AP: CHAPTER 24: THE ORIGIN OF SPECIES ANSWERS
1. A SPECIES is a population or group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring.
2. PATTERNS OF SPECIATION
ANAGENESIS – accumulation of changes that
gradually transform a given species into one with
different characteristics
CLADOGENESIS – splitting of a gene pool into
two or more separate pools, each giving rise to
one or more new species. This is the only
pattern of speciation that increases the
diversity and number of different species.
3. Reproductive isolation and/or geographical isolation, which causes a reduction in gene flow.
4. BARRIERS THAT CAUSE ISOLATION
PREZYGOTIC BARRIERS (“before
zygote”)
Barrier that prevents mating between
species or hinders fertilization of the egg if
different species attempt to mate
Temporal Isolation – species breed
during different times of the day, season,
years, therefore games cannot mix
Habitat Isolation – when two species
occupy different habitats within the same
area, therefore they will rarely encounter
one another (e.g. garter snakes, 1 species
lives in water, the other lives on land)
Behavioural Isolation – Courtship rituals
that attract mates and other behaviours
that are unique to a species (e.g. blue
footed boobies)
Mechanical Isolation – Morphological
differences prevent successful mating (e.g.
snails of Japan)
Gamete Isolation – Sperm of one species
may not be able to fertilize the egg of
another species. (e.g. sperm cannot
survive in the reproductive tract of females,
mechanisms prevent sperm from
penetrating egg’s outer membrane)
POSTZYGOTIC BARRIERS (“after
zygote”)
Barrier which prevents zygote from
developing into a viable, fertile adult.
Hybrid Inviability (or Reduced Viability)
– Genes of different parent species may
interact and impair hybrid’s development
(e.g. hybrids are weak, frail or die)
Hybrid Sterility – when hybrids produced
are sterile even if vigorous. Occurs when
chromosomes of parent species are
different in number and/or structure,
therefore meiosis does not produce normal
gametes (e.g. horse + donkey = mule)
5. MODES OF SPECIATION
ALLOPATRIC SPECIATION (“other country”) – gene flow disrupted when population is dived
into geographically isolated subpopulations. When the environment changes and these
populations come into contact, they cannot interbreed, which means they are now two different
species.
SYMPATRIC SPECIATION (“same country”) – geographically overlapping populations that still
experience speciation due to other barriers (e.g. habitat, temporal, mechanical, behavoural
isolation)
6. Harris’ Antelope Squirrel and the White-Tailed Antelope Squirrels demonstrate allopatric
speciation because are separated by the Grand Canyon. This causes the gene pools to change
and experience “forces” of evolution differently (e.g. mutations, sexual selection, etc.) until they
are no longer able to interbreed, even if they were to come into contact with one another.
8. ADAPTIVE RADIATION – the emergence of
numerous species from a common ancestor
introduced into a new environment. Each
newly emerging form specializes to fill an
ecological niche. E.g. Galapagos finches.
Island chains encourage adaptive radiation
because many (like the Hawaiian Islands)
were created by volcanoes. When these
formed, they became gradually populated by
organisms that traveled from other areas. This opened up ecological niches for these animals,
which provided many opportunities for natural selection and therefore a splitting of the gene
pool and thus the emergence of numerous species.
9. Reproductive isolation (without geographical isolation) and sexual selection are two intrinsic
factors that result in sympatric speciation.
10. POLYPOLOIDY – mutation that occurs resulting in an extra set of chromosomes (normally cells
have 2 sets – diploid).
HORSE  64 chromosomes (32 pairs) + DONKEY  62 chromosomes (31 pairs)
= MULE  63 chromosomes
(has one unpaired chromosome during meiosis
therefore mule is infertile).
Because mules cannot interbreed with either a horse or donkey, they are considered to be a
different species than a horse and donkey. And, because horses and donkeys cannot produce
fertile offspring, they are both considered to be a different species.
13. Bottleneck effect and sexual selection demonstrate sympatric speciation. The drying of the lake
caused a bottleneck effect. This drying formed new ecological niches for the “new species” of
cichlids.
14. RATE OF SPECIATION
GRADUALISM – species descend from a common
ancestor slowly and diverge more in their morphology
as they acquire new adaptations.
PUNCTUATED EQUILIBRIUM – a new species
changes most as it buts from “parent” species, and then
changes little for the rest of its existence (stasis).
15. MICROEVOLUTION – explains how adaptations evolve
in populations; is confined to a single gene pool
MACROEVOLUTION – explains how a new species originates and develops; describes the
divergence of gene pools
17. The Mollusk eye shows that complex structures (i.e. the human eye and the Mollusk eye) have
evolved in increments from much simpler versions (i.e. photoreceptor cells) that performed the
same basic function (e.g. vision). The eye of the Small Mollusk uses photoreceptors still because
it is adequate for its survival and reproduction, therefore it has not evolved a more complex eye
like ours.
19. EVOLUTIONARY TRENDS
CONVERGENT EVOLUTION – when unrelated species occupy the same
environment or habitat experience the same selective pressures and show
similar adaptations.
ANALOGOUS TRAITS – traits that have the same function, but different
structures
E.g. a whale looks like a fish because they both evolved in the same
environment, but the underlying bone structure of the whale reveals a common ancestry to mammals,
not to fish.
PARALLEL EVOLUTION – describes two related species that have made
similar evolutionary adaptations after the divergence from a common
ancestor
E.g. marsupial mammals of Australia and the placental mammals of North
America – there are striking similarities between them because they share a
common ancestor and evolved in similar environments
CO-EVOLUTION – two evolutionary set of adaptations of two interacting
species. All-predator prey relationships are an example.
E.g. the milkweed plant contains poisons that deter herbivores from eating
them. The butterfly lays its eggs in the weed and absorbs the poisonous
chemicals from the plant. They store the poison in their tissues. The poison,
which is present in the adult butterfly, makes the butterfly toxic to any animal
that eats it.