Evolution and Natural Selection

Inheritance: How Useful Traits Spread

If a trait helps an organism survive or reproduce - even slightly - that organism tends to leave more offspring. Those offspring inherit the helpful trait. Over many generations, even small advantages can have large effects.

Examples include:

• plants with deeper roots surviving droughts more reliably
• birds with better colour vision spotting food more easily
• bacteria with altered proteins resisting antibiotics

You might wonder whether tiny advantages matter. Imagine two versions of a gene: A and B. If individuals with A are only 1% more likely to leave surviving offspring, that small edge compounds. After about 100 generations, A could be much more common; after hundreds or thousands of generations, it might completely replace B. Evolution doesn't require dramatic leaps - just persistence and time.

Natural Selection: Nature's Filter

The peppered moth shows directional colour change driven by pollution: dark moths became common on soot-darkened trees, then declined again when the air became cleaner.

Natural selection is not conscious, purposeful, or trying to produce perfection. It is simply the consistent survival and reproduction of individuals whose traits work well in their environment.

• Camouflaged animals avoid predators more often.
• Plants with sturdier stems survive strong winds.
• Bacteria that tolerate heat survive where others die.

Mutations arise randomly, but selection is not random. It is a steady filter that keeps what works and reduces what doesn't.

Evolution Has No Final Goal

Evolution does not aim for "higher" or "more advanced." Bacteria are not "less evolved" than humans; they simply thrive in different ways. Sharks are not "primitive" - they are highly specialised for their niche. Many species look similar to ancient relatives because their environments haven't changed much, not because they failed to "progress."

Evolution is more like a branching tree than a ladder, with countless successful shapes, strategies, and lifestyles.

Big Changes Come From Many Small Steps

Complex organs never appeared fully formed. Each evolved through small, functional stages:

• Eyes began as simple light-sensitive cells that helped early organisms sense day and night.
• Wings began as small flaps that helped with gliding, balance, or body temperature control.
• Lungs began as air sacs in fish that gulped oxygen from stagnant water.
• Brains began as small clusters of nerve cells coordinating simple instincts and movements.

Every intermediate stage worked well enough for survival. Evolution builds step by step, never skipping ahead.

Evolution Is Not Pure Chance

It is common to hear "evolution is just random," but that's misleading.

• Mutations occur randomly.
• Selection is not random - it consistently favours traits that work.

The result is not chaos, but the gradual shaping of complex, functional structures through countless filtering steps.

Evolution Happens Over Vast Timescales

To understand evolution properly, you must appreciate its enormous timescale. The earliest life reproduced incredibly fast, producing immense numbers of generations long before animals existed. Over those millions of generations, simple cells evolved into more complex ones and, eventually, multicellular life.

Approximate generation counts:

• bacteria: ~1,000–50,000 generations per year (depending on species)
• simple single-celled eukaryotes: hundreds per year (estimated from modern examples)
• early animals: dozens per year
• mammals: one generation every 1–5 years
• humans: one generation every 20–30 years

Across Earth's history, that adds up to tens of trillions or even quadrillions of generations. Over such spans, even rare mutations and small advantages can transform life dramatically.

Key milestones:

• Possible earliest life: ~3.8 billion years ago
• Oldest confirmed microfossils: ~3.5 billion years ago
• First multicellular organisms: ~600–800 million years ago
• First animals with complex tissues: ~600 million years ago

A billion years is hard to imagine. Here are some intuitive comparisons:

• If 1 gram = 1 year, 3.5 billion years would weigh as much as around 875 elephants.
• If 1 glass of water = 1 year, the timeline would fill 350 Olympic swimming pools.
• If 1 step = 1 year, you would walk 2.6 million km - roughly six round trips to the Moon.

With time this vast, small evolutionary changes become mighty forces.

Speciation: How New Species Form

New species do not appear suddenly or from a single unusual birth. Instead, entire populations gradually split over thousands or millions of years.

• A population becomes divided (by geography, behaviour, climate, etc.).
• Each group accumulates different mutations and adaptations.
• Differences build up slowly.
• Eventually the groups can no longer interbreed.

No individual is ever the "first" of a new species. It's more like a colour gradient: red → orange → yellow. Adjacent generations are similar, but the ends become distinct.

Why Imperfect or "Stupid" Designs Persist

Not every feature in living organisms is elegant or efficient. Evolution does not design from scratch - it builds on whatever already exists. That means some structures are leftovers from our ancestors, shaped by history rather than by perfect engineering. Here are a few well-known examples.

• The appendix: In early primates, this was part of a much larger chamber for digesting tough, plant-heavy diets. Humans still retain a small version of it, and although it is no longer essential, it does contain immune tissue and may help maintain healthy gut bacteria. Removing it usually causes no harm, but keeping it is not harmful enough for evolution to eliminate the organ entirely.
• Wisdom teeth: Our ancestors had larger jaws and diets that caused more tooth wear, leaving space for a third set of molars. As human diets softened and jaw sizes shrank, there was less room. The teeth still grow because the underlying genetic programme hasn't been strongly selected against - before modern dentistry, many people survived long enough to pass on their genes even if the extra molars caused pain, infections, or complications later in life.
• The retina's "backwards wiring" and the blind spot: In vertebrate eyes, the nerves and blood vessels sit in front of the light-detecting cells. Because those nerves must pass through the retina to reach the brain, they create a small gap where no light is detected - this is the blind spot. Many people aren't aware they have one because the brain automatically fills in the missing information. This odd layout is a historical accident: early vertebrate eyes developed this way, and evolution could only refine what was already there. By contrast, octopuses evolved their eyes independently with the wiring behind the light-sensitive cells, so they have no blind spot at all.
• The recurrent laryngeal nerve: This nerve links the brain to the muscles of the voice box (larynx). In our fish ancestors, it took a short, direct route. As necks and throats evolved in early land vertebrates, the nerve became trapped behind a major artery near the heart. Because evolution cannot rebuild the entire layout, the nerve still follows this looping path: from the brain, down into the chest, under the aorta, and then back up to the larynx. In giraffes, this detour stretches more than four metres. It works, but it is a clear example of history shaping anatomy more than efficiency.

Evolution produces what works "well enough," not what an engineer would design from scratch.

Convergent Evolution: Nature Repeats Good Ideas

Different species often evolve similar traits because they face similar challenges. This shows that evolution is not random wandering - effective solutions tend to reappear.

• Eyes evolved independently in vertebrates, insects, and cephalopods.
• Wings evolved in insects, birds, bats, and pterosaurs.
• Streamlined bodies evolved in dolphins (mammals) and ichthyosaurs (reptiles).
• Venom evolved many times across snakes, lizards, fish, and even mammals.

Similar problems encourage similar evolutionary solutions.

Evolution of Behaviour and Instinct

Evolution shapes not only bodies but behaviour. Some behaviours are instinctive, some learned, and many are a mixture of both. The balance depends on what works best for survival.

Instincts: Behaviours You're Born With

Some behaviours are genuinely hard-wired. Animals perform them correctly even without practice or exposure to others. These instincts arise because, over many generations, natural selection favoured individuals whose nervous systems produced the correct behaviour automatically - especially when getting it wrong would be dangerous.

Examples include:

• spider web patterns
• sea turtle hatchlings crawling toward the sea
• the human suckling reflex
• chick pecking behaviour
• mouse freeze responses when sensing predators

These are not behaviours that are "just learned very quickly" - they are truly innate, produced by inherited neural and sensory circuits shaped by evolution.

The Baldwin Effect: How Learned Behaviours Become Easier Over Time

Sometimes a behaviour begins as something learned. Individuals who learn it faster do better, and genes that influence learning ability spread. Over many generations the species becomes better and better at learning the behaviour - so good, in fact, that it may look instinctive.

This is not the old idea of "inheriting learned skills," which was promoted by Jean-Baptiste Lamarck. Lamarck suggested that animals could pass on traits they acquired during their lifetimes (for example, a giraffe stretching its neck and producing longer-necked offspring). Modern biology shows this isn't how inheritance works. What evolution can pass on are genetic tendencies that make learning easier - not the learned knowledge itself.

Behaviours That Mix Instinct and Learning

Many behaviours blend inborn abilities with experience:

• Birdsong: birds are born with a rough template but refine it through learning.
• Human language: we have innate abilities for learning language, but rely on culture and environment to develop it.
• Migration: many species feel innate urges to migrate but learn specific routes.
• Tool use: curiosity and basic dexterity are innate; technique is learned.

Instinct gives reliability. Learning gives flexibility. Evolution tailors the balance to each species' needs, shaping behaviours that must be correct from birth while allowing others to develop through experience.

Evolution Is Ongoing – We See It Today

Evolution is not ancient history - it is happening constantly. Examples include:

• bacteria evolving antibiotic resistance
• viruses mutating into new strains
• animals shifting behaviour as climates change
• fish evolving tolerance to polluted waters
• insects developing resistance to pesticides

We live in a world where evolution is active every day.

Why Human Infants Are So Helpless

Compared with almost any other animal, human newborns are remarkably helpless. A foal can stand within hours; a chimpanzee infant can cling to its mother from birth. Humans, in contrast, take many months to walk, years to feed themselves properly, and almost two decades to reach full independence. This extreme dependency is not a flaw - it is the result of several powerful evolutionary pressures acting together.

1. The Obstetric Dilemma: Big Brains vs. a Narrow Birth Canal
Humans evolved exceptionally large brains. We also evolved upright walking, which reshaped the pelvis into a narrower, bowl-like structure. These two changes conflict with each other: a very large infant head cannot easily pass through a narrowed birth canal. Natural selection solved this by "shifting" birth earlier. Human babies are born while their brains are still small, soft, and relatively underdeveloped. This makes birth possible, but results in infants who are neurologically immature and highly dependent.

2. A "Fourth Trimester" Outside the Womb
Because humans must be born earlier than ideal, much of the brain development that occurs before birth in other mammals happens after birth in humans. This has led some researchers to describe the first 3–4 months of life as a "fourth trimester." During this period, infants cannot support their heads, regulate their posture, or coordinate movements well because the nervous system is still developing rapidly. Humans essentially complete part of gestation outside the womb.

3. Immense Brain Growth After Birth
A newborn chimpanzee's brain is roughly 40% of its adult size. A human newborn's is only about 25%. The remaining growth - massive by comparison - occurs in the first few years of life. This slow, extended development allows humans to form complex neural networks shaped by experience, culture, and social learning. The cost is long vulnerability, but the benefit is a brain capable of language, abstract reasoning, symbolic thought, and deep social understanding.

4. Learning Advantages: Flexibility Over Early Independence
Human infants are not born with many fixed instincts because natural selection favoured flexibility and learning. A rapidly changing social, ecological, and technological environment rewards brains that adapt rather than rely entirely on hard-wired behaviours. Long childhoods give humans time to explore, imitate, practise, acquire language, and absorb cultural knowledge - advantages that ultimately outweighed the short-term cost of extended dependency.

5. Cooperative Breeding: We Evolved to Raise Children Together
Humans also evolved in highly social groups where childcare was shared. Mothers, fathers, siblings, grandparents, and unrelated adults ("alloparents") all contributed to raising young. This cooperative system made it possible for babies to be born helpless because no single individual had to carry the entire burden of care. Many anthropologists argue that this social childcare helped drive the evolution of empathy, communication, bonding, and our unusually cooperative nature.

Overall, human helplessness at birth is an evolutionary trade-off: a narrower pelvis and a giant brain limit how developed a baby can be at birth, but being born early opens the door to extraordinary learning capacity. The result is a species whose young begin life very dependent, yet grow into the most cognitively flexible and culturally complex organisms on Earth.

Q&A: Common Questions and Misconceptions About Evolution