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Chapter 24 ~ The Origin of Species. Introduction. Evolutionary theory also explains macroevolution , the origin of new taxonomic groups (new species, new genera, new families, new kingdoms) Speciation is the keystone process in the origination of diversity of higher taxa.

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Chapter 24 ~ The Origin of Species

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  • Evolutionary theory also explains macroevolution, the origin of new taxonomic groups (new species, new genera, new families, new kingdoms)
  • Speciation is the keystone process in the origination of diversity of higher taxa.
  • Fossil records show two patterns of speciation: anagenesis and cladogenesis
  • Cladogenesis:
Traditionally, morphological differences have been used to distinguish species.

Today, differences in body function, biochemistry, behavior, and genetic makeup are also used to differentiate species.

the biological species concept emphasizes reproductive isolation
The biological species concept emphasizes reproductive isolation
  • Developed by Ernst Mayr in 1942
  • Species:
Species are based on interfertility, not physical similarity
  • i.e. the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species.
In contrast, humans haveconsiderable diversity,but we all belong to thesame species because ofour capacity to interbreed.
prezygotic and postzygotic barriers isolate the gene pools of biological species
Prezygotic and postzygotic barriers isolate the gene pools of biological species
  • Reproductive isolation prevents interbreeding, even if the ranges overlap
  • Prezygotic barriers:

5 types

Habitat isolation. organisms that use different habitats are unlikely to encounter each other to even attempt mating

ex. two species of garter snakes, genus Thamnophis, live in same areas but one lives in water and other is terrestrial

  • Behavioral isolation. Many species use elaborate behaviors unique to a species to attract mates

ex. female fireflies only flash back and attract males who first signaled to them with a species-specific rhythm of light signals

Temporal isolation. Two species that breed during different times of day, different seasons, or different years cannot mix gametes.

ex. Eastern and western spotted skunk- geographic ranges overlap, yet one mates in late summer, other in late winter

  • Mechanical isolation. species may attempt to mate but fail b/c they are anatomically incompatible

ex. reproductive isolation of flowering plants that are pollinated by insects/animals

ex. many insects, the male and female copulatory organs do not fit together, preventing sperm transfer

Gametic isolation. gametes do not form a zygote b/c of incompatibilities preventing fusion or other mechanisms
    • In internal fertilization, the environment of the female reproductive tract
    • For external fertilization, rely on presence of specific molecules on the egg’s coat that adhere only to sperm. Or, between pollen of flowers.
Postzygotic barriers:
    • Reduced hybrid viability. may abort the development at some embryonic stage or produce frail offspring
    • Reduced hybrid fertility. hybrid may be infertile and cannot backbreed with either parental species

ex. may be due to differences in chromosome number or structure

mule, the hybrid between a horse and donkey cannot mate (except very rarely) with either horses or donkeys

Hybrid breakdown. In some cases, first generation hybrids are viable and fertile. However, when they mate with either parent species or with each other, the next generation are feeble or sterile.

ex. cotton

    • Similar fossils cannot be studied, taxa based on morphology
    • Entirely asexual species (bacteria) are based on biochemical and structural differences
alternative concepts of species
Alternative concepts of species
  • Ecological species concept:
  • Pluralistic species concept:
Morphological species concept:
  • Genealogical species concept:

ex. The sequences of nucleic acids and proteins provide data that are used to define species by unique genetic markers.


Modes of speciation (based on how gene flow is interrupted)

  • Allopatric:populations segregated by a geographical barrier; can result in adaptive radiation (island species)
  • Sympatric:reproductively isolated subpopulation in the midst of its parent population (change in genome); polyploidy in plants; cichlid fishes
allopatric speciation other country
Allopatric Speciation“other country”
  • Mountain ranges, glaciers, land bridges, or splintering of lakes may divide one population into isolated groups or a small population may colonize new area
    • Significance depends on the organism ex. Squirrels vs birds at Grand Canyon

Can separated populations interbreed and produce fertile offspring when they come back in contact?


Adaptive radiation:

  • Numerous species originate from one parent species.
  • Species can exploit new resources
  • Ex. Hawaii islands, Galapogos islands

Diane Dodd studied Drosophila, demonstrating a prezygotic barrier to interbreeding developing from adaptive diveregence

sympatric speciation same country
Sympatric Speciation “same country”
  • Physical separation of population not required, occurs within a population
  • Genetic change in population
    • In plants, polyploidy (multiplication of the chromosome number) discovered by Hugo de Vries from experiments with the primrose flower
    • In animals, mate preference changes
Autopolyploid: 2n  4n
    • One species with 2x the normal chromosome number as original parent species due to failure in meiosis.
    • Cannot interbreed with the parent (4n + 2n = 0). Can self-fertilize or mate with other tetraploids (4n + 4n = 4n).
    • Uncommon in animals (whiptail lizard)
Allopolyploid:(2n + 4n = 6n)
    • Two different, but related species produce a polyploid hybrid. (aka. Hybridization)
    • Usually sterile. Meiosis will not pair up homologs so gametes cannot be formed in new species. Ex. oats, cotton, potatoes, tobacco, and wheat
    • Produce hearty species (hybrid vigor) that retains the best attributes of each parent stock. Ex. Rapid growth and abundant fruit production.
Individuals of two closely related sympatric cichlid species will not mate under normal light because females have specific color preferences and males differ in color.

However, under light conditions showing no color differences, females will mate with males of the other species

Evolutionist agree that natural selection is the mechanism for this process, but debate over the rate of this process:

Two schools

of thought

    • Gradualism
    • Punctuated Equilibrium
    • Evolution proceeds slowly and at a constant rate
    • Populations slowly diverge from one another through the gradual accumulation of adaptive characteristics in the population
    • Occurs over long periods of time
    • Darwin was a gradualist
    • Anagenesis or Phyletic evolution
Punctuated Equilibrium
    • Divergence in rapid bursts
    • Proposed by Niles Eldredge and Stephen Jay Gould (1972)
    • Helped explain non-gradual appearance of species in the fossil record
    • Rapid bursts separated by long periods w/out change, unique features originate “rapidly” at time of split
    • Changes due to mass extinction (Founders effect/bottleneck)
    • Cladogenesis (branching evolution)



Changes accumulated in

allele frequecies

resulting in speciation

Species seen in fossil record

Evolution is a response between organisms and their current environments, leading to changes in evolutionary trends as conditions change

How could a complex organ like the human eye be the product of gradual evolution, rather than a finished design created specially for humans?
  • The simplest eyes are just clusters of photoreceptors, pigmented cells sensitive to light.
  • Complex eyes have evolved in several organisms, from clusters of photoreceptors to camera-like eyes, can be seen in mollusks

Range of the eye complexity in mollusks

  • a simple patch of photoreceptors found in some limpets,
  • photoreceptors in an eye-cup,
  • a pinhole-camera-type eye in Nautilus,
  • an eye with a primitive lens in some marine snails, and
  • a complex camera-type eye in squid.
evo devo genes that control development
“Evo-devo”: Genes that control development
  • “Evo-devo” is a field that examines how slight genetic divergences can become magnified into major morphological differences between species.
  • Particularly, genes that program development by controlling the rate, timing, and spatial pattern of changes in form as an organism develops from a zygote to an adult.

ex. differences in the feet of tree-dwelling versus ground-dwelling salamanders

mutation in the alleles that control the timing of foot development

The relative timing of reproductive development and somatic development

Ex. a sexually mature stage can retain juvenile structures - a process called paedomorphosis

This axolotlsalamander hasthe typical externalgills and flattenedtail of an aquaticjuvenile but hasfunctioning gonads