Environmental Influences Natural Selection An idea formulated by Darwin, resting on three principles: Natural Variation Reproductive Success Environmental Influences

Natural Variation: Individuals are not all alike Variation is either advantageous, disadvantageous, or neutral, relative to a given environmental factor Without an environmental context, you can’t establish the value of any variation.

Reproductive Success You are reproductively successful if: You survive to reproductive age You can attract a mate You produce viable offspring Your offspring produce offspring

Environmental Influences Survival depends on a variety of environmental factors: resources, climate, predation, competition… Any inherited trait that you have that can help you survive is advantageous That trait is ‘selected for’ Disadvantageous traits are ‘selected against’

Summary of Natural Selection: In any population of living organisms, individuals are born with variable traits Also, if resources are unlimited, populations will tend to grow exponentially. However, resources are limited; not all individuals can survive, and a struggle for resources ensues. Those individuals with variations that give them a survival advantage will live longer, reproduce more often. Their advantageous traits will spread through the population. Over time, this can lead to a change in the population and, over even longer time, the species.

Summary of Natural Selection: Individuals are Selected Populations Change Species Evolve

What ISN’T Natural Selection: “Getting used to” new environmental changes: the individual can only adjust so much (i.e. homeostatic responses). Change in response to Want or Need (volition): organisms can’t just ‘try harder’ These ideas were part of the flawed theory of the Comte de LaMarck – the idea of Acquired Characteristics through Use & Disuse.

LaMarck’s Theory: Giraffe Example Long ago, Giraffes were short necked They grazed & browsed, competing with other herbivores. As grasses and shrubs dwindled, they reached up for leaves. Stretching their necks, they acquired a new characteristic. The next generation was born with longer necks; they continued stretching, and the repetition of this led to today’s Giraffes.

LaMarck’s Theory: Giraffe Example

Natural Selection explains Giraffes Long ago, Giraffes may have been short necked, but some were born with longer necks. As grasses and shrubs dwindled, only the longer necked ones could reach up for leaves in higher tree branches. They survived & stayed healthy, and when they reproduced, the long necked trait was passed on to offspring. Over many generations, long necks became the standard trait for giraffes.

Natural Selection explains Giraffes Population of Giraffes with variable lengths of neck Longer necked individuals get more food, are healthier, survive longer, and have more offspring: over extremely long periods of time, all giraffes become long necked.

Natural Selection: Cheetahs How would we explain the evolution of Cheetahs by Natural Selection? Not all cheetahs run equally fast. The fastest runners got the prey, were better fed & healthier. They reproduced more often, passing this trait on to their offspring. With time, all cheetahs got the need for speed.

Natural Selection Summary In its most basic form, Natural Selection always works in the same manner: Variation  Survival  Reproduction Which variations are favoured depends on the environmental factors. There are some twists and variations, though, that complicate natural selection somewhat…

Sexual Selection Species with sexual dimorphism Males compete, females choose Mating is non-random

Sexual Selection

Species with strong social structures Kin Selection Species with strong social structures Jeopardize their own reproduction & exclusive fitness to help relatives survive Inclusive fitness Altruism

Artificial Selection Domestic Species Humans = Selective Agents Traits aren’t always advantageous to the organism

Artificial Selection: Pigeons

Artificial Selection: Dogs

Selection Patterns Directional, Stabilizing, or Disruptive Which trait gets selected determines the overall phenotype of a species; selective pressures change as the environment changes. What’s advantageous now may not be advantageous over time… if your environment changes, you may be out of luck, but your species will survive: some individuals will still have favourable traits.

Selection Patterns: The Case of the Peppered Moths 19th Century Industrial Revolution Britain: melanistic (dark) moths selected for; pale moths stood out & were hunted by predators 20th Century = steam power era ends, less soot on trees: pale moths selected for; dark moths were selected against.

Selection Patterns: Darwin’s Finches Galapagos Island finches developed a variety of beak shapes to exploit a variety of food sources (fruit, seeds, insect, cacti)

Selection Patterns: Darwin’s Finches