Who wrote this (1858)? – “It occurred to me to ask the question, why do some die and some live. – And the answer was clearly, that on the whole the best fitted lived. From the effects of disease the most healthy escaped; from enemies, the strongest, the swiftest, or the most cunning; from famine, the best hunters or those with the best digestion; and so on. – Then I at once saw that the ever present variability of all living things would furnish the material from which, by the mere weeding out of those less adapted to the actual conditions, the fittest alone would continue the race. – There suddenly flashed upon me the idea of the survival of the fittest. – The more I thought over it, the more I became convinced that I had at length found the long-sought-for law of nature that solved the problem of the Origin of Species.”

Artificial selection

The premises 1. Phenotypic variation among a surplus of individuals. 2. Phenotypic variation has a genetic component 3. Differential reproductive success 4. Survival and reproduction is nonrandom 5. Phenotypic variation shifts between generations to reflect that of the reproducers.

What is selected?

Natural selection on mutations in the β-galactosidase gene of Escherichia coli in laboratory

Marine iguana body size and selection

Sexual Selection Differential reproductive success based on sexually dimorphic characters. Recognized by intersexual phenotypic differences that can’t be explained by natural selection Selection imposed on males (typically) by other males or by females of the same species. (1) can produce exaggerated secondary sexual characteristics (2) sexual dimorphism

In both NS and SS, the investigative challenge is determining how the selection improves fitness of individuals. If reproduction is sexual The investment is asymmetrical – A. energy expended in gamete production – B. energy expended in offspring; i.e.,parental investment – e.g., orangutans With exception of monogamous species, general pattern: – Males: low energy expenditure – Females: high energy expenditure

Different selection pressures on males and females. Males – fitness will be limited by number of mates. Females – fitness limited by – 1. s ufficient resources to put into reproduction – 2. Limits on reproduction imposed by care of offspring.

Two kinds of sexual selection: 1. Intrasexual selection With few exceptions, this is male-male competition for females Males control access to females 2. Intersexual selection With few exceptions, females choose mates based on variation in male secondary sexual characteristics or resources controlled by the male. Both require heritable, quantitative phenotypic variability.

Male-male combat Marine Iguana

Territory, body size, and mating success Largest male

The delimma * = max size at which body wt maintained. Largest males at a disadvantage

Sperm Competition 1. Number of sperm adjusted for environmental condition e.g., Mediterranean fruit flies Different physiological responses 1. Isolated males; isolated copulations 2. Male + male congener; male present during copulation Phenotypic plasticity?

Evolved structures enhancing fitness; e.g., damselflies

Intersexual selection Barn swallows

Tail signal modifications: Controls: 1. Cut and re-glue (N = 11) 2. Unaltered (N = 11) Treatments: 1. Shorten (N = 11) 2. Elongate (N = 11)

Hanging Fly: Bittacus sp.

A species of hanging fly

Assortative mating Gouldian finch Female mate choice Head-color match = healthier chicks Sex ratio 50:50 Shortage of compatible males Mismatched matings 70% chicks are male But – sex determined by female Therefore, she must be choosing what? Head-dying experiments Sex ratio 50:50