BIOLOGY 280 Study Guide - Midterm Guide: Hybrid Speciation, Heteroneura, Reproductive Isolation
16 Nov 2020
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1. Describe, in as much detail as you can, some evidence that allopatric divergence can lead
to partial reproductive isolation.
Partial reproductive isolation will occur when the barrier is allopatric divergence is removed
and the two species can produce hybrids. Unless reinforcement occurs, the reproductive isolation
will never reach 100%.
Initially, a single species of fruit flies is split into two groups. One group is fed starch-based
food and the other maltose-based food. After a couple of generations, the two populations
diverged into different species of fruit flies. The resulting populations showed mating
preferences for their own kind because they’ve lived in different niches in so many generations
that the only way to stay fit is to mate with their own kind. In another example, fruit flies from
populations reared for many generations in a warm, moist, dark environment breed readily with
other such flies, but not with flies from populations reared for many generations in a cool, dry,
light environment. The result shows that reproductive isolation can evolve as a side effect of
divergent natural selection. In fruit flies, data shows that there is less mating between the
populations in different environments after the experiment than when the fruits flies are placed in
the same environment. This shows that you can get some sort of reproductive isolation or at least
a start just as a side effect of evolving separately. In real life, different species of Darwin’s
finches, with different beak size, will make different songs. The ones with larger beaks can’t
move rapidly so their songs are longer with more space in between and vice versa with smaller
beaks. Because of this, females of one species may not recognize the song of another species and
will not mate with them.
2. Describe the role that sexual selection might play in speciation, and describe an example
in which it might have played that role
Sexual selection directly affects traits related to mating. There is a parallel change in mate
preference and sexual traits within a population that leads to prezygotic isolation between
populations. Evidence of sexual selection in speciation can be seen in fruit flies. Drosophila
heteroneura have wide heads that males use to battle with and Drosophila silvestris use their
back legs to battle. A strong sexual selection for wider heads in heteroneura prevents the 2
species from interbreeding because silvestris males don’t have the wide head to win over
females. Therefore, there is a reproductive isolation between the two species.
3. What is reinforcement? Describe some evidence that it has occurred.
Reinforcement is the third process of speciation (in some, but not all species) where natural
selection increases the reproductive isolation between two populations of species. This occurs
when two separated populations come back together from allopatric to sympatric phase (i.e. the
mountain range that separated the two eroded down) and reproductive isolation may be complete
or incomplete. If it is complete, speciation has occurred because reinforcement leads to 100%
reproductive isolation. If it is incomplete, hybrids of the two species will be produced. A lower
hybrid fitness compared to either parental forms will cause selection to increase the reproductive
isolation because each form will be more fit if they avoid mating with other species and produce
disadvantageous hybrids. A lower hybrid fitness also means that the parents have lower fitness
because they chose to mate with species of another population instead of sticking with their own.
The selection against the hybrids will cause the population to diverge even more until there is
100% isolation.
Evidence of this can be seen in Hawaiian fruit fly graphs of allopatric taxa and sympatric
taxa in a comparison between genetic distance (the differences in genome sequence that can
measure the time of separating) vs prezygotic isolation (measured by the likelihood of mating
with another species. A shorter genetic distance means the two species have been separated for a
short amount of time and vice versa. Looking at the allopatric taxa graph, the level of
reproductive isolation is based heavily on time. The longer the species have been separated, the
higher the prezygotic isolation. On the sympatric taxa graph, there is a high degree of isolation
when the genetic distance is low. This suggest that there may have been reinforcement.
Something must’ve happened that caused the species that lived together to evolve much faster.
This is supported because if reinforcement has occurred, reproductive isolation between the two
species should’ve evolved more rapidly in sympatric phases.
4. What is homoploid hybrid speciation? Describe some evidence that it has occurred.
Homoploid hybrid speciation is the formation of a new-hybrid species, independent from its
parents, via hybridization with no genome duplication and no increase in ploidy. There can be
different species in ecological spaces (difference in niches) and produce hybrids. The hybrid
swarm (the cloud of the middle) is a bunch of hybrids living in a novel habitat with different
genotypes. Some of the genotypes are better adapted to live in the habitats that their parents can’t
survive in. The specific hybrids are fit in their specific habitats, making them reproductively
isolated (very rare).
Evidence of this can be seen in species of sunflowers. If two difference species of
sunflowers, H. annuus (parent) and H. petiolaris (parent), had pollen reach one another, they will
hybridize and produce a newer species. Another species, Helianthus anomalous, lives in the
deserts where annuus and petiolaris can’t survive in. A genetic analysis of the 3rd species showed
similarities if there was a combination between annuus and petiolaris. An experiment was
conducted by breeding H. annuus and H. petiolaris. There were 3 lineages with 4 generations and
their genomes were compared. The results showed that all three lineages were similar and were
interfertile. The genome sequenced matched and was similar to the genome of wild H.
anomalous. The pollen fertility increased from 5% to 92%. This experiment was repeated
multiple times for accuracy. The results suggest that hybrid speciation is possible because they
can thrive in novel habitats that their parents cannot survive in. In reverse, these hybrids can’t
thrive in their parents’ habitat so there will be reproductive isolation leading to speciation.
5. What is an adaptive radiation? Describe some examples. What conditions favor the
evolution of adaptive radiations?
Adaptive radiation is a pattern of speciation that seems to have recurred many times in
evolutionary history. It is described as sudden bursts of speciation where species are getting
more and more different from one another. Meanwhile, there is an increase of morphological and
ecological divergence. For example, in Hawaii there are thousands of fruit fly species that vary
in size, shape, wing morphology, etc. It is believed that all those species descended from a single
species that arrived from a continental place in the Hawaiian Islands at the beginning of this
adaptive radiation. Another example are different species of honeycreepers that exhibit a wide
diversity on the Hawaiian Islands. An ancestor species colonized the islands around the mainland
and diversified rapidly to fill multiple niches. The most prominent morphological divergence are
beak shapes that were specialized to feed on certain resources on different islands. Some birds
had long, thin beaks to eat nectar from flowers and others had thick, wide beaks to crack open
Document Summary
Describe, in as much detail as you can, some evidence that allopatric divergence can lead to partial reproductive isolation. Partial reproductive isolation will occur when the barrier is allopatric divergence is removed and the two species can produce hybrids. Unless reinforcement occurs, the reproductive isolation will never reach 100%. Initially, a single species of fruit flies is split into two groups. One group is fed starch-based food and the other maltose-based food. After a couple of generations, the two populations diverged into different species of fruit flies. The resulting populations showed mating preferences for their own kind because they"ve lived in different niches in so many generations that the only way to stay fit is to mate with their own kind. In another example, fruit flies from populations reared for many generations in a warm, moist, dark environment breed readily with other such flies, but not with flies from populations reared for many generations in a cool, dry, light environment.
