Podstawy ekologii, materiały do konwersatoriów, TiR, sem. I, rok akademicki 2015/2016
Konwersatorium 8
Biotop a mechanizmy powstawania izolacji rozrodczej
Materiał źródłowy: http://evolution.berkeley.edu/evolibrary/news/090301_cichlidspeciation
[10.10.2015]
„…Where's the evolution?
The physics of light affects not just how blue water looks to us, but how the animals living in the world's
oceans, lakes, and rivers are able to find food and each other — and this, in turn, can impact their
evolution. Natural selection favors traits that perform well in local environmental conditions. Many
fish species, for example, have evolved vision that is specifically tuned to see well in the sort of light
available where they live. But even beyond simple adaptation, the physics of light can lead to speciation.
In fact, biologists recently demonstrated that the light penetrating to different depths of Africa's Lake
Victoria seems to have played a role in promoting a massive evolutionary radiation. More than 500
species of often brightly colored cichlid fish have evolved there in just a few hundred thousand years!
To understand how the physics of light can promote speciation, picture a lake with slightly cloudy water.
Near the surface, blue light dominates the visual environment, but in deeper waters, red light does. A
fish population lives along the lake's shore where it slopes from very shallow water to deeper water —
so some of the fish spend more of their time in blue light and some spend more of their time in red light.
Like all populations, the fish have genetic variation — that is, individual fish have
different gene versions from one another — and some of this variation affects the fishes' ability to see
different colors. Some fish have genes that enable them to see blue light better, while other fish have a
red light advantage. Because of the differential penetration of light into the lake, fish with gene versions
sensitizing them to blue light have an advantage in shallower waters because they can better find food
and spot predators there, while fish tuned to red light have an advantage in deeper waters. So in different
parts of the fishes' habitat, different color-sensitivity genes are favored by natural selection. Over many
generations, if the fish don't move too much within their range, blue sensitivity will evolve to be more
common among fish living near the surface and red sensitivity will become more common among fish
living further down the slope.
By itself, natural selection acting on light sensitivity can cause something of a rift in the population, but
when sexual selection is considered as well, the divergence is amplified. To see how, add the male fish
to the equation. They have some variation in color. Some males have genes for blue coloration, some
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have genes for red coloration. This matters because female fish are choosy about their mates and tend
to pick brightly colored males to father their offspring. In this scenario, blue males living in deep waters
would have trouble finding mates for two reasons: (1) there is little blue light around, so they look duller
than red males, and (2) the females living in deep waters tend to be less sensitive to blue light than they
are to red. On the other hand, red males living in deep water would be winners on both counts: their
coloration makes the most of the available red light, and the females living at those depths tend to carry
genes that make them extra-sensitive to red light. Bottom-dwelling blue fish face a long series of lonely
nights, while bottom-dwelling red fish get all the girls. And of course, the opposite is true near the
surface. Over many generations of sexual selection acting in this way, the two parts of the population
may diverge. Though they live right next door to one another, the fish will evolve to prefer to mate with
other fish that share their coloration, light-sensitivity, and habitat. Over time, the two sub-populations
may even cease to mate with one another entirely and evolve enough differences to be considered
separate species.
Although the scenario described above is a hypothetical one based on evolutionary theory, biologists
have now discovered strong evidence that this process has actually occurred in the cichlid fish of Lake
Victoria. They've observed, along with other lines of evidence, many neighboring species pairs in which
the surface-dwellers tend to be blue and blue-light-sensitive, while the deeper fish tend to be red and
red-light-sensitive. Biologists are particularly excited about this discovery because it may represent an
unusual mode of speciation. The sort of speciation that is easiest to gather evidence about is allopatric
speciation — speciation that occurs partly through geographic isolation of the emerging species. In
contrast, these cichlid species may have evolved without any physical separation at all. ...”
Niejednorodne typy ubarwienia to nie jedyny atrybut różnicujący pielęgnice. Kolejnym jest m.in.
budowa czaszki i żuchwy oraz ich wysoka plastyczność, dzięki której możliwe jest niejako
„dostosowywanie się” do nowego rodzaju pokarmu w nowych warunkach srodowiskowych (oczywiście
cały proces „przejścia” jednego typu pokarmu na inny musi trwać). Szczegóły opisywanego
zróżnicowania
zaprezentowano
na
poniższym
rysunku
(źródło:
https://openi.nlm.nih.gov/detailedresult.php?img=1797158_1471-2148-7-10-1&req=4 [10.10.2015]):
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Rys. Zróżnicowanie budowy czaszki i żuchwy u wybranych przedstawicieli pielęgnic (a-h). (a)
Rhamphochromis macrophthalmus, drapieżnik polujący na inne gat. ryb, (b) Haplochromis euchilus,
odżywia się kopiąc w piasku i poszukując tam pokarmu, (c) Labidochromis vellicans, odżywia się
poprzez chwytanie wodnych stawonogów, (d) Lethrinops brevis, kopie w piasku analogicznie jak
Haplochromis euchilus, (e) Petrotilapia tridentiger, zeskrobuje glony, humus i inne resztki organiczne
ze skał, (f) Labeotropheus fuelleborni, zjada jedynie glony, które zeskrobuje ze skał, (g) Haplochromis
similis, odżywia się liśćmi roślin wodnych, (h) Genyochromis mento, zjada łuski innych gat. ryb,
Problemy do rozwiązania
1. W oparciu o przedstawione powyżej materiały należy zdefiniować proces radiacji adaptacyjnej
(proces gwałtowanego różnicowania się jednostek taksonomicznych w wyniku nabywania
nowych adaptacji do środowiska). W tym przypadku należy koniecznie zastosować pojęcie
niszy ekologicznej.
2. Proszę wskazać potencjalne zagrożenia dla poziomu różnorodności gatunkowej pielęgnic
(wyrażonego określoną, dużą liczba odrębnych gatunków), a wynikające z:
a. zanieczyszczenia wód w jeziorach (np. zadziałał czynnik antropogeniczny, w wyniku którego
istotnie wzrosła mętność wody);
b. obniżenia poziomu lustra wody (np. zadziałał czynnik naturalny, w wyniku czego zasilanie
jeziora wodami z rzek oraz innymi wodami uległo znacznemu osłabieniu).
Dla każdego z punktów proszę zbudować logiczny i przyczynowo skutkowy ciąg wydarzeń
z uwzględnieniem wskazania możliwych kierunków ewolucji pielęgnic.
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