Lesson 5 - Speciation Flashcards
(16 cards)
What is a species?
physical similarity # same species
Need to consider: physiology, biochemistry, behaviour, and genetics when distinguishing between different species
Species = individuals that can interbreed to produce viable and fertile offspring, who can also interbreed
How do species form? (Speciation)
The formation of a new species from an existing population
aka. macroevolution
For a species to develop, interbreeding must be prevented: Two populations need to be reproductively isolated (little or no gene flow between them)
Pre-zygotic Isolating Mechanisms
Prevents mating or fertilization (if mating occurs between two different species)
Behavioural Isolation
Unique behavioural patterns, mating rituals, and chemical signals (pheromones)
- identifies & attracts members of the same species
- prevents interbreeding between related species
Ecological/Habitat Isolation
Species occupy different habitats within the same general area, so they rarely encounter each other
Temporal Isolation
Species may occupy the SAME habitat but be separated by timing (temporal) barriers
Ex. mate or flower at different times of the day, month, year, or seasons
Mechanical Isolation
Structural differences in sexual organs prevent fertilization
Gametic Isolation
If mating does occur, zygote formation fails
Chemical incompatibility 🡪 , Sperm cannot survive in the female reproductive tract to reach the eggs
Biochemical barrier 🡪 : Sperm cannot fertilize eggs (receptor recognition)
Post-zygotic Isolating Mechanisms
RARELY do zygotes form between different
species
If a zygote forms, hybrid offspring are prevented from developing into viable, fertile individuals
Types of Speciation
Populations must become isolated from each other, reproductively and/or geographically
The two differ in how gene flow is disrupted within a population.
i) Sympatric speciation
- live in the same area, but diverge genetically
ii) Allopatric speciation
- separated by a geographic variation and diverge genetically
Sympatric Speciation
Occurs due to chromosomal changes (in plants) or behavioural isolating mechanisms (ex, non-random mating in animals)
Genetic change in offspring causesa reproductive barrier between offspring and the parent population
More common in plants
Polyploidy
In flowering plants:
- Non-disjunction during meiosis forms diploid gametes
- Self-pollinate = Diploid gamete (2n) + diploid gamete (2n) 🡪 polyploids (4n)
- Tetraploid (4n) may produce diploid gametes and self-pollinate or reproduce with other tetraploids
- Tetraploid may reproduce with the original population (gametes 1n), but the offspring (3n) will be sterile
Allopatric speciation
A new species evolves when a population has been geographically isolated due to a physical barrier or variation in the environment
- Ex. Glacier, lava flow, and ocean fluctuations create islands
- Ex. peripheral isolation from the parent population, due to
environmental differences
After geographic isolation has
occurred:
- organisms become reproductively incompatible
- allele frequencies diverge due to mechanisms of microevolution
Many isolated populations do not become a new species simply because they could not survive
Adaptive radiation – a form of allopatric speciation where a common ancestor diversifies into a variety of different species in response to new environmental conditions
Example:
- Ancestral finch species got blown off course and landed on all the Galapagos Islands
- Islands had different selective pressures
- Finches had various niches to adapt to
- Over time, speciation occurred due to reproductive and geographic isolation
Phylogenetic Tree
shows the descent from a common ancestor
The length of each branch reflects how much the DNA has mutated from the group’s common ancestor
Patterns of Speciation
Divergent evolution – species that were once similar become increasingly distinct, as they adapt to different environmental conditions
Ex. Darwin’s Finches, homologous structures
Convergent Evolution – unrelated species have
independently been subjected to similar selective pressures, causing similar traits to arise
Ex. analogous structures, birds, bats,and insects do not share
a direct common ancestor, but all evolved wings at separate times
Gradualism vs. Punctuated Equilibrium
Gradualism
- Evolution occurs at a slow, steady rate
- Big changes are a result of small, adaptive changes gradually accumulating over time
- Evidence of transitional fossils
Punctuated Equilibrium
- rapid bursts of change
- long periods of little to no change (equilibrium)
- species undergo rapid change when they first get isolated from the main population
- After initial divergence, very little change occurs
- supported by fossil evidence
