Transcript Acquired characteristics - Queen Mary University of London

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Lars Chittka‘s bees on Van Gogh‘s Sunflowers
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Speciation 1 - Contents
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What is a species?
Allopatric speciation
Reproductive Isolating Mechanisms
Clines and ring species
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How many species are there?
There are about 1.5 million described species of animals (of which 1 million are insects)
About 400.000 species of plants
About 13.000 species discovered each year
Total of species including funghi, bacteria and protists: 13 to 30 million
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Felis
silvestris
catus
Panthera tigris
Panthera pardus
Panthera leo
Puma concolor
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The definition of biological species
- a reproductively isolated community in which all individuals potentially or actually
interbreed amongst themselves, but are genetically isolated from other groups (Ernst Mayr
1905 - 2005)
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Archaeopteryx (about 150 million years old)
(had several Dinosaur and several avian features, incl. Feathers)
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Problems with the biological species concept
- It cannot be applied to extinct species
- It cannot be applied to asexually reproducing species (of which there are many, for
example Cnemidophorus lizards, aphids (by Parthenogenesis), many plants by runners,
budding in Hydra, amoebas, etc. )
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Problems with the biological species concept
- It cannot be applied to extinct species / fossils
- It cannot be applied to asexually reproducing species
- Technically difficult to apply – can’t test 100s of 1000s of species (and populations within
them (for reproductive interactions and their success)
- There are many borderline cases: e.g. two North American tree species (Populus),
balsam & cottonwood. Fossils show that they have been morphologically distinct for over
12 million years. Yet they also hybridise occasionally, and hybrids are also fertile.
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Wolves & coyotes: another problem case
- distinct entities: have evolved in separate lineages for at least 500.000 years – yet they
occasionally hybridise in the wild.
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Morphospecies
Individuals that are similar or identical in important taxonomic traits (not just
morphology, but also e.g. physiological traits).
Problems – an alien might easily categorise different human (or dog) races as
different species!
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white tailed deer
elk
moose
FIGURE 9.1 Defining Species
(a) Morphospecies. Viewed today, at one moment in time, species A, C, and E are clearly
distinct, demarcated by current natural discontinuities between them. (b) Paleospecies
(chronospecies). Viewed historically, through time, discovered fossil intermediates (B and
D) fill in the missing gaps above, giving us a more or less continuous series with no
obvious discontinuities between them.
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Other species definitions
The evolutionary species concept: a species consists of all the individuals that share a
common history. Problem: how can we decide if they actually share a common history?
The recognition species concept (Paterson 1980s): “.. inclusive population of
individual biparental organisms which share a common fertilisation system”. Emphasis here
is on factors that keep a species as a unity, rather than those that keep species distinct.
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Allopatric Speciation (the most common scenario)
Four steps lead to allopatric speciation. First, a single species is an interbreeding
reproductive community. Second, a barrier develops, dividing the species. Third, separated
into different habitats, the divided populations become differentiated through the
accumulation of differences. Fourth, so different have the separate populations become,
that is when the barrier disappears and they overlap again. Interbreeding does not occur.
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Note: divergence in allopatric populations can occur through
adaptation to different habitat conditions OR through evolutionary
chance processes (genetic drift)
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Cotton top
Gold lion
Allopatric speciation
The populations of Tamarin monkeys are separated on the sides of the Amazon River.
Where the river tributary is wide and individuals on opposite banks do not interbreed, the
populations are diverging toward separate species. Where the river tributary is narrow, the
individuals still interbreed.
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Geographic isolation can occur
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through founder events: individuals get passively displaced to
islands (also called peripatric speciation)
new geographical barriers: mountain ranges fold up, continents
break apart, rivers or deserts form
glacial barriers
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Ring species—salamanders
The ensatina salamander (Ensatina eschscholtzii) occurs from Canada to Southern
California with interbreeding between adjacent populations through this range. The Central
Valley—a dry, hot lowland area—is divided into a coastal arm and inland arm. However,
where these two arms of the species meet again in Southern California, interbreeding does
not occur.
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FIGURE 9.4 Clinal Variation
In the leopard frog (Rana pipiens), tadpoles exhibit a range of temperature tolerances,
generally enduring colder temperatures in higher (northern) latitudes and warm
temperatures at lower (southern) latitudes. (From J. Moore)
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FIGURE 9.5 Reproductive Success of different pairings
In the leopard frog (Rana pipiens), eggs from females in the north were fertilized with
sperm from males progressively farther to the south. The degree of embryo or tadpole
abnormalities was scored, from A (normal young) through progressively more
abnormalities to F (high death rate). This study was done by J. Moore in 1949. Today, this
study and others prompt biologists to actually divide leopard frogs into subspecies or even
different species. (From J. Moore)
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FIGURE 9.6 Leopard Frogs Today-Zygotic Isolation
The four groups of leopard frogs resemble one another closely in their external appearance. But early
tests of interbreeding produced defective embryos in some combinations, leading biologists to suspect that
these might be different subspecies or even different species. Research on males’ mating calls indicates
that the various groups differ substantially, and that such prezygotic behavior separates and
reproductively isolates members of each group, producing four species: (1) Rana pipiens; (2) Rana blairi;
(3) Rana utricularia; (4) Rana berlandieri.
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When is there selection for reproductive isolation?
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When two populations become geographically isolated, they might diverge,
because they are now adapting to distinct habitat conditions (or they might
diverge by chance processes).
When they come into contact again, they might still be “sexually compatible“ –
but the offspring might be sterile (as in horses and donkeys), or less fit, for
example because it is not adapted to either of the habitats of parental
populations
In such conditions, there are high costs to “mistakes“ (mating with the wrong
partner)
Thus selection will favour individuals that don‘t make such mistakes, or where
the results of mistakes are sorted out early on.
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Pre-zygotic Reproductive Isolation
Ecological isolation, e.g. in the Malaria vectors of the genus Anopheles
Anopheles labranchiae – oviposits in brackish water
A. maculipennis – oviposits in running water
A. messeae – oviposits in stagnant water
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Pre-zygotic Reproductive Isolation
Seasonal isolation, e.g.
Pinus radiata sheds pollen in February
- Pinus muricata in April.
Hybrids do form on Monterey peninsula but show signs of inviability and infertility.
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Pre-zygotic Reproductive Isolation
Ethological isolation e.g.
Hawaiian picture-wing Drosophila silvestris & D. heteroneura are
genetically very similar but have different mating behaviour (different wing vibration patterns –
“songs”. The difference in head shape is due to a single gene with large effects
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Pre-zygotic Reproductive Isolation
Ethological isolation Single genes can sometimes have large effects. That applies also to behaviour
differences and sensory processing. For example, Drosophila melanogaster and D. simulans have
different “songs”. Identification of species membership in different Drosophila species occurs by “song” –
i.e. species specific patterns of wing vibration – that may be transmitted via the substrate. You can hear
some such songs at http://www.jason.org/expeditions/jason6/people/kaneshiro.html
If the two species don’t have a choice in the lab, they will mate – but will show preference for their own
species when given a choice.
The song durations and intervals between them are controlled by the per gene (that also controls
circadian rhythms). If a transgenic fly is produced which has the per gene from the respective other
species, the transgenic fly will mate with partners from the other species, showing that song and song
preference are controlled by the same gene!
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Pre-zygotic Reproductive Isolation
Mechanical isolation, e.g. in Partula snails on the island Moorea, where sympatric species have shells
that are coiled in opposite directions, resulting in mechanical difficulty in mating.
Genitalia of Culex pipiens & C. torrentium don’t fit. The species remained undetected for a long time
because they are otherwise so similar morphologically.
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Another case of prezygotic reproductive isolation: Mimulus flowers
and and pollinators
The two closely related flower species differ in multiple ways so that one species is best
adapted to bumblebee pollination, the other to hummingbird pollination; differences
concern morphology (including placement of reproductive organs), colour, nectar content.
Several of these may be genetically linked (after Bradshaw & Schemske)
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