Post on 08-Feb-2016
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CHAPTER 24THE ORIGIN OF
SPECIES
Question?
What is a species? Evolution theory must also
explain how species originate. Darwin’s “Mystery of mysteries” In the Galápagos Islands Darwin
discovered plants and animals found nowhere else on Earth
Overview: That “Mystery of Mysteries”
Speciation, the origin of new species, is at the focal point of evolutionary theory
Evolutionary theory must explain how new species originate and how populations evolve
Fig. 24-1
How did this flightless bird come to live on the isolated Galápagos Islands?
Microevolution consists of adaptations that evolve within a population, confined to one gene pool
Macroevolution refers to evolutionary change above the species levelExample – the appearance of feathers during the evolution of birds from one group of dinosaurs
Two Basic Patterns of Evolutionary Change”
1. Anagenesis ana = new, genos = raceAccumulation of heritable changes that gradually transforms a species into a species with different characteristics
Two Basic Patterns of Evolutionary Change”
2. Cladogenesis klados = branch, genos = race
Branching evolution in which a new species arises from a population that buds from a parent species
Concept 24.1: The biological species concept emphasizes
reproductive isolation Species is a Latin word meaning
“kind” or “appearance” Biologists compare morphology,
physiology, biochemistry, and DNA sequences when grouping organisms
Fig. 24-2a
Similarity between different species
Fig. 24-2b
(b) Diversity within a speciesDiversity within a species
Question
How many species of African Violets are here?
The Biological Species Concept
• The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
• Gene flow between populations holds the phenotype of a population together
Fig. 24-3c
Grey-crowned babblers
Does gene flow occur between widely separated populations?
Reproductive Isolation
Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring
Hybrids are the offspring of crosses between different species
Reproductive isolation can be classified by whether factors act before or after fertilization
Prezygotic barriers block fertilization from occurring by:Impeding different species from attempting to mate
Preventing the successful completion of mating
Hindering fertilization if mating is successful
Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:Reduced hybrid viabilityReduced hybrid fertilityHybrid breakdown
EXAMPLES OF PREZYGOTIC
BARRIERS
Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
Terrestrial Thamnophis
Water-dwelling Thamnophis
Habitat Isolation
Populations live in different habitats or ecological niches.
Ex – mountains vs lowlands.
Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Eastern spotted skunk(Spilogale putorius)
Western spotted skunk(Spilogale gracilis)
Temporal Isolation
Breeding seasons or time of day different.
Ex – flowers open in morning or evening.
Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Courtship ritual of blue-footed boobies
Behavioral Isolation
Mating or courtship behaviors different.
Different sexual attractions operating.
Ex – songs and dances in birds.
Mechanical isolation: Morphological differences can prevent successful mating
Bradybaena with shells spiraling in opposite directions
Mechanical Isolation Structural differences that prevent
gamete transfer. Ex – anthers not positioned to put
pollen on a bee, but will put pollen on a bird.
Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
Seaurchins
Gametic Isolation
Gametes fail to attract each other and fuse.
Ex – chemical markers on egg and sperm fail to match.
EXAMPLES OF POSTZYGOTIC
BARRIERS
Reduced Hybrid Viability
Zygote fails to develop or mature.
Ex – when different species of frogs or salamanders hybridize.
Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
Ensatina hybrid
Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
Hybrids are viable, but can't reproduce sexually.
Chromosome count often “odd” so meiosis won’t work.
Ex – donkeys and horses produce mules
Fig. 24-4m
(i)
Donkey Donkey
Fig. 24-4n
( j)
Horse
Fig. 24-4o
(k)
Mule (sterile hybrid)
Hybrids
‘Heaven Scent’
an F1 hybrid between 2 species, but sterile.
Hybrid Breakdown
Offspring are fertile, but can't compete as well as the “pure breeds”.
Ex – many plant hybrids
Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile
Hybrid cultivated rice plants withstunted offspring (center)
Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
Other Definitions of Species
Other species concepts emphasize the unity within a species rather than the separateness of different species
The morphological species concept defines a species by structural features It applies to sexual and asexual
species but relies on subjective criteria
Morphospecies
Organisms with very similar morphology or physical form.
Problem
Where does extensive phenotype variation fit?
The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual
species and emphasizes the role of disruptive selection
The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual
species, but it can be difficult to determine the degree of difference required for separate species
Question
How many species of African Violets are here?
African Violets 1950-60’s –20 species described 70,000 cultivars or human created hybrids
African Violets
2007 – lumping occurred to 6 species and 10 subspecies
2008 – AVSA now recognizes 9 species and 8 subspecies
point – species definitions can be fluid and change