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Natural Selection
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1Darwin and [blank_start]Wallace[blank_end] in 1858 proposed the theory of [blank_start]natural[blank_end] selection as a possible mechanism for [blank_start]evolutionary[blank_end] change
They made three observations
- reproduction produces [blank_start]more[blank_end] offspring than can survive = implies substantial [blank_start]mortality[blank_end]
- individuals within a species [blank_start]vary[blank_end] in their ability to survive and [blank_start]reproduce[blank_end]
- offspring [blank_start]inherit[blank_end] characteristics from their parents that affect their survival and reproduction
This lead to the conclusion that factors enhancing reproduction must be favoured by [blank_start]natural[blank_end] selection to produce an adaptive evolutionary change across [blank_start]generations[blank_end] and through time (descent with advantageous modification)- AWallace
- Bnatural
- Cevolutionary
- Dmore
- Emortality
- Fvary
- Greproduce
- Hinherit
- Inatural
- Jgenerations
2Natural selection can take place in different ways:
- might form a [blank_start]normal[blank_end] distribution around an average phenotype ([blank_start]genotypes[blank_end] code for different phenotypes)
-
[blank_start]stabilising[blank_end] selection = removes the extreme [blank_start]phenotypes[blank_end] thus narrows distribution
[blank_start]directional[blank_end] selection = pushes phenotypes in one direction > involved in adapting to a changing or [blank_start]new[blank_end] environment
divergent selection (also known as [blank_start]disruptive[blank_end]) = selection where both [blank_start]extremes[blank_end] of a phenotype are better adapted to opposite ends on an environmental gradient > [blank_start]middle[blank_end] phenotypes are not very well adapted; the two populations which are adapted to different niches can be very close in geographical location e.g. could be different [blank_start]heights[blank_end] on a shore line
Selection is about [blank_start]genotypes[blank_end] changing over generations, selection acts on the phenotype (the observable characteristics)
[blank_start]Balancing[blank_end] selection = different phenotypes are favoured at different points in space or [blank_start]time[blank_end] > within a population you can have differences over time which are often driven by [blank_start]predator[blank_end]-prey relationships e.g. predators may recognise brown clams as food and therefore the numbers decrease resulting them to predate on the higher abundance of white clams and the cycle [blank_start]repeats[blank_end]- Anormal
- Bgenotypes
- Cstabilising
- Dphenotypes
- Edirectional
- Fnew
- Gdisruptive
- Hextremes
- Imiddle
- Jheights
- Kgenotypes
- LBalancing
- Mtime
- Npredator
- Orepeats
3evolution in a biological context refers specifically to changes in [blank_start]genetic[blank_end] makeup of populations over [blank_start]time[blank_end]
- NOTE: individuals do not evolve; [blank_start]populations[blank_end] (a group of individuals of a single [blank_start]species[blank_end] that live and interbreed in a particular geographic area) do- Agenetic
- Btime
- Cpopulations
- Dspecies
4Natural selection is an important mechanism for evolution BUT it does not act alone: four additional processes that effect genetic makeup ([blank_start]genotypes[blank_end]) of populations over time and thus can result in evolution
(evolution is change in genetic makeup over [blank_start]time[blank_end] in a population)
- MUTUATION
- [blank_start]GENE[blank_end] FLOW
- GENETIC [blank_start]DRIFT[blank_end]
- NON-RANDOM [blank_start]MATING[blank_end]- Agenotypes
- Btime
- CGENE
- DDRIFT
- EMATING
5natural selection is not the only type of selection:
also...
- [blank_start]artificial[blank_end] selection
- sexual [blank_start]selection[blank_end]
however most species are shaped by [blank_start]natural[blank_end] selection- Aartificial
- Bselection
- Cnatural
6[blank_start]Artificial[blank_end] Selection
- Darwin kept and bred [blank_start]pigeons[blank_end] and used artificial selection to show [blank_start]evolutionary[blank_end] change takes place
- the 300 species of domesticated pigeons are all derived from the [blank_start]Rock[blank_end] dove due to pigeon fanciers (people who keep and breed pigeons)- AArtificial
- Bpigeons
- Cevolutionary
- DRock
7Wild mustard has been selectively bred into [blank_start]cabbage[blank_end], cauliflower, kale, broccoli, brussel [blank_start]sprouts[blank_end]; all very closely related but still considered to be the [blank_start]same[blank_end] species but look very different
Another example of [blank_start]artificial[blank_end] selection demonstrating [blank_start]evolution[blank_end]- Acabbage
- Bsprouts
- Csame
- Dartificial
- Eevolution
8Experimental Selection
- [blank_start]abdominal[blank_end] bristles in [blank_start]Drosophilia[blank_end] (fruit flies)
- original stock has [blank_start]30[blank_end]-50 bristles
- after 35 generations of selective breeding
- 2 distinct [blank_start]populations[blank_end] with 0-30 and 60-110 bristles
This is organic evolution in the lab
- demonstrates [blank_start]directional[blank_end] selection (in one direction) and [blank_start]divergent[blank_end]/disruptive (favours [blank_start]extreme[blank_end] phenotypes/middle phenotypes are not well adapted)- Aabdominal
- BDrosophilia
- C30
- Dpopulations
- Edirectional
- Fdivergent
- Gextreme
9Evidence for [blank_start]Darwin[blank_end] and Wallace's first observation: reproduction produces [blank_start]more[blank_end] offspring than can survive implying substantial [blank_start]mortality[blank_end]
- [blank_start]Annual[blank_end] plants produce two [blank_start]seeds[blank_end] per year which germinate into 2 plants etc. at the end of 20 years this would make 1 [blank_start]million[blank_end] plants if there was no mortality
common songbird and small [blank_start]rodent[blank_end] populations usually experience [blank_start]90[blank_end]%+ mortality within the [blank_start]first[blank_end] year and 50-75% thereafter- ADarwin
- Bmore
- Cmortality
- DAnnual
- Eseeds
- Fmillion
- Grodent
- H90
- Ifirst
10Evidence for Darwin and [blank_start]Wallace[blank_end]'s second observation: individuals within a [blank_start]species[blank_end] vary in their ability to [blank_start]survive[blank_end] and reproduce
Lifetime [blank_start]reproductive[blank_end] success is highly variable e.g. [blank_start]Red[blank_end] deer, Inner Hebrides
- in a sample size of 100s
Nearly [blank_start]half[blank_end] of both sexes produce no offspring! The remaining produce up to [blank_start]14[blank_end] for females and 24 for [blank_start]males[blank_end] (stag)
Sparrow hawks: females usually produce up to 8 fledgings per lifetime but some can produce more than [blank_start]20[blank_end] while 23% produce [blank_start]none[blank_end]!
in black/brown [blank_start]albatrosses[blank_end] (ancient mariner) mate for life in pairs: some pairs produce [blank_start]lots[blank_end] of offspring and others produce less, some produce only on [blank_start]good[blank_end] condition years and some never [blank_start]reproduce[blank_end]- AWallace
- Bspecies
- Csurvive
- Dreproductive
- ERed
- Fhalf
- G14
- Hmales
- I20
- Jnone
- Kalbatrosses
- Llots
- Mgood
- Nreproduce
11Evidence for Observation 3: offspring [blank_start]inherit[blank_end] characteristics from their parents that affect their ability to [blank_start]survive[blank_end] and reproduce
Characteristics affecting reproduction are heritable:
- Purple loosestrife plant
- number of [blank_start]seeds[blank_end] per capsule correlates between parents and [blank_start]offspring[blank_end]
[blank_start]Shell[blank_end] breadth of parents and offsprings in snails correlate and body size correlates with [blank_start]egg[blank_end] production
- [blank_start]larger[blank_end] snails produce more eggs = better reproductive success and fitness
BUT why do they not continue to evolve to bigger and bigger sizes?
- costly > face other limitations such as [blank_start]predation[blank_end] (easier to see if bigger), would need more food
other [blank_start]factors[blank_end] play a part- Ainherit
- Bsurvive
- Cseeds
- Doffspring
- EShell
- Fegg
- Glarger
- Hpredation
- Ifactors
12Diagram of Evolution
- orange frogs might act as camouflage > limited by factors in the [blank_start]environment[blank_end] e.g. food predators, space
- competition for resources > very important in evolution > leads to genetic variation in frogs which could lead to [blank_start]phenotypic[blank_end] variation in frogs
genetic variations could happen because of [blank_start]mutation[blank_end] in alleles which code for proteins or it may be that there is low background level of grey alleles in the population e.g. it may be recessive
if there is change in [blank_start]environmental[blank_end] conditions or they migrate then the other phenotype grey may be favoured- Aenvironment
- Bphenotypic
- Cmutation
- Denvironmental
13Selection for the Optimum
- [blank_start]human[blank_end] birth weight are distributed around the value associated with minimum neonatal mortality
- this is evidence for [blank_start]stabilising[blank_end] selection (not to low, not too high)
- why do big and small babies die more often?
In the middle there is an [blank_start]optimum[blank_end] birth weight with the least complications, therefore example of [blank_start]stabilising[blank_end] selection to remove the extremes (with more complications)- Ahuman
- Bstabilising
- Coptimum
- Dstabilising
14Optimal Reproductive Rate in Songbirds
- brood [blank_start]size[blank_end] has an effect of fledging [blank_start]weight[blank_end] because parents have to forage for food, more chicks = less food to go around
- as brood size [blank_start]increases[blank_end] the average weight of the fledglings [blank_start]decreases[blank_end]
- so there is an optimum number of offspring
- the proportion of young grey tits surviving after 3 months is greater for [blank_start]heavier[blank_end] fledglings > a 50% increase in size = [blank_start]300[blank_end]% increase in survivorship
The survival per brood (= reproductive success of parents) is greatest for [blank_start]intermediate[blank_end] brood sizes
- example of stabilising selection
But producing a large number of offsprings increases the chances of a [blank_start]higher[blank_end] number surviving
There is [blank_start]stabilising[blank_end] selection (removing extreme phenotypes) of reproductive rate because there is a trade off: producing more offspring so more survive vs. making offspring heavy enough to survive- Aweight
- Bsize
- Cincreases
- Ddecreases
- Eheavier
- F300
- Gintermediate
- Hhigher
- Istabilising
15Spatial Variation in Selection Pressure
anti [blank_start]predator[blank_end] defence
- three spine stickle backs in europe vary in the development of scutes and [blank_start]spines[blank_end] (variation in the amount of body [blank_start]armour[blank_end])
- some have more spikes than other > takes [blank_start]energy[blank_end] to produce spines and they are costly to carry out (drag = waste of [blank_start]energy[blank_end])
SO there is a [blank_start]trade[blank_end] off as to whether the energy expended and used having spines is worth the benefit they bring
number of predators are higher at mid latitudes, so there is more armour on stickle backs (UK)
at high latitudes the number of predators (Pike and perch) are reduced so number of spikes is [blank_start]reduced[blank_end]
this is an example of an [blank_start]evolutionary[blank_end] arms race
The saved energy from not having as many spikes can be put into [blank_start]egg[blank_end] production = more chance of genes being passed on- Apredator
- Bspines
- Carmour
- Denergy
- Eenergy
- Ftrade
- Greduced
- Hevolutionary
- Iegg
16Predation
- major selective force
- [blank_start]predator[blank_end]-prey co-evolution is an '[blank_start]arms[blank_end] race'
- prey evolve defence mechanisms and predators evolve ways to be more [blank_start]efficient[blank_end]
[blank_start]Strength[blank_end] of selection and rate of evolutionary arms race depends on the amount of [blank_start]specialisation[blank_end]
- more specialised = rate of evolution/arms race [blank_start]increases[blank_end]
snails vary in colour and banding pattern according the habitat they are in, this is called [blank_start]crypsis[blank_end] (camouflage) to avoid [blank_start]predation[blank_end] by thrushes
different morphs predominate in various habitats e.g. brown, unbanded shells beech woods- Apredator
- Barms
- Cefficient
- DStrength
- Especialisation
- Fincreases
- Gcrypsis
- Hpredation
17Some insects have evolved [blank_start]bright[blank_end] 'warning colouration' in association with accumulating poisons from their larval [blank_start]food[blank_end] plants
- this is effective in reducing [blank_start]predation[blank_end] by birds through facilitation of [blank_start]learning[blank_end] that toxicity is associated with a striking [blank_start]visual[blank_end] signal
this phenomenon is to be [blank_start]aposematic[blank_end] e.g. cinnabar [blank_start]moth[blank_end]
- Abright
- Bfood
- Cpredation
- Dlearning
- Evisual
- Faposematic
- Gmoth
18[blank_start]Convergent[blank_end] evolution = when different species look the same
- AConvergent
19[blank_start]Batesian[blank_end] mimicry = poisonous animals mimic a toxic model
[blank_start]mullerian[blank_end] mimicry = two poisonous species mimic one another to increase the rate of learning they are toxic- Amullerian
- BBatesian
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- Evolution Quiz on Natural Selection, created by sophie waddington on 19/04/2018.
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- sophie waddington
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