Speciation: How Species Form

Question 1

Speciation

Answer 1

Speciation:
The evolutionary formation of new species.
→ Also called macroevolution

Species: members of groups or
populations that interbreed or
have the ability to interbreed with
each other under natural conditions.

Question 2

Reproductive Isolation

Answer 2

• Two populations may become reproductively isolated over
  time → they become two distinct species
• A reproductive isolating mechanism is any behavioural, structural or biochemical trait that prevents individuals of different species from reproducing successfully together.

Question 3

Means of Reproductive Isolation:

Answer 3

PRE-ZYGOTIC (pre-fertilization barriers)
= prevention of interspecies mating or fertilization of eggs

POST-ZYGOTIC (post-fertilization barriers)
= prevention of zygotes from developing into viable, fertile
individuals

Question 4

Pre-zygotic Isolating Mechanisms

Answer 4

Prevent successful mating:
* Behavioural isolation
* Habitat isolation
* Temporal isolation
Prevent successful fertilization:
* Mechanical isolation
* Gametic isolation

Question 5

Pre-zygotic Isolating Mechanisms

(Behavioural Isolating Mechanism)

Answer 5

• Any special signals or behaviours that are species specific preventing
  interbreeding with closely related species

Examples:
* Insects releasing pheromones
* Courtship rituals of elk
* Songs of birds

Question 6

Pre-zygotic Isolating Mechanisms
(Habitat Isolating Mechanisms)

Answer 6

• Two species may live in the same general region but in different habitats, so they may
  rarely encounter each other

Example: Garter Snake
Two species of North American garter snakes (common garter snake and northwest garter snake) live in the same area Northwest prefers open areas, common garter snake found near water
→ They have become two different species because of the rare opportunities to mate

Question 7

Pre-zygotic Isolating Mechanisms

(Temporal Isolating Mechanisms)

Answer 7

*Barriers to mating caused by timing differences

Examples:
*Plants that live in the same habitat but flower at different times of the day, in different seasons, or in
different years
→ pollen can’t reach the egg

Question 8

Pre-zygotic Isolating Mechanisms

(Mechanical Isolating Mechanisms)

Answer 8

*structural differences in reproductive organs make them anatomically incompatible
Examples:
*Genitals of insects such as the damselflies
*Variation in flower structure to impede pollination → pollen won’t reach egg

Question 9

Pre-zygotic Isolating Mechanisms

(Gametic Isolating Mechanisms)

Answer 9

• If gametes from different species meet they will rarely fuse to form a zygote
• There are many forms of gametic
  isolation between species
  Example:
• In species where fertilization is
  internal, sperm may not survive in
  the female reproductive tract

Question 10

Post-zygotic Isolation Mechanisms

Answer 10

• In nature it is rare for sperm
  of one species to successfully fertilize an egg of another
• When these “hybrids” are produced various barriers prevent them from
  developing into viable, fertile
  individuals
  – Hybrid Inviability
  – Hybrid Sterility
  – Hybrid Breakdown

Question 11

Post-zygotic Isolating Mechanisms
(Hybrid Inviability)

Answer 11

• Genetic incompatibility of the interbred species
  stops zygote during its development

Example:
* Sheep + goat embryo dies in early
development before birth
Sheep = 54 chromosomes
Goats = 60 chromosomes

Question 12

Post-zygotic Isolating Mechanisms

(Hybrid Sterility)

Answer 12

• When two different species can mate and produce an offspring BUT the hybrid is sterile
• Meiosis in the hybrid fails

Examples:
* Horse + Donkey = Mule
* Lion + Tiger = Liger/Tigon

Question 13

Post-zygotic Isolating Mechanisms

(Hybrid Breakdown)

Answer 13

*First-generation of hybrids of crossed species are viable and fertile BUT when the hybrids mate with each other or one of the parent species the offspring is sterile
or weak
Example:
*Cotton plants – different species
can produce offspring, but their
hybrids cannot continue reproducing

Question 14

Reproductive Isolation and Speciation

Answer 14

• Reproductive isolation may lead to speciation.
• The gene pool is isolated, any mutation and selection that occurs is no longer shared
• Any significant evolutionary changes that occur in either population (new or old) will result in the formation of separate species.

Question 15

Sympatric Speciation

Answer 15

evolution of populations within the same geographic area into separate
species.
* Can be due to chromosomal
changes (plants) and non-random
mating (animals) that alter gene
flow

Question 16

Allopatric Speciation

Answer 16

= evolution of populations into
separate species as a result of
geographic isolation
→ water, canyon, mountain range, human construction (dams, highways, canals)