior? How these
groups arise is the problem of speciation; the origin of species
- Darwin didn’t think of the discontinuities of nature as a problem to be solved, or thought that
these discontinuities would somehow be favoured by natural selection he couldn’t explain
nature’s clusters in a coherent way
- The idea of splitting is as important as understanding how a single species evolves diversity of
nature encompasses millions of species, each with its own unique set of traits; all of diversity
came from a single ancient ancestor
- If speciation didn’t occur, there’d be no biodiversity at all – only a single, long-evolved
descendant of that very first species
- Species are recognized as a group of individuals resembling one another more than they resemble
members of other groups this definition has some problems because sexually dimorphic
species can differ physically between females and males, and there’s also a problem of variation
within an interbreeding group, like humans
- At what point are differences between populations large enough to make us call them different
species? This concept makes the designation of species an arbitrary exercise, but we know that
species but we know that species have an objective reality and aren’t simply arbitrary human
constructs
- Why do similar looking forms turn out to be different species? they coexist in the same
location but never exchange genes: the members of one species don’t just hybridize with
members of another; the groups are reproductively isolated from one another: the constitute
distinct “gene pools” that don’t intermingle, so its reasonable to assume that under any realistic
view of what makes a group distinct in nature, these cryptic forms are distinct
- Members of the same species can mate with each other and produce offspring that contain
combinations of their genes
- Species are distinct not merely because they look different, but because there are barriers between
them that prevent interbreeding
- Mayr, a Russian geneticist, defined species as a group of interbreeding natural populations that
are reproductively isolated from other such groups this is known as the biological species
concept, or BSC
- “reproductively isolated” means that members of different species have traits – differences in
appearance, behavior, or physiology that prevent them from successfully interbreeding, while
members of the same species can interbreed readily
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- There’s many different reproductive barriers; species might not interbreed simply because their
mating or flowering seasons don’t overlap
- Closely related species living in same area remain distinct because their peak spawning periods
are several hours apart, preventing eggs of one species from meeting sperm from another.
- Animal species have different mating displays or pheromones, and don’t find one another
sexually attractive; species can also be isolated by preferring different habitats, so they don’t
encounter one another
- Isolating barriers can also act after mating; if fetuses are formed, they might die before birth, or
even if hybrids survive, they may be sterile “Species that produce sterile hybrids certainly
can’t exchange genes”; several of these barriers can act together
- BSC takes care of many problems that appearance-based species concepts can’t handle
- There’s potential gene flow from one population to the other through intermediate geographical
areas, and little doubt that if they did mate they’d produce fertile offspring
- A species is a reproductive community a gene pool; meaning that a species is also an
evolutionary community
- If a “good mutation” crops up within a species, the gene containing that mutations will spread
throughout the species, but it won’t go any further to any other species because one species won’t
exchange genes with another species
- Biological species are the unit of evolution to a large extent, the thing that evolves
- Members of all species generally look and behave pretty much alike because they all share genes,
respond in the same way to evolutionary forces; and lack of interbreeding between species living
in an area maintain species’ differences in appearance and behavior, but also allows them to
continue diverging without limits
- Organisms that don’t sexually reproduce don’t fit the criteria for BSC
- “sister species” species that are each other’s closes relatives; often separated in nature by
geographical barriers
- It was proposed that the combined effects of evolution and geography could explain how one
species divided into two through reproductive barriers; Mayr argued that these barriers are just
by-products of natural or sexual selection that caused geographically isolated populations to
evolve in different directions
- Divergent selection is selection that drives different populations in different evolutionary
directions
- Different mutations affecting male behaviours or traits could appear in different places, and
sexual selection might then drive the populations in different directions results in females in
one population having preference for a specific trait in males, and females in another population
preferring a different trait in males; this prevents the two populations from ever mating and
mixing their genes, so they’re considered different species
- If two geographically isolated populations evolve along different pathways long enough, their
genomes can become so different that when they’re put together in a hybrid, they don’t work well
together; so development is disrupted and hybrids either die prematurely or if they live, they are
sterile
- Species don’t arise for the purpose of filling up empty niches in nature; we don’t have different
species because nature somehow needs them species are evolutionary accidents, and the
‘clusters’ don’t evolve because they increase diversity or provide balanced ecosystems; they’re
the inevitable result of genetic barriers that arise when spatially isolated populations evolve in
different directions
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