lecture 11 Evolution focusing on Amphibians Flashcards
what occurs during natural selection
Natural selection
*Individuals do not evolve –populations evolve over time
*Natural selection can increase the frequency of adaptations that are favourable in a particular environment
*Freshwater inherently unstable (evaporation and depletion)
*Freshwater fish groups made partial moves to land
*The ancestors of amphibians had paired fins that allowed for support and movement underwater
*These then gained enough strength to walk on land
*Natural selection may result in adaptation to new conditions, sometimes giving rise to new species
*In tetrapods-ancestors varied in inherited traits –individuals who have inherited traits that give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals = the unequal ability of individuals to survive
*In the Devonian period the move to land happened once providing the lineage to all tetrapods
what are homologies and evolutionary trees?
*Evolutionary trees are hypotheses that summarise our current understanding of patterns of descent
*Tetrapodsand their closest living relatives, the lungfishes
*Each branch point represents the most recent common ancestor
*Three homologies shown on the tree—limbs with digits, the amnion, feathers—form a nested pattern
how have amphibians adapted and diversified?
*Amphibians inhabit all continents except Antarctica
*The life cycle can be complex with metamorphosis but some have lost this
*We have fossorial/ walking/ swimming/ climbing/ “flying”/ jumping all demonstrated
*Huge variation in life histories/ ecology and behaviour
*Huge variation in reproductive strategies
Adaptation and the diversity of amphibians’ Reproductive strategies - Anura
*Anura
*Mostly external fertilization *Exceptions -Nectophrynoides–Kihansi spray toad Internal fertilization and internal development of the embryo
Gymnophiona/ Apoda
*Fertilisation internal
*Either deposit eggs in damp ground or guard eggs in folds of body or have aquatic larvae or viviparous
Urodela/ Caudata
*Mostly internal fertilization (female picks up a spermatophore in vent)
*Some deposit eggs in water
*Some deposit eggs in damp earth or under logs
*Many species guard eggs
*Some exhibit direct development missing out the juvenile form completely and hatch as mini adults
*Complicated versions e.g. Ambystoma tigrinum
The race against Chytrid
*Amphibian species are at risk of extinction because of human actions, such as habitat destruction, climate change, and pollution
*Chytrid is a fungus that has decimated some amphibian populations
*It is thought to pass via sharing pools of water or direct contact
*Some amphibians seem to be able to tolerate or resist the infection
*In Panama there were huge declines and some (20%) species have recovered
*Some recoveries could be explained by upslope dispersal and recolonisation of frogs from lowland populations
*Plausible mechanism is that some species persisted at low numbers and subsequently increased in abundance, possibly because of evolution in host defences.
The biological species concept
*The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with members of other such groups.
*Gene flow between populations holds a species together genetically
*Re-worded
*Biological species concept: the most widely used species concept, which defines species in terms of interbreeding. Mayr proposed that ‘species are groups of interbreeding natural populations that are reproductively isolated from other such groups’
Speciation
*Speciation can take place with or without geographic separation
*Speciation can occur in two ways:
*Allopatric speciation
*Sympatric speciation
*(Peripatricspeciation
Sympatric speciation
*Same geographic area
*Gene flow is reduced by
*Polyploidy
*Sexual selection
*Habitat differentiation
Test yourself
Q1: Are Mammals more closely related to amphibians or birds? Why?
Q2: How does the concept of descent with modification explain both the unity and diversity of life? Use Amphibian examples to explain
Q3: If you discovered a fossil of an extinct amphibian that lived high in the Andes, would you predict that it would more closely resemble present-day amphibians from South American jungles or present-day amphibians that live high in Asian mountains? Explain.
Q4: Discuss the statement, chytrid has created chytrid resistance in amphibians
