Evolution Darwin and Selection

I. Evolution

II. Darwin and Selection

A. Transitional Observations

1. ‘Artificial Selection’ and Domesticated Animals and Plants

Selection can create phenotypes beyond the initial range of expression.. There are no adult wolves as small as chihuahuas.



“In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population…” - The Autobiography of Charles Darwin 1809-1882 (Barlow 1958).



2. Reading Malthus

Thomas Malthus (1766-1834)

Essay On the Principle of Population (1798)



2. Reading Malthus

Thomas Malthus (1766-1834)

Essay On the Principle of Population (1798)

P1: All populations have the capacity to ‘over-reproduce’

P2: Resources are finite

C: There will be a “struggle for existence”… most offspring born will die before reaching reproductive age.



2. Reading Malthus

“In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work; but I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it. In June 1842 I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil in 35 pages; and this was enlarged during the summer of 1844 into one of 230 pages, which I had fairly copied out and still possess.” - The Autobiography of Charles Darwin 1809-1882 (Barlow 1958).



2. Reading Malthus

B. Natural Selection

P1: All populations have the capacity to ‘over-reproduce’

P2: Resources are finite

C: There will be a “struggle for existence”… most offspring born will die before reaching reproductive age.

P3: Organisms in a population vary, and some of this variation is heritable C2: As a result of this variation, some organisms will be more likely to survive and reproduce than others – there will be differential reproductive success.C3: The population change through time, as adaptive traits accumulate in the population.Corollary: Two populations, isolated in different environments, will diverge from one another as they adapt to their own environments. Eventually, these populations may become so different from one another that they are different species.

B. Natural Selection

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved". - The Origin of Species (Darwin 1859).

III. Natural SelectionA.The Nature of Selection - Types of Selection


Stabilizing selection for size at birth in humans


Directional selection for beak size


Disruptive selection for beak width in African finches feeding on two species of sedges with soft and hard seeds.

Can result in sympatric speciation – speciation within a reproducing population, in which different subpopulations become adapted to different niches in the same environment and produce poorly adapted hybrids when they breed with other subpopulations.

III. Natural SelectionA.The Nature of Selection - Modes of Selection

Natural SelectionSexual Selection: fitness of a genotype/trait depends on

organism’s sex


Natural SelectionSexual Selection: fitness of a genotype/trait depends on sexKin Selection: fitness depends on relatedness within group



Frequency-dependent Selection: fitness depends on genotype frequency

Papilio memnon females have many possible phenotypes that mimic toxic species. Abundant morphs are selected against; because bird predators learn they are tasty and prey on them. Negative frequency dependence.




Heliconius erato is a toxic species in Central America. Whichever morph becomes more abundant (even by chance) becomes EVEN MORE abundant because birds learn to avoid it more readily. Positive frequency dependence.




Female guppies prefer to mate with the rarest male phenotype; selects for polymorphism over time.



Frequency-dependent Selection: fitness depends on genotype frequencyArtificial Selection

III. Natural SelectionA.The Nature of SelectionB. Testing for Selection/Adaptation1. Phenotypic Plasticity

Why do populations differ?

1)Genetic adaptation2)Evolutionary diverge due to other agents of change3)Phenotypic plasticity – the direct effect of environment on phenotype

Daphnia grow spines and head-spines when developing in the presence of fish (predators)

Gastrimargus grasshoppers developing in different seasons (with predictable sustrate colors) develop into different colored nymphs and adults

III. Natural SelectionA.The Nature of SelectionB. Testing for Selection/Adaptation1.Phenotypic Plasticity2.Genetics, environment, or genetic adaptation to environment?

So, suppose we observe variation in plant size among genetically different plants growing in a field:

This variation in phenotype might be due to a combination of genetic and environmental differences between them.

V(phen) = V(env) + V(gen)


IF these plants were all grown under the same environmental conditions (‘common garden’ experiment), then there is no variation in the environment and the variation we observe can be attributed to genetic differences.

V(phen) = 0 + V(gen)


(whether these genetic differences are ADAPTIVE is another question…)

IF these plants were all grown under the same environmental conditions (‘common garden’ experiment), then there is no variation in the environment and the variation we observe can be attributed to genetic differences.

V(phen) = 0 + V(gen)

IIi. Natural SelectionA.The Nature of SelectionB. Testing for Selection/Adaptation1.Phenotypic Plasticity2.Genetics, environment, or genetic adaptation to environment?

(But this is only true for this environment!)

In a different environment, phenotypic and genetic variation may be expressed differently.

III. Natural SelectionB. Testing for Selection/Adaptation1.Phenotypic Plasticity2.Genetics, environment, or genetic adaptation to environment?

So, in a large population experiencing a range of environments:

V(phen) = V(env) + V(gen) + V(g*e)

V(g*e) is a genotype by environment interaction; reflecting the fact that genotypes may respond in different ways to changes in the environment.


Hei

ght

Environment

Suppose we had populations of each genotype, and these were the mean heights of these populations.

Stanford Mather

GenotypeCF


Hei

ght

Environment


Stanford Mather

GenotypeCF

XS

XM

V(env)


Hei

ght

Environment


Stanford Mather

GenotypeCF

XC

XF

V(gen)


Hei

ght

Environment


Stanford Mather

GenotypeCF

XCS XCM XFS XFM=The effect of environment IS THE SAME for the two genotypes:(g*e) = 0.


So, in this comparison:


Sig. Sig. ns


Hei

ght

Environment

Suppose we compare B and E.

Stanford Mather

GenotypeBE


Hei

ght

Environment

Environmental effects are significant

Stanford Mather

GenotypeBE

XS

XM

V(env)


Hei

ght

Environment

Genetic effects are insignificant; means don’t differ.

Stanford Mather

GenotypeBE

XC XE

V(gen) = 0


Hei

ght

Environment

There is a significant ‘G x E’ interaction.

Stanford Mather

GenotypeBE

XCS XCM XFS XFM>>The effect of environment IS NOT THE SAME for the two genotypes!!


In a different environment, phenotypic and genetic variation may be expressed differently.

So, in this comparison:


Sig. ns Sig.


We should expect adaptations to specific environmental conditions to be reflected by such “genotype by environment” interactions in characters that affect reproductive success.

III. Natural SelectionA.The Nature of SelectionB. Testing for Selection/Adaptation1.Phenotypic Plasticity2.Genetics, environment, or genetic adaptation to environment?3.Selection for Acclimation Ability

III. Natural SelectionA.The Nature of SelectionB. Testing for Selection/Adaptation1.Phenotypic Plasticity2.Genetics, environment, or genetic adaptation to environment?3.Selection for Acclimation Ability

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Evolution Darwin and Selection

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