Term 2 Lecture 9: Convergent Evolution Flashcards
Definition
Convergent evolution is the independent evolution of analogous features in different lineages, separated by either space or time. Features that were absent in the last common ancestor
Analogous means similar function or form of structures
Examples of convergent evolution
The eyes of vertebrates and cephalopods
The wings of birds, pterosaurs, flying insects and bats:
Analogous organ that is anatomically different. All have an acrofoil: feathers in birds, skin in bats and pterosaurs and in flying insects wings are formed from completely different organs unrelated to limbs
The long beaks and tongues of hummingbirds and songbirds:
Evolved because they are both nectivores - hummingbirds in the Americas (Neotropical) and sunbirds in Asia and Africa (paleo tropical)
Hummingbird and sunbird food plants: also show convergent coevolution - their flowers are often red and tubular and have a nectar reward. They are often scentless as bird pollinators cannot ‘smell’ - resulting in two completely different plant families developing similar features
Pitcher plants:
Such as Nepenthes (paleo tropical)
And saracenia ( N America)
All have developed:
- carnivorous ‘pitcher’ (jug shaped) structure from modified leaves
- pitfall traps with waxy surfaces, colourful slippery lips (peristomes), downward pointing hairs in the tube, a water filled cavity with digestive enzymes and a ‘lid’ (operculum) to prevent enzyme dilution by rainfall
→these features evolved independently in several plant lineages due to similar selection pressures of nutrient limited environments where soil is particularly lacking in nitrogen
Placental wolf and fox vs. marsupial wolf: outward appearance differs but skull and skeleton show great similarity in morphology
Why does convergent evolution occur?
Similar environments exert similar pressures.
Abiotic environmental conditions underpin biomes causing similar ecosystems determined by climate (temperature, precipitation and seasons) across different continents driving convergence in different lineages.
Biomes can often be defined by the vegetation occurring within the ecosystem determined by abiotic environmental conditions.
Convergent evolution of plants within biomes : cushion plants
Cushion plants occur in high altitude (alpine tundra) up mountains and in temperate tropical regions.
Silene acaelis
(caryophyllaceae, Svalbard)
Donatia novae-zealande
(Donatiaceae, Tasmania)
Thylacospernum caespitosum
(Caryophyllaceae, Himalayas)
The cushions resemble moss growing on bare rock. Flowers grow close to the surface of the cushion. All have a similar appearance suited it high altitude harsh environment. The compact structure reduced wind velocity acting as a heat trap and limiting tissue damage whilst also avoiding excess transpiration.
This extra heat is visible on infrared imagin. Shoots are kept warmest for growth allowing regular photosynthesis and preventing tissues from freezing.
Cushion plants dampen diurnal fluctuations in temperature - avoiding high temperature that can cause drought stress from transpiration.
Similarly at night they retain heat to prevent tissues from freezing.
This also benefits other plants that can live in amongst them.
Convergent evolution of plants within biomes : the challenging tropical montane environment of giant rosettes
Tropical mountains like Mt Kenya in E. Africa and Mt Kilimanjaro in neighbouring Tanzania are said to have ‘4 seasons in one day’
- high solar radiation (including UV) during the day lead to high evapotranspiration and drought stress
- frozen ground during the night, ice crystals form causing ‘frost heave’ a phenomenon where the top layer of soil is pushed up and turned over making it hard for soil to form and a risk of frost damage
- in the morning the ground is still frozen but once the sun has begun to rise the plants begin to transpire losing water so there is drought from the cold as well as the heat.
Due to frost heave the soil substrate is low in water and nutrients
Where giant rosettes are found
Hawaii - silver sword occurs >2100m above sea level (asl)
Many giant rosettes grow in the Andean mountains (Colombia, Ecuador, Venezuela) species of genus Espeletia (Asteraceae) in tropical high altitude moorland e.g. Espeletia timotensis and Espeletia pycnophylla
In Peru there are rosettes of genus Puya (Bromeliaceae) which are bromeliads but not epiphytes.
The Andean and Peruvian rosettes occur in the ‘Paramo’ (3500-4800 asl)
In the East African mountains Dendrosenelios (Asteraceae) and Lobelias (Lobeliaceae) giant rosettes occur at 3500-4000m asl e.g. lobelia deckenii and lobelia telekii on My Kenya
Why are giant rosettes so successful in stressful tropical montane environments? Height
Height - especially Espeletia in the Andes and Dendrosenecio in east Africa.
Why? Because temperature at ground level increases strongly from dawn to midday causing low leaves to transpire whilst ground and roots are still frozen resulting in drought stress.
Also frost is more likely to damage ground level buds.
Temperature difference between day and night reduces with increasing height above ground protecting buds from frost and preventing drought stress.
This is because different in air temperatures are smaller.
So mortality risk declines with height
Why are giant rosettes so successful in stressful tropical montane environments? retention of dead leaves and hairy leaves
Retention of dead leaves around the stem (aka marcescence)
At high altitude risk of tissue freezing ( and fluids in xylem and phloem)
Spongy pith in some rosettes is used to retain water and is protected by marcescent leaves.
Pith acts as a water reservoir for transpiration before the ground thaws whilst the sun is rising.
Pubescence of some leaves and flowering stems - dense ‘hairs’ (trichomes) on leaves stems and flowers act as insulation reducing water loss by transpiration by reducing relative humidity gradient between plant tissues inside the leaf and the outside air.
The insulation provides warmth that helps the leaf to continue to grow and photosynthesise it also aids transport of photosynthate in the phloem.
Trichomes also absorb UVB radiation which can cause damage to tissues/DNA/ cell membranes and proteins
Why are giant rosettes so successful in stressful tropical montane environments? Leaf colour, antifreeze and nyctanistic leaves
Leaf ‘colour’ white/silver surfaces reflect light and heat to prevent overheating.
Antifreeze e.g. Lobelia leaves have a high sucrose concentration to protect membranes from freezing. The leaves also contain and secrete mucilage containing hydrophilic carbohydrate molecules into the pool of water that collects at the centre of the rosette drawing in the water molecules and inhibiting ice crystal formation on/ within the leaves
Nyctanistic leaves close at night pushing mucilage up and out of the centre to coat leaves and prevent overnight frost damage.
Closing the leaves also protects the central bud from freezing temperatures (in Lobelias and Dendrosenecios)
